ABCMdb

ABCC7 p.Thr338Ala

ClinVar:	c.1012A>G , p.Thr338Ala ? , not provided c.1013C>T , p.Thr338Ile D , Pathogenic
CF databases:	c.1013C>T , p.Thr338Ile D , CF-causing ; CFTR1: A nucleotide change C->T at position 1145 which causes the replacement of a Threonine by Isoleucine residue in codon 338 of exon 7. c.1012A>G , p.Thr338Ala (CFTR1) ? , This mutation was identified in one Iranian CBAVD patient.
Predicted by SNAP2:	A: D (85%), C: D (91%), D: D (95%), E: D (95%), F: D (95%), G: D (95%), H: D (95%), I: D (53%), K: D (95%), L: D (95%), M: D (95%), N: D (91%), P: D (95%), Q: D (95%), R: D (95%), S: D (91%), V: D (85%), W: D (95%), Y: D (95%),
Predicted by PROVEAN:	A: N, C: D, D: D, E: D, F: D, G: D, H: D, I: D, K: D, L: D, M: D, N: N, P: N, Q: D, R: D, S: N, V: N, W: D, Y: D,

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[hide] Insight in eukaryotic ABC transporter function by ... FEBS Lett. 2006 Feb 13;580(4):1064-84. Epub 2006 Jan 19. Frelet A, Klein M
Insight in eukaryotic ABC transporter function by mutation analysis.
FEBS Lett. 2006 Feb 13;580(4):1064-84. Epub 2006 Jan 19., 2006-02-13 [PMID:16442101]

Abstract [show]
With regard to structure-function relations of ATP-binding cassette (ABC) transporters several intriguing questions are in the spotlight of active research: Why do functional ABC transporters possess two ATP binding and hydrolysis domains together with two ABC signatures and to what extent are the individual nucleotide-binding domains independent or interacting? Where is the substrate-binding site and how is ATP hydrolysis functionally coupled to the transport process itself? Although much progress has been made in the elucidation of the three-dimensional structures of ABC transporters in the last years by several crystallographic studies including novel models for the nucleotide hydrolysis and translocation catalysis, site-directed mutagenesis as well as the identification of natural mutations is still a major tool to evaluate effects of individual amino acids on the overall function of ABC transporters. Apart from alterations in characteristic sequence such as Walker A, Walker B and the ABC signature other parts of ABC proteins were subject to detailed mutagenesis studies including the substrate-binding site or the regulatory domain of CFTR. In this review, we will give a detailed overview of the mutation analysis reported for selected ABC transporters of the ABCB and ABCC subfamilies, namely HsCFTR/ABCC7, HsSUR/ABCC8,9, HsMRP1/ABCC1, HsMRP2/ABCC2, ScYCF1 and P-glycoprotein (Pgp)/MDR1/ABCB1 and their effects on the function of each protein.

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405 Alanine substitutions of these residues has been shown to strongly affect conductance, which is greatly reduced in F337A [190] and S341A [46] and significantly increased in T338A [187]. Login to comment

[hide] Relationship between anion binding and anion perme... J Physiol. 2001 Feb 15;531(Pt 1):51-66. Linsdell P
Relationship between anion binding and anion permeability revealed by mutagenesis within the cystic fibrosis transmembrane conductance regulator chloride channel pore.
J Physiol. 2001 Feb 15;531(Pt 1):51-66., 2001-02-15 [PMID:11179391]

Abstract [show]
1. Anion binding within the pores of wild-type and mutant cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channels, expressed in two different mammalian cell lines, was assayed using patch clamp recording. Specifically, experiments measured both the conductance of different anions and the ability of other permeant anions to block Cl- permeation through the pore. 2. Under symmetrical ionic conditions, wild-type CFTR channels showed the conductance sequence Cl- > NO3- > Br- > or = formate > F- > SCN- congruent to ClO4-. 3. High SCN- conductance was not observed, nor was there an anomalous mole fraction effect of SCN- on conductance under the conditions used. Iodide currents could not be measured under symmetrical ionic conditions, but under bi-ionic conditions I- conductance appeared low. 4. Chloride currents through CFTR channels were blocked by low concentrations (10 mM) of SCN-, I- and ClO4-, implying relatively tight binding of these anions within the pore. 5. Two mutations in CFTR which alter the anion permeability sequence, F337S and T338A, also altered the anion conductance sequence. Furthermore, block by SCN-, I- and ClO4- were weakened in both mutants. Both these effects are consistent with altered anion binding within the pore. 6. The effects of mutations on anion permeability and relative anion conductance suggested that, for most anions, increased permeability was associated with increased conductance. This indicates that the CFTR channel pore does not achieve its anion selectivity by selective anion binding within the mutated region. Instead, it is suggested that entry of anions into the region around F337 and T338 facilitates their passage through the pore. In wild-type CFTR channels, anion entry into this crucial pore region is probably dominated by anion hydration energies.

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19 Two mutations in CFTR which alter the anion permeability sequence, F337S and T338A, also altered the anion conductance sequence. Login to comment
38 The present study seeks to shed new light on the relationship between anion binding and anion permeability in CFTR channels by comparing the anion binding properties of wild-type CFTR with two mutants with altered anion selectivity, F337S and T338A. Login to comment
42 The CFTR mutants F337A, L, S, W, Y and T338A were constructed and transfected into CHO and BHK cells by Alexandra Evagelidis and Shu-Xian Zheng in the laboratory of Dr John Hanrahan (McGill University, Montreal, Quebec, Canada), as described previously (Linsdell et al. 1998, 2000). Login to comment
43 In the present study, the permeation properties of two mutants, F337S and T338A, have been examined in detail. Login to comment
48 In contrast to mutations at F337, all mutations previously examined at T338 (T338A, N, S, V) significantly altered anion selectivity and Cl¦ conductance (Linsdell et al. 1998). Login to comment
49 For the present study, T338A was examined because: (1) it has a high single channel conductance, allowing single channel currents to be resolved, (2) its anion selectivity strongly follows the lyotropic sequence, in fact more strongly than that of wild-type CFTR, such that its effects on anion permeability might be considered 'opposite` to the effects of F337S, (3) replacing the threonine with a small, 'neutral` alanine is considered less likely to cause large changes in transmembrane helix structure, and (4) T338A is well expressed in both CHO and BHK cells (see Linsdell et al. 1998, for a full description of the permeation phenotype of T338A). Login to comment
83 Unitary properties of F337S and T338A CFTR A, single channel currents carried by wild-type, F337S and T338A, expressed in CHO cells, with symmetrical 150 mÒ NaCl, at a membrane potential of -50 mV. Login to comment
85 B, mean i-V relationships under these ionic conditions, wild-type 0, F337S 1 and T338A ±; mean of data from 3-9 patches. Login to comment
93 Estimation of unitary current amplitude from macroscopic current variance A, activation of macroscopic wild-type, F337S and T338A CFTR currents in BHK cell patches at +50 mV, in the symmetrical presence of 150 mÒ NaCl, by addition of PKA in the presence of ATP (see Methods). Login to comment
95 All three have been fitted by eqn (1) (see Methods), giving i = 0·118 pA and N = 374 for wild-type, i = 0·0387 pA and N = 1359 for F337S and i = 0·279 pA and N = 175 for T338A. Login to comment
105 Unitary Cl¦ currents carried by wild-type, F337S and T338A CFTR are compared in Fig. 3A. Mean slope conductance (Fig. 3B) was reduced in F337S (from 7·59 ± 0·10 pS (n = 12) to 1·76 ± 0·03 pS (n = 7)), and significantly increased in T338A (to 9·94 ± 0·14 pS, n = 6). Login to comment
112 Macroscopic current variance analysis of relative anion conductance in wild-type, F337S and T338A Macroscopic currents were activated in BHK cell patches at +50 mV, in the symmetrical presence of the anion named on the far left, by addition of PKA in the presence of ATP (see Fig. 4). Login to comment
117 Relative anion permeabilities and conductances for wild-type and mutant CFTR ------------------------------------------------------------ PXÏPCl gXÏgCl ------------------ --------------------------- WT a F337S a T338A b WT c WT d F337S d T338A d ------------------------------------------------------------ Cl 1·00 ± 0·01 1·00 ± 0·08 1·00 ± 0·02 1·00 ± 0·01 1·00 ± 0·10 1·00 ± 0·16 1·00 ± 0·09 (10) (3) (11) (12) (9) (7) (4) Br 1·37 ± 0·07 0·50 ± 0·04 1·74 ± 0·04 0·48 ± 0·01 0·48 ± 0·13 0·16 ± 0·03* 0·44 ± 0·02 (8) (4) (3) (6) (4) (4) (3) I 0·83 ± 0·03 0·23 ± 0·02 2·09 ± 0·16 - - - - (6) (4) (5) F 0·10 ± 0·01 0·43 ± 0·02 0·12 ± 0·02 - 0·094 ± 0·017 0·76 ± 0·19* 0·054 ± 0·011 (9) (4) (4) (3) (4) (3) SCN 3·55 ± 0·26 0·93 ± 0·10 5·85 ± 0·27 - 0·060 ± 0·012 0·17 ± 0·04* 0·085 ± 0·007 (7) (5) (4) (5) (3) (4) NO× 1·58 ± 0·04 1·08 ± 0·02 2·04 ± 0·08 0·60 ± 0·02 0·73 ± 0·07 0·29 ± 0·08* 0·96 ± 0·05 (10) (4) (3) (5) (5) (3) (3) ClOÚ 0·25 ± 0·01 0·19 ± 0·00 1·35 ± 0·08 - 0·059 ± 0·014 0·041 ± 0·008 0·082 ± 0·011 (8) (3) (3) (6) (2) (4) Formate 0·24 ± 0·01 0·27 ± 0·02 0·45 ± 0·04 0·35 ± 0·01 0·49 ± 0·01 0·17 ± 0·02** 0·46 ± 0·07 (9) (3) (3) (6) (5) (3) (3) ------------------------------------------------------------a From Linsdell et al. (2000); b from Linsdell et al. (1998); c by single channel recording, d by current variance analysis. Login to comment
124 Macroscopic current variance analysis was also used to compare the relative conductances of different anions in wild-type, F337S and T338A (Fig. 6; Table 1). Login to comment
127 Relative conductances in wild-type estimated by these two methods (Fig. 7), as well as those in F337S and T338A estimated from macroscopic current variance analysis, are summarised in Table 1. Login to comment
129 Although smaller differences are apparent for T338A, in no case were these significantly different from wild-type (Table 1). Login to comment
130 The conductance sequences for wild-type, F337S and T338A are summarised in Table 2. Login to comment
143 Permeability and conductance sequences for wild-type and mutant CFTR ------------------------------------------------------------ Wild-Type F337S T338A ------------------------------------------------------------ Permeability sequence SCN > NO× > Br > Cl > NO× > Cl ü SCN > Br > SCN > I ü NO× > Br > I > ClOÚ formate > F F > formate > I > ClOÚ ClOÚ > Cl > formate > F Conductance sequence Cl > NO× > Br ü formate > Cl > F > NO× > SCN Cl ü NO× > formate Br > F > SCN ClOÚ formate Br > ClOÚ SCN ClOÚ > F ------------------------------------------------------------ Permeability and conductance sequences are derived from data given in Table 1. Login to comment
159 The ability of permeant anions to block Cl¦ currents was also examined in selectivity altering mutants, using macroscopic current variance experiments (for the low-conductance F337S) or single channel recording (for the high-conductance T338A), at -50 mV (Fig. 11). Login to comment
162 T338A Cl¦ currents were only weakly blocked by SCN¦ and ClOÚ¦, and not significantly blocked by other permeant anions including I¦ (Fig. 11C). Login to comment
164 T338A Cl¦ currents were significantly increased in the presence of 25 mÒ intracellular NO×¦, suggesting that the high permeability and conductance of NO×¦ in this mutant (Table 1) allowed some degree of summation of Cl¦ and NO×¦ currents through the channel. Login to comment
165 The relationship between anion permeability and anion conductance The results summarised in Tables 1 and 2 indicate that the CFTR mutation F337S, which virtually abolishes the lyotropic pattern of anion permeability (Linsdell et al. 2000), also alters the relative conductance of different anions in the pore, whereas T338A, which strengthens the lyotropic nature of anion permeability (Linsdell et al. 1998), has no significant effect on relative conductance. Login to comment
171 Anion permeability in wild-type and T338A CFTR follows an approximate lyotropic sequence, with relative anion permeability being correlated with energy of hydration (Linsdell & Hanrahan, 1998; Linsdell et al. 1998; Fig. 12A). Login to comment
172 In contrast, there is no obvious relationship between relative anion conductance and energy of hydration in either wild-type or T338A (Fig. 12B). Login to comment
175 The relationship between anion permeability and energy of hydration observed in wild-type and T338A is lost in F337S (Fig. 12A), which we previously suggested reflected a reduction in the relative importance of anion dehydration in determining anion P. Linsdell J. Physiol. 531.160 Figure 11. Login to comment
176 Block of wild-type, F337S and T338A CFTR by intracellular permeant anions Relative current amplitudes (at -50 mV) in the presence of different intracellular permeant anions were estimated from single channel recording (for wild-type, see Fig. 10, and also for T338A), or from macroscopic current variance analysis for F337S. Login to comment
184 Interestingly, Cl¦ remains the anion with the highest conductance in this mutant, and the difference in conductance between Cl¦ and anions with similar energies of hydration (Br¦ and NO×¦) is greater than for wild-type or T338A (Fig. 12B). Login to comment
186 This applies both to lyotropic anions with increased permeability in T338A (Br¦, NO×¦, ClOÚ¦) and the kosmotropic anion F¦, which shows increased permeability in F337S. Login to comment
189 The conductance ratio, gIÏgCl, was 0·20 ± 0·03 (n = 4) for wild-type, 0·14 ± 0·05 (n = 4) for F337S, and 0·59 ± 0·09 (n = 4) for T338A. Login to comment
197 Dependence of relative anion permeability (A) and relative anion conductance (B) on anion-free energy of hydration in wild-type, F337S and T338A CFTR Values of PXÏPCl and gXÏgCl in each case are as given in Table 1. Login to comment
218 Macroscopic I-V relationships under bi-ionic conditions (intracellular I¦, extracellular Cl¦), for wild-type, F337S and T338A CFTR expressed in BHK cells The different degrees of outward rectification suggest different relative I¦ conductances in these three CFTR variants. Login to comment
242 In the present study, the anion binding properties of two mutants, F337S and T338A, were examined in detail. Login to comment
245 In contrast, T338A did not significantly affect the relative conductance of different anions (Table 1), although some small changes in the conductance sequence were apparent (Table 2). Login to comment
249 Block by SCN¦, I¦ and ClOÚ¦ was significantly weakened in both F337S and T338A, and F337S showed significant block by high concentrations of Br¦ not evident in wild-type (Fig. 11). Login to comment
250 These results suggest that lyotropic anion binding is weakened in both F337S and T338A, in spite of the 'opposite` effect of these two mutations on anion selectivity. Login to comment
256 Implications for the mechanism of anion selectivity The mutations F337S and T338A caused co-ordinated changes in the relative permeability and relative conductance of Br¦, F¦, NO×¦, ClOÚ¦ and I¦ ions, with high permeability being associated with high conductance (Table 1; Fig. 13). Login to comment
261 Thus, not only in wild-type CFTR but also both F337S and T338A, Cl¦ is the anion with the highest conductance (Table 1), even though other anions may have higher permeabilities. Login to comment
264 Optimization of Cl¦ conductance, rather than Cl¦ permeability, is likely to be of greater importance to the physiological function of CFTR and other Cl¦ channels, although in CFTR the mutations T338A and T338S increase Cl¦ conductance, apparently without adversely affecting other channel properties (Linsdell et al. 1998). Login to comment

[hide] Asymmetric structure of the cystic fibrosis transm... Biochemistry. 2001 Jun 5;40(22):6620-7. Gupta J, Evagelidis A, Hanrahan JW, Linsdell P
Asymmetric structure of the cystic fibrosis transmembrane conductance regulator chloride channel pore suggested by mutagenesis of the twelfth transmembrane region.
Biochemistry. 2001 Jun 5;40(22):6620-7., 2001-06-05 [PMID:11380256]

Abstract [show]
The cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel contains 12 membrane-spanning regions which are presumed to form the transmembrane pore. Although a number of findings have suggested that the sixth transmembrane region plays a key role in forming the pore and determining its functional properties, the role of other transmembrane regions is currently not well established. Here we assess the functional importance of the twelfth transmembrane region, which occupies a homologous position in the carboxy terminal half of the CFTR molecule to that of the sixth transmembrane region in the amino terminal half. Five residues in potentially important regions of the twelfth transmembrane region were mutated individually to alanines, and the function of the mutant channels was examined using patch clamp recording following expression in mammalian cell lines. Three of the five mutations significantly weakened block of unitary Cl(-) currents by SCN(-), implying a partial disruption of anion binding within the pore. Two of these mutations also caused a large reduction in the steady-state channel mean open probability, suggesting a role for the twelfth transmembrane region in channel gating. However, in direct contrast to analogous mutations in the sixth transmembrane region, all mutants studied here had negligible effects on the anion selectivity and unitary Cl(-) conductance of the channel. The relatively minor effects of these five mutations on channel permeation properties suggests that, despite their symmetrical positions within the CFTR protein, the sixth and twelfth transmembrane regions make highly asymmetric contributions to the functional properties of the pore.

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No. Sentence Comment
126 Although no information on the voltage dependence of SCN- block is obtained in this way, we have previously used this same protocol to show directly that intrapore anion binding is altered in the TM6 mutants F337S and T338A (21). Login to comment
156 Alanine substitution for these three TM6 residues has been shown to strongly affect conductance, which is greatly reduced in F337A (21) and S341A (13), and significantly increased in T338A (16). Login to comment
164 However, as summarized in Table 1, TM12 mutations M1137A and N1138A did not alter the anion selectivity sequence, in stark contrast to the corresponding TM6 mutations F337A (20) and T338A (16). Login to comment
177 Similar weakening of SCN- block was previously observed in the TM6 mutants F337S and T338A (21). Login to comment

[hide] Thiocyanate as a probe of the cystic fibrosis tran... Can J Physiol Pharmacol. 2001 Jul;79(7):573-9. Linsdell P
Thiocyanate as a probe of the cystic fibrosis transmembrane conductance regulator chloride channel pore.
Can J Physiol Pharmacol. 2001 Jul;79(7):573-9., [PMID:11478590]

Abstract [show]
Immediately following exposure to thiocyanate (SCN-)-containing solutions, the cystic fibrosis conductance regulator Cl- channel exhibits high unitary SCN conductance and anomalous mole fraction behaviour, suggesting the presence of multiple anion binding sites within the channel pore. However, under steady-state conditions SCN-conductance is very low. Here I show, using patch clamp recording from CFTR-transfected mammalian cell lines, that under steady-state conditions neither SCN- conductance nor SCN- permeability show anomalous mole fraction behaviour. Instead, SCN conductance, permeability, and block of Cl- permeation can all be reproduced by a rate theory model that assumes only a single intrapore anion binding site. These results suggest that under steady-state conditions the interaction between SCN- and the CFTR channel pore can be understood by a simple model whereby SCN- ions enter the pore more easily than Cl-, and bind within the pore more tightly than Cl-. The implications of these findings for investigating and understanding the mechanism of anion permeation are discussed.

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131 In spite of this caveat, it is interesting to note that the effects of mutations within the pore that drastically alter SCN- permeability but have only small effects on SCN- binding (F337S, T338A; Linsdell 2001) may result in such a model from alterations in barrier height, implying that the effects of these mutations may result primarily from changes in anion access to the pore. Login to comment

[hide] Identification of a region of strong discriminatio... Am J Physiol Lung Cell Mol Physiol. 2001 Oct;281(4):L852-67. McCarty NA, Zhang ZR
Identification of a region of strong discrimination in the pore of CFTR.
Am J Physiol Lung Cell Mol Physiol. 2001 Oct;281(4):L852-67., [PMID:11557589]

Abstract [show]
The variety of methods used to identify the structural determinants of anion selectivity in the cystic fibrosis transmembrane conductance regulator Cl(-) channel has made it difficult to assemble the data into a coherent framework that describes the three-dimensional structure of the pore. Here, we compare the relative importance of sites previously studied and identify new sites that contribute strongly to anion selectivity. We studied Cl(-) and substitute anions in oocytes expressing wild-type cystic fibrosis transmembrane conductance regulator or 12-pore-domain mutants to determine relative permeability and relative conductance for 9 monovalent anions and 1 divalent anion. The data indicate that the region of strong discrimination resides between T338 and S341 in transmembrane 6, where mutations affected selectivity between Cl(-) and both large and small anions. Mutations further toward the extracellular end of the pore only strongly affected selectivity between Cl(-) and larger anions. Only mutations at S341 affected selectivity between monovalent and divalent anions. The data are consistent with a narrowing of the pore between the extracellular end and a constriction near the middle of the pore.

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No. Sentence Comment
60 Mutants K335E, K335F, T338A, T339A, S341A, S341T, T1134A, and T1134F were prepared as previously described (33). Login to comment
143 Relative permeabilities for WT and mutant CFTRs for monovalent anions CFTR n NO3 Br SCN I ClO4 Acetate Isethionate Glutamate Gluconate WT 16 1.35Ϯ0.01 1.19Ϯ0.02 2.42Ϯ0.06 0.36Ϯ0.01 0.10Ϯ0.01 0.15Ϯ0.00* 0.24Ϯ0.01 0.24Ϯ0.01 0.18Ϯ0.01 K335A 5 1.35Ϯ0.01 1.36Ϯ0.03 3.10Ϯ0.11† 0.75Ϯ0.02† 0.12Ϯ0.01 0.06Ϯ0.01† 0.07Ϯ0.01† 0.07Ϯ0.01† 0.08Ϯ0.01† K335F 7 1.51Ϯ0.03† 1.36Ϯ0.02† 2.73Ϯ0.14 0.99Ϯ0.03† 0.20Ϯ0.02† 0.13Ϯ0.01 0.18Ϯ0.03 0.30Ϯ0.02 0.20Ϯ0.02 K335E 5 1.24Ϯ0.04 1.17Ϯ0.02 2.60Ϯ0.06 1.10Ϯ0.03† 0.23Ϯ0.01† 0.10Ϯ0.01† 0.11Ϯ0.01† 0.10Ϯ0.01† 0.11Ϯ0.01† T338A 5 1.74Ϯ0.07† 1.59Ϯ0.02† 4.35Ϯ0.24† 2.56Ϯ0.13† 1.84Ϯ0.08† 0.07Ϯ0.01† 0.06Ϯ0.01† 0.08Ϯ0.01† 0.08Ϯ0.01† T338E 3 3.65Ϯ0.19† 1.94Ϯ0.04† 4.29Ϯ0.13† 2.41Ϯ0.24† 1.18Ϯ0.06† 0.16Ϯ0.03 0.37Ϯ0.05† 0.36Ϯ0.01† 0.22Ϯ0.03 T339A 5 1.47Ϯ0.01 1.29Ϯ0.03 2.65Ϯ0.06 0.57Ϯ0.02† 0.24Ϯ0.04 0.10Ϯ0.02 0.19Ϯ0.02 0.18Ϯ0.01 0.15Ϯ0.01 S341A 6 1.91Ϯ0.02† 1.42Ϯ0.01† 3.10Ϯ0.09† 0.59Ϯ0.00*† 0.09Ϯ0.00* 0.11Ϯ0.01† 0.12Ϯ0.00*† 0.11Ϯ0.00*† 0.12Ϯ0.00*† S341E 12 2.01Ϯ0.10† 1.46Ϯ0.05† 2.81Ϯ0.18 0.84Ϯ0.00*† 0.31Ϯ0.03† 0.20Ϯ0.01 0.23Ϯ0.02 0.19Ϯ0.01 0.19Ϯ0.02 S341T 5 1.81Ϯ0.05† 1.39Ϯ0.03 3.15Ϯ0.15† 0.41Ϯ0.01 0.07Ϯ0.00* 0.05Ϯ0.00*† 0.06Ϯ0.00*† 0.03Ϯ0.01† 0.06Ϯ0.01† T1134A 6 1.43Ϯ0.02 1.30Ϯ0.02 2.66Ϯ0.02 0.46Ϯ0.00*† 0.06Ϯ0.00*† 0.08Ϯ0.01† 0.10Ϯ0.01† 0.11Ϯ0.01† 0.10Ϯ0.00*† T1134F 5 1.31Ϯ0.07 1.17Ϯ0.05 2.50Ϯ0.10 0.63Ϯ0.01† 0.08Ϯ0.00* 0.13Ϯ0.01 0.09Ϯ0.01† 0.18Ϯ0.02 0.13Ϯ0.01 T1134E 4 1.68Ϯ0.02† 1.39Ϯ0.05† 2.37Ϯ0.18 0.19Ϯ0.03† 0.20Ϯ0.03 0.06Ϯ0.01† 0.09Ϯ0.01† 0.08Ϯ0.01† 0.10Ϯ0.01† Values are means Ϯ SE with only data from the hyperpolarizing ramp protocol; n, no. of oocytes. Relative permeability, permeability of anion x to that of Cl. Anions are listed in order of increasing ionic radius. Login to comment
167 Selectivity sequences for WT and mutant CFTRs CFTR Selectivity Sequence by Relative Permeability WT SCNϾϾNO3 ϾBrϾClϾϾIϾisethionateϭglutamateϾgluconateϭacetateϾClO4 K335A SCNϾϾBrϭNO3 ϾClϾIϾϾClO4 Ͼgluconateϭisethionateϭglutamateϭacetate K335F SCNϾϾNO3 ϾBrϾClϭIϾϾglutamateϾgluconateϭClO4 ϭisethionateϾacetate K335E SCNϾϾNO3 ϾBrϭIϾClϾϾClO4 Ͼgluconateϭisethionateϭglutamateϭacetate T338A SCNϾϾIϾϾClO4 ϭNO3 ϾBrϾClϾϾgluconateϭisethionateϭglutamateϭacetate T338E SCNϾNO3 ϾIϾBrϾClO4 ϾClϾϾisethionateϭglutamateϾgluconateϭacetate T339A SCNϾϾNO3 ϾBrϾClϾϾIϾϾClO4 ϭisethionateϭglutamateϭgluconateϾacetate S341A SCNϾNO3 ϾBrϾClϾϾIϾϾgluconateϭisethionateϭglutamateϭacetateϭClO4 S341E SCNϾNO3 ϾBrϾClϾIϾϾClO4 Ͼisethionateϭacetateϭglutamateϭgluconate S341T SCNϾϾNO3 ϾBrϾClϾϾIϾϾClO4 ϭisethionateϭgluconateϭacetateϭglutamate T1134A SCNϾϾNO3 ϾBrϾClϾϾIϾϾglutamateϭisethionateϭgluconateϭacetateϭClO4 T1134F SCNϾϾNO3 ϾBrϾClϾϾIϾϾglutamateϾacetateϭgluconateϾisethionateϭClO4 T1134E SCNϾNO3 ϾBrϾClϾϾClO4 ϭIϾgluconateϭisethionateϭglutamateϭacetate L856 A REGION OF STRONG DISCRIMINATION IN THE CFTR PORE AJP-Lung Cell Mol Physiol • VOL 281 • OCTOBER 2001 • www.ajplung.org out propagation to distant sites. Login to comment
191 Relative conductances for WT and mutant CFTRs for monovalent anions CFTR n NO3 Br SCN I ClO4 Acetate Isethionate Glutamate Gluconate WT 16 0.87Ϯ0.01 0.77Ϯ0.01 0.18Ϯ0.01 0.25Ϯ0.01 0.23Ϯ0.01 0.55Ϯ0.01 0.50Ϯ0.01 0.57Ϯ0.02 0.56Ϯ0.02 K335A 5 0.88Ϯ0.04 0.77Ϯ0.02 0.30Ϯ0.02† 0.35Ϯ0.02 0.24Ϯ0.02 0.33Ϯ0.01† 0.32Ϯ0.02† 0.37Ϯ0.02† 0.38Ϯ0.02† K335F 7 1.21Ϯ0.05† 0.87Ϯ0.02† 0.55Ϯ0.02† 0.36Ϯ0.01† 0.19Ϯ0.01 0.34Ϯ0.01† 0.34Ϯ0.01† 0.41Ϯ0.01† 0.37Ϯ0.01† K335E 5 1.16Ϯ0.05† 0.91Ϯ0.02† 0.59Ϯ0.02† 0.51Ϯ0.02† 0.28Ϯ0.01 0.22Ϯ0.01† 0.25Ϯ0.01† 0.22Ϯ0.01† 0.24Ϯ0.01† T338A 5 1.20Ϯ0.13† 1.03Ϯ0.06† 0.98Ϯ0.12† 0.82Ϯ0.02† 0.50Ϯ0.04† 0.18Ϯ0.05† 0.08Ϯ0.01† 0.31Ϯ0.05† 0.29Ϯ0.05† T338E 3 3.66Ϯ0.36† 1.53Ϯ0.09† 1.80Ϯ0.12† 1.39Ϯ0.11† 0.87Ϯ0.03† 0.36Ϯ0.04† 0.56Ϯ0.17 0.44Ϯ0.03† 0.48Ϯ0.03† T339A 5 1.01Ϯ0.02† 0.77Ϯ0.03 0.22Ϯ0.01 0.31Ϯ0.03 0.23Ϯ0.01 0.38Ϯ0.02† 0.48Ϯ0.01 0.48Ϯ0.01 0.52Ϯ0.01 S341A 6 1.67Ϯ0.01† 1.08Ϯ0.01† 0.63Ϯ0.03† 0.26Ϯ0.00* 0.15Ϯ0.01† 0.63Ϯ0.01† 0.54Ϯ0.02 0.63Ϯ0.01 0.63Ϯ0.01 S341E 12 1.74Ϯ0.11† 1.14Ϯ0.02† 1.81Ϯ0.06† 0.48Ϯ0.01† 0.35Ϯ0.02† 0.28Ϯ0.01† 0.69Ϯ0.02† 0.65Ϯ0.01† 0.68Ϯ0.01† S341T 5 0.85Ϯ0.02 0.82Ϯ0.01 0.29Ϯ0.01† 0.22Ϯ0.01 0.13Ϯ0.01† 0.48Ϯ0.01 0.45Ϯ0.02 0.43Ϯ0.02 0.55Ϯ0.01 T1134A 6 0.83Ϯ0.02 0.78Ϯ0.01 0.24Ϯ0.01† 0.21Ϯ0.01 0.09Ϯ0.01† 0.39Ϯ0.01† 0.38Ϯ0.01† 0.39Ϯ0.01† 0.40Ϯ0.01 T1134F 5 0.68Ϯ0.03† 0.69Ϯ0.03† 0.36Ϯ0.01† 0.07Ϯ0.01† 0.16Ϯ0.01 0.48Ϯ0.02 0.30Ϯ0.02† 0.22Ϯ0.01† 0.32Ϯ0.02† T1134E 4 0.99Ϯ0.02† 1.00Ϯ0.02† 0.50Ϯ0.02† 0.20Ϯ0.03 0.26Ϯ0.02 0.32Ϯ0.03† 0.34Ϯ0.01† 0.34Ϯ0.03† 0.34Ϯ0.03† Values are means Ϯ SE with only data from the hyperpolarizing ramp protocol; n, no. of oocytes. Relative conductance, conductance of anion x to that of Cl. Anions are listed in order of increasing ionic radius. Login to comment
197 The shape of the I-V curve between -80 and ϩ60 mV was not affected by the K335A, T338A, T339A, or T1134A mutations, whereas S341A CFTR exhibited less outward rectification than WT CFTR. Login to comment
213 Vrev Cl in ND96 bath solution for WT and mutant CFTRs CFTR n Vrev Cl WT 16 -21.24Ϯ0.59 K335A 5 -22.12Ϯ0.35 K335F 7 -21.92Ϯ0.90 K335E 5 -22.88Ϯ0.36 T338A 5 -26.97Ϯ0.79* T338E 3 -20.58Ϯ1.07 T339A 5 -22.21Ϯ0.98 S341A 6 -21.21Ϯ0.56 S341E 12 -28.77Ϯ1.36* S341T 5 -26.62Ϯ1.43* T1134A 6 -28.33Ϯ1.23* T1134F 5 -19.74Ϯ0.73 T1134E 4 -27.54Ϯ1.27* Values are means Ϯ SE; n, no. of oocytes. Login to comment
221 Px/PCl values for small anions were increased in T338A CFTR, whereas Px/PCl values for large anions were decreased. Login to comment
224 Interestingly, Px/PCl for SCN- , the most permeant anion tested in WT CFTR, was nearly doubled in T338A CFTR, whereas PI/PCl was increased sevenfold. Login to comment
225 Gx/GCl values for large anions were also decreased in T338A CFTR, whereas Gx/GCl values for small anions were increased (Table 4). Login to comment
227 The pattern was similar to that for T338A CFTR in that Px/PCl values for large anions were decreased in S341A CFTR, whereas Px/PCl values for small anions were increased. Login to comment
228 Except for PNO3/PCl, the magnitude of the effect in S341A CFTR was less than that seen in T338A CFTR. Login to comment
231 Interestingly, although GClO4/GCl was increased in T338A CFTR, GClO4/GCl was decreased in S341A CFTR. Login to comment
232 The pattern in S341A CFTR was similarly inverted for Gacetate/GCl compared with that in T338A CFTR. Login to comment
256 For anions larger than isethionate (glutamate and gluconate), even the T338A mutation had only a very small effect on Gx/GCl. Login to comment
268 Px/PCl and Gx/GCl values for each anion x in each mutant were calculated for T1134A, K335A, T338A, T339A, and S341A CFTRs and normalized to Px/PCl and Gx/GCl values for WT CFTR. Login to comment
284 As in T338A CFTR, PClO4/PCl stands out as most sensitive to mutation at this position because PClO4/PCl was increased 18-fold in T338A CFTR and 11-fold in T338E CFTR. Login to comment
289 Comparing S341E with S341A CFTR and T338E with T338A CFTR, we can see that the introduction of a negative charge at S341 more strongly destabilized the binding of SCN- (which is pronounced in WT CFTR) than did the equivalent mutation at T338. Login to comment
330 In contrast, mutation T338A induced significant hysteresis for all three of the large anions studied. Login to comment
359 Even mutation T338A, which had the most pronounced effects on selectivity between monovalent anions, did not affect selectivity between Cl- and S2O3 2- . Login to comment
388 Selectivity between Cl- and the divalent anion S2O3 2CFTR n GS2O3/GCl WT 16 0.39Ϯ0.01 K335A 5 0.37Ϯ0.01 K335F 7 0.39Ϯ0.01 K335E 5 0.34Ϯ0.01* T338A 5 0.38Ϯ0.01 T338E 3 0.70Ϯ0.08* T339A 5 0.39Ϯ0.02 S341A 6 0.27Ϯ0.01* S341E 12 0.54Ϯ0.01* S341T 5 0.38Ϯ0.01 T1134A 6 0.34Ϯ0.02 T1134F 5 0.33Ϯ0.01* T1134E 4 0.44Ϯ0.05 Values are means Ϯ SE; n, no. of oocytes. Login to comment
392 However, a subsequent study (24) found that the single mutation T338A led to an even larger single-channel conductance and that mutations here did not strongly affect the functional minimum pore diameter. Login to comment
398 McDonough et al. (33) previously showed that T339A CFTR was identical to WT CFTR with respect to blockade by DPC, whereas T338A CFTR exhibited identical affinity for DPC (at -100 mV) but slightly altered voltage dependence. Login to comment
454 1) Selectivity between Cl- and NO3 - as well as between Cl- and Br- was affected more in S341A CFTR than in T338A CFTR. Login to comment
455 2) Although relative permeabilities for the largest anions (acetate and larger) were affected greatly in T338A CFTR, they were also reduced significantly in S341A CFTR. Login to comment
456 3) GClO4/GCl and Gacetate/GCl were oppositely affected by mutations T338A and S341A, as if these amino acids lie on opposite sides of a barrier that determines selectivity between these anions of very similar size. Login to comment

[hide] Point mutations in the pore region directly or ind... Pflugers Arch. 2002 Mar;443(5-6):739-47. Epub 2001 Dec 8. Gupta J, Linsdell P
Point mutations in the pore region directly or indirectly affect glibenclamide block of the CFTR chloride channel.
Pflugers Arch. 2002 Mar;443(5-6):739-47. Epub 2001 Dec 8., [PMID:11889571]

Abstract [show]
The sulfonylurea glibenclamide is a relatively potent inhibitor of the CFTR Cl(-) channel. This inhibition is thought to be via an open channel block mechanism. However, nothing is known about the physical nature of the glibenclamide-binding site on CFTR. Here we show that mutations in the pore-forming 6th and 12th transmembrane regions of CFTR affect block by intracellular glibenclamide, confirming previous suggestions that glibenclamide enters the pore in order to block the channel. Two mutations in the 6th transmembrane region, F337A and T338A, significantly weakened glibenclamide block, consistent with a direct interaction between glibenclamide and this region of the pore. Interestingly, two mutations in the 12th transmembrane region (N1138A and T1142A) significantly strengthened block. These two mutations also abolished the dependence of block on the extracellular Cl(-) concentration, which in wild-type CFTR suggests an interaction between Cl(-) and glibenclamide within the channel pore that limits block. We suggest that mutations in the 12th transmembrane region strengthen glibenclamide block not by directly altering interactions between glibenclamide and the pore walls, but indirectly by reducing interactions between Cl(-) ions and glibenclamide within the pore. This work demonstrates that glibenclamide binds within the CFTR channel pore and begins to define its intrapore binding site.

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No. Sentence Comment
4 Two mutations in the 6th transmembrane region, F337A and T338A, significantly weakened glibenclamide block, consistent with a direct interaction between glibenclamide and this region of the pore. Login to comment
63 While block of the TM12 mutants S1141A (Fig. 1) and T1134A and M1137A (data not shown) was indistinguishable from wild-type, block was significantly weakened in the TM6 mutants F337A and T338A, and significantly strengthened in the TM12 mutants N1138A and T1142A (Fig. 1). Login to comment
69 Mean fraction of control current remaining following addition of 60 µM glibenclamide (I/I0) is shown as a function of voltage for wild-type (q), T338A (s), N1138A (s), F337A (ss) and T1142A (xx). Login to comment
70 Mean of data from 5-10 patches, fitted by Eq. according to the mean parameters shown in Fig. 3 rent remaining following addition of glibenclamide (I/I0) was significantly reduced at all voltages in N1138A and T1142A (P<0.05), and significantly increased in F337A and T338A at negative membrane potentials (P<0.05). Login to comment
75 Consistent with the results shown in Fig. 2, Kd(0) was significantly increased in F337A and T338A, and significantly decreased in N1138A and T1142A (Fig. 3A). Login to comment
83 The extracellular Cl-concentration had a similar effect on Kd(0) in the TM6 mutants F337A (Fig. 5A) and T338A (Figs. 4, 5A). Login to comment
98 A Example I-V relationships for wild-type (left), T338A (center) and T1142A (right), recorded with 10 mM extracellular Clas described in Materials and methods, before (Ctrl) and immediately following addition of 60 µM glibenclamide (+Glib) to the intracellular solution. B Mean fraction of control current remaining following addition of glibenclamide, with 154 mM (q) or 10 mM (qq) extracellular Cl-, for wild-type (left), T338A (center) and T1142A (right). Login to comment

[hide] Molecular determinants of Au(CN)(2)(-) binding and... J Physiol. 2002 Apr 1;540(Pt 1):39-47. Gong X, Burbridge SM, Cowley EA, Linsdell P
Molecular determinants of Au(CN)(2)(-) binding and permeability within the cystic fibrosis transmembrane conductance regulator Cl(-) channel pore.
J Physiol. 2002 Apr 1;540(Pt 1):39-47., 2002-04-01 [PMID:11927667]

Abstract [show]
Lyotropic anions with low free energy of hydration show both high permeability and tight binding in the cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel pore. However, the molecular bases of anion selectivity and anion binding within the CFTR pore are not well defined and the relationship between binding and selectivity is unclear. We have studied the effects of point mutations throughout the sixth transmembrane (TM6) region of CFTR on channel block by, and permeability of, the highly lyotropic Au(CN)(2)(-) anion, using patch clamp recording from transiently transfected baby hamster kidney cells. Channel block by 100 microM Au(CN)(2)(-), a measure of intrapore anion binding affinity, was significantly weakened in the CFTR mutants K335A, F337S, T338A and I344A, significantly strengthened in S341A and R352Q and unaltered in K329A. Relative Au(CN)(2)(-) permeability was significantly increased in T338A and S341A, significantly decreased in F337S and unaffected in all other mutants studied. These results are used to define a model of the pore containing multiple anion binding sites but a more localised anion selectivity region. The central part of TM6 (F337-S341) appears to be the main determinant of both anion binding and anion selectivity. However, comparison of the effects of individual mutations on binding and selectivity suggest that these two aspects of the permeation mechanism are not strongly interdependent.

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No. Sentence Comment
12 Channel block by 100 mM Au(CN)2 _ , a measure of intrapore anion binding affinity, was significantly weakened in the CFTR mutants K335A, F337S, T338A and I344A, significantly strengthened in S341A and R352Q and unaltered in K329A. Login to comment
13 Relative Au(CN)2 _ permeability was significantly increased in T338A and S341A, significantly decreased in F337S and unaffected in all other mutants studied. Login to comment
42 Some of these have previously been associated with altered anion selectivity (F337S, T338A; Linsdelletal.1998,2000),alteredanion:cationselectivity(R352Q; Guinamard & Akabas, 1999), or disrupted open channel blocker binding (S341A; McDonough et al. 1994). Login to comment
78 Currents carried by the CFTR mutants K329A, K335A, T338A, S341A and I344A were also stimulated an average of 2_3-fold by PPi (Fig. 2B). Login to comment
87 Comparison between different channel variants at _100 mV reveals the sensitivity to this concentration of Au(CN)2 _ is R352Q > S341A > wild-type, K329A > I344A > K335A = F337S > T338A. Login to comment
100 Relative Au(CN)2 _ permeability was significantly decreased in F337S and significantly increased in T338A (Fig. 4), consistent with the previously described opposite effects of these mutants on CFTR lyotropic anion selectivity (Linsdell et al. 1998, 2000). Login to comment
101 Interestingly, S341A also significantly increased PAu(CN)2/PCl (Fig. 4), although to a lesser extent than T338A. Login to comment
116 Au(CN)2 _ permeability of different CFTR variants A, example CFTR I-V relationships recorded with 150 m KAu(CN)2 in the extracellular solution and 150 m KCl in the intracellular solution, for wild-type, F337S and T338A. Login to comment
123 At this voltage, block by 100 mM Au(CN)2 _ was significantly weakened in K335A, F337S, T338A and I334A, significantly strengthened in S341A and R352Q and unaffected in K329A (Fig. 3). Login to comment
124 The sequence of relative sensitivity to block by 100 mM Au(CN)2 _ at _100 mV (R352Q > S341A > wild-type, K329A > I344A > K335A = F337S > T338A) suggests that T338 normally makes the strongest contribution to Au(CN)2 _ binding within the pore, with nearby residues K335 and F337 also making large contributions. Login to comment
139 The effects of F337S and T338A on PAu(CN)2/PCl are consistent with the disruption (F337S; Linsdell et al. 2000) and strengthening (T338A; Linsdell et al. 1998) of lyotropic anion selectivity previously described in these two mutants. Login to comment
142 Multiple TM6 residues contribute to anion binding, as determined by Au(CN)2 _ block of Cl_ permeation; the dramatic weakening of Au(CN)2 _ block in T338A suggests a particularly strong role of this residue in binding. Login to comment
147 Only mutations in the central portion of TM6 (F337S, T338A, S341A) affected both Au(CN)2 _ binding and Au(CN)2 _ permeability (Figs 3_5). Login to comment
157 Nevertheless, there does not seem to be a strong correlation between these two aspects of pore function, such that they may be controlled independently by the same structural featuresofthepore.Thus,F337Sisassociatedwithweakened Au(CN)2 _ binding and decreased Au(CN)2 _ permeability, T338A with weakened Au(CN)2 _ binding and increased Au(CN)2 _ permeability and S341A with strengthened Au(CN)2 _ bindingandincreasedpermeability(Figs3and4). Login to comment

[hide] Mechanism of lonidamine inhibition of the CFTR chl... Br J Pharmacol. 2002 Nov;137(6):928-36. Gong X, Burbridge SM, Lewis AC, Wong PY, Linsdell P
Mechanism of lonidamine inhibition of the CFTR chloride channel.
Br J Pharmacol. 2002 Nov;137(6):928-36., [PMID:12411425]

Abstract [show]
1. The cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel is blocked by a broad range of organic anionic compounds. Here we investigate the effects of the indazole compound lonidamine on CFTR channels expressed in mammalian cell lines using patch clamp recording. 2. Application of lonidamine to the intracellular face of excised membrane patches caused a voltage-dependent block of CFTR currents, with an apparent K(d) of 58 micro M at -100 mV. 3. Block by lonidamine was apparently independent of channel gating but weakly sensitive to the extracellular Cl(-) concentration. 4. Intracellular lonidamine led to the introduction of brief interruptions in the single channel current at hyperpolarized voltages, leading to a reduction in channel mean open time. Lonidamine also introduced a new component of macroscopic current variance. Spectral analysis of this variance suggested a blocker on rate of 1.79 micro M(-1) s(-1) and an off-rate of 143 s(-1). 5. Several point mutations within the sixth transmembrane region of CFTR (R334C, F337S, T338A and S341A) significantly weakened block of macroscopic CFTR current, suggesting that lonidamine enters deeply into the channel pore from its intracellular end. 6. These results identify and characterize lonidamine as a novel CFTR open channel blocker and provide important information concerning its molecular mechanism of action.

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No. Sentence Comment
7 5 Several point mutations within the sixth transmembrane region of CFTR (R334C, F337S, T338A and S341A) signi®cantly weakened block of macroscopic CFTR current, suggesting that lonidamine enters deeply into the channel pore from its intracellular end. Login to comment
116 As shown in Figure 7a, 55 mM lonidamine inhibited currents carried by R334C, K335A, F337S, T338A and S341A-CFTR. Login to comment
118 The eect of these mutations on block by lonidamine is more clearly seen in the dose-response curves shown in Figure 7b. Fits of these mean data by equation 1 suggests a Kd (at 7100 mV) of 58.5 mM for wild-type, 65.6 mM for K335A, 90.0 mM for T338A, 186 mM for F337S, 206 mM for S341A, and 338 mM for R334C. Login to comment
119 Similar analyses at other potentials showed a similar increase in Kd in R334C, F337S, S341A and (to a far lesser extent) T338A (Figure 7c). Login to comment
120 Fitting data from individual patches with equation 2 gave similar and, except in the case of K335A, signi®cant changes in Kd(-100): wild-type 60.6+5.2 mM (n=5), K335A 63.1+7.4 mM (n=5) (P40.05), T338A 93.4+4.1 mM (n=5) (P50.002), F337S 166+18 mM (n=5) (P50.0005), S341A 169+25 mM (n=5) (P50.005), R334C 260+19 mM (n=4) (P50.00001). Login to comment
121 These same ®ts also revealed changes in the voltage dependence of block, as judged by changes in d, although this was only statistically signi®cant in the case of R334C: wild-type 0.426+0.033 (n=5), K335A 0.484+0.024 (n=5) (P40.05), T338A 0.410+0.045 (n=5) (P40.05), F337S 0.365+0.015 (n=5) (P40.05), S341A 0.285+0.061 (n=5) (P40.05), R334C 0.233+0.066 (n=4) (P50.05). Login to comment
143 (a) Example I-V relationships for R334C, K335A, F337S, T338A and S341A-CFTR, before (solid lines) and following (dotted lines) addition of 55 mM lonidamine to the intracellular solution. Login to comment
145 (b) Concentration dependence of block at 7100 mV for wild-type, R334C, K335A, F337S, T338A and S341A. Login to comment
147 Each has been ®tted by equation 1, giving Kds of 58.5 mM (wild-type), 65.6 mM (K335A), 90.0 mM (T338A), 186 mM (F337S), 206 mM (S341A) and 338 mM (R334C). Login to comment

[hide] Molecular determinants and role of an anion bindin... J Physiol. 2003 Jun 1;549(Pt 2):387-97. Epub 2003 Apr 4. Gong X, Linsdell P
Molecular determinants and role of an anion binding site in the external mouth of the CFTR chloride channel pore.
J Physiol. 2003 Jun 1;549(Pt 2):387-97. Epub 2003 Apr 4., 2003-06-01 [PMID:12679372]

Abstract [show]
Chloride permeation through the cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channel is blocked by highly lyotropic permeant anions which bind tightly within the pore. Here we show that several different substitutions of a positively charged amino acid residue, arginine R334, in the putative outer mouth of the CFTR pore, greatly reduce the block caused by lyotropic Au(CN)2- ions applied to the intracellular side of the channel. Fixed positive charge at this site appears to play a role in Au(CN)2- binding, as judged by multiple substitutions of differently charged amino acid side chains and also by the pH dependence of block conferred by the R334H mutant. However, non-charge-dependent effects also appear to contribute to Au(CN)2- binding. Mutation of R334 also disrupts the apparent electrostatic interaction between intracellular Au(CN)2- ions and extracellular permeant anions, an interaction which normally acts to relieve channel block. All six mutations studied at R334 significantly weakened this interaction, suggesting that arginine possesses a unique ability to coordinate ion-ion interactions at this site in the pore. Our results suggest that lyotropic anions bind tightly to a site in the outer mouth of the CFTR pore that involves interaction with a fixed positive charge. Binding to this site is also involved in coordination of multiple permeant anions within the pore, suggesting that anion binding in the outer mouth of the pore is an important aspect in the normal anion permeation mechanism.

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40 conditions of high extracellular Cl_ concentration, Au(CN)2 _ block is weakened in the CFTR pore mutants K335A, F337S and T338A (Gong et al. 2002a), suggesting that these pore residues may contribute to lyotropic anion binding site(s) within the pore. Login to comment
60 In wild-type, K335A, F337S and T338A, high extracellular Cl_ significantly weakens Au(CN)2 _ block and (except in F337S) increases the fraction of the transmembrane electric field apparently experienced by the blocker. Login to comment
131 In contrast, mutation of other nearby TM6 residues associated with weakened Au(CN)2 _ binding (K335A, F337S, T338A) showed similar sensitivity to extracellular Cl_ concentration to that seen in wild-type (Figs 1 and 2). Login to comment

[hide] Extent of the selectivity filter conferred by the ... Mol Membr Biol. 2003 Jan-Mar;20(1):45-52. Gupta J, Lindsell P
Extent of the selectivity filter conferred by the sixth transmembrane region in the CFTR chloride channel pore.
Mol Membr Biol. 2003 Jan-Mar;20(1):45-52., [PMID:12745925]

Abstract [show]
Point mutations within the pore region of the cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel have previously been shown to alter the selectivity of the channel between different anions, suggesting that part of the pore may form an anion 'selectivity filter'. However, the full extent of this selectivity filter region and the location of anion binding sites in the pore are currently unclear. As a result, comparisons between CFTR and other classes of Cl(-) channel of known structure are difficult. We compare here the effects of point mutations at each of eight consecutive amino acid residues (arginine 334-serine 341) in the crucial sixth transmembrane region (TM6) of CFTR. Anion selectivity was determined using patch-clamp recording from inside-out membrane patches excised from transiently transfected mammalian cell lines. The results suggest that selectivity is predominantly controlled by a single site involving adjacent residues phenylalanine 337 and threonine 338, and that the selectivity conferred by this 'filter' region is modified by anion binding to flanking sites involving the more extracellular arginine 334 and the more intracellular serine 341. Other residues within this part of the pore play only minor roles in controlling anion permeability and conductance. Our results support a model in which specific TM6 residues make important contributions to a single, localized anion selectivity filter in the CFTR pore, and also contribute to multiple anion binding sites both within and on either side of the filter region.

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41 Example leak-subtracted I Á/V relationships obtained with different intracellular anions are shown for wild-type, R334C, F337A, T338A, T339V and S341A in Figure 2. Login to comment
44 Of eight mutants studied, only T339V was without any significant effect on anion permeability (Table 1), and five mutations (R334C, K335A, F337A, T338A, I340A) led to changes in the permeability sequence among halides (Figure 2 and Table 2). Login to comment
45 The relative permeability of the lyotropic SCN( anion, which is high in the wild-type (PSCN/PCl 0/4.759/0.30, n0/6) (Table 1) was significantly altered in six out of eight mutants studied (Table 1 and Figure 3), with PSCN/PCl being greatly reduced in F337A and most strongly increased in T338A and S341A. Login to comment
59 Wild type R334C K335A I336A F337A T338A T339V I340A S341A Cl 1.009/0.00 (6) 1.009/0.01 (6) 1.009/0.05 (5) 1.009/0.01 (5) 1.009/0.02 (6) 1.009/0.02 (8) 1.009/0.03 (6) 1.009/0.02 (5) 1.009/0.01 (6) Br 1.479/0.06 (6) 0.969/0.00 (5)** 1.529/0.03 (5) 1.359/0.05 (5) 0.669/0.03 (6)** 2.209/0.05 (5)** 1.829/0.24 (5) 1.409/0.09 (6) 2.459/0.20 (5)** I 0.819/0.04 (6) 0.729/0.05 (3) 1.579/0.06 (4)** 0.589/0.02 (4)* 0.389/0.15 (3)* 2.799/0.26 (7)** 0.769/0.02 (6) 1.249/0.07 (6)** 0.739/0.06 (6) F 0.119/0.01 (6) 0.099/0.01 (3) 0.139/0.02 (3) 0.079/0.01 (5) 0.409/0.02 (4)** 0.139/0.01 (6) 0.079/0.00 (5) 0.069/0.01 (5) 0.059/0.01 (6)* SCN 4.759/0.30 (6) 2.769/0.38 (6)** 3.989/0.16 (5) 3.709/0.11 (5)* 1.269/0.12 (5)** 7.509/0.29 (6)** 4.829/0.40 (5) 4.189/0.14 (7)* 10.09/1.8 (6)* Relative permeabilities for different anions present in the intracellular solution under bi-ionic conditions were calculated from macroscopic current reversal potentials according to Eq. (1) (see Experimental procedures). Login to comment
65 Wild-type R334C K335A I336A F337A T338A T339V I340A S341A Cl (G(50/G'50) 1.039/0.09 (6) 4.509/0.60 (6)** 1.399/0.09 (5)** 1.519/0.14 (5)* 1.189/0.22 (6) 1.779/0.25 (8)* 1.199/0.06 (7)* 1.419/0.11 (5)* 1.809/0.18 (5)** Cl (GCl/GCl) 1.009/0.08 (6) 1.009/0.13 (6) 1.009/0.07 (5) 1.009/0.09 (5) 1.009/0.22 (6) 1.009/0.14 (8) 1.009/0.06 (7) 1.009/0.09 (5) 1.009/0.10 (5) Br 0.649/0.05 (6) 0.329/0.02 (6)** 0.669/0.05 (5) 1.079/0.10 (5)* 0.359/0.06 (6)** 0.499/0.03 (5) 0.659/0.09 (5) 0.669/0.08 (6) 1.529/0.30 (4)* I 0.299/0.05 (6) 0.749/0.02 (3)* 0.279/0.01 (4) 0.109/0.02 (4)* 0.349/0.08 (3) 0.389/0.03 (5) 0.309/0.05 (7) 0.279/0.03 (6) 1.049/0.16 (7)** F 0.379/0.04 (6) 0.329/0.04 (3) 0.349/0.03 (3) 0.709/0.10 (4)* 0.129/0.02 (3)* 0.239/0.02 (6)* 0.509/0.10 (4) 0.309/0.02 (5) 0.519/0.07 (6) SCN 0.389/0.02 (6) 0.339/0.03 (6) 0.669/0.10 (5)* 0.279/0.02 (6)* 0.399/0.04 (5) 0.269/0.02 (5)* 0.269/0.02 (4)* 0.359/0.04 (6) 0.839/0.14 (6)* Relative conductances for different anions were calculated from the slope of the macroscopic I Á/V relationship for inward versus outward currents (see Experimental procedures). Login to comment
72 This halide selectivity sequence is changed to Eisenman sequence II in I340A, and Eisenman sequence I in both K335A and T338A (Table 2), consistent with a strengthening of lyotropic anion selectivity in these mutants. Login to comment
76 In the present study, large increases in the permeability of the lyotropic SCN( anion were observed in both T338A and S341A, and a dramatic decrease in SCN( permeability was observed in F337A (Figure 3), consistent with previous results with Au(CN)2 ( which suggest these residues are the main determinants of the permeability of strongly lyotropic anions [15]. Login to comment
78 Taken together, these anion permeability data suggest a relative loss of lyotropic anion selectivity in F337A and (to a lesser extent) R334C, strengthening of lyotropic selectivity in T338A and S341A, and only minor effects at other positions. Login to comment
86 Halide permeability sequence Eisenman sequence CFTR variants I( !/Br( !/Cl( !/F( I K335A, T338A Br( !/I( !/Cl( !/F( II I340A Br( !/Cl( !/I( !/F( III wild-type, I336A, T339V, S341A Cl( !/Br( !/I( !/F( IV R334C Cl( !/Br( !/F( !/I( V F337A Sequences were derived from the relative permeabilities given in table 1. Login to comment
109 Lyotropic anion selectivity is disrupted in F337A and modified in R334C, T338A and S341A. Login to comment

[hide] CFTR: what's it like inside the pore? J Exp Zool A Comp Exp Biol. 2003 Nov 1;300(1):69-75. Liu X, Smith SS, Dawson DC
CFTR: what's it like inside the pore?
J Exp Zool A Comp Exp Biol. 2003 Nov 1;300(1):69-75., 2003-11-01 [PMID:14598388]

Abstract [show]
The Cystic Fibrosis Conductance Regulator (CFTR) functions as a cAMP-activated, anion-selective channel, but the structural basis for anion permeation is not well understood. Here we summarize recent studies aimed at understanding how anions move through the CFTR channel, and the nature of the environment anions experience inside the pore. From these studies it is apparent that anion permeability selectivity and anion binding selectivity of the pore are consistent with a model based on a "dielectric tunnel." The selectivity pattern for halides and pseudohalides can be predicted if it is assumed that permeant anions partition between bulk water and a polarizable space that is characterized by an effective dielectric constant of about 19. Covalent labeling of engineered cysteines and pH titration of engineered cysteines and histidines lead to the conclusion that the CFTR anion conduction path includes a positively charged outer vestibule. A residue in transmembrane segment 6 (TM6) (R334) appears to reside in the outer vestibule of the CFTR pore where it creates a positive electrostatic potential that enhances anion conduction.

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114 T338C CFTR undergoes pH-dependent changes in gCl and I-V shape that are not seen in wild type, T338A or T338S CFTR. Login to comment

[hide] Mutation-induced blocker permeability and multiion... J Gen Physiol. 2003 Dec;122(6):673-87. Epub 2003 Nov 10. Gong X, Linsdell P
Mutation-induced blocker permeability and multiion block of the CFTR chloride channel pore.
J Gen Physiol. 2003 Dec;122(6):673-87. Epub 2003 Nov 10., [PMID:14610019]

Abstract [show]
Chloride permeation through the cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channel is blocked by a broad range of anions that bind tightly within the pore. Here we show that the divalent anion Pt(NO2)42- acts as an impermeant voltage-dependent blocker of the CFTR pore when added to the intracellular face of excised membrane patches. Block was of modest affinity (apparent Kd 556 microM), kinetically fast, and weakened by extracellular Cl- ions. A mutation in the pore region that alters anion selectivity, F337A, but not another mutation at the same site that has no effect on selectivity (F337Y), had a complex effect on channel block by intracellular Pt(NO2)42- ions. Relative to wild-type, block of F337A-CFTR was weakened at depolarized voltages but strengthened at hyperpolarized voltages. Current in the presence of Pt(NO2)42- increased at very negative voltages in F337A but not wild-type or F337Y, apparently due to relief of block by permeation of Pt(NO2)42- ions to the extracellular solution. This "punchthrough" was prevented by extracellular Cl- ions, reminiscent of a "lock-in" effect. Relief of block in F337A by Pt(NO2)42- permeation was only observed for blocker concentrations above 300 microM; as a result, block at very negative voltages showed an anomalous concentration dependence, with an increase in blocker concentration causing a significant weakening of block and an increase in Cl- current. We interpret this effect as reflecting concentration-dependent permeability of Pt(NO2)42- in F337A, an apparent manifestation of an anomalous mole fraction effect. We suggest that the F337A mutation allows intracellular Pt(NO2)42- to enter deeply into the CFTR pore where it interacts with multiple binding sites, and that simultaneous binding of multiple Pt(NO2)42- ions within the pore promotes their permeation to the extracellular solution.

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98 Block of R334C and S341A appeared somewhat weaker than for wild-type CFTR, whereas K335A and T338A showed a similar degree of block as wild-type (Fig. 5, A-C). Login to comment
105 However, when we investigated the block at the most negative voltages that we were able to keep membrane patches (-150 mV) with a low extracellular Cl-concentration (4 mM), we noticed an anomalous voltage-dependent increase in Pt(NO2)4 2--blocked current in F337A but not in wild-type, F337Y or T338A (Fig. 6). Login to comment
106 Under these conditions, the strength of Pt(NO2)4 2- block in wild-type, F337Y, and T338A increases with increasingly negative voltages, eventually leading to a negative slope of the current-voltage relationship in the presence of blocker (Fig. 6 B). Login to comment
116 Thus, at very negative voltages, Pt(NO2)4 2- ions can escape from the F337A channel pore, but apparently not from the pore of wild-type, F337Y or T338A, by passing through the channel and into the extracellular solution-a process previously termed "punchthrough" (Nimigean and Miller, 2002). Login to comment
145 (A) Example macroscopic currents carried by the CFTR mutants R334C, K335A, F337A, T338A, and S341A before (Control) and after addition of 300 ␮M Pt(NO2)4 2to the intracellular solution. Login to comment
147 Each plot has been fitted by Eq. 2; this provides a good fit of R334C (Kd(0) ϭ 2080 ␮M, z␦ ϭ -0.174), K335A (Kd(0) ϭ 418 ␮M, z␦ ϭ -0.317), T338A (Kd(0) ϭ 626 ␮M, z␦ ϭ -0.351) and S341A (Kd(0) ϭ 1362 ␮M, z␦ ϭ -0.249), but a poor fit of F337A. Login to comment
190 In contrast, mutations of the adjacent TM6 residue (T338), including T338A, altered the selectivity between different lyotro- Figure 9. Login to comment
201 The slight Pt(NO2)4 2- permeability of F337A therefore suggests that this divalent anion might normally be prevented from passing through the pore for similar reasons that limit the permeability of kosmotropic anions like F-. In contrast, the T338A mutation appears to enhance unblock by permeation of the lyotropic Au(CN)2 - ion (Gong and Linsdell, 2003b). Login to comment

[hide] Steady-state interactions of glibenclamide with CF... J Membr Biol. 2004 May 1;199(1):15-28. Zhang ZR, Zeltwanger S, McCarty NA
Steady-state interactions of glibenclamide with CFTR: evidence for multiple sites in the pore.
J Membr Biol. 2004 May 1;199(1):15-28., 2004-05-01 [PMID:15366420]

Abstract [show]
The objective of the present study was to clarify the mechanism by which the sulfonylurea drug, glibenclamide, inhibits single CFTR channels in excised patches from Xenopus oocytes. Glibenclamide blocks the open pore of the channel via binding at multiple sites with varying kinetics. In the absence of glibenclamide, open-channel bursts exhibited a flickery intraburst closed state (C1); this is due to block of the pore by the pH buffer, TES. Application of 25 microM glibenclamide to the cytoplasmic solution resulted in the appearance of two drug-induced intraburst closed states (C2, C3) of widely different duration, which differed in pH-dependence. The kinetics of interaction with the C3 state, but not the C2 state, were strongly voltage-dependent. The durations of both the C2 and C3 states were concentration-dependent, indicating a non-linear reaction scheme. Application of drug also increased the burst duration, which is consistent with an open-channel blocking mechanism. A kinetic model is proposed. These results indicate that glibenclamide interacts with open CFTR channels in a complex manner, involving interactions with multiple binding sites in the channel pore.

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309 The most significant change was seen with the T338A mutant in transmembrane domain 6, although this only reflected a two-fold decrease in affinity. Login to comment

[hide] Direct comparison of the functional roles played b... J Biol Chem. 2004 Dec 31;279(53):55283-9. Epub 2004 Oct 25. Ge N, Muise CN, Gong X, Linsdell P
Direct comparison of the functional roles played by different transmembrane regions in the cystic fibrosis transmembrane conductance regulator chloride channel pore.
J Biol Chem. 2004 Dec 31;279(53):55283-9. Epub 2004 Oct 25., 2004-12-31 [PMID:15504721]

Abstract [show]
The cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel contains 12 transmembrane (TM) regions that are presumed to form the channel pore. However, little is known about the relative functional contribution of different TM regions to the pore. We have used patch clamp recording to investigate the functional consequences of point mutations throughout the six transmembrane regions in the N-terminal part of the CFTR protein (TM1-TM6). A range of specific functional assays compared the single channel conductance, anion binding, and anion selectivity properties of different channel variants. Overall, our results suggest that TM1 and -6 play dominant roles in forming the channel pore and determining its functional properties, with TM5 perhaps playing a lesser role. In contrast, TM2, -3, and -4 appear to play only minor supporting roles. These results define transmembrane regions 1 and 6 as major contributors to the CFTR channel pore and have strong implications for emerging structural models of CFTR and related ATP-binding cassette proteins.

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78 Only one mutation studied, T338A, led to a significant increase in channel conductance (Figs. 3 and 4) as described previously (35). Login to comment
93 Note that in the final panel, showing data from the high conductance mutant T338A, the current (i) scale is increased. Login to comment
102 The weakest block by Au(CN)2 - was observed in K95Q, T338A, R334K, and Q98A, consistent with these residues perhaps being associated with permeant anion binding sites inside the pore. Login to comment
109 Thiocyanate permeability was strongly increased in A96V and T338A, suggesting enhancement of lyotropic selectivity in these mutants, and dramatically reduced in F337A, which we previously suggested reflects the role of Phe-337 in contributing to an anion selectivity filter in the pore (11, 36). Login to comment

[hide] Determination of the functional unit of the cystic... J Biol Chem. 2005 Jan 7;280(1):458-68. Epub 2004 Oct 25. Zhang ZR, Cui G, Liu X, Song B, Dawson DC, McCarty NA
Determination of the functional unit of the cystic fibrosis transmembrane conductance regulator chloride channel. One polypeptide forms one pore.
J Biol Chem. 2005 Jan 7;280(1):458-68. Epub 2004 Oct 25., 2005-01-07 [PMID:15504728]

Abstract [show]
The magnitudes and distributions of subconductance states were studied in chloride channels formed by the wild-type cystic fibrosis transmembrane conductance regulator (CFTR) and in CFTRs bearing amino acid substitutions in transmembrane segment 6. Within an open burst, it was possible to distinguish three distinct conductance states referred to as the full conductance, subconductance 1, and subconductance 2 states. Amino acid substitutions in transmembrane segment 6 altered the duration and probability of occurrence of these subconductance states but did not greatly alter their relative amplitudes. Results from real time measurements indicated that covalent modification of single R334C-CFTR channels by [2-(trimethylammonium)ethyl]methanethiosulfonate resulted in the simultaneous modification of all three conductance levels in what appeared to be a single step, without changing the proportion of time spent in each state. This behavior suggests that at least a portion of the conduction path is common to all three conducting states. The time course for the modification of R334C-CFTR, measured in outside-out macropatches using a rapid perfusion system, was also consistent with a single modification step as if each pore contained only a single copy of the cysteine at position 334. These results are consistent with a model for the CFTR conduction pathway in which a single anion-conducting pore is formed by a single CFTR polypeptide.

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93 Fig. 1 also contains records illustrating the subconductance behavior of two mutant CFTRs: R334C and T338A. Login to comment
94 The former exhibits a full conductance that is less than that of WT-CFTR under comparable conditions (14), and the latter exhibits an increased full conductance (9.8 pS in T338A-CFTR). Login to comment
97 This result suggests that neither the R334C nor T338A mutation, although they involve residues reputed to lie within the CFTR pore (14, 18), greatly altered the relative magnitude of the subconductance states. Login to comment
124 A-C, records for WT-, R334C-, and T338A-CFTR, respectively, were generated in excised, inside-out mode with asymmetrical [Cl- ], where the pipette was filled with 40 mM [Cl- ] and bath (cytoplasmic) solution contained 302 mM [Cl- ] in order to potentiate the single channel amplitude. Login to comment

[hide] Variable reactivity of an engineered cysteine at p... J Biol Chem. 2006 Mar 24;281(12):8275-85. Epub 2006 Jan 24. Liu X, Alexander C, Serrano J, Borg E, Dawson DC
Variable reactivity of an engineered cysteine at position 338 in cystic fibrosis transmembrane conductance regulator reflects different chemical states of the thiol.
J Biol Chem. 2006 Mar 24;281(12):8275-85. Epub 2006 Jan 24., 2006-03-24 [PMID:16436375]

Abstract [show]
In a previous study of T338C CFTR (cystic fibrosis transmembrane conductance regulator) we found that protons and thiol-directed reagents modified channel properties in a manner consistent with the hypothesis that this residue lies within the conduction path, but the observed reactivity was not consistent with the presence of a single thiolate species in the pore. Here we report results consistent with the notion that the thiol moiety can exist in at least three chemical states, the simple thiol, and two altered states. One of the altered states displays reactivity toward thiols like dithiothreitol and 2-mercaptoethanol as well as reagents: mixed disulfides (methanethiosulfonate reagents: MTSET+, MTSES-) and an alkylating agent (iodoacetamide). The other altered state is unreactive. The phenotype associated with the reactive, altered state could be replicated by exposing oocytes expressing T338C CFTR to CuCl2, but not by glutathionylation or nitrosylation of the thiol or by oxidation with hydrogen peroxide. The results are consistent with the hypothesis that substituting a cysteine at 338 can create an adventitious metal binding site. Metal liganding alters thiol reactivity and may, in some cases, catalyze oxidation of the thiol to an unreactive form such as a sulfinic or sulfonic acid.

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53 RESULTS T338C/WT CFTR Conductance Was Markedly Altered by 2-ME or DTT Prior to Exposure to Exogenous Thiol-directed Reagents5 - Exposing oocytes expressing T338C/WT CFTR to 2-ME or DTT during steady state activation led to increases in conductance (without any discernable change in reversal potential) that were rapid (t1/2 ϭ 20 s), and of variable amplitude and were not seen in oocytes expressing CFTR constructs lacking the cysteine at 338, such as WT, T338A, T338H, T338S CFTR, or Cys-less CFTR. Login to comment
102 This 6 Similarly, exposure to reducing agents was without effect on oocytes expressing either T338A or T338S CFTR, constructs that retain the 18 endogenous cysteines (see below). Login to comment
111 It should be noted that the cysteine at position 338 is essential for the effects of MTS reagents as well as 2-ME and DTT shown above, because neither the conductance due to T338A or T338S CFTR was sensitive to reducing agents or thiol-directed reagents.7 Trapping Thiols with an Alkylating Agent, IAM-The results presented so far are compatible with a scheme in which the total conductance of an oocyte expressing T338C/WT CFTR or T338C/Cys-less CFTR comprises at least three components that we will label as gSH, gSX1, and gSX2, where the total conductance, gCl, is given by Equation 1. gCl ϭ gSH ϩ gSX1 ϩ gSX2 (Eq. 1) 7 X. Liu and D. C. Dawson, unpublished observation. Login to comment
162 C, oocytes expressing T338A CFTR were first exposed to: 1 mM Au(CN)2 - (black circles), 1 mM IAM, 1 mM Au(CN)2 - . Login to comment
167 Fig. 4C contains the result of an experiment showing that IAM did not affect the function of T338A CFTR (n ϭ 3). Login to comment
169 Block of T338A CFTR was unaffected by exposure to IAM. Login to comment
192 Diamide-GSH had no discernable effect on conductance of oocytes expressing T338A CFTR.7 Oxidation by NO or H2O2 Did Not Reproduce the Signature Behavior of Spontaneously Oxidized T338C CFTR Channels-Fig. 7A depicts a typical experiment (n ϭ 4) in which an oocyte expressing T338C CFTR was first exposed to 1 mM DTT to increase the number of cysteines in the simple thiol form. Exposure to 1 mM SNAP, a commonly used NO donor (46, 65), produced a minimal effect on the conductance, but largely blocked the subsequent reaction with MTSES- , indicating oxida- tionofthecysteinetothenitrosothiol.Thisapparentoxidationwaswithout effect on the macroscopic conductance but was readily reversed by exposing oocytes to 1 mM DTT, as indicated by an 80% decrease in conductance followingthesecondexposuretoMTSES- .SNAPhadnodiscernableeffect on conductance of oocytes expressing T338A CFTR.7 Fig. 7B depicts a typical experiment (n ϭ 2) in which an oocyte expressing T338C CFTR was first exposed to 1 mM DTT to increase the number of cysteines in the simple thiol form. Exposure to 5 mM H2O2 FIGURE 5. Login to comment
214 At 1 ␮M, copper induced an 80% (Ϯ5%, n ϭ 5) decrease in T338C CFTR conductance, but was without effect on T338A or WT CFTR conductance.9 Washing often produced a slow recovery from inhibition that could vary from near zero to about 32% of the inhibited conductance. Login to comment

[hide] Two novel missense and one novel nonsense CFTR mut... Mol Hum Reprod. 2006 Nov;12(11):717-21. Epub 2006 Sep 14. Radpour R, Gourabi H, Gilani MA, Dizaj AV, Rezaee M, Mollamohamadi S
Two novel missense and one novel nonsense CFTR mutations in Iranian males with congenital bilateral absence of the vas deferens.
Mol Hum Reprod. 2006 Nov;12(11):717-21. Epub 2006 Sep 14., [PMID:16973827]

Abstract [show]
Congenital bilateral absence of the vas deferens (CBAVD) is a frequent cause of obstructive azoospermia. Nearly 75% of men with CBAVD have at least one detectable common cystic fibrosis (CF) transmembrane conductance regulator (CFTR) mutation. To study the involvement of CFTR mutations in the Iranian population with presumed low CF frequency, we analysed 112 Iranian CBAVD males. Three Iranian CBAVD males with no clinical CF phenotype indicated by a normal karyotype, normal pancreatic function and sweat chloride concentration and no Y chromosome microdeletions were studied for CFTR mutations, IVS8-5T mutations and M470V exon 10 missense polymorphism. The entire coding sequence of each gene was analysed using a combination of the denaturing gradient-gel electrophoresis or by single-strand conformation analysis and direct DNA sequencing. Also, 52 fertile males were tested as controls to rule out polymorphism. This approach allowed us to detect one novel nonsense mutation (K536X) in the nucleotide-binding domain 1 (NBD1) region and two novel missense mutations (Y122H and T338A) in the M2 and M6 regions of CFTR gene in our studied population, which were not reported previously. Also, the conservation of changed nucleotide and amino acid in mutated regions was analysed by aligning with nine different species. K536X nonsense mutation (transversion) was found in the first NBD (NBF1), which plays an important regulatory role in CFTR function. It was, therefore, considered as a severe allele responsible for elevated sweat chloride levels and obstructive azoospermia. Because Y122H and T338A mutations were compound heterozygote with the IVS8-5T, it is difficult to judge the severity of these mutations and their role in the CBAVD phenotype.

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10 This approach allowed us to detect one novel nonsense mutation (K536X) in the nucleotide-binding domain 1 (NBD1) region and two novel missense mutations (Y122H and T338A) in the M2 and M6 regions of CFTR gene in our studied population, which were not reported previously. Login to comment
14 Because Y122H and T338A mutations were compound heterozygote with the IVS8-5T, it is difficult to judge the severity of these mutations and their role in the CBAVD phenotype. Login to comment
45 Mutation Location Mutation type Nucleotide alteration Allele frequency (%) K536X Exon 11 Nonsense 1738 A to T 1/224 (0.45) Y122H Exon 4 Missense 496 T to C 1/224 (0.45) T338A Exon 7 Missense 1144 A to G 1/224 (0.45) Figure 1. Login to comment
48 (C) Transabdominal ultrasonography in Patient no. 49 with T338A and IVS8-5T mutations. Login to comment
73 mutation of 1144A→G in exon 7 (Figure 2) which causes an amino acid change of threonine to alanine at position 338 of CFTR polypeptide. Login to comment
78 The numbering of the reported mutations is as follows: c.1738A>T or p.Lys536Stop (K536X), c.496T>C or p.Tyr122His (Y122H) and c.1144A>G or p.Thr338Ala (T338A). Login to comment
104 T338A An exon 7 missense mutation in TMD-M6 was found with 1144A→G in a CBAVD phenotype (Patient no. 49). Login to comment
105 T338A was Table II. Login to comment
110 Mutation type IVS8-(TG)mTn M470V n (%) K536X (TG)10 9T / (TG)10 9T M/V 1 (0.89) Y122H (TG)11 7T / (TG)13 5T V/V 1 (0.89) T338A (TG)11 7T / (TG)13 5T M/V 1 (0.89) Figure 3. Login to comment
121 Because Y122H and T338A mutations were compound heterozygote with the IVS8-5T, it is difficult to judge the severity of these mutations and their role in the CBAVD phenotype. Login to comment

[hide] Molecular study of (TG)m(T)n polymorphisms in Iran... J Androl. 2007 Jul-Aug;28(4):541-7. Epub 2007 Feb 21. Radpour R, Gourabi H, Gilani MA, Dizaj AV
Molecular study of (TG)m(T)n polymorphisms in Iranian males with congenital bilateral absence of the vas deferens.
J Androl. 2007 Jul-Aug;28(4):541-7. Epub 2007 Feb 21., [PMID:17314234]

Abstract [show]
Congenital bilateral absence of the vas deferens (CBAVD) is a frequent cause of obstructive azoospermia. Nearly 75% of men with CBAVD have at least 1 detectable common mutation in the cystic fibrosis transmembrane conductance regulator (CFTR) gene. The different alleles at the (TG)(m)(T)(n) polymorphic locus at the 3' end of human CFTR intron 8 determine the efficiency of exon 9 splicing. To study the CFTR gene mutations and (TG)(m)(T)(n) polymorphisms in Iranian CBAVD patients with presumed low CF frequency and to better understand the complex regulation of exon 9 splicing among our study population, we analyzed CFTR mutations and (TG)(m)(T)(n) polymorphisms in 112 Iranian CBAVD, 7 congenital unilateral absence of the vas deferens males from Iran, and 84 fertile males as controls. Moreover, we compared the rate of CFTR transcripts with exon 9 (9+) with reduction of the (T)(n) repeat in our study population. Our study showed that the 5T mutation was present with high frequency in our patients. Longer (TG)(m) polymorphic tracts increase the proportion of exon 9 deletion transcripts but only when activated by the 5T allele. The combination of the 5T allele in 1 copy of the CFTR gene with a CF mutation in the other copy is the most common cause of CBAVD in the Iranian population. We also observed the highest level of exon 9+ splicing efficiency among the tested samples with the (TG)(12)(T)(7) allele, which represents the most common intron 8 splice variant allele in the general population. Our results support the idea that a putative role of the (T)(n) repeat is to distance the (TG)(m) repeat from the 3' splice site and that the different alleles at the (T)(n) locus affect the efficiency by which the splice acceptor consensus sequence is recognized.

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77 CFTR gene mutations in 112 CBAVD patients and 7 CBAVD patients* Samples Mutation genotype3 (TG)m(T)n n (%) CBAVD Two mutations detected (5 /112 5 4.46%) F508del / R117H (TG)10 9T / (TG)10 9T 1 (0.89) F508del / 621+1G.T (TG)11 7T / (TG)11 7T 1 (0.89) 1540A/G / 1540A/G (TG)11 7T / (TG)11 7T 2 (1.79) R347H / R117H (TG)10 9T / (TG)11 7T 1 (0.89) One mutation detected with one 5T allele (32 / 112 5 28.57%) G551D / - (TG)10 7T/ (TG)13 5T 2 (1.79) F508del / - (TG)12 7T/ (TG)13 5T 8 (7.14) (TG)11 9T/ (TG)13 5T 6 (5.36) 1717-1G.A / - (TG)11 7T/ (TG)12 5T 4 (3.57) R117H / - (TG)12 7T/ (TG)13 5T 2 (1.79) 621+1G.T / - (TG)11 7T/ (TG)13 5T 3 (2.68) 2 (1.79) 1540A/G / - (TG)11 7T/ (TG)13 5T 2 (1.79) R553X / - (TG)12 7T/ (TG)13 5T 1 (0.89) Y122H / -4 (TG)11 7T / (TG)13 5T 1 (0.89) T338A / -4 (TG)10 7T / (TG)13 5T 1 (0.89) No mutation detected with two 5T alleles (11 / 112 5 9.82%) - / - (TG)12 5T / (TG)13 5T 3 (2.68) - / - (TG)13 5T / (TG)13 5T 8 (7.14) One mutation detected without 5T allele (35 / 112 5 31.25%) G85E / - (TG)11 7T / (TG)11 7T 2 (1.79) G551D / - (TG)10 9T / (TG)12 7T1 1 (0.89) 621+1G.T / - (TG)11 7T / (TG)11 7T 2 (1.79) (TG)10 9T / (TG)11 7T 1 (0.89) R334W / - (TG)12 7T / (TG)10 7T 1 (0.89) F508del / - (TG)11 7T / (TG)11 7T 7 (6.25) (TG)11 9T / (TG)12 7T 3 (2.68) (TG)10 9T / (TG)10 9T 2 (1.79) 1717-1G.A / - (TG)11 7T / (TG)12 7T 3 (2.68) (TG)10 9T / (TG)11 7T 2 (1.79) R117H/- (TG)12 7T / (TG)12 7T 2 (1.79) (TG)10 9T / (TG)11 7T 1 (0.89) 2789+5G.A / - (TG)10 7T / (TG)11 7T 1 (0.89) 3120+1G.A / - (TG)10 9T / (TG)11 7T 2 (1.79) R560T / - (TG)10 9T / (TG)11 7T 1 (0.89) N1303K / - (TG)10 9T / (TG)11 7T 1 (0.89) 1651A/G / - (TG)11 7T / (TG)12 7T 1 (0.89) R553X / - (TG)10 9T / (TG)10 7T 1 (0.89) K536X / -4 (TG)10 9T / (TG)10 9T 1 (0.89) No mutation detected with one 5T alleles (7 / 112 5 6.25%) - / - (TG)13 5T / (TG)12 7T 3 (2.68) - / - (TG)13 5T / (TG)10 9T 4 (3.57) No mutation detected (22 / 112 5 19.64%) - / - (TG)11 7T / (TG)11 7T 12 (10.71) - / - (TG)11 7T / (TG)12 7T 1 (1.79) - / - (TG)10 9T / (TG)10 9T 3 (2.68) - / - (TG)10 9T / (TG)11 7T 6 (5.36) CUAVD One mutation detected without 5T allele (2 / 7 5 28.57%) R334W / - (TG)10 9T / (TG)11 7T 1 (14.29) R117H / - (TG)11 7T / (TG)11 7T 1 (14.29) No mutation detected with one 5T alleles (3 / 7 5 42.86%) - / - (TG)11 9T / (TG)13 5T 2 (28.57) - / - (TG)10 7T / (TG)13 5T 1 (14.29) No mutation detected (2 / 7 5 28.57%) - / - (TG)10 9T / (TG)12 7T 2 (28.57) * CBAVD indicates congenital bilateral absence of the vas deferens; CUAVD, congenital unilateral absence of the vas deferens. Login to comment

[hide] A possible role for intracellular GSH in spontaneo... Biometals. 2008 Jun;21(3):277-87. Epub 2007 Sep 12. Liu X
A possible role for intracellular GSH in spontaneous reaction of a cysteine (T338C) engineered into the Cystic Fibrosis Transmembrane Conductance Regulator.
Biometals. 2008 Jun;21(3):277-87. Epub 2007 Sep 12., [PMID:17849169]

Abstract [show]
The conductance of oocytes expressing T338C CFTR (Cystic Fibrosis Transmembrane Conductance Regulator) exhibits variable responses to dithiothreitol (DTT) and 2-mercaptoethanol (2-ME) that we proposed might be due to the extraction of copper from an adventitious binding site (Liu et al. J Biol Chem 281(12):8275-8285, 2006). In order to study the origins of variability in chemical reactivity of T338C CFTR channels, oocytes expressing T338C CFTR were exposed to BCNU (bischloroethylnitrosourea), an inhibitor of glutathione reductase. BCNU treatment caused a significant reduction of initial conductance and an increase in the response to 2-ME or DTT, suggesting a direct or indirect influence of intracellular glutathione (GSH), a major determinant of the disposition of intracellular copper. Single-channel recordings indicated that T338C CFTR channels not exposed to 2-ME or DTT exhibited multiple conductance levels not seen in T338A CFTR channels. Exposure to BCNU shifted the distribution of single-channel current amplitudes towards lower values, whereas exposure to DTT favored higher amplitudes. These results suggest that the altered chemical state of T338C channels is associated with a decreased single-channel conductance and that intracellular factors (most likely GSH) may modulate the propensity of the channel to form these altered states.

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4 Single-channel recordings indicated that T338C CFTR channels not exposed to 2-ME or DTT exhibited multiple conductance levels not seen in T338A CFTR channels. Login to comment
51 Summarized in Fig. 1 are results obtained from oocytes expressing T338C or T338A CFTRs that were either untreated, or exposed to 100 lM BCNU for 72 h prior to electrophysiological recording. Login to comment
59 (B) The initial steady state conductance of oocytes expressing T338A CFTR (black bars) and the conductance after exposure to 1 mM 2-ME (white bars) were summarized for the control oocytes and oocytes maintained in the storage solution (MBSH) containing 100 lM BCNU since injection of cRNA significantly from untreated controls, suggesting that the treatment may mimic the ''naturally modified`` state. Login to comment
60 Oocytes expressing T338A CFTR (Fig. 1B) exhibited no response to 2-ME with or without exposure to BCNU. Login to comment
61 The mean initial conductance of T338A CFTR was slightly lower in BCNU treated oocytes (67 ± 6 lS) than untreated ones (52 ± 9 lS), but the difference was not statistically significant. Login to comment
90 To verify that a cysteine was required for the multiple current amplitudes observed in T338C CFTR, I recorded single-channel currents of T338A CFTR. Login to comment
93 Thus exposure to BCNU did not result in any significant change in the fractional distribution of 0.9 pA events in T338A CFTR channels. Login to comment
94 No difference was detected among the apparent open probabilities (NPo /N) of T338A CFTR channels (pH 7.4) under control or BCNU treated conditions using records obtained at 500 lM intracellular ATP. Login to comment
116 Regardless, a cysteine at position 338 was essential for the GSH effect because at concentrations as high as 10 mM, GSH had no effect on conductance of oocytes expressing Cys-less CFTR (Fig. 5B, n = 2) or T338A CFTR (Fig. 5C, n = 2). Login to comment
120 These results indicate that although GSH is capable of breaking a mixed disulfide bond at 338, the reaction precedes at a much lower rate and required a much higher Fig. 4 BCNU had no effect on single T338A CFTR conductance. Login to comment
121 Fractional distribution of single-channel current amplitudes at pH 7.4 from patches obtained from T338A CFTR expressing oocytes that were: (A) incubated in MBSH, (B) incubated in MBSH containing 100 lM BCNU since injection of cRNA. Login to comment
122 Fractional distribution of single-channel current amplitudes at pH 6.0 from patches obtained from T338A CFTR expressing oocytes that were: (C) incubated in MBSH, (D) incubated in MBSH containing 100 lM BCNU since injection of cRNA. Login to comment
136 (C) Following activation (hatched bar and crosshair), an oocyte expressing T338A CFTR was exposed to 10 mM GSH (open triangles) et al. 1993; Ferreira et al. 1993). Login to comment

[hide] Evidence for direct CFTR inhibition by CFTR(inh)-1... Biochem J. 2008 Jul 1;413(1):135-42. Caci E, Caputo A, Hinzpeter A, Arous N, Fanen P, Sonawane N, Verkman AS, Ravazzolo R, Zegarra-Moran O, Galietta LJ
Evidence for direct CFTR inhibition by CFTR(inh)-172 based on Arg347 mutagenesis.
Biochem J. 2008 Jul 1;413(1):135-42., 2008-07-01 [PMID:18366345]

Abstract [show]
CFTR (cystic fibrosis transmembrane conductance regulator) is an epithelial Cl- channel inhibited with high affinity and selectivity by the thiazolidinone compound CFTR(inh)-172. In the present study, we provide evidence that CFTR(inh)-172 acts directly on the CFTR. We introduced mutations in amino acid residues of the sixth transmembrane helix of the CFTR protein, a domain that has an important role in the formation of the channel pore. Basic and hydrophilic amino acids at positions 334-352 were replaced with alanine residues and the sensitivity to CFTR(inh)-172 was assessed using functional assays. We found that an arginine-to-alanine change at position 347 reduced the inhibitory potency of CFTR(inh)-172 by 20-30-fold. Mutagenesis of Arg347 to other amino acids also decreased the inhibitory potency, with aspartate producing near total loss of CFTR(inh)-172 activity. The results of the present study provide evidence that CFTR(inh)-172 interacts directly with CFTR, and that Arg347 is important for the interaction.

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110 T338A and R347A were similar to wild-type CFTR. Login to comment
127 CFTR form CFTRinh-172 Ki (μM) Hill coefficient I- influx (mM/s) n Wild-type 1.32 + - 0.25 1.03 + - 0.07 0.1336 + - 0.0107 10 S341A 0.57 + - 0.17 1.21 + - 0.37 0.0297 + - 0.0064 4 T338A 3.20 + - 0.86 1.13 + - 0.20 0.1260 + - 0.0225 4 R347A 44.98 + - 4.71** 0.91 + - 0.04 0.1288 + - 0.0154 7 R334A 2.39 + - 0.74 0.93 + - 017 0.0313 + - 0.062 4 A349S 1.23 + - 0.41 1.11 + - 0.25 0.1500 + - 0.011 4 R347D >50 Not determined 0.1160 + - 0.0136 7 R347D/D924R >50 Not determined 0.1008 + - 0.0504 4 R347C >50 Not determined 0.1437 + - 0.0123 4 Mock 0.003 + - 0.001 10 introduced a mutation at position 349 (an alanine residue replaced by a serine residue). Login to comment

[hide] Mutations at arginine 352 alter the pore architect... J Membr Biol. 2008 Mar;222(2):91-106. Epub 2008 Apr 18. Cui G, Zhang ZR, O'Brien AR, Song B, McCarty NA
Mutations at arginine 352 alter the pore architecture of CFTR.
J Membr Biol. 2008 Mar;222(2):91-106. Epub 2008 Apr 18., [PMID:18421494]

Abstract [show]
Arginine 352 (R352) in the sixth transmembrane domain of the cystic fibrosis transmembrane conductance regulator (CFTR) previously was reported to form an anion/cation selectivity filter and to provide positive charge in the intracellular vestibule. However, mutations at this site have nonspecific effects, such as inducing susceptibility of endogenous cysteines to chemical modification. We hypothesized that R352 stabilizes channel structure and that charge-destroying mutations at this site disrupt pore architecture, with multiple consequences. We tested the effects of mutations at R352 on conductance, anion selectivity and block by the sulfonylurea drug glipizide, using recordings of wild-type and mutant channels. Charge-altering mutations at R352 destabilized the open state and altered both selectivity and block. In contrast, R352K-CFTR was similar to wild-type. Full conductance state amplitude was similar to that of wild-type CFTR in all mutants except R352E, suggesting that R352 does not itself form an anion coordination site. In an attempt to identify an acidic residue that may interact with R352, we found that permeation properties were similarly affected by charge-reversing mutations at D993. Wild-type-like properties were rescued in R352E/D993R-CFTR, suggesting that R352 and D993 in the wild-type channel may interact to stabilize pore architecture. Finally, R352A-CFTR was sensitive to modification by externally applied MTSEA+, while wild-type and R352E/D993R-CFTR were not. These data suggest that R352 plays an important structural role in CFTR, perhaps reflecting its involvement in forming a salt bridge with residue D993.

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105 Transitions to these subconductance levels occur rarely in WT-CFTR but more frequently in some pore-domain mutants, such as R334C and T338A, although the relative conductances between levels s1, s2, and f are maintained (Zhang et al. 2005a). Login to comment

[hide] Atomic model of human cystic fibrosis transmembran... Cell Mol Life Sci. 2008 Aug;65(16):2594-612. Mornon JP, Lehn P, Callebaut I
Atomic model of human cystic fibrosis transmembrane conductance regulator: membrane-spanning domains and coupling interfaces.
Cell Mol Life Sci. 2008 Aug;65(16):2594-612., [PMID:18597042]

Abstract [show]
We describe herein an atomic model of the outward-facing three-dimensional structure of the membrane-spanning domains (MSDs) and nucleotide-binding domains (NBDs) of human cystic fibrosis transmembrane conductance regulator (CFTR), based on the experimental structure of the bacterial transporter Sav1866. This model, which is in agreement with previous experimental data, highlights the role of some residues located in the transmembrane passages and directly involved in substrate translocation and of some residues within the intracellular loops (ICL1-ICL4) making MSD/NBD contacts. In particular, our model reveals that D173 ICL1 and N965 ICL3 likely interact with the bound nucleotide and that an intricate H-bond network (involving especially the ICL4 R1070 and the main chain of NBD1 F508) may stabilize the interface between MSD2 and the NBD1F508 region. These observations allow new insights into the ATP-binding sites asymmetry and into the molecular consequences of the F508 deletion, which is the most common cystic fibrosis mutation.

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194 Two mutations involving these residues (F337A and T338A) also significantly weakened the glibenclamide-mediated blocking of the channel [69], suggesting a direct interaction between the inhibitor and this region of the pore. Login to comment

[hide] Non-pore lining amino acid side chains influence a... J Physiol. 1998 Oct 1;512 ( Pt 1):1-16. Linsdell P, Zheng SX, Hanrahan JW
Non-pore lining amino acid side chains influence anion selectivity of the human CFTR Cl- channel expressed in mammalian cell lines.
J Physiol. 1998 Oct 1;512 ( Pt 1):1-16., 1998-10-01 [PMID:9729613]

Abstract [show]
1. The effects of individually mutating two adjacent threonine residues in the sixth membrane-spanning region (TM6) of the cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channel on permeation properties were examined using patch clamp recording from mammalian cell lines stably expressing human CFTR. 2. A number of mutations of T338 significantly affected the permeation properties of the channel. Increases and decreases in single channel conductance were observed for different mutants. Anion selectivity was strongly affected, with no two channel variants sharing the same selectivity sequence. Several mutations led to strong inward rectification of the macroscopic current-voltage relationship. The effects of these mutations on permeation properties were correlated with the size of the amino acid side chain substituted, rather than its chemical nature. 3. Most mutations of T339 resulted in a lack of functional channel expression and apparent misprocessing of the protein. One mutant, T339V, was characterized in detail; its permeation properties were significantly altered, although these effects were not as strong as for T338 mutations. 4. These results suggest an important role for T338 in controlling the permeation properties of the CFTR Cl- channel. It is suggested that mutation of this residue alters the interaction between permeating anions and the channel pore via an indirect effect on the orientation of the TM6 helix.

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91 Significant rectification resulting in a -I+50ÏI-0 ratio significantly less than one was observed in all mutants except T338A (Fig. 3). Login to comment
93 Single CFTR channel currents in inside-out patches excised from CHO cells stably expressing wild-type, T338A or T338S are shown in Fig. 4A. Login to comment
97 Mean slope conductance was increased from 7·9 ± 0·1 pS (n = 18) for wild-type to 10·4 ± 0·1 pS (n = 9) for T338A and 11·3 ± 0·2 pS (n = 5) for T338S. Login to comment
99 Although the gating of T338A and T338S channels was not studied in detail, no striking differences from wild-type were noted. Login to comment
107 Unitary properties of T338A and T338S CFTR A, examples of single channel activity for wild-type, T338A and T338S, recorded at -50 mV. Login to comment
109 B and C, mean single channel current-voltage relationships for wild-type, T338A (B) and T338S (C). Login to comment
142 Permeability of intracellular anions in wild-type and mutant CFTR Cl¦ channels ------------------------------------------------------------ Anion WT T338A T338S T338N T338V T339V ------------------------------------------------------------ Thiocyanate 2·63 ± 0·13 (6) 5·85 ± 0·27 (4)* 4·80 ± 0·19 (3)* 8·72 ± 1·03 (4)* 1·92 ± 0·35 (4)* 3·28 ± 0·08 (4)* Nitrate 1·53 ± 0·04 (7) 2·04 ± 0·08 (3)* 1·82 ± 0·03 (4)* 4·22 ± 0·22 (3)* 6·84 ± 1·18 (7)* 1·61 ± 0·02 (3) Bromide 1·23 ± 0·03 (5) 1·74 ± 0·04 (3)* 1·47 ± 0·07 (3)* 1·66 ± 0·15 (3)* 1·04 ± 0·09 (5) 1·39 ± 0·06 (4)* Chloride 1·00 ± 0·01 (10) 1·00 ± 0·02 (11) 1·00 ± 0·02 (6) 1·00 ± 0·03 (10) 1·00 ± 0·04 (11) 1·00 ± 0·06 (10) Iodide 0·84 ± 0·03 (5) 2·09 ± 0·16 (5)* 1·76 ± 0·09 (3)* 1·03 ± 0·05 (3)* 0·79 ± 0·11 (3) 0·84 ± 0·02 (3) Perchlorate 0·25 ± 0·02 (6) 1·35 ± 0·08 (3)* 0·66 ± 0·06 (3)* 0·41 ± 0·03 (3)* 0·54 ± 0·00 (3)* 0·24 ± 0·01 (4) Benzoate 0·069 ± 0·006 (6) 0·17 ± 0·03 (4)* 0·091 ± 0·019 (3) 0·089 ± 0·015 (4) 0·15 ± 0·02 (4)* 0·097 ± 0·014 (4) Hexafluorophosphate < 0·019 (4) 0·53 ± 0·01 (3)* 0·31 ± 0·02 (3)* 0·68 ± 0·02 (3)* 0·39 ± 0·05 (3)* 0·051 ± 0·010 (4)* Fluoride 0·11 ± 0·01 (7) 0·12 ± 0·02 (4) 0·095 ± 0·012 (4) 0·11 ± 0·01 (4) 0·093 ± 0·009 (3) 0·17 ± 0·02 (4)* Formate 0·25 ± 0·01 (8) 0·45 ± 0·04 (3)* 0·43 ± 0·03 (3)* 0·35 ± 0·04 (4)* 0·22 ± 0·01 (3) 0·28 ± 0·02 (3) Acetate 0·090 ± 0·004 (8) 0·19 ± 0·01 (3)* 0·18 ± 0·01 (3)* 0·10 ± 0·02 (5) 0·11 ± 0·02 (3) 0·16 ± 0·01 (3)* Propanoate 0·14 ± 0·01 (3) 0·18 ± 0·02 (4) 0·098 ± 0·010 (4)* 0·077 ± 0·013 (3)* 0·13 ± 0·02 (3) - Pyruvate 0·10 ± 0·01 (5) 0·20 ± 0·01 (3)* 0·13 ± 0·02 (3) 0·075 ± 0·015 (3) 0·17 ± 0·03 (3)* - Methane sulphonate 0·077 ± 0·005 (5) 0·14 ± 0·02 (4)* 0·079 ± 0·014 (3) 0·038 ± 0·004 (3)* 0·088 ± 0·007 (3) - Glutamate 0·096 ± 0·008 (4) 0·082 ± 0·009 (3) 0·080 ± 0·008 (3) 0·060 ± 0·012 (5)* 0·11 ± 0·01 (3) - Isethionate 0·13 ± 0·01 (4) 0·11 ± 0·01 (3) 0·086 ± 0·012 (5)* 0·043 ± 0·007 (3)* 0·067 ± 0·005 (3)* - Gluconate 0·068 ± 0·004 (36) 0·10 ± 0·01 (3)* 0·060 ± 0·004 (3) 0·044 ± 0·004 (3) 0·077 ± 0·009 (3) 0·088 ± 0·021 (5) ------------------------------------------------------------ Relative permeabilities of different anions present in the intracellular solution under biionic conditions were calculated from macroscopic current reversal potentials (e.g. Figs 7 and 10) as described in Methods. Login to comment
150 Note that all lyotropic anions showed the permeability sequence T338A > T338S > wild-type, again suggesting that the effects of these mutations on pore properties are correlated with the size of the amino acid side chain substituted. Login to comment
153 Pore diameter of T338 mutants Previously, we suggested that the double mutant channel, TT338,339AA, had an increased minimum functional pore diameter, based on its increased permeability to extracellular formate, acetate, propanoate and pyruvate ions (Linsdell et P. Linsdell, S.-X. Zheng and J. W. Hanrahan J. Physiol. 512.18 -------------------------------------------------------------------------------------------- Table 2 ---------------------------------------------- Wild-type SCN¦ > NO×¦ > Br¦ > Cl¦ > I¦ > ClOÚ¦ > formate > F¦ > PFÜ¦ T338A SCN¦ > I¦ ü NO×¦ > Br¦ > ClOÚ¦ > Cl¦ > PFÜ¦ > formate > F¦ T338S SCN¦ > NO×¦ ü I¦ > Br¦ > Cl¦ > ClOÚ¦ > formate > PFÜ¦ > F¦ T338N SCN¦ > NO×¦ > Br¦ > I¦ = Cl¦ > PFÜ¦ > ClOÚ¦ > formate > F¦ T338V NO×¦ > SCN¦ > Br¦ = Cl¦ > I¦ > ClOÚ¦ > PFÜ¦ > formate > F¦ -------------------------------------------------------------------------------------------- Figure 7. Login to comment
164 In each case the data have been fitted by eqn (2), giving minimum functional pore diameters of 0·528 nm (wild-type), 0·576 nm (T338A), 0·549 nm (T338S), 0·510 nm (T338N) and 0·540 nm (T338V). Login to comment
168 In each case the data have been fitted using eqn (2) (see Methods), giving estimates of the functional pore diameter (d) of 0·528 nm for wild-type, 0·576 nm for T338A, 0·549 nm for T338S, 0·510 nm for T338N and 0·540 nm for T338V. Anions examined (in order of increasing diameter) were: SCN¦, Cl¦, NO×¦, Br¦, I¦, ClOÚ¦, benzoate and PFÜ¦. Login to comment
171 In this case, fits by eqn (2) suggested minimum pore diameters of 0·535 nm (wild-type), 0·615 nm (T338A), 0·505 nm (T338S), 0·503 nm (T338N) and 0·530 nm (T338V). Login to comment
179 As with T338A and T338S, T339V showed apparently normal channel gating, with open probability being time and P. Linsdell, S.-X. Zheng and J. W. Hanrahan J. Physiol. 512.110 Figure 9. Login to comment
181 In each case the data have been fitted using eqn (2) (see Methods), giving estimates of the functional pore diameter (d) of 0·535 nm for wild type, 0·615 nm for T338A, 0·505 nm for T338S, 0·503 nm for T338N and 0·530 nm for T338V. Anions examined (in order of increasing diameter) were: formate, acetate, propanoate, pyruvate, methane sulphonate and gluconate.) Login to comment
196 Conversely, the elevated conductance of T338A and T338S might be advantageous in gene or protein replacement therapies for Alteration of CFTR anion selectivityJ. Physiol. 512.1 11 Figure 10. Login to comment
205 Interestingly, both T338A (10·4 pS; Fig. 4B) and T338S (11·3 pS; Fig. 4C) have higher conductances than that we reported previously for TT338,339AA (9·9 pS; Linsdell et al. 1997b). Login to comment
232 Thus in T338A, for example, the relationship between permeability and ionic hydration energy appeared much more linear than for wild-type (Fig. 12B). Login to comment
242 Relationship between relative permeability and free energy of hydration for different intracellular anions A, wild-type; B, T338A. Login to comment

[hide] Differential contribution of TM6 and TM12 to the p... Pflugers Arch. 2012 Mar;463(3):405-18. Epub 2011 Dec 13. Cui G, Song B, Turki HW, McCarty NA
Differential contribution of TM6 and TM12 to the pore of CFTR identified by three sulfonylurea-based blockers.
Pflugers Arch. 2012 Mar;463(3):405-18. Epub 2011 Dec 13., [PMID:22160394]

Abstract [show]
Previous studies suggested that four transmembrane domains 5, 6, 11, 12 make the greatest contribution to forming the pore of the CFTR chloride channel. We used excised, inside-out patches from oocytes expressing CFTR with alanine-scanning mutagenesis in amino acids in TM6 and TM12 to probe CFTR pore structure with four blockers: glibenclamide (Glyb), glipizide (Glip), tolbutamide (Tolb), and Meglitinide. Glyb and Glip blocked wildtype (WT)-CFTR in a voltage-, time-, and concentration-dependent manner. At V (M) = -120 mV with symmetrical 150 mM Cl(-) solution, fractional block of WT-CFTR by 50 muM Glyb and 200 muM Glip was 0.64 +/- 0.03 (n = 7) and 0.48 +/- 0.02 (n = 7), respectively. The major effects on block by Glyb and Glip were found with mutations at F337, S341, I344, M348, and V350 of TM6. Under similar conditions, fractional block of WT-CFTR by 300 muM Tolb was 0.40 +/- 0.04. Unlike Glyb, Glip, and Meglitinide, block by Tolb lacked time-dependence (n = 7). We then tested the effects of alanine mutations in TM12 on block by Glyb and Glip; the major effects were found at N1138, T1142, V1147, N1148, S1149, S1150, I1151, and D1152. From these experiments, we infer that amino acids F337, S341, I344, M348, and V350 of TM6 face the pore when the channel is in the open state, while the amino acids of TM12 make less important contributions to pore function. These data also suggest that the region between F337 and S341 forms the narrow part of the CFTR pore.

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151 The surprising finding that mutations at six adjacent positions Q353A R352A T351A V350A A349S M348A R347A L346A V345A I344A C343A F342A S341A I340A T339A T338A F337A I336A K335A R334A WT ** ** ** ** ** ** * * * 0.8 0.6 0.4 0.2 0 Fractional block by Glyb50 μM Q353A R352A T351A V350A A349S M348A R347A L346A V345A I344A C343A F342A S341A I340A T339A T338A F337A I336A K335A R334A WT ** ** ** ** ** ** ** ** * * * * * * ** ** Fractional block by Tolb300 μM 0.8 0.6 0.4 0.2 0 Q353A R352A T351A V350A A349S M348A R347A L346A V345A I344A C343A F342A S341A I340A T339A T338A F337A I336A K335A R334A WT * ** ** ** ** ** ** ** ** Fractional block by Glip200 μM 0.8 0.6 0.4 0.2 0 Fig. 3 Alanine-scanning in TM6 to identify the amino acids that interact with the three blockers. Login to comment
166 Double asterisks indicate significantly different compared to WT-CFTR (p<0.01) Q353A R352A T351A V350A A349S M348A R347A L346A V345A I344A C343A F342A S341A I340A T339A T338A F337A I336A K335A R334A WT 0.3 0.2 0.1 0 * * ** ** 0.4 Initial block by 50 μM Glyb Q353A R352A T351A V350A A349S M348A R347A L346A V345A I344A C343A F342A S341A I340A T339A T338A F337A I336A K335A R334A WT 0.4 0.3 0.2 0.1 0 ** ** * Initial block by 200 μM Glip Fig. 5 Initial block of WT-CFTR and selected TM6 mutants by 50 μM Glyb (left) and 200 μM Glip (right) in symmetrical 150 mM Cl- solution. Data are shown only for those mutants which exhibited significant changes in steady-state fractional block according to Fig. 3 (bars show mean±SEM, n=5-10). Login to comment
193 Probable orientation of drugs in the pore Glyb and Glip are identical molecules along most of their lengths, differing only in the substituents on the ring at the Q353A R352A T351A V350A A349S M348A R347A L346A V345A I344A C343A F342A S341A I340A T339A T338A F337A I336A K335A R334A WT 0.8 0.6 0.2 0 ** ** ** ** Time-dependent block by 50 μμM Glyb Q353A R352A T351A V350A A349S M348A R347A L346A V345A I344A C343A F342A S341A I340A T339A T338A F337A I336A K335A R334A WT ** ** * ** * Time-dependent block by 200 μM Glip 0.4 0.8 0.6 0.2 00.4 Fig. 6 Time-dependent block of WT-CFTR and selected TM6 mutants by 50 μM Glyb (left) and 200 μM Glip (right) in symmetrical 150 mM Cl- solution. Data are shown only for those mutants which exhibited significant changes in fractional block according to Fig. 3 (bars show mean±SEM, n=5-10). Login to comment
219 Gupta and Linsdell reported that mutation T338A reduced block by Glyb [21], but this was not the case in our experiments with either Glyb or Glip. Login to comment

[hide] Divergent CFTR orthologs respond differently to th... Am J Physiol Cell Physiol. 2012 Jan 1;302(1):C67-76. doi: 10.1152/ajpcell.00225.2011. Epub 2011 Sep 21. Stahl M, Stahl K, Brubacher MB, Forrest JN Jr
Divergent CFTR orthologs respond differently to the channel inhibitors CFTRinh-172, glibenclamide, and GlyH-101.
Am J Physiol Cell Physiol. 2012 Jan 1;302(1):C67-76. doi: 10.1152/ajpcell.00225.2011. Epub 2011 Sep 21., [PMID:21940661]

Abstract [show]
Comparison of diverse orthologs is a powerful tool to study the structure and function of channel proteins. We investigated the response of human, killifish, pig, and shark cystic fibrosis transmembrane conductance regulator (CFTR) to specific inhibitors of the channel: CFTR(inh)-172, glibenclamide, and GlyH-101. In three systems, including organ perfusion of the shark rectal gland, primary cultures of shark rectal gland tubules, and expression studies of each ortholog in cRNA microinjected Xenopus laevis oocytes, we observed fundamental differences in the sensitivity to inhibition by these channel blockers. In organ perfusion studies, shark CFTR was insensitive to inhibition by CFTR(inh)-172. This insensitivity was also seen in short-circuit current experiments with cultured rectal gland tubular epithelial cells (maximum inhibition 4 +/- 1.3%). In oocyte expression studies, shark CFTR was again insensitive to CFTR(inh)-172 (maximum inhibition 10.3 +/- 2.5% at 25 muM), pig CFTR was insensitive to glibenclamide (maximum inhibition 18.4 +/- 4.4% at 250 muM), and all orthologs were sensitive to GlyH-101. The amino acid residues considered responsible by previous site-directed mutagenesis for binding of the three inhibitors are conserved in the four CFTR isoforms studied. These experiments demonstrate a profound difference in the sensitivity of different orthologs of CFTR proteins to inhibition by CFTR blockers that cannot be explained by mutagenesis of single amino acids. We believe that the potency of the inhibitors CFTR(inh)-172, glibenclamide, and GlyH-101 on the CFTR chloride channel protein is likely dictated by the local environment and the three-dimensional structure of additional residues that form the vestibules, the chloride pore, and regulatory regions of the channel.

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219 Gupta et al. (21) observed that two mutations in the 6th transmembrane region, F337A and T338A, significantly weakened glibenclamide block. Login to comment

[hide] CFTR: a cysteine at position 338 in TM6 senses a p... Biophys J. 2004 Dec;87(6):3826-41. Epub 2004 Sep 10. Liu X, Zhang ZR, Fuller MD, Billingsley J, McCarty NA, Dawson DC
CFTR: a cysteine at position 338 in TM6 senses a positive electrostatic potential in the pore.
Biophys J. 2004 Dec;87(6):3826-41. Epub 2004 Sep 10., [PMID:15361410]

Abstract [show]
We investigated the accessibility to protons and thiol-directed reagents of a cysteine substituted at position 338 in transmembrane segment 6 (TM6) of CFTR to test the hypothesis that T338 resides in the pore. Xenopus oocytes expressing T338C CFTR exhibited pH-dependent changes in gCl and I-V shape that were specific to the substituted cysteine. The apparent pKa of T338C CFTR was more acidic than that expected for a cysteine or similar simple thiols in aqueous solution. The pKa was shifted toward alkaline values when a nearby positive charge (R334) was substituted with neutral or negatively charged residues, consistent with the predicted influence of the positive charge of R334, and perhaps other residues, on the titration of a cysteine at 338. The relative rates of chemical modification of T338C CFTR by MTSET+ and MTSES- were also altered by the charge at 334. These observations support a model for CFTR that places T338 within the anion conduction path. The apparent pKa of a cysteine substituted at 338 and the relative rates of reaction of charged thiol-directed reagents provide a crude measure of a positive electrostatic potential that may be due to R334 and other residues near this position in the pore.

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64 The single channel current for T338A CFTR was measured using 150 mM symmetrical [Clÿ ]. Login to comment
121 (C) The I-V plots obtained at pH 9 (dashed line), 7.4 (dotted line), and 6 (solid line) from an oocyte expressing T338A CFTR. Login to comment
124 Changing the bath pH had essentially no effect on the conductances of oocytes expressing T338A CFTR (Fig. 2 C, n ¼ 3), nor did the same maneuver alter the conductances of oocytes expressing T338S (Fig. 3) or wt CFTR (Smith et al., 2001), consistent with the idea that the pH-dependent change in conductance of T338C CFTR was due to the titration of the cysteine substituted at 338. Login to comment
125 As an additional test of the hypothesis that the pH-induced response seen in T338C CFTR was due to the titration of the engineered cysteine, we exposed oocytes expressing T338C CFTR to NEM, a reagent that forms a thioether bond with the cysteine, and thereby blocks titration of the thiol group. Login to comment
155 Single-channel recording also indicated that the conductance of T338A CFTR channels was not sensitive to changes in bath pH (Fig. 5 C, inset). Login to comment
156 It is also of interest to note that the ratio of the single channel conductance at pH 6 and pH 7.4 for T338C CFTR was ;1.8, a value comparable to the ratio observed for the macroscopic conductances (1.7). Login to comment
164 No pH-induced change in Po* was observed in T338A CFTR, which averaged 0.85 6 1.2 at pH 6 and averaged 0.74 6 0.15 at pH 7.4 ([ATP] ¼ 1 mM). Login to comment
205 (Inset) T338A CFTR conductance was insensitive to changes in bath pH. Login to comment
206 Shown are examples of single-channel i-V plots obtained from inside-out patches detached from oocytes expressing T338A CFTR at pH 6 (solid circles) and pH 7.4 (shaded triangles). Login to comment
207 All the recordings for T338A CFTR were done in the presence of 150 mM symmetrical [Clÿ ]. Login to comment
208 NEM- or MTSESÿ -modified T338H/R334C CFTR were more titratable and the apparent pKa values were shifted toward more basic values. Login to comment
122 (C) The I-V plots obtained at pH 9 (dashed line), 7.4 (dotted line), and 6 (solid line) from an oocyte expressing T338A CFTR. Login to comment
165 No pH-induced change in Po* was observed in T338A CFTR, which averaged 0.85 6 1.2 at pH 6 and averaged 0.74 6 0.15 at pH 7.4 ([ATP] &#bc; 1 mM). Login to comment

[hide] Biochemical implications of sequence comparisons o... Arch Biochem Biophys. 2002 May 15;401(2):215-22. Tan AL, Ong SA, Venkatesh B
Biochemical implications of sequence comparisons of the cystic fibrosis transmembrane conductance regulator.
Arch Biochem Biophys. 2002 May 15;401(2):215-22., [PMID:12054472]

Abstract [show]
The cystic fibrosis transmembrane conductance regulator (CFTR) is a chloride channel that is both of medical significance in humans and of interest with regard to osmoregulation in aquatic organisms. CFTR is composed of five domains: two membrane-spanning domains, two nucleotide-binding domains, and a regulatory domain. Notwithstanding the plethora of information concerning the structure and function of CFTR, the biochemistry of many facets of CFTR are not completely understood. In this regard, we have performed a sequence alignment of representative vertebrate CFTR with the aim of generating hypotheses on the functional significance of conserved and variable residues. Postulates on function common to all organisms are: (i) Thr338 in the sixth transmembrane segment could have a function related to that of the pore-lining residue Lys335, and it is possible that Thr338 hydrogen bonds to Lys335, thus indirectly affecting anion permeability; (ii) the fragment (111)PDNKE could be an ion sensor; (iii) motifs in the two nucleotide-binding domains reflect differential ATP binding and hydrolysis; and (iv) an interaction in the R domain involving (765)RRQSVL and the C terminal end of the domain results in an inhibitory conformation. Major adaptations in fishes include variations in the postulated ion sensor (111)PDNKE, and the absence of a proline residue in the R domain with consequent higher chloride efflux.

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70 Hence it is conceivable that since the mutation T338A removes the hydrogen bond, the conformation of the hydrophilic Lys335 is perturbed, and as a result the permeability is altered. Login to comment

[hide] Cystic fibrosis transmembrane conductance regulato... J Gen Physiol. 1997 Oct;110(4):337-9. Dawson DC, Smith SS
Cystic fibrosis transmembrane conductance regulator. Permeant ions find the pore.
J Gen Physiol. 1997 Oct;110(4):337-9., [PMID:9379166]

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37 Of particular interest in this paper is the behavior of a mutant CFTR, T338A, T339A, in which two threonines in TM6 were substituted with alanines. Login to comment

[hide] Novel pore-lining residues in CFTR that govern per... Neuron. 1994 Sep;13(3):623-34. McDonough S, Davidson N, Lester HA, McCarty NA
Novel pore-lining residues in CFTR that govern permeation and open-channel block.
Neuron. 1994 Sep;13(3):623-34., [PMID:7522483]

Abstract [show]
The cystic fibrosis transmembrane conductance regulator (CFTR) is both a member of the ATP-binding cassette superfamily and a Cl(-)-selective ion channel. We investigated the permeation pathway of human CFTR with measurements on conduction and open-channel blockade by diphenylamine-2-carboxylic acid (DPC). We used site-directed mutagenesis and oocyte expression to locate residues in transmembrane domain (TM) 6 and TM 12 that contact DPC and control rectification and single-channel conductances. Thus, TM 12 and the previously investigated TM 6 line the CFTR pore. In each TM, residues in contact with DPC are separated by two turns of an alpha helix. The contributions of TM 6 and TM 12 to DPC block and Cl- permeation, however, are not equivalent. The resulting structural model for the conduction pathway may guide future studies of permeation in other Cl- channels and ATP-binding cassette transporters.

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78 Affinity and Voltage Dependence for Block of CFTR Variants by DPC Construct TM Ko( - 100) (PM) 0 I-V Relation n Properties Wild type Wild type low [Cl-], (10 mM) K335E 6 K335F 6 T338A 6 T339A 6 S341A 6 S341T 6 S1118A 11 T1134A 12 T1134F 12 S1141A 12 Triple 6,12 276 f 14 181 f 13" 303 -t 14 351 * 15' 220 * 14 284 * 47 1251 f 116a 530 f 80" 243 * 37 230 * 20 74 * 3" 220 * 13 325 * 26b 0.41 f 0.01 0.32 f 0.02" 0.42 f 0.01 0.42 f 0.02 0.36 f 0.02" 0.44 * 0.12 0.49 * 0.03" 0.35 f 0.09 0.40 f 0.02 0.35 * 0.02" 0.41 f 0.01 0.42 f 0.03 0.21 * O.Ol",b Linear, E,,, = -8 f 1 mV Ere\ = +48+2mV Inward rectification Linear Linear Linear Strong inward rectification Inward rectification Linear Linear Linear Linear Strong inward rectification Affinity for DPC was determined empirically at -100 mV, from whole-cell currents measured in the presence of 200 uM DPC (see Experimental Procedures). Login to comment
226 In addition, other mutations of hydroxylated residues in TM 6, namely T338A and T339A, affect neither DPC block nor rectification (Table 1). Login to comment

[hide] Molecular determinants of anion selectivity in the... Biophys J. 2000 Jun;78(6):2973-82. Linsdell P, Evagelidis A, Hanrahan JW
Molecular determinants of anion selectivity in the cystic fibrosis transmembrane conductance regulator chloride channel pore.
Biophys J. 2000 Jun;78(6):2973-82., [PMID:10827976]

Abstract [show]
Ionic selectivity in many cation channels is achieved over a short region of the pore known as the selectivity filter, the molecular determinants of which have been identified in Ca(2+), Na(+), and K(+) channels. However, a filter controlling selectivity among different anions has not previously been identified in any Cl(-) channel. In fact, because Cl(-) channels are only weakly selective among small anions, and because their selectivity has proved so resistant to site-directed mutagenesis, the very existence of a discrete anion selectivity filter has been called into question. Here we show that mutation of a putative pore-lining phenylalanine residue, F337, in the sixth membrane-spanning region of the cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel, dramatically alters the relative permeabilities of different anions in the channel. Specifically, mutations that reduce the size of the amino acid side chain present at this position virtually abolish the relationship between anion permeability and hydration energy, a relationship that characterizes the anion selectivity not only of wild-type CFTR, but of most classes of Cl(-) channels. These results suggest that the pore of CFTR may indeed contain a specialized region, analogous to the selectivity filter of cation channels, at which discrimination between different permeant anions takes place. Because F337 is adjacent to another amino acid residue, T338, which also affects anion selectivity in CFTR, we suggest that selectivity is predominantly determined over a physically discrete region of the pore located near these important residues.

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157 Under the macroscopic current recording conditions used here, the mutation T338A changes the halide selectivity from Eisenman sequence III to sequence I, consistent with the strengthening of lyotropic selectivity in this mutant (Linsdell et al., 1998). Login to comment

[hide] Interaction between permeation and gating in a put... Biophys J. 2000 Jul;79(1):298-313. Zhang ZR, McDonough SI, McCarty NA
Interaction between permeation and gating in a putative pore domain mutant in the cystic fibrosis transmembrane conductance regulator.
Biophys J. 2000 Jul;79(1):298-313., [PMID:10866956]

Abstract [show]
The cystic fibrosis transmembrane conductance regulator (CFTR) is a chloride channel with distinctive kinetics. At the whole-cell level, CFTR currents in response to voltage steps are time independent for wild type and for the many mutants reported so far. Single channels open for periods lasting up to tens of seconds; the openings are interrupted by brief closures at hyperpolarized, but not depolarized, potentials. Here we report a serine-to-phenylalanine mutation (S1118F) in the 11th transmembrane domain that confers voltage-dependent, single-exponential current relaxations and moderate inward rectification of the macroscopic currents upon expression in Xenopus oocytes. At steady state, the S1118F-CFTR single-channel conductance rectifies, corresponding to the whole-cell rectification. In addition, the open-channel burst duration is decreased 10-fold compared with wild-type channels. S1118F-CFTR currents are blocked in a voltage-dependent manner by diphenylamine-2-carboxylate (DPC); the affinity of S1118F-CFTR for DPC is similar to that of the wild-type channel, but blockade exhibits moderately reduced voltage dependence. Selectivity of the channel to a range of anions is also affected by this mutation. Furthermore, the permeation properties change during the relaxations, which suggests that there is an interaction between gating and permeation in this mutant. The existence of a mutation that confers voltage dependence upon CFTR currents and that changes kinetics and permeation properties of the channel suggests a functional role for the 11th transmembrane domain in the pore in the wild-type channel.

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338 Mutation T338A altered the voltage dependence of block by DPC without affecting affinity at afa;100 mV (McDonough et al., 1994). Login to comment
343 With respect to block by DPC, S1118F-CFTR had an effect much like that of T338A-CFTR, wherein affinity at afa;100 mV was not changed significantly but the voltage dependence was reduced. Login to comment

[hide] CFTR: Ligand exchange between a permeant anion ([A... Biophys J. 2006 Sep 1;91(5):1737-48. Epub 2006 Jun 9. Serrano JR, Liu X, Borg ER, Alexander CS, Shaw CF 3rd, Dawson DC
CFTR: Ligand exchange between a permeant anion ([Au(CN)2]-) and an engineered cysteine (T338C) blocks the pore.
Biophys J. 2006 Sep 1;91(5):1737-48. Epub 2006 Jun 9., [PMID:16766608]

Abstract [show]
Previous attempts to identify residues that line the pore of the cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel have utilized cysteine-substituted channels in conjunction with impermeant, thiol-reactive reagents like MTSET+ and MTSES-. We report here that the permeant, pseudohalide anion [Au(CN)2]- can also react with a cysteine engineered into the pore of the CFTR channel. Exposure of Xenopus oocytes expressing the T338C CFTR channel to as little as 100 nM [Au(CN)2]- produced a profound reduction in conductance that was not reversed by washing but was reversed by exposing the oocytes to a competing thiol like DTT (dithiothreitol) and 2-ME (2-mercaptoethanol). In detached, inside out patches single-channel currents were abolished by [Au(CN)2]- and activity was not restored by washing [Au(CN)2]- from the bath. Both single-channel and macroscopic currents were restored, however, by exposing [Au(CN)2]- -blocked channels to excess [CN]-. The results are consistent with the hypothesis that [Au(CN)2]- can participate in a ligand exchange reaction with the cysteine thiolate at 338 such that the mixed-ligand complex, with a charge of -1, blocks the anion conduction pathway.

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69 Similar responses to [Au(CN)2] were observed using oocytes expressing T338A CFTR (Fig. 1 C), and the apparent dissociation constants (in mM) were similar for the three constructs: Kwt &#bc; 0.754, KCys-less &#bc; 0.813, KT338A &#bc; 0.754. Login to comment
85 (C) A representative experiment showing the effect of [Au(CN)2] on T338A CFTR conductance. Login to comment
86 After activation by a stimulatory cocktail (Isop1IBMX, hatched bar), an oocyte expressing T338A CFTR channels was exposed to 1 mM DTT (open circles) and then to 10 mM, 100 mM, 1 mM, and 10 mM of [Au(CN)2] (solid circles). Login to comment
121 Block of the residual conductance by [Au(CN)2] was consistent with an apparent dissociation constant of ;0.8 mM, comparable to that seen with wt, T338A, and Cys-less CFTR. Login to comment
143 Exposure of an oocyte expressing a construct bearing a non-Cys substitution at position 338 (T338A CFTR) to IAM (2 mM) was without effect on reversible, lyotropic block by [Au(CN)2] (13). Login to comment

[hide] Two salt bridges differentially contribute to the ... J Biol Chem. 2013 Jul 12;288(28):20758-67. doi: 10.1074/jbc.M113.476226. Epub 2013 May 24. Cui G, Freeman CS, Knotts T, Prince CZ, Kuang C, McCarty NA
Two salt bridges differentially contribute to the maintenance of cystic fibrosis transmembrane conductance regulator (CFTR) channel function.
J Biol Chem. 2013 Jul 12;288(28):20758-67. doi: 10.1074/jbc.M113.476226. Epub 2013 May 24., [PMID:23709221]

Abstract [show]
Previous studies have identified two salt bridges in human CFTR chloride ion channels, Arg(352)-Asp(993) and Arg(347)-Asp(924), that are required for normal channel function. In the present study, we determined how the two salt bridges cooperate to maintain the open pore architecture of CFTR. Our data suggest that Arg(347) not only interacts with Asp(924) but also interacts with Asp(993). The tripartite interaction Arg(347)-Asp(924)-Asp(993) mainly contributes to maintaining a stable s2 open subconductance state. The Arg(352)-Asp(993) salt bridge, in contrast, is involved in stabilizing both the s2 and full (f) open conductance states, with the main contribution being to the f state. The s1 subconductance state does not require either salt bridge. In confirmation of the role of Arg(352) and Asp(993), channels bearing cysteines at these sites could be latched into a full open state using the bifunctional cross-linker 1,2-ethanediyl bismethanethiosulfonate, but only when applied in the open state. Channels remained latched open even after washout of ATP. The results suggest that these interacting residues contribute differently to stabilizing the open pore in different phases of the gating cycle.

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20 Infrequent subconductance behavior is seen in some CFTR mutants, such as T338A/Cand S1141A-CFTR (7, 12). Login to comment
213 We conclude that the subconductance states in CFTR probably also represent pore conformational change for the following reasons: 1) the CFTR channel pore forms from one polypeptide as a monomer and only bears one permeation pathway (12); 2) the s1 and s2 states occur as rare events in some point mutations, such as T338A/Cand K335A/C-CFTR, which do not appear to affect gross pore architecture, whereas they are frequent events in CFTR channels bearing salt bridge mutations, such as R352A- and R347A-CFTR, as discussed above; 3) mutations at sites involved in salt bridges (such as Arg347 , Arg352 , Asp924 , and Asp993 ) result in much more frequent occupancy of subconductance states; 4) mutations at sites involved in salt bridges (such as Arg347 and Arg352 ) lead to greatly altered sensitivity to pore blockers (7, 13); and 5) the subconductance behavior is not affected by different concentrations of Clafa; or by changes in membrane potential (12, 16). Login to comment

[hide] Relative contribution of different transmembrane s... Pflugers Arch. 2014 Mar;466(3):477-90. doi: 10.1007/s00424-013-1317-x. Epub 2013 Aug 20. Wang W, El Hiani Y, Rubaiy HN, Linsdell P
Relative contribution of different transmembrane segments to the CFTR chloride channel pore.
Pflugers Arch. 2014 Mar;466(3):477-90. doi: 10.1007/s00424-013-1317-x. Epub 2013 Aug 20., [PMID:23955087]

Abstract [show]
The membrane-spanning part of the cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel comprises 12 transmembrane (TM) alpha-helices, arranged in 2 symmetrical groups of 6. However, those TMs that line the channel pore are not completely defined. We used patch clamp recording to compare the accessibility of cysteine-reactive reagents to cysteines introduced into different TMs. Several residues in TM11 were accessible to extracellular and/or intracellular cysteine reactive reagents; however, no reactive cysteines were identified in TMs 5 or 11. Two accessible residues in TM11 (T1115C and S1118C) were found to be more readily modified from the extracellular solution in closed channels, but more readily modified from the intracellular solution in open channels, as previously reported for T338C in TM6. However, the effects of mutagenesis at S1118 (TM11) on a range of pore functional properties were relatively minor compared to the large effects of mutagenesis at T338 (TM6). Our results suggest that the CFTR pore is lined by TM11 but not by TM5 or TM7. Comparison with previous works therefore suggests that the pore is lined by TMs 1, 6, 11, and 12, suggesting that the structure of the open channel pore is asymmetric in terms of the contributions of different TMs. Although TMs 6 and 11 appear to undergo similar conformational changes during channel opening and closing, the influence of these two TMs on the functional properties of the narrowest region of the pore is clearly unequal.

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31 Overall, the effects of the T338A mutation have been interpreted as reflecting an increase in functional diameter of the narrowest part of the pore [26] and a reduced barrier to the movement of permeant ions inside the pore [11]. Login to comment
70 Because the properties of T338A-CFTR channels have been investigated in detail previously, simplified protocols were used to facilitate comparison of the effects of the T338A mutation with those of other mutations (Figs. 5, 6, 7, and 8). Login to comment
87 T338A, as well as other substitutions at this position, cause complex changes in the relative permeability of different anions in CFTR [26]; however, the major changes observed in T338A can be summarized as (a) an increase in the relative permeability of lyotropic anions such as SCN- , NO3 - , Br- , I- , and ClO4 - , and (b) an increase in the relative permeability of extracellular organic (kosmotropic) monvalent anions including formate, acetate, propanoate, and pyruvate [26]. Login to comment
150 In addition, possible additive effects of reducing the side chain volumes of these two nearby residues was investigated using a T338A/S1118A double mutant. Login to comment
155 The depolarizing (rightward) shift in the current reversal potential indicates an increased PSCN/PCl in the T338A/S1118A double mutant. Login to comment
156 b Mean PSCN/PCl values calculated from reversal potential measurements under these conditions as described in the "Materials and methods" section. Mean of data from three to six patches. Asterisks indicate a significant difference from wild type (P<0.01), while hashtag indicates a significant difference from the T338A mutant (P<0.0002) Fig. 8 Acetate permeability of mutants. Login to comment
158 The hypolarizing (leftward) shift in the current reversal potential indicates an increased Pacetate/PCl in the T338A/S1118A double mutant. Login to comment
159 b Mean Pacetate/PCl values calculated from reversal potential measurements under these conditions as described in the "Materials and methods" section. Mean of data from three to five patches. Asterisks indicate a significant difference from wild type, while hashtag indicates a significant difference from the T338A mutant (P<0.01) very small (<5 %) reductions in conductance (Fig. 5). Login to comment
162 These very minor effects on conductance are in contrast with the large (>25 %) increase seen in T338A (Fig. 5), as reported previously [26]. Login to comment
163 The double mutant T338A/S1118A had a similarly elevated conductance that was not significantly different from that of T338A alone (P>0.75; Fig. 5). Login to comment
164 Altered movement of permeant ions in the pore of T338A-CFTR is also reflected by changes in the voltage-dependent block of Cl-currents by low concentrations of permeant Au(CN)2 - ions [11, 14, 15]. Login to comment
165 As shown in Fig. 6, channel block by intracellular Au(CN)2 - ions is altered in two distinct ways by the T338A mutation: (a) block is significantly weakened and (b) the I-V relationship in the presence of Au(CN)2 - shows an unusual "N"-shape (Fig. 6a), resulting in a "U"- shaped fractional current-voltage relationship (Fig. 6c) that indicates strongest block close to 0 mV membrane potential that is weakened at both more hyperpolarized and more depolarized voltages. Login to comment
166 This unusual shape is thought to reflect increased unblock by blocker permeation at hyperpolarized voltages, which may reflect a reduced barrier to Au(CN)2 - movement inside the pore in T338A [11]. Login to comment
168 Interestingly, block of the T338A/S1118A double mutant was slightly weaker than for T338A alone (Fig. 6c, d), suggesting that these two mutations might have additive effects on Au(CN)2 - binding in the pore. Login to comment
180 As proposed previously for TM6 residue T338 [42], the channel is shown as being in an "outward facing" configuration when closed (with T1115 and S1118 accessible from the outside), and switching to an "inward facing" configuration on opening (with T1115 and S1118 accessible from the inside) S1118A and T1115A, although this increase was much less than that observed in T338A (Fig. 7b). Login to comment
181 Interestingly, SCN- permeability was further increased in the T338A/S1118A double mutant (Fig. 7). Login to comment
184 Again, the increase in acetate permeability seen in T338A was significantly augmented in the T338A/S1118A mutation (Fig. 8), suggesting an additive effect of these two mutations on organic anion permeability. Login to comment
206 However, mutagenesis of S1118 to residues with smaller (alanine) or larger (glutamine, valine) residues had surprisingly small effects on channel functional properties, in particular compared to those of mutagenesis of T338A. Login to comment
207 The effects of the S1118A mutation on permeant anion (Au(CN)2 - ) binding (Fig. 6), permeability of the lyotropic SCN- anion (Fig. 7), and permeability of the organic acetate anion (Fig. 8) were qualitatively similar to, but generally smaller than, those of T338A, and in fact similar effects were seen in T1115A. Login to comment
211 Reduction of side chain volume in S1118A and T1115A, like T338A, led to an increase in the relative permeability of the small organic anion acetate, consistent with an increase in the apparent diameter of the narrowest region of the pore [25, 26]; however, introduction of side chains with larger volume (S1118Q, S1118V) did not lead to a decrease in acetate permeability (Fig. 8). Login to comment
213 Simultaneous mutagenesis of T338 and S1118 to small alanine residues also had only small additional effects compared to the T338A mutation alone (Figs. 5, 6, 7, and 8). Login to comment
214 Most striking here were a significantly increased permeability of the T338A/S1118A double mutant both to SCN- (Fig. 7) and to acetate (Fig. 8). Login to comment
215 Permeability of small lyotropic anions like SCN- might be influenced by interactions throughout the pore [18, 38] or might be determined predominantly at a localized "selectivity filter" [18, 24] and so the apparently additive effects of the T338A and S1118A mutations is difficult to interpret in terms of the relative roles or locations of these two residues. Login to comment
216 Permeability of large anions such as acetate is thought to be determined predominantly by steric factors at the narrowest part of the pore [25], and so the increase in acetate permeability in T338A/S1118A compared to either mutation alone might be considered evidence that these two mutations impact the dimensions of a common, narrow region of the pore. Login to comment
218 Nevertheless, T338A (which results in a similarly small reduction in side chain volume) is associated with much greater changes in pore properties. Login to comment

[hide] Murine and human CFTR exhibit different sensitivit... Am J Physiol Lung Cell Mol Physiol. 2015 Oct 1;309(7):L687-99. doi: 10.1152/ajplung.00181.2015. Epub 2015 Jul 24. Cui G, McCarty NA
Murine and human CFTR exhibit different sensitivities to CFTR potentiators.
Am J Physiol Lung Cell Mol Physiol. 2015 Oct 1;309(7):L687-99. doi: 10.1152/ajplung.00181.2015. Epub 2015 Jul 24., [PMID:26209275]

Abstract [show]
Development of therapeutic molecules with clinical efficacy as modulators of defective CFTR includes efforts to identify potentiators that can overcome or repair the gating defect in mutant CFTR channels. This has taken a great leap forward with the identification of the potentiator VX-770, now available to patients as "Kalydeco." Other small molecules with different chemical structure also are capable of potentiating the activity of either wild-type or mutant CFTR, suggesting that there are features of the protein that may be targeted to achieve stimulation of channel activity by structurally diverse compounds. However, neither the mechanisms by which these compounds potentiate mutant CFTR nor the site(s) where these compounds bind have been identified. This knowledge gap partly reflects the lack of appropriate experimental models to provide clues toward the identification of binding sites. Here, we have compared the channel behavior and response to novel and known potentiators of human CFTR (hCFTR) and murine (mCFTR) expressed in Xenopus oocytes. Both hCFTR and mCFTR were blocked by GlyH-101 from the extracellular side, but mCFTR activity was increased with GlyH-101 applied directly to the cytoplasmic side. Similarly, glibenclamide only exhibited a blocking effect on hCFTR but both blocked and potentiated mCFTR in excised membrane patches and in intact oocytes. The clinically used CFTR potentiator VX-770 transiently increased hCFTR by approximately 13% but potentiated mCFTR significantly more strongly. Our results suggest that mCFTR pharmacological sensitivities differ from hCFTR, which will provide a useful tool for identifying the binding sites and mechanism for these potentiators.

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131 WT 0.0 0.1 0.2 0.3 Fractional inhibition by 2.5 &#b5;M GlyH-101 # R334C R334A T338A R352A # # # 0.4 0.5 0.4 &#b5;A 50 s ND96 ISO ISO+ GlyH ND96 ISO R334C- hCFTR A B C D ND96 ISO ISO+ GlyH ND96 ISO T338A-hCFTR 1 &#b5;A 50 s 1.0 &#b5;A 50 s ND96 ISO ISO+ GlyH ND96 ISO WT-hCFTR Fig. 5. Login to comment
132 Effects of GlyH-101 on wild-type (WT)- (A), R334C- (B), and T338A-hCFTR (C). Login to comment
163 Extracellular 25 òe;M GlyH-101 failed to block but only potentiated R334A-mCFTR. Summary data for fractional inhibition (E) and fractional potentiation (F) of WT-, V100L-, R147H-, I201V-, R334A-, and T338A-mCFTR with 25 òe;M GlyH-101. Login to comment
181 In contrast, 2.5 òe;M GlyH-101 exhibited strengthened block of both T338A- and R352A-hCFTR (Fig. 5, C and D). Login to comment