ABCMdb

ABCC7 p.Leu102Cys

ClinVar:	c.305T>C , p.Leu102Pro ? , not provided
CF databases:	c.305T>C , p.Leu102Pro (CFTR1) ? , The L102P mutation was detected in the CFTR gene by DGGE and identified by direct sequencing. This mutation has been found in a child with severe CF. The other allele carries a R553X mutation. c.305T>G , p.Leu102Arg (CFTR1) ? ,
Predicted by SNAP2:	A: D (59%), C: N (53%), D: D (85%), E: D (80%), F: D (66%), G: D (80%), H: D (80%), I: N (82%), K: D (85%), M: N (57%), N: D (80%), P: D (85%), Q: D (71%), R: D (80%), S: D (66%), T: D (66%), V: N (61%), W: D (75%), Y: D (75%),
Predicted by PROVEAN:	A: D, C: D, D: D, E: D, F: D, G: D, H: D, I: N, K: D, M: N, N: D, P: D, Q: D, R: D, S: D, T: D, V: N, W: D, Y: D,

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[hide] Functional arrangement of the 12th transmembrane r... Pflugers Arch. 2011 Oct;462(4):559-71. Epub 2011 Jul 28. Qian F, El Hiani Y, Linsdell P
Functional arrangement of the 12th transmembrane region in the CFTR chloride channel pore based on functional investigation of a cysteine-less CFTR variant.
Pflugers Arch. 2011 Oct;462(4):559-71. Epub 2011 Jul 28., [PMID:21796338]

Abstract [show]
The membrane-spanning part of the cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel comprises 12 transmembrane (TM) alpha-helices, arranged into two pseudo-symmetrical groups of six. While TM6 in the N-terminal TMs is known to line the pore and to make an important contribution to channel properties, much less is known about its C-terminal counterpart, TM12. We have used patch clamp recording to investigate the accessibility of cytoplasmically applied cysteine-reactive reagents to cysteines introduced along the length of TM12 in a cysteine-less variant of CFTR. We find that methanethiosulfonate (MTS) reagents irreversibly modify cysteines substituted for TM12 residues N1138, M1140, S1141, T1142, Q1144, W1145, V1147, N1148, and S1149 when applied to the cytoplasmic side of open channels. Cysteines sensitive to internal MTS reagents were not modified by extracellular [2-(trimethylammonium)ethyl] MTS, consistent with MTS reagent impermeability. Both S1141C and T1142C could be modified by intracellular [2-sulfonatoethyl] MTS prior to channel activation; however, N1138C and M1140C, located deeper into the pore from its cytoplasmic end, were modified only after channel activation. Comparison of these results with previous work on CFTR-TM6 allows us to develop a model of the relative positions, functional contributions, and alignment of these two important TMs lining the CFTR pore. We also propose a mechanism by which these seemingly structurally symmetrical TMs make asymmetric contributions to the functional properties of the channel pore.

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140 In this respect, the slow rate of modification observed in N1138C (Fig. 3b) is similar to that we reported for P99C and L102C in TM1 [41] and T338C and S341C in TM6 [9], and the much higher modification rate constant for T1142C, S1141C, and (to a lesser extent) M1140C is closer to that reported for K95C in TM1 [41] and I344C, V345C, and M348C in TM6 [9]. Login to comment

[hide] Alternating access to the transmembrane domain of ... J Biol Chem. 2012 Mar 23;287(13):10156-65. Epub 2012 Feb 1. Wang W, Linsdell P
Alternating access to the transmembrane domain of the ATP-binding cassette protein cystic fibrosis transmembrane conductance regulator (ABCC7).
J Biol Chem. 2012 Mar 23;287(13):10156-65. Epub 2012 Feb 1., [PMID:22303012]

Abstract [show]
The cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel is a member of the ATP-binding cassette (ABC) protein family, most members of which act as active transporters. Actively transporting ABC proteins are thought to alternate between "outwardly facing" and "inwardly facing" conformations of the transmembrane substrate pathway. In CFTR, it is assumed that the outwardly facing conformation corresponds to the channel open state, based on homology with other ABC proteins. We have used patch clamp recording to quantify the rate of access of cysteine-reactive probes to cysteines introduced into two different transmembrane regions of CFTR from both the intracellular and extracellular solutions. Two probes, the large [2-sulfonatoethyl]methanethiosulfonate (MTSES) molecule and permeant Au(CN)(2)(-) ions, were applied to either side of the membrane to modify cysteines substituted for Leu-102 (first transmembrane region) and Thr-338 (sixth transmembrane region). Channel opening and closing were altered by mutations in the nucleotide binding domains of the channel. We find that, for both MTSES and Au(CN)(2)(-), access to these two cysteines from the cytoplasmic side is faster in open channels, whereas access to these same sites from the extracellular side is faster in closed channels. These results are consistent with alternating access to the transmembrane regions, however with the open state facing inwardly and the closed state facing outwardly. Our findings therefore prompt revision of current CFTR structural and mechanistic models, as well as having broader implications for transport mechanisms in all ABC proteins. Our results also suggest possible locations of both functional and dysfunctional ("vestigial") gates within the CFTR permeation pathway.

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42 Interestingly, cysteines substituted into another pore-lining TM, TM1, did not show access to both sides of the membrane, with the outermost site in this TM that could be modified by intracellular MTS reagents (L102C) being insensitive to extracellular MTS reagents (18). Login to comment
43 In the present work, we have compared changes in the accessibility of T338C in TM6 with L102C in TM1 to both intracellular and extracellular cysteine-reactive reagents, both large, impermeant [2-sul- fonatoethyl] MTS (MTSES) and smaller, permeant Au(CN)2 - ions, under conditions in which ATP-dependent channel gating is altered. Login to comment
50 Two reporter cysteines in the pore were studied: T338C in TM6, which is modified by both intracellular and extracellular MTS reagents (17), and L102C in TM1, which is modified by intracellular, but not extracellular MTS reagents (18). Login to comment
52 These two reporter cysteine substitutions were combined with mutations in the NBDs that affect ATP-dependent channel gating: K464A (NBD1) and E1371Q (NBD2). Login to comment
79 Fig. 1 shows the influence of these NBD mutations on the rate of modification of two cysteines introduced deep into the channel pore from the inside, T338C in TM6 and L102C in TM1. Login to comment
82 Rate of modification of T338C and L102C by internal MTSES. Login to comment
86 The decline in current amplitude following MTSES application has been fitted by a single exponential function. C, average modification rate constants (k) for MTSES, calculated from fits to data such as those shown in A and B. Asterisks indicate a significant difference from the cysteine mutants T338C and L102C (black bars) (p Ͻ 0.05). Login to comment
89 For modification of T338C, the mean modification rate constant was decreased ϳ2.4-fold in a K464A background and increased ϳ3.9-fold in E1371Q, whereas the modification rate constant for L102C was decreased by ϳ26% in K464A and increased ϳ2.0-fold in E1371Q. Login to comment
90 As in our previous work (22), the effects of the E1371Q mutation were mimicked by locking channels open by treatment with 2 mM pyrophosphate (data not shown; ϳ3.9-fold increase for T338C and ϳ1.6-fold increase for L102C). Login to comment
92 To investigate whether permeant anions show the same regulated access from the cytoplasm to T338C and L102C, we investigated channel modification by Au(CN)2 - , a highly permeant anion that has been used previously to modify cysteine side chains in the CFTR pore (14, 22). Login to comment
93 As shown in Fig. 2, application of a low concentration of Au(CN)2 - (2 ␮M) caused a rapid inhibition of current carried by both T338C and L102C. Login to comment
94 As with MTSES, the rate of modification by Au(CN)2 - was significantly decreased by the K464A mutation (by ϳ1.8-fold for T338C and ϳ3.4-fold for L102C) and significantly increased by the E1371Q mutation (by ϳ5.6-fold for T338C and ϳ1.8-fold for L102C), as well as by pyrophosphate treatment (by ϳ6.0-fold for T338C and ϳ2.0-fold for L102C; data not shown). Login to comment
96 Regulated Access from Extracellular Solution to Pore- T338C is modified not only by intracellular, but also by extracellular MTS reagents (14, 17, 26), whereas L102C was reported to be insensitive to extracellular MTS reagents (18). Login to comment
98 Expression of all CFTR constructs (except those containing the E1371Q mutation, see below) in baby hamster kidney cells led to the appearance of cAMP-activated whole cell currents that were inhibited by the specific CFTR inhibitor GlyH-101 (Fig. 3 and supplemental Fig. S2) and which were not observed in cells transfected with vector alone (supplemental Fig. S2). Login to comment
99 Expression of all E1371Q-CFTR constructs led to the appearance of constitutive, cAMP-insensitive but GlyH-101-inhibited whole cell currents (supplemental Fig. S2). Login to comment
101 In contrast, T338C was strongly inhibited by very much lower concentrations of MTSES (1 ␮M) and Au(CN)2 - (200 nM) (Fig. 3B). Login to comment
102 L102C was insensitive to 2 mM MTSES, but was strongly inhibited by intermediate concentrations of Au(CN)2 - (10 ␮M) (Fig. 3C). Login to comment
109 The decline in current amplitude following Au(CN)2 - application has been fitted by a single exponential function. C, average modification rate constants(k)forAu(CN)2 - ,calculatedfromfitstodatasuchasthoseshowninAandB.AsterisksindicateasignificantdifferencefromthecysteinemutantsT338C and L102C (black bars) (p Ͻ 0.02). Login to comment
112 L102C was not apparently modified by extracellular MTSES (Fig. 3C), consistent with previous findings (18). Login to comment
113 Fig. 5 shows a similar analysis of the rate of modification by extracellular Au(CN)2 - , both for T338C (Fig. 5A; 200 nM Au(CN)2 - ) and for L102C (Fig. 5B; 10 ␮M Au(CN)2 - ). Login to comment
114 Quantification of the rate constant for modification (Fig. 5C) suggests, for modification of T338C, an increase of ϳ5.7-fold in K464A and a decrease of ϳ150-fold in E1371Q, and for modification of L102C, an increase of ϳ2.3-fold in K464A and a decrease of ϳ2.7-fold in E1371Q. Login to comment
116 Changing Patterns of Accessibility Suggest Marker Cysteine Residues "Switch Sides" of Membrane during Gating-The effects of NBD mutations on the rate of modification of T338C and L102C by internal cysteine-reactive reagents (estimated from experiments on inside-out membrane patches) and by external cysteine-reactive reagents (estimated from whole cell current recording experiments) are compared in Fig. 6. Login to comment
120 In each panel, it can be seen that the rate of modification by internal MTSES and Au(CN)2 - increases in the order K464A Ͻ Cys-less Ͻ E1371Q, whereas modification by extracellular MTSES (in T338C) and Au(CN)2 - shows the opposite pattern, K464A Ͼ Cys-less Ͼ E1371Q. Login to comment
122 This suggests that the reporter cysteines in the pore that we have used, T338C and L102C, are capable of "moving" from a relatively internally accessible position to a relatively externally accessible position. Login to comment
127 Sample whole cell currents were recorded at ϩ30 mV for Cys-less (A), T338C (B), and L102C (C). Login to comment
131 C, L102C currents were insensitive to MTSES (2 mM) but were inhibited by GlyH-101 (50 ␮M) and Au(CN)2 - (10 ␮M). Login to comment
151 For example, L102C in TM1 is modified by internal, but not external MTS reagents (18), a result confirmed by the present results (Figs. 1 and 3), whereas R104C, only 2 residues closer to the external end of TM1, is modified by external, but not internal MTS reagents (28). Login to comment
152 L102C, like T338C, becomes apparently more accessible to internal cysteine reactive reagents in open channels (Fig. 6B), but is inaccessible to extracellular MTSES (Fig. FIGURE 4. Login to comment
157 Modification rate constant for T338C/E1371Q was quantified from experiments using a higher concentration of MTSES (200 ␮M). Login to comment
158 Asterisks indicate a significant difference from T338C alone (p Ͻ 0.0005). Login to comment
160 L102C is accessible to permeant Au(CN)2 - ions applied to either side of the membrane, as expected for a permeant probe that ought to access the entire permeation pathway, and as with T338C access from the outside decreases as access from the inside increases (Fig. 6D), again consistent with easier access from the cytoplasm in open channels and from the extracellular solution in closed channels. Login to comment
161 One possible explanation for the difference in external accessibility of L102C in TM1 and T338C in TM6 is that T338C is located in a more superficial position in the outer mouth of the pore (at least in closed channels), such that it can be accessed by large extracellular MTS reagents that cannot penetrate further into the pore from the outside to modify L102C (Fig. 7A). Login to comment
162 Consistent with this differential access from the outside, the rate of modification by extracellular Au(CN)2 - is approximately 35 times greater for T338C than for L102C in a Cys-less background (Fig. 5C). Login to comment
170 Rate of modification of T338C and L102C by external Au(CN)2 - . Login to comment
174 Modification rate constants for T338C/E1371Q and L102C/E1371Q were quantified from experiments using a higher concentration of Au(CN)2 - (100 ␮M). Login to comment
175 Asterisks indicate a significant difference from T338C and L102C alone as applicable (black bars) (p Ͻ 0.01).Data are mean from three or four patches. Login to comment
178 Each panel illustrates the change in modification rate constant for the same reporter cysteine (T338C in A and C, L102C in B and D) in three different backgrounds (K464A, Cys-less, and E1371Q), for modification by MTSES (A and B) or Au(CN)2 - (C and D) applied to the intracellular (●, inside) or extracellular (E, outside) side of the membrane. Login to comment
180 Note that L102C was not apparently modified by extracellular MTSES (B; see Fig. 3C). Login to comment
191 In this model, reduced access from the extracellular solution in open channels is due to partial closure of a vestigial open gate, which decreases the rate of entry of extracellular MTSES and Au(CN)2 - to T338C and L102C, although not completely occluding the pore and thus allowing Cl- permeation. Login to comment
201 FIGURE7.ModelsofCFTRporestructureduringgating.Theimplicationsof the current experimental findings, that T338C and L102C in the CFTR pore show increased access to the extracellular solution in the closed state and increased access to the intracellular solution in the open state, can be interpreted according to a number of different simple diagram models of channel function. Login to comment
44 Interestingly, cysteines substituted into another pore-lining TM, TM1, did not show access to both sides of the membrane, with the outermost site in this TM that could be modified by intracellular MTS reagents (L102C) being insensitive to extracellular MTS reagents (18). Login to comment
45 In the present work, we have compared changes in the accessibility of T338C in TM6 with L102C in TM1 to both intracellular and extracellular cysteine-reactive reagents, both large, impermeant [2-sul- fonatoethyl] MTS (MTSES) and smaller, permeant Au(CN)2 afa; ions, under conditions in which ATP-dependent channel gating is altered. Login to comment
85 Rate of modification of T338C and L102C by internal MTSES. Login to comment
95 As in our previous work (22), the effects of the E1371Q mutation were mimicked by locking channels open by treatment with 2 mM pyrophosphate (data not shown; b03;3.9-fold increase for T338C and b03;1.6-fold increase for L102C). Login to comment
97 To investigate whether permeant anions show the same regulated access from the cytoplasm to T338C and L102C, we investigated channel modification by Au(CN)2 afa; , a highly permeant anion that has been used previously to modify cysteine side chains in the CFTR pore (14, 22). Login to comment
107 L102C was insensitive to 2 mM MTSES, but was strongly inhibited by intermediate concentrations of Au(CN)2 afa; (10 òe;M) (Fig. 3C). Login to comment
118 L102C was not apparently modified by extracellular MTSES (Fig. 3C), consistent with previous findings (18). Login to comment
119 Fig. 5 shows a similar analysis of the rate of modification by extracellular Au(CN)2 afa; , both for T338C (Fig. 5A; 200 nM Au(CN)2 afa; ) and for L102C (Fig. 5B; 10 òe;M Au(CN)2 afa; ). Login to comment
128 This suggests that the reporter cysteines in the pore that we have used, T338C and L102C, are capable of "moving" from a relatively internally accessible position to a relatively externally accessible position. Login to comment
133 Sample whole cell currents were recorded at af9;30 mV for Cys-less (A), T338C (B), and L102C (C). Login to comment
137 C, L102C currents were insensitive to MTSES (2 mM) but were inhibited by GlyH-101 (50 òe;M) and Au(CN)2 afa; (10 òe;M). Login to comment
167 L102C is accessible to permeant Au(CN)2 afa; ions applied to either side of the membrane, as expected for a permeant probe that ought to access the entire permeation pathway, and as with T338C access from the outside decreases as access from the inside increases (Fig. 6D), again consistent with easier access from the cytoplasm in open channels and from the extracellular solution in closed channels. Login to comment
168 One possible explanation for the difference in external accessibility of L102C in TM1 and T338C in TM6 is that T338C is located in a more superficial position in the outer mouth of the pore (at least in closed channels), such that it can be accessed by large extracellular MTS reagents that cannot penetrate further into the pore from the outside to modify L102C (Fig. 7A). Login to comment
169 Consistent with this differential access from the outside, the rate of modification by extracellular Au(CN)2 afa; is approximately 35 times greater for T338C than for L102C in a Cys-less background (Fig. 5C). Login to comment
177 Rate of modification of T338C and L102C by external Au(CN)2 d1a; . Login to comment
181 Modification rate constants for T338C/E1371Q and L102C/E1371Q were quantified from experiments using a higher concentration of Au(CN)2 afa; (100 òe;M). Login to comment
182 Asterisks indicate a significant difference from T338C and L102C alone as applicable (black bars) (p b0d; 0.01).Data are mean from three or four patches. Login to comment
185 Each panel illustrates the change in modification rate constant for the same reporter cysteine (T338C in A and C, L102C in B and D) in three different backgrounds (K464A, Cys-less, and E1371Q), for modification by MTSES (A and B) or Au(CN)2 afa; (C and D) applied to the intracellular (cf;, inside) or extracellular (E, outside) side of the membrane. Login to comment
187 Note that L102C was not apparently modified by extracellular MTSES (B; see Fig. 3C). Login to comment
198 In this model, reduced access from the extracellular solution in open channels is due to partial closure of a vestigial open gate, which decreases the rate of entry of extracellular MTSES and Au(CN)2 afa; to T338C and L102C, although not completely occluding the pore and thus allowing Clafa; permeation. Login to comment
208 FIGURE7.ModelsofCFTRporestructureduringgating.Theimplicationsof the current experimental findings, that T338C and L102C in the CFTR pore show increased access to the extracellular solution in the closed state and increased access to the intracellular solution in the open state, can be interpreted according to a number of different simple diagram models of channel function. Login to comment

[hide] Alignment of transmembrane regions in the cystic f... J Gen Physiol. 2011 Aug;138(2):165-78. Epub 2011 Jul 11. Wang W, El Hiani Y, Linsdell P
Alignment of transmembrane regions in the cystic fibrosis transmembrane conductance regulator chloride channel pore.
J Gen Physiol. 2011 Aug;138(2):165-78. Epub 2011 Jul 11., [PMID:21746847]

Abstract [show]
Different transmembrane (TM) alpha helices are known to line the pore of the cystic fibrosis TM conductance regulator (CFTR) Cl(-) channel. However, the relative alignment of these TMs in the three-dimensional structure of the pore is not known. We have used patch-clamp recording to investigate the accessibility of cytoplasmically applied cysteine-reactive reagents to cysteines introduced along the length of the pore-lining first TM (TM1) of a cysteine-less variant of CFTR. We find that methanethiosulfonate (MTS) reagents irreversibly modify cysteines substituted for TM1 residues K95, Q98, P99, and L102 when applied to the cytoplasmic side of open channels. Residues closer to the intracellular end of TM1 (Y84-T94) were not apparently modified by MTS reagents, suggesting that this part of TM1 does not line the pore. None of the internal MTS reagent-reactive cysteines was modified by extracellular [2-(trimethylammonium)ethyl] MTS. Only K95C, closest to the putative intracellular end of TM1, was apparently modified by intracellular [2-sulfonatoethyl] MTS before channel activation. Comparison of these results with recent work on CFTR-TM6 suggests a relative alignment of these two important TMs along the axis of the pore. This alignment was tested experimentally by formation of disulfide bridges between pairs of cysteines introduced into these two TMs. Currents carried by the double mutants K95C/I344C and Q98C/I344C, but not by the corresponding single-site mutants, were inhibited by the oxidizing agent copper(II)-o-phenanthroline. This inhibition was irreversible on washing but could be reversed by the reducing agent dithiothreitol, suggesting disulfide bond formation between the introduced cysteine side chains. These results allow us to develop a model of the relative positions, functional contributions, and alignment of two important TMs lining the CFTR pore. Such functional information is necessary to understand and interpret the three-dimensional structure of the pore.

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71 In contrast, macroscopic currents carried by four mutants, K95C, Q98C, P99C, and L102C, were found to be significantly and rapidly sensitive to the application of both MTSES and MTSET (Figs. 1-3). Login to comment
98 As shown in Fig. 3 A, MTSES modification was rapid in K95C, even when a low concentration of MTSES (20 µM) was used, and considerably slower in L102C (using 200 µM MTSES). Login to comment
105 (B) Example leak-subtracted I-V relationships for cys-less CFTR, K95C, Q98C, P99C, L102C, and R104C, recorded from inside-out membrane patches after maximal channel activation with 20 nM PKA, 1 mM ATP, and 2 mM PPi. Login to comment
112 As shown in Fig. 4 A, patches excised from MTSET-pretreated cells expressing K95C, Q98C, P99C, or L102C all gave macroscopic currents that were increased in amplitude after the addition of 2 mM constants was that modification was faster for cysteines introduced closer to the intracellular end of TM1, and slower for cysteines located more deeply along the axis of TM1 (Fig. 3 B). Login to comment
123 (A) Example time courses of macroscopic currents (measured at 50 mV during brief voltage excursions from a holding potential of 0 mV) carried by K95C (left) and L102C (right) as indicated, in inside-out membrane patches. Login to comment
125 In each case, MTSES (20 µM for K95C and 200 µM for L102C) was applied to the cytoplasmic face of the patch at time zero (as indicated by the hatched bar at the bottom of each panel). Login to comment
138 These results suggest that none of K95C, Q98C, P99C, or L102C can be modified covalently by extracellular MTSET. Login to comment
141 We used a similar approach to determine if K95C, Q98C, P99C, and L102C could be modified by MTSES pretreatment. Login to comment
149 In TM6, I344 and V345 were proposed to lie on either side of this barrier, based on similar and L102C were all strongly inhibited by the application of the test dose of MTSES, suggesting that they were not covalently modified by MTSES pretreatment. Login to comment
150 These results, which are summarized quantitatively in Fig. 5 C, suggest that although K95C can be modified by MTSES before channel activation, Q98C, P99C, and L102C are modified by MTSES only very slowly, if at all, in channels that have not been activated by PKA and ATP. Login to comment
214 In open CFTR channels, internally applied MTS reagents can penetrate far enough into the pore as to modify L102C in TM1 and F337C in TM6. Login to comment
228 Thus, the side chains of TM1 mutants K95C, Q98C, P99C, and L102C that we identified as accessible to MTS reagents applied from the inside (Fig. 2) were not accessible to MTSET applied to the outside (Fig. 4), whereas R104C, previously shown to be modified by external MTS reagents (Zhou et al., 2008), was not modified by internal MTSES or MTSET (Fig. 2). Login to comment
238 Although we have not investigated the state dependence of MTSES modification in TM1 in such great detail, our present results suggest a similar arrangement in which K95C can readily be modified before channel activation (Fig. 5), whereas Q98C, P99C, and L102C are modified rapidly after channel activation (Fig. 3) but very slowly if at all before activation (Fig. 5). Login to comment
256 For comparison, the MTSES modification rate constant for P99C and L102C (Fig. 3) was similar to that of T338C and S341C in TM6 (El Hiani and Linsdell, 2010) (all between 100 and 150 M1 s1 ), and the modification rate constant for K95C was comparable to, or slightly greater than, that of I344C, V345C, and M348C (El Hiani and Linsdell, 2010) (all between 2,000 and 4,000 M1 s1 ). Login to comment

[hide] Cysteine scanning of CFTR's first transmembrane se... Biophys J. 2013 Feb 19;104(4):786-97. doi: 10.1016/j.bpj.2012.12.048. Gao X, Bai Y, Hwang TC
Cysteine scanning of CFTR's first transmembrane segment reveals its plausible roles in gating and permeation.
Biophys J. 2013 Feb 19;104(4):786-97. doi: 10.1016/j.bpj.2012.12.048., [PMID:23442957]

Abstract [show]
Previous cysteine scanning studies of the cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel have identified several transmembrane segments (TMs), including TM1, 3, 6, 9, and 12, as structural components of the pore. Some of these TMs such as TM6 and 12 may also be involved in gating conformational changes. However, recent results on TM1 seem puzzling in that the observed reactive pattern was quite different from those seen with TM6 and 12. In addition, whether TM1 also plays a role in gating motions remains largely unknown. Here, we investigated CFTR's TM1 by applying methanethiosulfonate (MTS) reagents from both cytoplasmic and extracellular sides of the membrane. Our experiments identified four positive positions, E92, K95, Q98, and L102, when the negatively charged MTSES was applied from the cytoplasmic side. Intriguingly, these four residues reside in the extracellular half of TM1 in previously defined CFTR topology; we thus extended our scanning to residues located extracellularly to L102. We found that cysteines introduced into positions 106, 107, and 109 indeed react with extracellularly applied MTS probes, but not to intracellularly applied reagents. Interestingly, whole-cell A107C-CFTR currents were very sensitive to changes of bath pH as if the introduced cysteine assumes an altered pKa-like T338C in TM6. These findings lead us to propose a revised topology for CFTR's TM1 that spans at least from E92 to Y109. Additionally, side-dependent modifications of these positions indicate a narrow region (L102-I106) that prevents MTS reagents from penetrating the pore, a picture similar to what has been reported for TM6. Moreover, modifications of K95C, Q98C, and L102C exhibit strong state dependency with negligible modification when the channel is closed, suggesting a significant rearrangement of TM1 during CFTR's gating cycle. The structural implications of these findings are discussed in light of the crystal structures of ABC transporters and homology models of CFTR.

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11 Moreover, modifications of K95C, Q98C, and L102C exhibit strong state dependency with negligible modification when the channel is closed, suggesting a significant rearrangement of TM1 during CFTR`s gating cycle. Login to comment
52 (B) Cytoplasmic application of MTSES dramatically reduced ATP-gated L102C/Cysless channel currents in an excised inside-out membrane patch. Login to comment
81 Fig. 1 B shows a representative recording of L102C mutant channels in response to the application of MTSES. Login to comment
108 We next tested the accessibility to external MTSES on three positions identified by experiments with inside-out patches, namely K95C, Q98C, and L102C, in the same manner and all three positions turned out nonreactive (data not shown). Login to comment
149 For instance, for L102C mutant channels, we found that macroscopic currents were increased by 108.5 5 9.5% (n &#bc; 6) after modification by MTSETapplied to the cytoplasmic side of the channel in excised inside-out patches (Fig. 5 A). Login to comment
150 Also similar to what has been reported for positions I344 and M348 in TM6, following MTSET modification of L102C-CFTR, robust activity was observed even in the complete absence of ATP. Login to comment
155 The remarkable effects of MTSET on L102C-CFTR currents shown in Fig. 5 A prompted us to examine this effect more closely with single-channel recordings. Login to comment
156 Fig. 5 B shows single-channel traces before and after MTSET modification of L102C-CFTR. Login to comment
159 Although previous studies have provided evidence that these short-lived closures are ATP independent (43,44) and could result from voltage-dependent block of the pore by large anions from the cytoplasmic side of the channel (45), it is noted that these events are abundantly present in the double mutant L102C/E1371Q at both negative and positive membrane potentials in the absence of ATP (see Fig. S3). Login to comment
165 State-dependent modification of E92C-, K95C-, Q98C-, and L102C-CFTR The observation that MTSET modification of Q98C and L102C alters CFTR gating suggests that TM1 indeed participates in gating motions of CFTR. Login to comment
174 Similar results were obtained from experiments on L102C (Fig. 6 B) and Q98C mutants. Login to comment
175 These data suggest that the modification rate of cysteines on these positions is FIGURE 5 Gating of MTSET-modified L102C/Cysless channel. Login to comment
176 (A) A continuous current recording of L102C/Cysless channels showing that MTSET modification increases the macroscopic current and renders the current ATP-independent. Login to comment
177 (B) Single-channel recording of the L102C/ Cysless-CFTR. Login to comment
196 Third, in the report by Wang et al. (35), K95C, but not Q98C, P99C, or L102C, can react with internal MTSES even before the channel is activated by PKA and ATP, implying a regulated barrier between positions 95 and 98. Login to comment
204 (B) A real-time recording with a similar protocol shown in (A) for L102C/Cysless channels. Login to comment
206 (C) Summary of the modification rates for K95C, Q98C, and L102C in the presence of ATP (solid squares). Login to comment
274 Once we accept the possibility that TM1 undergoes major conformational changes during gating transitions, it is not surprising to see alterations in gating by the mutation itself (e.g., L102C), as well as by subsequent chemical modifications (Fig. 5). Login to comment
275 It is nevertheless interesting to note that in L102C/E1371Q-CFTR, the gate can open and close repeatedly even when the NBDs are in a dimeric configuration (Fig. S3). Login to comment
276 On the other hand, modification of L102C by MTSET literally renders the channel permanently open even long after ATP is removed and presumably NBDs have been separated. Login to comment