ABCMdb

ABCC7 p.Tyr1424Ala

ClinVar:	c.4272C>T , p.Tyr1424= N , Benign
CF databases:	c.4272C>G , p.Tyr1424* (CFTR1) ? ,
Predicted by SNAP2:	A: D (80%), C: D (63%), D: D (91%), E: D (91%), F: N (78%), G: D (91%), H: D (85%), I: D (71%), K: D (91%), L: D (71%), M: D (66%), N: D (85%), P: D (95%), Q: D (85%), R: D (91%), S: D (85%), T: D (85%), V: D (71%), W: D (85%),
Predicted by PROVEAN:	A: D, C: D, D: D, E: D, F: N, G: D, H: D, I: D, K: D, L: D, M: D, N: D, P: D, Q: D, R: D, S: D, T: D, V: D, W: N,

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[hide] The carboxyl terminus of the cystic fibrosis trans... J Biol Chem. 2000 Feb 4;275(5):3655-60. Weixel KM, Bradbury NA
The carboxyl terminus of the cystic fibrosis transmembrane conductance regulator binds to AP-2 clathrin adaptors.
J Biol Chem. 2000 Feb 4;275(5):3655-60., 2000-02-04 [PMID:10652362]

Abstract [show]
The cystic fibrosis transmembrane conductance regulator (CFTR) undergoes rapid and efficient endocytosis. Since functionally active CFTR is found in purified clathrin-coated vesicles isolated from both cultured epithelial cells and intact epithelial tissues, we investigated the molecular mechanisms whereby CFTR could enter such endocytic clathrin-coated vesicles. In vivo cross-linking and in vitro pull-down assays show that full-length CFTR binds to the endocytic adaptor complex AP-2. Fusion proteins containing the carboxyl terminus of CFTR (amino acids 1404-1480) were also able to bind AP-2 but did not bind the Golgi-specific adaptor complex AP-1. Substitution of an alanine residue for tyrosine at position 1424 significantly reduced the ability of AP-2 to bind the carboxyl terminus of CFTR; however, mutation to a phenylalanine residue (an amino acid found at position 1424 in dogfish CFTR) did not perturb AP-2 binding. Secondary structure predictions suggest that Tyr(1424) is present in a beta-turn conformation, a conformation disrupted by alanine but not phenylalanine. Together, these data suggest that the carboxyl terminus of CFTR contains a tyrosine-based internalization signal that interacts with the endocytic adaptor complex AP-2 to facilitate efficient entry of CFTR into clathrin-coated vesicles.

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No. Sentence Comment
4 Substitution of an alanine residue for tyrosine at position 1424 significantly reduced the ability of AP-2 to bind the carboxyl terminus of CFTR; however, mutation to a phenylalanine residue (an amino acid found at position 1424 in dogfish CFTR) did not perturb AP-2 binding. Login to comment
93 To examine the contribution of Tyr1424 to the association between the carboxyl terminus of CFTR and AP-2 adaptors, Tyr1424 was mutated to Ala in the context of the GST fusion protein GST-CT (GST-CT-Y1424A). Login to comment
94 GST-CT-Y1424A preabsorbed to glutathione-Sepharose was incubated with purified adaptors, and after extensive washing, bound proteins were analyzed by immunoblot as before. Login to comment
95 Although GST-CT-Y1424A was capable of binding AP-2 complexes (Fig. 6A), its capacity for binding was significantly reduced compared with wild-type GST-CT. Login to comment
96 Densitometric analysis of the immunoblot indicates that the Y1424A mutation reduces binding to AP-2 complexes compared with the wild-type carboxyl terminus construct, GST-CT, even at higher amounts of GST-CT-Y1424A (Fig. 6B). Login to comment
100 When the Student`s t test was applied to the data in Fig. 6, GST-CF- Y1424A showed significant reduction of adaptor binding compared with wild-type constructs, with a p value of Ͻ0.02. Login to comment
109 Mutation of tyrosine 1424 to an alanine causes marked loss of beta-structure at this site (Fig. 8). Login to comment
148 Mutation of tyrosine 1424 to alanine but not phenylalanine inhibits AP-2 binding to CFTR. Login to comment
150 10 or 20 ␮g of wild-type (WT, lanes 1 and 2) or Y1424A (lanes 3 and 4) constructs were incubated with purified adaptor complexes. Login to comment
152 The asterisk indicates p Ͻ 0.02 for difference between Y1424A and wild type by Student`s t test. Login to comment
154 20 ␮g of Y1424F protein (lane 1), Y1424A (lane 2), wild-type (lane 3), or GST alone (lane 4) were incubated with purified adaptors as described. Login to comment
162 Mutation of Tyr1424 to Ala resulted in a significant inhibition of AP-2 binding to the CFTR-GST fusion protein, indicating the importance of this residue in AP-2 binding. Login to comment
163 However, binding of AP-2 to Y1424A-CFTR was not completely abolished. Login to comment
166 Of note are the observations of Collawn and colleagues (25), who have shown that a Y1424A mutation in transiently expressed CFTR results in a 40% inhibition of CFTR endocytosis rates, a finding consistent with the extent to which Y1424A mutations inhibit AP-2 binding. Login to comment

[hide] Inhibition of cystic fibrosis transmembrane conduc... J Clin Invest. 2000 Jun;105(12):1711-21. Hallows KR, Raghuram V, Kemp BE, Witters LA, Foskett JK
Inhibition of cystic fibrosis transmembrane conductance regulator by novel interaction with the metabolic sensor AMP-activated protein kinase.
J Clin Invest. 2000 Jun;105(12):1711-21., [PMID:10862786]

Abstract [show]
The cystic fibrosis transmembrane conductance regulator (CFTR) is an ATP-gated Cl(-) channel that regulates other epithelial transport proteins by uncharacterized mechanisms. We employed a yeast two-hybrid screen using the COOH-terminal 70 residues of CFTR to identify proteins that might be involved in such interactions. The alpha1 (catalytic) subunit of AMP-activated protein kinase (AMPK) was identified as a dominant and novel interacting protein. The interaction is mediated by residues 1420-1457 in CFTR and by the COOH-terminal regulatory domain of alpha1-AMPK. Mutations of two protein trafficking motifs within the 38-amino acid region in CFTR each disrupted the interaction. GST-fusion protein pull-down assays in vitro and in transfected cells confirmed the CFTR-alpha1-AMPK interaction and also identified alpha2-AMPK as an interactor with CFTR. AMPK is coexpressed in CFTR-expressing cell lines and shares an apical distribution with CFTR in rat nasal epithelium. AMPK phosphorylated full-length CFTR in vitro, and AMPK coexpression with CFTR in Xenopus oocytes inhibited cAMP-activated CFTR whole-cell Cl(-) conductance by approximately 35-50%. Because AMPK is a metabolic sensor in cells and responds to changes in cellular ATP, regulation of CFTR by AMPK may be important in inhibiting CFTR under conditions of metabolic stress, thereby linking transepithelial transport to cell metabolic state.

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131 Interestingly, the mutation of Y1424A substantially diminished the interaction strength. Mutation of another possible trafficking motif in the vicinity, the di-leucine at 1430 and 1431 (40) (LL-1430,1431-AA) also dramatically reduced the interaction strength. Login to comment

[hide] Localization of sequences within the C-terminal do... J Biol Chem. 2001 Jan 12;276(2):1291-8. Gentzsch M, Riordan JR
Localization of sequences within the C-terminal domain of the cystic fibrosis transmembrane conductance regulator which impact maturation and stability.
J Biol Chem. 2001 Jan 12;276(2):1291-8., 2001-01-12 [PMID:11022033]

Abstract [show]
Some disease-associated truncations within the 100-residue domain C-terminal of the second nucleotide-binding domain destabilize the mature protein (Haardt, M., Benharouga, M., Lechardeur, D., Kartner, N., and Lukacs, G. L. (1999) J. Biol. Chem. 274, 21873-21877). We now have identified three short oligopeptide regions in the C-terminal domain which impact cystic fibrosis transmembrane conductance regulator (CFTR) maturation and stability in different ways. A highly conserved hydrophobic patch (region I) formed by residues 1413-1416 (FLVI) was found to be crucial for the stability of the mature protein. Nascent chain stability was severely decreased by shortening the protein by 81 amino acids (1400X). This accelerated degradation was sensitive to proteasome inhibitors but not influenced by brefeldin A, indicating that it occurred at the endoplasmic reticulum. The five residues at positions 1400 to 1404 (region II) normally maintain nascent CFTR stability in a positional rather than a sequence-specific manner. A third modulating region (III) constituted by residues 1390 to 1394 destabilizes the protein. Hence the nascent form regains stability on further truncation back to residues 1390 or 1380, permitting some degree of maturation and a low level of cyclic AMP-stimulated chloride channel activity at the cell surface. Thus while not absolutely essential, the C-terminal domain strongly modulates the biogenesis and maturation of CFTR.

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178 In addition to the lack of influence of truncation at residue 1420, alanine substitution of Tyr1424 and Leu1430 -Leu1431 also did not alter the steady state amount of nascent or mature CFTR (not shown). Login to comment

[hide] Multiple endocytic signals in the C-terminal tail ... Biochem J. 2001 Mar 15;354(Pt 3):561-72. Hu W, Howard M, Lukacs GL
Multiple endocytic signals in the C-terminal tail of the cystic fibrosis transmembrane conductance regulator.
Biochem J. 2001 Mar 15;354(Pt 3):561-72., 2001-03-15 [PMID:11237860]

Abstract [show]
The cystic fibrosis transmembrane conductance regulator (CFTR) is a cAMP-dependent protein kinase (PKA)-activated chloride channel that is localized to the plasma membrane and endosomal compartment. Endosomal targeting of CFTR is attributed to the Tyr(1424)-based internalization signal, identified in the C-terminal tail of the channel. Mutation of the Tyr(1424) residue could partly inhibit the endocytosis of CFTR and its association with the adapter protein AP-2. To reveal additional endosomal targeting signals, site-directed mutagenesis of both a chimaera, composed of a truncated form of interleukin 2 receptor alpha chain (TacT) and the C-terminal tail of CFTR (Ct), and the full-length CFTR was performed. Morphological and functional assays revealed the presence of multiple internalization motifs at the C-terminus, consisting of a phenylalanine-based motif (Phe(1413)) and a bipartite endocytic signal, comprising a tyrosine (Tyr(1424)) and a di-Leu-based (Leu(1430)-Leu) motif. Whereas the replacement of any one of the three internalization motifs with alanine prevented the endocytosis of the TacT-Ct chimaera, mutagenesis of Phe(1413)-Leu impaired the biosynthetic processing of CFTR, indicating that Phe(1413) is indispensable for the native structure of CFTR. In contrast, replacement of Leu(1430)-Leu- and Tyr(1424)-based signals with alanine increased the cell-surface density of both the chimaeras and CFTR in an additive manner. These results suggest that the internalization of CFTR is regulated by multiple endocytic sorting signals.

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No. Sentence Comment
75 Alanine substitutions in the following mutants were generated by site-directed mutagenesis: K2, F1413A,L1414A; K3, Y1424A,L1430A,L1431A; K6, F1413A; K7, L1414A; K8, Y1424A; K9, L1430A; K10, F16A,F17A (see also Figure 5), by using either single-stranded (for K2, K3, K8 and K9; Muta Gene in itro mutagenesis from Bio-Rad) or double-stranded (for K10; QuickChange site-directed mutagenesis from Stratagene [35]) pcDNA3-TacT-Ct as template. Login to comment
172 The membrane-distal signals, consisting of a canonical Tyr-based motif and a di-Leu motif, were mutated together and designated as mutant K3 (Y1424A,L1430A,L1431A) (Figure 5). Login to comment
200 To evaluate the involvement of single amino acid residues in the membrane-proximal and membrane-distal internalization signals, four additional constructs were prepared that disrupted the three putative endocytic signals individually, as follows: K6, F1413A; K7, L1414A; K8, Y1424A; K9, L1430A (see also Figure 5). Login to comment
231 Mutation of the membrane-distal internalization signals (K8, Y1424A; K9, L1430A) did not interfere with the processing of CFTRM2, as demonstrated by the large-amplitude cAMP-dependent iodide release from cells expressing K3CFTRM2, K8CFTRM2 or K9CFTRM2 (Figure 8A). Login to comment
269 Alanine substitutions of individual residues in the membrane-proximal (K6, F1413A; K7, L1414A) or the membrane-distal (K8, Y1424A; K9, L1430A) stretch revealed an additive effect on the internalization activity (Figure 7D). Login to comment
279 In contrast, the disruption of individual components of the bipartite membrane-distal signal caused a 4-fold (K8, Y1424A) and a 3-fold (K9, L1430A) increase in the cell-surface density of the CFTRM2 (Figure 8B; see also Figure 7D). Login to comment

[hide] Mu 2 binding directs the cystic fibrosis transmemb... J Biol Chem. 2001 Dec 7;276(49):46251-9. Epub 2001 Sep 17. Weixel KM, Bradbury NA
Mu 2 binding directs the cystic fibrosis transmembrane conductance regulator to the clathrin-mediated endocytic pathway.
J Biol Chem. 2001 Dec 7;276(49):46251-9. Epub 2001 Sep 17., 2001-12-07 [PMID:11560923]

Abstract [show]
The cystic fibrosis transmembrane conductance regulator (CFTR) contains a conserved tyrosine-based internalization motif, (1424)YDSI, which interacts with the endocytic clathrin adaptor complex, AP-2, and is required for its efficient endocytosis. Although direct interactions between several endocytic sequences and the medium chain and endocytic clathrin adaptor complexes have been shown by protein-protein interaction assays, whether all these interactions occur in vivo or are physiologically important has not always been addressed. Here we show, using both in vitro and in vivo assays, a physiologically relevant interaction between CFTR and the mu subunit of AP-2. Cross-linking experiments were performed using photoreactive peptides containing the YDSI motif and purified adaptor complexes. CFTR peptides cross-linked a 50-kDa subunit of purified AP-2 complexes, the apparent molecular mass of mu 2. Furthermore, isolated mu 2 bound to the sorting motif, YDSI, both in cross-linking experiments and glutathione S-transferase pull-down experiments, confirming that mu 2 mediates the interaction between CFTR and AP-2 complexes. Inducible overexpression of dominant-negative mu 2 in HeLa cells results in AP-2 complexes that fail to interact with CFTR. Moreover, internalization of CFTR in mutant cells is greatly reduced compared with wild type HeLa cells. These results indicate that the AP-2 endocytic complex selectively interacts with the conserved tyrosine-based internalization signal in the carboxyl terminus of CFTR, YDSI. Furthermore, this interaction is mediated by the mu 2 subunit of AP-2 and mutations in mu 2 that block its interaction with YDSI inhibit the incorporation of CFTR into the clathrin-mediated endocytic pathway.

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155 Furthermore, peptides containing alanine substitution at tyrosine 1424 and isoleucine 1427, *ADSA, abolished the ability of the peptide to cross-link to AP-2 (Fig. 2A, lane 3). Login to comment
331 These results strongly correlate with the reduction in endocytosis observed in CFTR Y1424A mutants (10, 35), suggesting that the interaction specifically affected is that between ␮2 and the 1424 YDSI endocytosis signal. Login to comment
339 Moreover, the only mutation identified that affected CFTR internalization was the YXX⌽ motif in the carboxyl terminus of CFTR, Y1424A. Login to comment

[hide] Ablation of internalization signals in the carboxy... J Biol Chem. 2002 Dec 20;277(51):49952-7. Epub 2002 Oct 9. Peter K, Varga K, Bebok Z, McNicholas-Bevensee CM, Schwiebert L, Sorscher EJ, Schwiebert EM, Collawn JF
Ablation of internalization signals in the carboxyl-terminal tail of the cystic fibrosis transmembrane conductance regulator enhances cell surface expression.
J Biol Chem. 2002 Dec 20;277(51):49952-7. Epub 2002 Oct 9., 2002-12-20 [PMID:12376531]

Abstract [show]
The cystic fibrosis transmembrane conductance regulator (CFTR) is a chloride channel that undergoes endocytosis through clathrin-coated pits. Previously, we demonstrated that Y1424A is important for CFTR endocytosis (Prince, L. S., Peter, K., Hatton, S. R., Zaliauskiene, L., Cotlin, L. F., Clancy, J. P., Marchase, R. B., and Collawn, J. F. (1999) J. Biol. Chem. 274, 3602-3609). Here we show that a second substitution in the carboxyl-terminal tail of CFTR, I1427A, on Y1424A background more than doubles CFTR surface expression as monitored by surface biotinylation. Internalization assays indicate that enhanced surface expression of Y1424A,I1427A CFTR is caused by a 76% inhibition of endocytosis. Patch clamp recording of chloride channel activity revealed that there was a corresponding increase in chloride channel activity of Y1424A,I1427A CFTR, consistent with the elevated surface expression, and no change in CFTR channel properties. Y14124A showed an intermediate phenotype compared with the double mutation, both in terms of surface expression and chloride channel activity. Metabolic pulse-chase experiments demonstrated that the two mutations did not affect maturation efficiency or protein half-life. Taken together, our data show that there is an internalization signal in the COOH terminus of CFTR that consists of Tyr(1424)-X-X-Ile(1427) where both the tyrosine and the isoleucine are essential residues. This signal regulates CFTR surface expression but not CFTR biogenesis, degradation, or chloride channel function.

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No. Sentence Comment
1 Previously, we demonstrated that Y1424A is important for CFTR endocytosis (Prince, L. S., Peter, K., Hatton, S. R., Zaliauskiene, L., Cotlin, L. F., Clancy, J. P., Marchase, R. B., and Collawn, J. F. (1999) J. Biol. Chem. 274, 3602-3609). Login to comment
2 Here we show that a second substitution in the carboxyl-terminal tail of CFTR, I1427A, on Y1424A background more than doubles CFTR surface expression as monitored by surface biotinylation. Login to comment
3 Internalization assays indicate that enhanced surface expression of Y1424A,I1427A CFTR is caused by a 76% inhibition of endocytosis. Login to comment
4 Patch clamp recording of chloride channel activity revealed that there was a corresponding increase in chloride channel activity of Y1424A,I1427A CFTR, consistent with the elevated surface expression, and no change in CFTR channel properties. Login to comment
18 We find that the substitution of Tyr1424 and Ile1427 with alanine residues resulted in a 2-fold increase in surface expression, whereas the single Y1424A mutation shows an intermediate phenotype. Login to comment
20 Because the chloride channel activity and relative surface expression of Y1424A and I1427A CFTR are elevated to a similar extent, we propose that these substitutions affect protein trafficking but not CFTR chloride channel function. Login to comment
23 The construction of the Y1424A mutant was described previously (3). Login to comment
24 For construction of the Y1424A,I1427A mutant, a BstXI-SgrAI fragment that coded for the COOH-terminal tail region of Y1424A CFTR was subcloned into pSK-Bluescript (Stratagene). Login to comment
77 COS-7 cells expressing wild-type, Y1424A, and Y1424A,I1427A CFTR were surface-biotinylated and lysed in RIPA buffer (see "Materials and Methods"). Login to comment
82 The levels of expression of wild-type (Wt), Y1424A, and Y1424A,I1427A CFTR were analyzed in COS-7 cells 48 h after transfection. Login to comment
90 The relative amounts of wild-type (lanes 2 and 6), Y1424A (lanes 3 and 7), and Y1424A,I1427A CFTR (lanes 4 and 8) are shown. Login to comment
94 The percentage CFTR at the cell surface was markedly increased for Y1424A,I1427A CFTR compared with both wild-type (108% increase, n ϭ 10, p Ͻ 0.001) and Y1424A CFTR (59% increase, n ϭ 10, p Ͻ 0.001) (Fig. 1, bottom panel). Login to comment
97 Mutations at Tyr1424 and Ile1427 Do Not Alter CFTR Maturation Efficiency or Protein Half-life-To test the effects of these mutations on maturation efficiency and protein half-life, we performed metabolic pulse-chase experiments on COS-7 cells expressing wild-type, Y1424A, and Y1424A,I1427A CFTR. Login to comment
99 The results in Fig. 2 show that the half-lives for wild-type (Wt), Y1424A, and Y1424A,I1427A CFTR were 10.3 Ϯ 2.3, 11.3 Ϯ 2.6, and 11.3 Ϯ 1.5 h (mean Ϯ S.D.). Login to comment
102 The average maturation efficiency for wild-type (Wt), Y1424A, and Y1424A,I1427A CFTR were 32, 31, and 31%, respectively (bottom right panel). Login to comment
103 This finding demonstrated that elevated surface expression of Y1424A,I1427A CFTR was not because of alterations in maturation efficiency. Login to comment
114 COS-7 cells transfected with wild-type, Y1424A, or Y1424A,I1427A CFTR were analyzed 48-h post-transfection. Login to comment
119 The percentage of wild-type, Y1424A, and Y1424A,I1427A CFTR internalized after 2.5 min was 34, 18, and 8 respectively. Login to comment
122 attributed to alterations in the internalization rate of CFTR, we performed internalization assays on COS-7 cells expressing wild-type, Y1424A, and Y1424A,I1427A CFTR. Login to comment
128 For Y1424A and Y1424A,I1427A CFTR, internalization dropped to 21 and 8%, respectively, during the same time period. Login to comment
130 The Y1424A and Y1424A,I1427A CFTR Have Normal Chloride Channel Properties-Because the biochemical data suggested that a specific motif in the CFTR COOH terminus dramatically affected endocytosis and because point mutations in the NH2 terminus lead to both disruption of binding to docking machinery and changes in CFTR ion channel function, we tested whether the mutation of Tyr1424 and Ile1427 affected chloride channel function. Login to comment
136 In agreement with the surface biotinylation assays, CFTR whole cell Cl- currents in Y1424A CFTR and Y1424A,I1427A CFTR-transfected cells were elevated compared with wild-type CFTR-expressing cells (Table I), suggesting that the elevated Cl-channel activity was the result of the elevated surface expression of CFTR. Login to comment
140 Fig. 4, C and D, show the Y1424A and Y1424A,I1427A Cl- current- voltage relationships, respectively, and indicate that although the sensitivities to DIDS and glibenclamide remain similar to wild-type (Fig. 4B), the total current is elevated in the single and double mutants. Login to comment
142 Single channel biophysical properties of wild-type, Y1424A, and Y1424A,I1427A CFTR were also assessed. Login to comment
145 Representative recordings of wild-type, Y1424A, and Y1424A,I1427A CFTR at 50-60 mV (negative to pipette potential) are shown in Fig. 4E. Login to comment
150 Nevertheless, the whole cell and single channel recordings together show that the difference in Cl-channel activity is attributed to elevated surface expression without a significant change in CFTR chloride channel properties among wild-type, Y1424A, and Y1424A,I1427A CFTR. Login to comment
154 In examining the mechanism for the elevated surface expression of CFTR, we first showed that total expression levels of wild-type, Y1424A, and Y1424A,I1427A were the same. Login to comment
155 We next demonstrated that maturation efficiency and protein half-life were unaffected, suggesting that a primary alteration caused by these substitutions involved changes in distribution TABLE I Summary of whole cell patch clamp recordings for wild-type CFTR and for CFTR mutants shows elevated activity in the mutants relative to wild type Transient transfection cAMP-activated chloride currenta Non-green Green Control Wild type Y1424A Y1424A/I1427A pA at ϩ100 mV Set 1 255 Ϯ 36b (13)c 1070 Ϯ 95 (5) 1650 Ϯ 200* (5) 2125 Ϯ 195† (5) (Fold-difference) 1.0 1.54 1.99 Set 2 201 Ϯ 68 (3) 738 Ϯ 52 (5) 986 Ϯ 24* (5) 1451 Ϯ 35† (5) (Fold-difference) 1.0 1.34 1.97 Set 3 393 Ϯ 140 (4) 1193 Ϯ 55 (7) 1767 Ϯ 164* (7) 3424 Ϯ 205† (6) (Fold-difference) 1.0 1.48 2.87 Fold-difference average 1.0 1.45 Ϯ 0.06* 2.28 Ϯ 0.30† a Three sets of transiently transfected COS-7 cells were analyzed in parallel with the protein biochemistry. Login to comment
161 Moreover, we showed that Y1424A,I1427A CFTR was internalized much more slowly than the native protein (76% inhibition at 2.5 min) with an internalization rate of ϳ2%/min. Login to comment
165 Second, the internalization rate of Y1424A,I1427A CFTR is comparable with the rate of bulk flow lipid uptake via the endocytic pathway (ϳ2%/min.) Login to comment
175 Typical whole-cell I-V plots for wild-type CFTR (panel B), Y1424A (panel C), and Y1424A,I1427A (panel D) showing cyclic AMP-stimulated chloride currents in the absence of blockers (squares), presence of DIDS (upward triangles), and presence of glibenclamide (inverted triangles). Login to comment

[hide] The role of regulated CFTR trafficking in epitheli... Am J Physiol Cell Physiol. 2003 Jul;285(1):C1-18. Bertrand CA, Frizzell RA
The role of regulated CFTR trafficking in epithelial secretion.
Am J Physiol Cell Physiol. 2003 Jul;285(1):C1-18., [PMID:12777252]

Abstract [show]
The focus of this review is the regulated trafficking of the cystic fibrosis transmembrane conductance regulator (CFTR) in distal compartments of the protein secretory pathway and the question of how changes in CFTR cellular distribution may impact on the functions of polarized epithelial cells. We summarize data concerning the cellular localization and activity of CFTR and attempt to synthesize often conflicting results from functional studies of regulated endocytosis and exocytosis in CFTR-expressing cells. In some instances, findings that are inconsistent with regulated CFTR trafficking may result from the use of overexpression systems or nonphysiological experimental conditions. Nevertheless, judging from data on other transporters, an appropriate cellular context is necessary to support regulated CFTR trafficking, even in epithelial cells. The discovery that disease mutations can influence CFTR trafficking in distal secretory and recycling compartments provides support for the concept that regulated CFTR recycling contributes to normal epithelial function, including the control of apical CFTR channel density and epithelial protein secretion. Finally, we propose molecular mechanisms for regulated CFTR endocytosis and exocytosis that are based on CFTR interactions with other proteins, particularly those whose primary function is membrane trafficking. These models provide testable hypotheses that may lead to elucidation of CFTR trafficking mechanisms and permit their experimental manipulation in polarized epithelial cells.

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195 Cells expressing either a dominant negative ␮2 or a CFTR lacking the tyrosine-based internalization motif at the COOH terminus (Y1424A) fail to endocytose CFTR efficiently. Login to comment

[hide] Efficient endocytosis of the cystic fibrosis trans... J Biol Chem. 1999 Feb 5;274(6):3602-9. Prince LS, Peter K, Hatton SR, Zaliauskiene L, Cotlin LF, Clancy JP, Marchase RB, Collawn JF
Efficient endocytosis of the cystic fibrosis transmembrane conductance regulator requires a tyrosine-based signal.
J Biol Chem. 1999 Feb 5;274(6):3602-9., 1999-02-05 [PMID:9920908]

Abstract [show]
We previously demonstrated that the cystic fibrosis transmembrane conductance regulator (CFTR) is rapidly endocytosed in epithelial cells (Prince, L. S., Workman, R. B., Jr., and Marchase, R. B. (1994) Proc. Natl. Acad. Sci. U. S. A. 91, 5192-5196). To determine the structural features of CFTR required for endocytosis, we prepared chimeric molecules consisting of the amino-terminal (residues 2-78) and carboxyl-terminal tail regions (residues 1391-1476) of CFTR, each fused to the transmembrane and extracellular domains of the transferrin receptor. Functional analysis of the CFTR-(2-78) and CFTR-(1391-1476) indicated that both chimeras were rapidly internalized. Deletion of residues 1440-1476 had no effect on chimera internalization. Mutations of potential internalization signals in both cytoplasmic domains reveal that only one mutation inhibits internalization, Y1424A. Using a surface biotinylation reaction, we also examined internalization rates of wild type and mutant CFTRs expressed in COS-7 cells. We found that both wild type and A1440X CFTR were rapidly internalized, whereas the Y1424A CFTR mutant, like the chimeric protein, had approximately 40% reduced internalization activity. Deletions in the amino-terminal tail region of CFTR resulted in defective trafficking of CFTR out of the endoplasmic reticulum to the cell surface, suggesting that an intact amino terminus is critical for biosynthesis. In summary, our results suggest that both tail regions of CFTR are sufficient to promote rapid internalization of a reporter molecule and that tyrosine 1424 is required for efficient CFTR endocytosis.

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No. Sentence Comment
6 Mutations of potential internalization signals in both cytoplasmic domains reveal that only one mutation inhibits internalization, Y1424A. Login to comment
8 We found that both wild type and A1440X CFTR were rapidly internalized, whereas the Y1424A CFTR mutant, like the chimeric protein, had ϳ40% reduced internalization activity. Login to comment
103 Next, we analyzed CFTR-TR chimeras that contained point mutations in potential internalization signals in both cytoplasmic tail regions: Y38A, L69A, Y1424A, and L1430A (Fig. 1). Login to comment
104 Analysis of these mutants in internalization assays indicated that only one mutation, Y1424A, affected the internalization rate of the chimeras (Fig. 2B; ϳ40% loss of internalization activity, p Ͻ 0.05), suggesting that this residue might be a part of an internalization signal. Login to comment
116 A1440X contains a stop mutation at residue 1440, and Y1424A contains an alanine substitution for tyrosine. Login to comment
123 Comparison of the CFTR-(1391-1440) with the Y1424A mutant showed little or no difference (not shown). Login to comment
134 Tyrosine 1424 Is Important for CFTR Endocytosis-Next, we tested the only point mutation that affected internalization of the chimeras, Y1424A. Login to comment
144 B, internalization of CFTR and Y1424A in COS-7 cells. Login to comment
145 Cells transfected with wild-type CFTR (pGT-CFTR) or Y1424A were analyzed as described in A for percentage CFTR internalized from the cell surface during the warm-up period. Login to comment
146 C, percentage of CFTR or Y1424A at the cell surface under steady-state conditions. Login to comment
147 Cells transfected with CFTR or Y1424A were analyzed for total CFTR expression by performing the two-step biotinylation reaction without a warm-up step. Login to comment
148 Biotinylated and nonbiotinylated CFTR or Y1424A was separated on a monovalent avidin column and quantitated as described for A. Login to comment
156 tion would imply that the steady-state distribution of Y1424A might favor a higher cell surface distribution pattern, we determined the percentage of CFTR at the cell surface using surface biotinylation at 4 °C. Login to comment
157 As predicted, the percentage of Y1424A at the cell surface was higher than the wild-type CFTR protein (36.1 versus 23.1% (p Ͻ 0.01), respectively; Fig. 6C), supporting the idea that tyrosine 1424 was important for CFTR endocytosis. Login to comment
163 A similar comparison between wild type CFTR (pGT) and Y1424A (pGT) revealed that the expression levels were similar (Fig. 5B). Login to comment
178 The only mutation that we identified in our studies that affected CFTR internalization was found in the carboxyl-terminal tail, Y1424A. Login to comment

[hide] Keratin K18 increases CFTR surface expression by b... J Biol Chem. 2012 Oct 8. Duan Y, Sun Y, Zhang F, Zhang WK, Wang D, Wang Y, Cao X, Hu W, Xie C, Cuppoletti J, Magin TM, Wang H, Wu Z, Li N, Huang P
Keratin K18 increases CFTR surface expression by binding to its C-terminal hydrophobic patch.
J Biol Chem. 2012 Oct 8., [PMID:23045527]

Abstract [show]
Malfunction of the cystic fibrosis transmembrane conductance regulator (CFTR) leads to cystic fibrosis, but the regulation of CFTR is not fully understood. Here, we identified the intermediate filament protein keratin K18 (K18) as a CFTR binding protein by various approaches. We mapped a highly conserved hydrophobic patch (F1413LVI) in the CFTR carboxy-terminus, known to determine plasmalemmal CFTR stability, as the K18 binding site. On the other hand, the C-terminal tail of K18 was found to be a critical determinant for binding CFTR. Overexpression of K18 in cells robustly increased the surface expression of wild-type CFTR, whereas depletion of K18 through RNA interference specifically diminished it. K18 binding increased the surface expression of CFTR by accelerating its apical recycling rate without altering CFTR's biosynthesis, maturation or internalization. Importantly, CFTR surface expression was markedly reduced in duodenal and gallbladder epithelia of K18-/- mice. Taken together, our results suggest that K18 increases the cell surface expression of CFTR by interacting with CFTR's C-terminal hydrophobic patch. These findings offer novel insights into the regulation of CFTR and suggest that K18 and its dimerization partner K8 may be modifier genes in cystic fibrosis.

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160 Mutation of Tyr1424 to alanine did not alter CFTR-C-K18 binding, arguing against a role of AP2 in mediating the interaction between CFTR and K18. Login to comment
151 Mutation of Tyr1424 to alanine did not alter CFTR-C-K18 binding, arguing against a role of AP2 in mediating the interaction between CFTR and K18. Login to comment
253 In agreement with previous studies (4), substitution of Tyr1424 with an alanine in the CFTR C terminus (M4 mutation) strongly suppressed AP2-binding (Fig. 6A). Login to comment
297 Panel a, AP2 (top blot) and K18 (middle blot) from Calu-3 cells pulled down by GST alone (GST), GST-CFTR-(1407-1480) (WT), and GST-CFTR-(1407-1480) with 1413 FLVI1416 -AAAA (M1) or Y1424A (M4) mutations; the bottom blot shows loading of GST proteins. Login to comment

[hide] Disabled-2 protein facilitates assembly polypeptid... J Biol Chem. 2012 Apr 27;287(18):15087-99. Epub 2012 Mar 7. Cihil KM, Ellinger P, Fellows A, Stolz DB, Madden DR, Swiatecka-Urban A
Disabled-2 protein facilitates assembly polypeptide-2-independent recruitment of cystic fibrosis transmembrane conductance regulator to endocytic vesicles in polarized human airway epithelial cells.
J Biol Chem. 2012 Apr 27;287(18):15087-99. Epub 2012 Mar 7., [PMID:22399289]

Abstract [show]
Cystic fibrosis transmembrane conductance regulator (CFTR) is a cAMP-activated Cl(-) channel expressed in the apical plasma membrane of fluid-transporting epithelia, where the plasma membrane abundance of CFTR is in part controlled by clathrin-mediated endocytosis. The protein networks that control CFTR endocytosis in epithelial cells have only been partially explored. The assembly polypeptide-2 complex (AP-2) is the prototypical endocytic adaptor critical for optimal clathrin coat formation. AP-2 is essential for recruitment of cargo proteins bearing the YXXPhi motif. Although AP-2 interacts directly with CFTR in vitro and facilitates CFTR endocytosis in some cell types, it remains unknown whether it is critical for CFTR uptake into clathrin-coated vesicles (CCVs). Disabled-2 (Dab2) is a clathrin-associated sorting protein (CLASP) that contributes to clathrin recruitment, vesicle formation, and cargo selection. In intestinal epithelial cells Dab2 was not found to play a direct role in CFTR endocytosis. By contrast, AP-2 and Dab2 were shown to facilitate CFTR endocytosis in human airway epithelial cells, although the specific mechanism remains unknown. Our data demonstrate that Dab2 mediates AP-2 independent recruitment of CFTR to CCVs in polarized human airway epithelial cells. As a result, it facilitates CFTR endocytosis and reduces CFTR abundance and stability in the plasma membrane. These effects are mediated by the DAB homology domain. Moreover, we show that in human airway epithelial cells AP-2 is not essential for CFTR recruitment to CCVs.

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89 Plasmids and Transient Transfection-The cDNAs encoding human full-length WT-CFTR and the CFTR Y1424A mutant in pS65T-C1 vector (GFP-CFTR WT and GFP-CFTR Y1424A, respectively) were generous gifts from Dr. Bruce A. Stanton, Department of Physiology, Dartmouth Medical School, Hanover, NH. Human Dab2 (Uniprot accession number P98082-1) was obtained from Origene (SC321375) Technologies, Inc., Rockville, MD). Login to comment
255 First, we examined whether the Y1424A substitution in CFTR, previously shown to disrupt the CFTR-␮2 interaction, affects CFTR recruitment to CCVs. Login to comment
256 Parental CFBE41o-cells transiently transfected with the GFP-CFTR WT or the GFP-CFTR Y1424A mutant were grown in plastic tissue culture dishes for 72 h. Login to comment
258 As illustrated in Fig. 8, the abundance of the GFP-CFTR WT and GFP-CFTR Y1424A in CCVs was similar. Login to comment
259 Next, we examined the effect of the Y1424A substitution on CFTR endocytosis. Login to comment
260 The biotinylation-based endocytic assays were conducted in parental CFBE41o-cells expressing GFP-CFTR WT or the GFP-CFTR Y1424A mutant. Login to comment
261 As illustrated in Fig. 9, endocytosis of the GFP-CFTR WT and GFP-CFTR Y1424A was similar in the linear uptake phase, consistent with steady-state measurements (Figs. 8 and 10). Login to comment
262 Finally, studies were conducted to determine the effect of the Y1424A substitution on CFTR stability in the plasma membrane. Login to comment
275 Error bars, S.E. Dab2 Facilitates CFTR Endocytosis APRIL 27, 2012•VOLUME 287•NUMBER 18 JOURNAL OF BIOLOGICAL CHEMISTRY 15093 membrane biotinylation at °C, the disappearance of the GFP-CFTR WT or GFP-CFTR Y1424A mutant from the plasma membrane was monitored as a function of time at 37 °C. Login to comment
276 As shown in Fig. 10, the plasma membrane stability of the GFP-CFTR WT and GFP-CFTR Y1424A was similar. Login to comment
277 The above data demonstrate that the Y1424A mutation affects neither CFTR recruitment to CCVs, CFTR endocytosis, nor CFTR stability in the plasma membrane and suggest that the CFTR interaction with ␮2 adaptin is not essential for CFTR endocytosis in human airway epithelial cells. Login to comment
327 Subcellular fractionation experiments performed to determine the effect of the Y1424A mutation on CFTR recruitment CCV in CFBE41o-cells. Login to comment
328 The CCV fractions were prepared by density gradient and differential centrifugation in parental CFBE41o-cells 72 h after transfection with the GFP-CFTR WT or GFP-CFTR Y1424A mutant. Login to comment
330 A representative Western blot (B) and summary of experiments (C) demonstrate that the Y1424A mutation did not decrease the abundance of CFTR in CCVs. Login to comment
356 Endocytic assays performed to determine the effect of the Y1424A mutation on CFTR endocytosis. Login to comment
357 Endocytic assays were performed in parental CFBE41o-cells 72 h after transfection with the GFP-CFTR WT or GFP-CFTR Y1424A mutant. Login to comment
359 A representative Western blot (IB; A) and summary of experiments (B) demonstrates that the Y1424A mutation did not inhibit CFTR endocytosis. Login to comment
366 Biotinylation experiments performed to determine the effects of the Y1424A mutation on CFTR expression in the plasma membrane of CFBE41o-cells as a function of time. Login to comment
367 Biotinylation experiments were performed in parental CFBE41o-cells 72 h after transfection with the GFP-CFTR WT or GFP-CFTR Y1424A mutant. Login to comment
369 A representative Western blot (IB; A) and summary of experiments (B) demonstrate that the Y1424A mutation did not increase CFTR stability in the plasma membrane. Login to comment
88 Plasmids and Transient Transfection-The cDNAs encoding human full-length WT-CFTR and the CFTR Y1424A mutant in pS65T-C1 vector (GFP-CFTR WT and GFP-CFTR Y1424A, respectively) were generous gifts from Dr. Bruce A. Stanton, Department of Physiology, Dartmouth Medical School, Hanover, NH. Human Dab2 (Uniprot accession number P98082-1) was obtained from Origene (SC321375) Technologies, Inc., Rockville, MD). Login to comment
253 First, we examined whether the Y1424A substitution in CFTR, previously shown to disrupt the CFTR-òe;2 interaction, affects CFTR recruitment to CCVs. Login to comment
254 Parental CFBE41o-cells transiently transfected with the GFP-CFTR WT or the GFP-CFTR Y1424A mutant were grown in plastic tissue culture dishes for 72 h. Login to comment
257 Next, we examined the effect of the Y1424A substitution on CFTR endocytosis. Login to comment
273 Error bars, S.E. Dab2 Facilitates CFTR Endocytosis APRIL 27, 2012ߦVOLUME 287ߦNUMBER 18 JOURNAL OF BIOLOGICAL CHEMISTRY 15093 at SEMMELWEIS UNIV OF MEDICINE on December 5, membrane biotinylation at &#b0;C, the disappearance of the GFP-CFTR WT or GFP-CFTR Y1424A mutant from the plasma membrane was monitored as a function of time at 37 &#b0;C. Login to comment
274 As shown in Fig. 10, the plasma membrane stability of the GFP-CFTR WT and GFP-CFTR Y1424A was similar. Login to comment
325 Subcellular fractionation experiments performed to determine the effect of the Y1424A mutation on CFTR recruitment CCV in CFBE41o-cells. Login to comment
326 The CCV fractions were prepared by density gradient and differential centrifugation in parental CFBE41o-cells 72 h after transfection with the GFP-CFTR WT or GFP-CFTR Y1424A mutant. Login to comment
354 Endocytic assays performed to determine the effect of the Y1424A mutation on CFTR endocytosis. Login to comment
355 Endocytic assays were performed in parental CFBE41o-cells 72 h after transfection with the GFP-CFTR WT or GFP-CFTR Y1424A mutant. Login to comment
364 Biotinylation experiments performed to determine the effects of the Y1424A mutation on CFTR expression in the plasma membrane of CFBE41o-cells as a function of time. Login to comment
365 Biotinylation experiments were performed in parental CFBE41o-cells 72 h after transfection with the GFP-CFTR WT or GFP-CFTR Y1424A mutant. Login to comment

[hide] Regulated trafficking of the CFTR chloride channel... Eur J Cell Biol. 2000 Aug;79(8):544-56. Kleizen B, Braakman I, de Jonge HR
Regulated trafficking of the CFTR chloride channel.
Eur J Cell Biol. 2000 Aug;79(8):544-56., [PMID:11001491]

Abstract [show]
The cystic fibrosis transmembrane conductance regulator (CFTR), the ABC transporter encoded by the cystic fibrosis gene, is localized in the apical membrane of epithelial cells where it functions as a cyclic AMP-regulated chloride channel and as a regulator of other ion channels and transporters. Whereas a key role of cAMP-dependent phosphorylation in CFTR-channel gating has been firmly established, more recent studies have provided clear evidence for the existence of a second level of cAMP regulation, i.e. the exocytotic recruitment of CFFR to the plasma membrane and its endocytotic retrieval. Regulated trafficking of the CFTR Cl- channel has sofar been demonstrated only in a subset of CFTR-expressing cell types. However, with the introduction of more sensitive methods to measure CFTR cycling and submembrane localization, it might turn out to be a more general phenomenon that could contribute importantly to both the regulation of CFTR-mediated chloride transport itself and to the regulation of other transporters and CFTR-modulated cellular functions. This review aims to summarize the present state of knowledge regarding polarized and regulated CFTR trafficking and endosomal recycling in epithelial cells, to discuss present gaps in our understanding of these processes at the cellular and molecular level, and to consider its possible implications for cystic fibrosis.

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118 Construction of chimeras consisting of the C-terminal or N-terminal CFTR tail regions fused to the transferrin receptor (TR) followed by mutational analysis in chicken embryo fibroblasts revealed that one specific mutation within the C-terminal CFTR-TR, Y1424A, reduced the internalization rate by 40% (Prince et al., 1999). Login to comment
120 As expected, the C-terminal CFTR mutation Y1424A significantly reduced the AP-2 association with CFTR, confirming that the tyrosine-based signal is required for the formation of the AP-2 complex (Weixel and Bradbury, 2000) (Fig. 1). Login to comment