Efficient Endocytosis of the Cystic Fibrosis Transmembrane Conductance Regulator Requires a Tyrosine-based Signal

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 ∼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. 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 ∼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. Cystic fibrosis is caused by mutations in the gene encoding the cystic fibrosis transmembrane conductance regulator (CFTR) 1The abbreviations used are: CFTR, cystic fibrosis transmembrane conductance regulator; TR, transferrin receptor; SPQ, 6-methoxy-N-(3-sulfopropyl)quinolinium; BSA, bovine serum albumin; PBS, phosphate-buffered saline. (1Riordan J.R. Rommens J.M. Kerem B. Alon N. Rozmahel R. Grzelczak Z. Zielenski J. Lok S. Plavsic N. Chou J. Drumm M.L. Iannuzzi M.C. Collins F.S. Tsui L. Science. 1989; 245: 1066-1073Crossref PubMed Scopus (5977) Google Scholar), which functions as a chloride channel on the apical surface of epithelial cells (2Anderson M.P. Rich D.P. Gregory R.J. Smith A.E. Welsh M.J. Science. 1991; 251: 679-682Crossref PubMed Scopus (433) Google Scholar, 3Bear C.E. Li C. Kartner N. Bridges R.J. Jensen T.J. Ramjeesingh M. Riordan J.R. Cell. 1992; 68: 809-818Abstract Full Text PDF PubMed Scopus (779) Google Scholar). The most common mutation in CF, ΔF508, is a temperature-sensitive mutant that fails to exit the endoplasmic reticulum, presumably because of a protein folding defect (4Denning G.M. Anderson M.P. Amara J.F. Marshall J. Smith A.E. Welsh M.J. Nature. 1992; 358: 761-764Crossref PubMed Scopus (1063) Google Scholar). Previous studies have demonstrated that CFTR is endocytosed (5Prince L.S. Workman Jr., R.B. Marchase R.B. Proc. Natl. Acad. Sci. U. S. A. 1994; 91: 5192-5196Crossref PubMed Scopus (120) Google Scholar, 6Lukacs G.L. Segal G. Kartner N. Grinstein S. Zhang F. Biochem. J. 1997; 328: 353-361Crossref PubMed Scopus (124) Google Scholar) through clathrin-coated vesicles (6Lukacs G.L. Segal G. Kartner N. Grinstein S. Zhang F. Biochem. J. 1997; 328: 353-361Crossref PubMed Scopus (124) Google Scholar, 7Bradbury N.A. Cohn J.A. Venglarik C.J. Bridges R.J. J. Biol. Chem. 1994; 269: 8296-8302Abstract Full Text PDF PubMed Google Scholar), suggesting that CFTR internalization may provide a mechanism for controlling the cAMP-stimulated chloride channel activity at the cell surface (6Lukacs G.L. Segal G. Kartner N. Grinstein S. Zhang F. Biochem. J. 1997; 328: 353-361Crossref PubMed Scopus (124) Google Scholar). Others have suggested that CFTR may play additional roles by regulating plasma membrane recycling (5Prince L.S. Workman Jr., R.B. Marchase R.B. Proc. Natl. Acad. Sci. U. S. A. 1994; 91: 5192-5196Crossref PubMed Scopus (120) Google Scholar, 8Bradbury N.A. Jilling T. Berta G. Sorscher E.J. Bridges R.J. Kirk K.L. Science. 1992; 256: 530-532Crossref PubMed Scopus (298) Google Scholar) and in clearance of Pseudomonas aeruginosa from the respiratory tract (9Pier G.B. Grout M. Zaidi T.S. Olsen J.C. Johnson L.G. Yankaskas J.R. Goldberg J.B. Science. 1996; 271: 64-67Crossref PubMed Scopus (368) Google Scholar, 10Pier G.B. Grout M. Zaidi T.S. Proc. Natl. Acad. Sci. U. S. A. 1997; 94: 12088-12093Crossref PubMed Scopus (278) Google Scholar). The purpose of this study was to determine the structural features of the CFTR protein required for internalization. Internalization signals identified to date include tyrosine-based motifs (YXXφ or NPXY, where X is any amino acid and φ is a bulky hydrophobic residue), dileucine motifs, and acidic cluster/casein kinase II-based motifs (11Collawn J.F. Stangel M. Kuhn L.A. Esekogwu V. Jing S. Trowbridge I.S. Tainer J.A. Cell. 1990; 63: 1061-1072Abstract Full Text PDF PubMed Scopus (394) Google Scholar, 12Trowbridge I.S. Collawn J.F. Hopkins C.R. Annu. Rev. Cell Biol. 1993; 9: 129-161Crossref PubMed Scopus (704) Google Scholar, 13Letourneur F. Klausner R.D. Cell. 1992; 69: 1143-1157Abstract Full Text PDF PubMed Scopus (461) Google Scholar, 14Voorhees P. Deignan E. Donselaar E.V. Humphrey J. Marks M.S. Peters P.J. Bonifacino J.S. EMBO J. 1995; 14: 4961-4975Crossref PubMed Scopus (187) Google Scholar, 15Mauxion F. Le Borgne R. Munier-Lehmann H. Hoflack B. J. Biol. Chem. 1996; 271: 2171-2178Abstract Full Text Full Text PDF PubMed Scopus (133) Google Scholar). Initial studies of type III membrane proteins indicate that the targeting signals occur in the amino- and carboxyl-terminal cytoplasmic tail regions (16Piper R.C. Tai C. Kulesza P. Pang S. Warnock D. Baenziger J. Slot J.W. Geuze H.J. Puri C. James D.E. J. Cell Biol. 1993; 121: 1221-1232Crossref PubMed Scopus (97) Google Scholar, 17Corvera S. Chawla A. Chakrabarti R. Joly M. Buxton J. Czech M.P. J. Cell Biol. 1994; 126: 979-989Crossref PubMed Scopus (103) Google Scholar, 18Garippa R.J. Judge T.W. James D.E. McGraw T.E. J. Cell Biol. 1994; 124: 705-715Crossref PubMed Scopus (70) Google Scholar, 19Tan P.K. Waites C. Liu Y. Krantz D.E. Edwards R.H. J. Biol. Chem. 1998; 273: 17351-17360Abstract Full Text Full Text PDF PubMed Scopus (102) Google Scholar). Our initial studies on the identification of CFTR internalization signals focused on the two tail regions of the CFTR molecule. Here we show that both the amino- and carboxyl-terminal cytoplasmic tail regions of CFTR, residues 2–78 and 1391–1476, are individually sufficient to promote rapid internalization of a reporter molecule, the transferrin receptor (TR). We also demonstrate both in the context of chimeric and native proteins that tyrosine 1424 is important for CFTR endocytosis. Furthermore, we show that the intracellular distribution of the CFTR-TR chimeras is similar to that of the TR, suggesting that endocytosis may regulate CFTR activity at the cell surface. The CFTR-TR chimeras were constructed using the polymerase chain reaction as described previously (20Collawn J.F. Lai A. Domingo D. Fitch M. Hatton S. Trowbridge I.S. J. Biol. Chem. 1993; 268: 21686-21692Abstract Full Text PDF PubMed Google Scholar). A polymerase chain reaction was performed on pKCTR-CFTR cDNA (also referred to as pGT-CFTR), and unique NheI andAflII sites were introduced in the 5′ and 3′ primers, respectively. For the amino-terminal CFTR tail, the 5′ and 3′ primers were 5′-AA-GCT-AGC-CAG-AGG-TCG-CCT-CTG-GAA-AA-3′ and 5′-AA-CTT-AAG-GAA-AAAACA-TCG-CCG-AAG-GGC, respectively. For the carboxyl-terminal CFTR tail, the 5′ and 3′ primers were 5′-GCT-AGC-GCA-TTT-GCT-GAT-TGC-ACA-GTA-ATT-3′ and 5′-CTT-AAG-TTG-CAC-CTC-TTC-TTCTGT-CTC-CTC-3′, respectively. The polymerase chain reaction-generated fragment was then subcloned into pBluescript SK+ with a human TR insert containing these two sites (ATG-ATG-GCT-AGC-CTT-AAG-AGG) encoding a seven-residue cytoplasmic tail with the A. Trowbridge I.S. J. Biol. Chem. 1995; Full Text Full Text PDF PubMed Scopus Google Scholar). The of the two sites to the TR S. T. Hopkins C. Trowbridge I.S. J. Cell Biol. 1990; PubMed Scopus Google Scholar), and the amino- and carboxyl-terminal regions of CFTR were the and residues Mutations were introduced into the amino- and carboxyl-terminal tail regions using the The mutations were by F. S. R. Proc. Natl. Acad. Sci. U. S. A. PubMed Scopus Google Scholar, S. Biochem. J. Scholar) of the using the TR and CFTR-TR chimeras were expressed in as described previously Trowbridge I.S. L. Hopkins C.R. Collawn J.F. J. Cell Biol. 1994; 126: PubMed Scopus Google Scholar) using the C.J. P. J. PubMed Google Scholar, C.J. M.J. P. S. J.A. J. 1990; Scholar). The of transferrin internalization was using the J. Biol. Chem. Full Text PDF PubMed Google Scholar) as described previously S. L. Collawn J.F. J. Biol. Chem. 1998; 273: Full Text Full Text PDF PubMed Scopus Google Scholar). and were by Rich D.P. Marshall J. Gregory R.J. Welsh M.J. Smith Cell. 1991; Full Text PDF PubMed Scopus Google Scholar). pKCTR-CFTR was by Sorscher and the Gregory James Cystic of at Z. S. J. M. S. J.R. M. Sorscher E.J. 1995; Google Scholar). For of the amino-terminal region of CFTR, a fragment of pKCTR-CFTR was subcloned into For of the carboxyl-terminal region of CFTR, fragment from pKCTR-CFTR was subcloned into CFTR mutations or in the amino- or carboxyl-terminal tail regions were prepared from the or from as described previously (11Collawn J.F. Stangel M. Kuhn L.A. Esekogwu V. Jing S. Trowbridge I.S. Tainer J.A. Cell. 1990; 63: 1061-1072Abstract Full Text PDF PubMed Scopus (394) Google Scholar) by the of Proc. Natl. Acad. Sci. U. S. A. PubMed Scopus Google Scholar). were by or and then subcloned into or of The mutations were by S. Biochem. J. Scholar) using the to the of wild type or mutant CFTR in COS-7 cells was performed as described by Rich D.P. Marshall J. Gregory R.J. Welsh M.J. Smith Cell. 1991; Full Text PDF PubMed Scopus Google Scholar). The cells were in with serum and at in with for CFTR in cells was using the Rich D.P. Marshall J. Gregory R.J. Welsh M.J. Smith Cell. 1991; Full Text PDF PubMed Scopus Google Scholar). cells were for with by and then in a for of cells was with a a and was at and was studies were at the of the cells were in a and of a were to a at were with indicated at and then to the was to the from to with as of was from the of cells by in of A CFTR, and CFTR and or the of cells a in in the are the cells and the of cells analysis was by the of cells in each with the CFTR or the The used in the were and to that Cell surface CFTR biotinylation was performed as described previously (5Prince L.S. Workman Jr., R.B. Marchase R.B. Proc. Natl. Acad. Sci. U. S. A. 1994; 91: 5192-5196Crossref PubMed Scopus (120) Google Scholar). and proteins were on an as described previously (5Prince L.S. Workman Jr., R.B. Marchase R.B. Proc. Natl. Acad. Sci. U. S. A. 1994; 91: 5192-5196Crossref PubMed Scopus (120) Google Scholar). The and were then as described or mutant CFTRs were from the or with or by The CFTR were with and protein kinase and then by and as described previously (5Prince L.S. Workman Jr., R.B. Marchase R.B. Proc. Natl. Acad. Sci. U. S. A. 1994; 91: 5192-5196Crossref PubMed Scopus (120) Google Scholar). was performed using a The of cell surface CFTR was by the of in the CFTR of the by the of CFTR found in both the and (5Prince L.S. Workman Jr., R.B. Marchase R.B. Proc. Natl. Acad. Sci. U. S. A. 1994; 91: 5192-5196Crossref PubMed Scopus (120) Google Scholar). COS-7 cells or mutant CFTRs were and the were in acid for at with in for at and with or in for at were then for in in with in PBS, BSA, for at and then for in in were in in a of and with were a with a and a were prepared on at the CFTR-TR chimeras and TR were and The were then with and in in for at with with in and with for at The were then with TR in and with TR in for at and in at The were with in and with in for at and then and as described To the regions of CFTR important for endocytosis, we prepared chimeric molecules consisting of the amino terminus (residues 2–78) and the terminus of CFTR (residues each fused to the transmembrane and extracellular domains of the human TR chimeras with a TR and TR mutant TR that is (11Collawn J.F. Stangel M. Kuhn L.A. Esekogwu V. Jing S. Trowbridge I.S. Tainer J.A. Cell. 1990; 63: 1061-1072Abstract Full Text PDF PubMed Scopus (394) Google were expressed in using a from the C.J. P. J. PubMed Google Scholar, C.J. M.J. P. S. J.A. J. 1990; Scholar). Cell surface of both chimeras was using transferrin at Internalization rates of the CFTR-TR chimeras were using the of and J. Biol. Chem. Full Text PDF PubMed Google Scholar). of 2–78 CFTR-TR and CFTR-TR indicated that both were rapidly and similar in to the TR For the TR an internalization (11Collawn J.F. Stangel M. Kuhn L.A. Esekogwu V. Jing S. Trowbridge I.S. Tainer J.A. Cell. 1990; 63: 1061-1072Abstract Full Text PDF PubMed Scopus (394) Google Scholar) was suggested that both cytoplasmic tail regions of CFTR were sufficient to promote TR endocytosis. To determine the potential acidic cluster/casein kinase region of CFTR (residues in the carboxyl-terminal tail was important for endocytosis, we prepared a mutant this and the internalization rates of the two The results indicate that and CFTR-(1391–1476) chimeras were with similar suggesting that the of the carboxyl-terminal tail of CFTR was required for efficient endocytosis. we CFTR-TR chimeras that mutations in potential internalization signals in both cytoplasmic tail and of these in internalization indicated that only one the internalization of the chimeras ∼40% of internalization suggesting that this a of an internalization of the human wild type transferrin receptor to the internalization of the receptor in this cell type the for analysis of the CFTR we were to determine the CFTR chimeras for the as the TR To the intracellular distribution of the we distribution to that of the transferrin receptor using CFTR-(2–78) was to that with the native TR receptor is similar to of the human TR expressed in these cells with the receptor that the surface of the TR to in that of CFTR-(2–78) vesicles were or were vesicles containing only TR in The intracellular distribution of the chimera similar to that of the CFTR-(2–78) chimera from a TR that intracellular signals and is to the cell surface of the with the Y1424A mutant or no In the wild type and are by whereas the two CFTR chimeras are only transferrin at indicated that the surface of both CFTR chimeras is the TR results suggest that both chimeras with the TR and were a of the recycling as is the for the native that both cytoplasmic tail regions were sufficient for endocytosis, we were in the context of the CFTR CFTR, the TR, is a type III membrane protein with In to the and the two both amino- and carboxyl-terminal of CFTR are cytoplasmic in Using a cell surface biotinylation to CFTR endocytosis (5Prince L.S. Workman Jr., R.B. Marchase R.B. Proc. Natl. Acad. Sci. U. S. A. 1994; 91: 5192-5196Crossref PubMed Scopus (120) Google Scholar) that on biotinylation of the found in extracellular we the internalization of CFTR to a previously described mutant, D.P. Gregory R.J. Smith A.E. Welsh M.J. 1993; Google Scholar). we that A1440X expressed in COS-7 cells is by using the surface biotinylation we CFTR and A1440X clearance from the cell surface the A1440X mutant was the CFTR, as had for the chimeric protein, that the residues in CFTR were for rapid of CFTR and CFTR expressed in COS-7 cells. CFTR and CFTR expressed in COS-7 cells were from cell The CFTR or CFTR were then in with protein kinase A and and by and as described previously (5Prince L.S. Workman Jr., R.B. Marchase R.B. Proc. Natl. Acad. Sci. U. S. A. 1994; 91: 5192-5196Crossref PubMed Scopus (120) Google Scholar). The of and are indicated on the of the internalization rates of CFTR and CFTR internalization of CFTR and A1440X in COS-7 cells. COS-7 cells with CFTR or A1440X were Cell surface CFTR or CFTR were using a cell surface biotinylation previously described (5Prince L.S. Workman Jr., R.B. Marchase R.B. Proc. Natl. Acad. Sci. U. S. A. 1994; 91: 5192-5196Crossref PubMed Scopus (120) Google Scholar). both were at to the surface of Internalization is by a of biotinylation of the cell surface by a on the as in and and proteins were on a and CFTR and in and by and to the of CFTR on the cell surface the the of for CFTR and for the internalization of CFTR and Y1424A in COS-7 cells. with CFTR or Y1424A were as described in A for CFTR from the cell surface the of CFTR or Y1424A at the cell surface with CFTR or Y1424A were for CFTR by the biotinylation reaction a and CFTR or Y1424A was on a and as described for A. The of CFTR at the cell surface the of CFTR in COS-7 cells using and COS-7 cells with CFTR using the and were and CFTR was on a and CFTR were then from the two in with protein kinase A and and by and The of and are indicated on the The of CFTR expressed using the was the we the only mutation that internalization of the Y1424A. of this mutation in CFTR that was the CFTR protein internalization that the distribution of Y1424A a cell surface distribution we the of CFTR at the cell surface using surface biotinylation at the of Y1424A at the cell surface was the CFTR protein the that tyrosine 1424 was important for CFTR endocytosis. In our analysis of CFTR internalization, we were to that the internalization rates on the cells were with the Rich D.P. Marshall J. Gregory R.J. Welsh M.J. Smith Cell. 1991; Full Text PDF PubMed Scopus Google Scholar) or the Z. S. J. M. S. J.R. M. Sorscher E.J. 1995; Google Scholar). The CFTR internalization in COS-7 cells with was whereas the CFTR internalization in cells with was the were in the two we the by CFTR from cells with the two with and protein kinase and by and (5Prince L.S. Workman Jr., R.B. Marchase R.B. Proc. Natl. Acad. Sci. U. S. A. 1994; 91: 5192-5196Crossref PubMed Scopus (120) Google Scholar). is in the protein in cells using the were suggesting that CFTR internalization rates were by protein A similar the transferrin receptor is in cells J. Biol. Chem. 1997; Full Text Full Text PDF PubMed Scopus Google Scholar), suggesting that the for surface is and A similar wild type CFTR and Y1424A that the were similar In to the potential internalization signals in the amino terminus of CFTR, we prepared of and of each of these in COS-7 cells only the of CFTR protein was to and at the cell surface To determine the intracellular of the CFTR we examined the cells using is in the CFTR protein and A1440X A and a distribution and of surface of the cell whereas of the had a and any of surface The results that of the were expressed to exit the endoplasmic reticulum and the cell surface. as that the CFTR to the cell surface, we each of these for CFTR channel activity using the A that CFTR channel activity with and of each of the to CFTR that the CFTR were chloride or that to the cell surface or The A1440X mutant, the amino-terminal had chloride channel activity the amino-terminal of CFTR to the cell surface, we were to determine this is for CFTR endocytosis. The of this study are as the amino and of CFTR are each sufficient to promote the endocytosis of the transferrin receptor; the carboxyl-terminal amino of CFTR are required for tyrosine 1424 is a critical of an internalization and in the amino-terminal tail region of CFTR in of CFTR surface Our results indicate that both cytoplasmic tail domains of CFTR are sufficient to promote internalization of a reporter molecule, suggesting that these domains are of with the indicate that both domains with proteins at the cell surface, suggesting that CFTR internalization may by these The only mutation that we identified in our studies that CFTR internalization was found in the carboxyl-terminal tail, Y1424A. mutation endocytosis by in both the native protein and the For a similar mutation in the tyrosine-based internalization of the TR results in an of internalization activity (11Collawn J.F. Stangel M. Kuhn L.A. Esekogwu V. Jing S. Trowbridge I.S. Tainer J.A. Cell. 1990; 63: 1061-1072Abstract Full Text PDF PubMed Scopus (394) Google Scholar). the a effect on CFTR internalization TR internalization, is to that the CFTR protein one internalization as is the for the F. Klausner R.D. Cell. 1992; 69: 1143-1157Abstract Full Text PDF PubMed Scopus (461) Google Scholar). To determine the tyrosine was in we CFTR carboxyl-terminal tail from In the tyrosine is In this a is found in of the tyrosine studies have demonstrated that for tyrosine in the TR internalization a of internalization activity T.E. Cell 1990; PubMed Scopus Google Scholar). In of CFTR the is suggesting that this to the of internalization and may a CFTR internalization Our studies using analysis of the CFTR amino terminus that this region of the molecule is required for folding of of the that the to the cell surface. each mutant was only the of CFTR was we were to CFTR from the surface of cells any of the studies demonstrated that the protein was was to a analysis of COS-7 cells demonstrated that cells the CFTR had chloride channel whereas the cells and cells with each of the an intact amino terminus is required for CFTR as is suggested by the that mutations in the amino-terminal tail in cystic fibrosis For only one mutation in the carboxyl-terminal tail to in CF, and our results on the are the that the amino terminus may with domains of CFTR for CFTR and we were to show that the amino-terminal tail was sufficient to promote TR endocytosis, we were to show that this was for CFTR endocytosis. of CFTR internalization, the is that this in of the two tail the of the amino terminus in CFTR endocytosis this results suggest that the amino terminus with Collawn J.F. Kirk K.L. Proc. Natl. Acad. Sci. U. S. A. 1998; PubMed Scopus Google Scholar), and this CFTR activity B. J. Kirk K.L. Nature. 1997; PubMed Scopus Google Scholar). is to a of the of chloride channel activity. the trafficking of CFTR the channel activity of CFTR Collawn J.F. Kirk K.L. Proc. Natl. Acad. Sci. U. S. A. 1998; PubMed Scopus Google Scholar). the amino terminus of CFTR with domains of CFTR, then the is this for with the and the molecules at the cell surface. is that is in and that the of of CFTR from the cell surface through the targeting found in the CFTR amino-terminal The amino terminus may the only at the cell surface. studies have also that the residues of the carboxyl-terminal tail of CFTR, with and that this CFTR to the D. A. R.C. M.J. J. Biol. Chem. 1998; 273: Full Text Full Text PDF PubMed Scopus Google S. P.J. Li M. 1998; PubMed Scopus Google Scholar). through to regulate the of the CFTR protein at the cell surface or regulate CFTR channel or both D. A. R.C. M.J. J. Biol. Chem. 1998; 273: Full Text Full Text PDF PubMed Scopus Google Scholar). the is at the apical surface in human epithelial cells and with CFTR in in a in a of the internalization rates of CFTR and A1440X mutant in a epithelial cell required to the of the terminus in membrane endocytosis. for a protein to the to endocytosed through the this is the for the receptor Y. J.A. R.J. J. Biol. Chem. 1997; Full Text Full Text PDF PubMed Scopus Google Scholar), which to with the of J.A. A. R.H. L.S. S. E.J. Grinstein S. R.J. Nature. 1998; PubMed Scopus Google Scholar). Our results demonstrate that CFTR signals that to rapidly from the cell surface, is the of studies indicate that the chloride channel in may in of A. F. A. T.J. Proc. Natl. Acad. Sci. U. S. A. 1998; PubMed Scopus Google Scholar, Proc. Natl. Acad. Sci. U. S. A. 1998; PubMed Scopus Google Scholar). a was for CFTR, this was to the distribution of CFTR and the of G.L. Kartner N. Riordan J.R. Grinstein S. J. Biol. Chem. 1992; Full Text PDF PubMed Google Scholar, J. J. 1994; PubMed Google Scholar). CFTR is important for then The is that endocytosis may provide a mechanism for regulating the of CFTR at the cell surface at any one CFTR is to the D. A. R.C. M.J. J. Biol. Chem. 1998; 273: Full Text Full Text PDF PubMed Scopus Google S. P.J. Li M. 1998; PubMed Scopus Google Scholar), then CFTR out of the recycling the at the is in any then of in rapid clearance from the cell surface through with the the that of the CFTR is in a rapidly recycling and is with the and that the distribution of these two is studies of CFTR in epithelial cells required we the trafficking and of the CFTR molecule. We and for critical of the