Conformational and Temperature-sensitive Stability Defects of the ΔF508 Cystic Fibrosis Transmembrane Conductance Regulator in Post-endoplasmic Reticulum Compartments

Deletion of phenylalanine at position 508 (ΔF508) is the most common cystic fibrosis (CF)-associated mutation in the CF transmembrane conductance regulator (CFTR), a cAMP-regulated chloride channel. The consensus notion is that ΔF508 imposes a temperature-sensitive folding defect and targets newly synthesized CFTR for degradation at endoplasmic reticulum (ER). A limited amount of CFTR activity, however, appears at the cell surface in the epithelia of homozygous ΔF508 CFTR mice and patients, suggesting that the ER retention is not absolute in native tissues. To further elucidate the reasons behind the inability of ΔF508 CFTR to accumulate at the plasma membrane, its stability was determined subsequent to escape from the ER, induced by reduced temperature and glycerol. Biochemical and functional measurements show that rescued ΔF508 CFTR has a temperature-sensitive stability defect in post-ER compartments, including the cell surface. The more than 4–20-fold accelerated degradation rate between 37 and 40 °C is, most likely, due to decreased conformational stability of the rescued ΔF508 CFTR, demonstrated by in situ protease susceptibility and SDS-resistant thermoaggregation assays. We propose that the decreased stability of the spontaneously or pharmacologically rescued mutant may contribute to its inability to accumulate at the cell surface. Thus, therapeutic efforts to correct the folding defect should be combined with stabilization of the native ΔF508 CFTR. Deletion of phenylalanine at position 508 (ΔF508) is the most common cystic fibrosis (CF)-associated mutation in the CF transmembrane conductance regulator (CFTR), a cAMP-regulated chloride channel. The consensus notion is that ΔF508 imposes a temperature-sensitive folding defect and targets newly synthesized CFTR for degradation at endoplasmic reticulum (ER). A limited amount of CFTR activity, however, appears at the cell surface in the epithelia of homozygous ΔF508 CFTR mice and patients, suggesting that the ER retention is not absolute in native tissues. To further elucidate the reasons behind the inability of ΔF508 CFTR to accumulate at the plasma membrane, its stability was determined subsequent to escape from the ER, induced by reduced temperature and glycerol. Biochemical and functional measurements show that rescued ΔF508 CFTR has a temperature-sensitive stability defect in post-ER compartments, including the cell surface. The more than 4–20-fold accelerated degradation rate between 37 and 40 °C is, most likely, due to decreased conformational stability of the rescued ΔF508 CFTR, demonstrated by in situ protease susceptibility and SDS-resistant thermoaggregation assays. We propose that the decreased stability of the spontaneously or pharmacologically rescued mutant may contribute to its inability to accumulate at the cell surface. Thus, therapeutic efforts to correct the folding defect should be combined with stabilization of the native ΔF508 CFTR. cystic fibrosis cystic fibrosis transmembrane conductance regulator endoplasmic reticulum brefeldin A cycloheximide endoglycosidase H peptide N-glycanase hemagglutinin monoclonal antibody transmembrane nucleotide binding domain wild type enhanced chemiluminescence sulfo-succinimidyl-2-(biotinamido)ethyl-1,2-dithiopropionate Cystic fibrosis (CF)1 is one of the most prevalent lethal genetic disorders among Caucasian populations (1Kerem B. Rommens J.M. Buchanan J.A. Markiewicz D. Cox T.K. Chakravarti A. Buchwald M. Tsui L.-C. Science. 1989; 245: 1073-1080Crossref PubMed Scopus (3248) Google Scholar). The CF gene encodes the cystic fibrosis transmembrane conductance regulator (CFTR), a cAMP-regulated Cl− channel and conductance regulator, expressed at the apical membrane of secretory epithelia (2Rommens J.M. Iannuzzi M.C. Kerem B. Drumm M.L. Melmer G. Dean M. Rosmahel R. Cole J.L. Kennedy D. Hidaka N. Zsiga M. Buchwald M. Riordan J.R. Tsui L.-C. Collins F.S. Science. 1989; 245: 1059-1065Crossref PubMed Scopus (2554) Google Scholar, 3Riordan J.R. Rommens J.M. Kerem B. Alon M. Rosmahel R. Grzelchak Z. Zielenski J. Lok S. Plavsic N. Chou J.-L. Drumm M.L. Iannuzzi M.C. Collins F.S. Tsui L.-C. Science. 1989; 245: 1066-1073Crossref PubMed Scopus (5976) Google Scholar). CFTR, a member of the ABC transporter family, consists of two structurally homologous halves, each comprised of six transmembrane (TM) helices and a nucleotide binding domain (NBD1 and NBD2), which are connected by the regulatory (R) domain (3Riordan J.R. Rommens J.M. Kerem B. Alon M. Rosmahel R. Grzelchak Z. Zielenski J. Lok S. Plavsic N. Chou J.-L. Drumm M.L. Iannuzzi M.C. Collins F.S. Tsui L.-C. Science. 1989; 245: 1066-1073Crossref PubMed Scopus (5976) Google Scholar). This complex, multidomain structure conceivably renders the posttranslational folding of wild type (wt) CFTR inefficient. More than 50% of the newly synthesized wt CFTR remains incompletely folded and is degraded at the endoplasmic reticulum (ER), whereas the remaining 25–50% undergoes an ATP-dependent conformational maturation and is exported to the cis/medial-Golgi, where its complex glycosylation can occur (4Lukacs G.L. Mohamed A. Kartner N. Chang X.-B. Riordan J.R. Grinstein S. EMBO J. 1994; 13: 6076-6086Crossref PubMed Scopus (342) Google Scholar, 5Ward C.L. Kopito R.R. J. Biol. Chem. 1994; 269: 25710-25718Abstract Full Text PDF PubMed Google Scholar). The hallmark of CF is the loss of cAMP-activated chloride conductance in the epithelial plasma membrane of airways, intestine, and exocrine glands (6Quinton P.M. FASEB J. 1990; 4: 2709-2717Crossref PubMed Scopus (406) Google Scholar, 7Zielenski J. Tsui L.-C. Annu. Rev. Genet. 1995; 29: 777-807Crossref PubMed Scopus (521) Google Scholar, 8Welsh M.J. Smith A. Cell. 1993; 73: 1251-1254Abstract Full Text PDF PubMed Scopus (1235) Google Scholar). More than 900 mutations have been identified in the CF gene, leading to impaired biosynthesis, processing, activation, and/or stability of CFTR (7Zielenski J. Tsui L.-C. Annu. Rev. Genet. 1995; 29: 777-807Crossref PubMed Scopus (521) Google Scholar, 8Welsh M.J. Smith A. Cell. 1993; 73: 1251-1254Abstract Full Text PDF PubMed Scopus (1235) Google Scholar). The most frequent mutation, deletion of phenylalanine at position 508 (ΔF508) in the NBD1, is found in >90% of the patients and detected in ∼70% of CF chromosomes (1Kerem B. Rommens J.M. Buchanan J.A. Markiewicz D. Cox T.K. Chakravarti A. Buchwald M. Tsui L.-C. Science. 1989; 245: 1073-1080Crossref PubMed Scopus (3248) Google Scholar, 7Zielenski J. Tsui L.-C. Annu. Rev. Genet. 1995; 29: 777-807Crossref PubMed Scopus (521) Google Scholar). It is believed that deletion of Phe-508 interrupts the posttranslational folding of CFTR (4Lukacs G.L. Mohamed A. Kartner N. Chang X.-B. Riordan J.R. Grinstein S. EMBO J. 1994; 13: 6076-6086Crossref PubMed Scopus (342) Google Scholar, 5Ward C.L. Kopito R.R. J. Biol. Chem. 1994; 269: 25710-25718Abstract Full Text PDF PubMed Google Scholar, 9Cheng S.H. Gregory R.J. Marshall J. Paul S. Souza D.W. White G.A. O'Riordan C.R. Smith A.E. Cell. 1990; 63: 827-834Abstract Full Text PDF PubMed Scopus (1427) Google Scholar, 10Denning G.M. Anderson M.P. Amara J.F. Marshall J. Smith A.E. Welsh M.J. Nature. 1992; 350: 761-764Crossref Scopus (1063) Google Scholar, 11Zhang F. Kartner N. Lukacs G.L. Nat. Struct. Biol. 1998; 5: 180-183Crossref PubMed Scopus (131) Google Scholar) and targets the core-glycosylated folding intermediate for degradation, predominantly via the ubiquitin-proteasome pathway at the ER (12Ward C.L. Omura S. Kopito R.R. Cell. 1995; 83: 121-128Abstract Full Text PDF PubMed Scopus (1133) Google Scholar, 13Jensen T.J. Loo M.A. Pind S. Williams D.B. Goldberg A.L. Riordan J.R. Cell. 1995; 83: 129-135Abstract Full Text PDF PubMed Scopus (775) Google Scholar). Exposure of ER-retention signals may contribute to the inability of folding intermediate(s) to exit the ER (14Chang X.B. Cui L. Hou Y.X. Jensen T.J. Aleksandov A.A. Mengos A. Riordan J.R. Mol. Cell. 1999; 4: 137-142Abstract Full Text Full Text PDF PubMed Scopus (127) Google Scholar). Accordingly, negligible expression of ΔF508 CFTR could be detected at the cell surface by immunochemical techniques in recombinant cells, CF primary airway cells, and CF tissues (9Cheng S.H. Gregory R.J. Marshall J. Paul S. Souza D.W. White G.A. O'Riordan C.R. Smith A.E. Cell. 1990; 63: 827-834Abstract Full Text PDF PubMed Scopus (1427) Google Scholar, 15Denning G.M. Ostedgaard L.S. Welsh M.J. J. Cell Biol. 1992; 118: 551-559Crossref PubMed Scopus (158) Google Scholar, 16Kartner N. Augustinas O. Jensen T.J. Naismith A.L. Riordan J.R. Nat. Genet. 1992; 1: 321-327Crossref PubMed Scopus (328) Google Scholar). The recognition that the ΔF508 CFTR channel is functional bothin vivo (17Pasyk E.A. Foskett J.K. J. Biol. Chem. 1995; 270: 12347-12350Abstract Full Text Full Text PDF PubMed Scopus (128) Google Scholar, 18F. R. Drumm M.L. Wilkinson D.J. Smit L.S. Worrell R.T. Strong T.V. Frizzell R.A. Dawson D.C. Collins F.S. Science. 1991; 254: 1797-1799Crossref PubMed Scopus (422) Google Scholar, 19Dalemans W. Barbry P. Champigny G. Jallat S. Dott K. Dreyer D. Crystal R.G. Pavirani A. Lecocq J. Lazdunski M. Nature. 1991; 354: 526-528Crossref PubMed Scopus (572) Google Scholar) and after its reconstitution into the phospholipid bilayer (20Li C. Ramjeesingh M. Reyes E. Jensen T. Chang X. Rommens J.M. Bear C.E. Nat. Genet. 1993; 3: 311-316Crossref PubMed Scopus (155) Google Scholar) suggested that the CF phenotype could be alleviated by relocating the mutant CFTR from the ER to the plasma membrane. Reduced temperature (10Denning G.M. Anderson M.P. Amara J.F. Marshall J. Smith A.E. Welsh M.J. Nature. 1992; 350: 761-764Crossref Scopus (1063) Google Scholar, 21French P.J. van Doorninck J.H. Peters R.H. Verbeek E. Ameen N.A. Marino C.R. de Jonge H.R. Bijman J. Scholte B.J. J. Clin. Invest. 1996; 98: 1304-1312Crossref PubMed Scopus (124) Google Scholar, 22Egan M.E. Schweibert E.M. Guggino W.B. Am. J. Physiol. 1995; 268: C243-C251Crossref PubMed Google Scholar), chemical chaperones (23Brown C.R. Hong-Brown L.Q. Biwersi J. Verkman A.S. Cell Stress Chaperones. 1996; 1: 117-125Crossref PubMed Scopus (362) Google Scholar, 24Sato S. Ward C.L. Krouse M.E. Wine J.J. Kopito R.R. J. Biol. Chem. 1996; 271: 635-638Abstract Full Text Full Text PDF PubMed Scopus (468) Google Scholar, 25Bebok Z. Venglarik C.J. Panczel Z. Jilling T. Kirk K.L. Sorcher E.J. Am. J. Physiol. 1998; 275: C599-C607Crossref PubMed Google Scholar), and down-regulation of Hsp70 (26Rubenstein R.C. Zeitlin P.L. Am. J. Physiol. 2000; 278: C259-C267Crossref PubMed Google Scholar, 27Jiang C. Fang S.L. Xiao Y.F. O'Connor S.P. Nadler S.G. Lee D.W. Jefferson D.M. Kaplan J.M. Smith A.E. Cheng S.H. Am. J. Physiol. 1998; 275: C171-C178Crossref PubMed Google Scholar) activity are thought to partially revert the folding defect of ΔF508 CFTR and the of the functional channel at the cell surface. more of ΔF508 CFTR was in the plasma membrane of primary epithelia from ΔF508 homozygous mice P.J. van Doorninck J.H. Peters R.H. Verbeek E. Ameen N.A. Marino C.R. de Jonge H.R. Bijman J. Scholte B.J. J. Clin. Invest. 1996; 98: 1304-1312Crossref PubMed Scopus (124) Google Scholar, Drumm M.L. 1999; Full Text Full Text PDF PubMed Scopus Google Scholar) and in the and epithelia of homozygous ΔF508 patients F. Bijman J. M. J. J. R. de Jonge H.R. B. 2000; Full Text Full Text PDF PubMed Scopus Google Scholar, N. Paul A. D. M. J. O. C. 1999; 29: PubMed Scopus Google Scholar). the of to and that the defect of the ΔF508 CFTR is N. Paul A. D. M. J. O. C. 1999; 29: PubMed Scopus Google Scholar, N. A. M. E. B. J. Clin. Invest. 1999; PubMed Scopus Google Scholar, N. A. C. E. A. B. R. 2000; PubMed Scopus Google Scholar, D. F. S. C. P. P. J. R. Invest. 2000; PubMed Scopus Google Scholar). the ER retention of the mutant is not accelerated from the post-ER could contribute to its inability to at the in tissues. that the rescued ΔF508 CFTR has a at the cell surface of ΔF508 CFTR G.L. Chang Bear C. Kartner N. Mohamed A. Riordan J.R. Grinstein S. J. Biol. Chem. 1993; 268: Full Text PDF PubMed Google Scholar). has or the of ΔF508 CFTR its escape from and functional for the of the ΔF508 CFTR. show that the stability defect is which is due to an conformational stability of the native demonstrated by protease susceptibility and the thermoaggregation of the rescued mutant to its wt The of are with to the phenotype and more therapeutic in A of cells, wt and ΔF508 CFTR with a hemagglutinin was and (4Lukacs G.L. Mohamed A. Kartner N. Chang X.-B. Riordan J.R. Grinstein S. EMBO J. 1994; 13: 6076-6086Crossref PubMed Scopus (342) Google Scholar). of the CFTR be of ER, and plasma from was and F. Kartner N. Lukacs G.L. Nat. Struct. Biol. 1998; 5: 180-183Crossref PubMed Scopus (131) Google Scholar). the core-glycosylated wt or mutant CFTR was from the a in the of cycloheximide The was in and or after in from ΔF508 and wt CFTR and at a of and in the of or for at °C in was by the of to and in at 37 °C for To between and of CFTR, cell with endoglycosidase H and at 37 °C for The of CFTR was with CFTR was with with the monoclonal and to the of the of CFTR the of and and to the of the of CFTR the of and F. Kartner N. Lukacs G.L. Nat. Struct. Biol. 1998; 5: 180-183Crossref PubMed Scopus (131) Google Scholar). of and was with the and monoclonal with and by the of and at and (4Lukacs G.L. Mohamed A. Kartner N. Chang X.-B. Riordan J.R. Grinstein S. EMBO J. 1994; 13: 6076-6086Crossref PubMed Scopus (342) Google Scholar). and at the temperature for the for each in of and with protease and and with or and by and The was a with (4Lukacs G.L. Mohamed A. Kartner N. Chang X.-B. Riordan J.R. Grinstein S. EMBO J. 1994; 13: 6076-6086Crossref PubMed Scopus (342) Google Scholar). and with for at 37 the of the in with and with and The plasma membrane conductance of was determined with J.A. Hou Jensen T.J. Kartner N. Alon N. Riordan J.R. J. Biol. Chem. 1993; 268: Full Text PDF PubMed Google Scholar). the chloride was with by the in for at was by the with of in of The was with a was the was by and to of ΔF508 CFTR, and of the for an The amount of in each was determined with an the of in the of wt and mutant CFTR was by the to from 37 to °C for SDS-resistant with for mutant and wt CFTR, and remaining in the with the and To the ΔF508 and wt CFTR expressed in that the ΔF508 CFTR appears an endoglycosidase H and core-glycosylated with an at 37 °C (9Cheng S.H. Gregory R.J. Marshall J. Paul S. Souza D.W. White G.A. O'Riordan C.R. Smith A.E. Cell. 1990; 63: 827-834Abstract Full Text PDF PubMed Scopus (1427) Google Scholar). The defect of the ΔF508 CFTR could be partially by the of and temperature to its in a of (10Denning G.M. Anderson M.P. Amara J.F. Marshall J. Smith A.E. Welsh M.J. Nature. 1992; 350: 761-764Crossref Scopus (1063) Google Scholar, C.R. Hong-Brown L.Q. Biwersi J. Verkman A.S. Cell Stress Chaperones. 1996; 1: 117-125Crossref PubMed Scopus (362) Google Scholar, 24Sato S. Ward C.L. Krouse M.E. Wine J.J. Kopito R.R. J. Biol. Chem. 1996; 271: 635-638Abstract Full Text Full Text PDF PubMed Scopus (468) Google Scholar). the of the of the ΔF508 CFTR was by the of with an (9Cheng S.H. Gregory R.J. Marshall J. Paul S. Souza D.W. White G.A. O'Riordan C.R. Smith A.E. Cell. 1990; 63: 827-834Abstract Full Text PDF PubMed Scopus (1427) Google Scholar). The and the of the rescued ΔF508 CFTR are to that of the wt CFTR The stability of the wt and rescued ΔF508 CFTR was determined of with or from ER to with brefeldin A the of the ΔF508 CFTR at in the of or at 37 and the remaining CFTR was with a of and that the of the ΔF508 CFTR is at than that of the wt CFTR at 37 of the of the ΔF508 CFTR could be in the of or after the temperature from to 37 °C the To that the deletion of Phe-508 the CFTR, wt or rescued ΔF508 CFTR that the ΔF508 was than wt CFTR at 37 °C the mutant with or reduced temperature and wt CFTR, that or can for the not The accelerated of ΔF508 CFTR be to the loss of the with to in NBD1, or at the not epithelia the ΔF508 CFTR that the mutation the stability of CFTR in epithelia to a with that found in not the notion that the is an of the rescued ΔF508 CFTR than or To the of the plasma mutant CFTR is to that of the ΔF508 CFTR which secretory and the of the rescued ΔF508 CFTR was with cell surface and The plasma membrane of cells, rescued or wt CFTR, with and with the rescued ΔF508 and the wt CFTR are to in to the core-glycosylated ΔF508 CFTR that of the ΔF508 CFTR after and by of at 37 °C the of wt CFTR was more than not than the rescued ΔF508 CFTR with the wt CFTR and The of conductance of to the of functional ΔF508 CFTR to the plasma membrane by the the was with the of the to ΔF508 CFTR to accumulate at the cell surface for the temperature to 37 °C a of the The amount of by decreased by 50% after and after of at 37 °C and in rescued whereas was in wt CFTR the cAMP-activated could not be detected in rescued and not the functional and the that the ΔF508 CFTR are at the cell surface in the post-ER the and measurements the that the functional and of the ΔF508 CFTR is than its wt at 37 suggesting that may between the rescued ΔF508 and wt CFTR at the plasma membrane. To the stability defect of the ΔF508 CFTR at 37 °C is to its at from to 40 of wt and rescued ΔF508 CFTR at °C a at ΔF508 CFTR was at more than its wt at 40 °C a and could be in the of the core-glycosylated wt and ΔF508 CFTR between and 40 °C with the notion that the core-glycosylated a common folding which is to (4Lukacs G.L. Mohamed A. Kartner N. Chang X.-B. Riordan J.R. Grinstein S. EMBO J. 1994; 13: 6076-6086Crossref PubMed Scopus (342) Google Scholar, 5Ward C.L. Kopito R.R. J. Biol. Chem. 1994; 269: 25710-25718Abstract Full Text PDF PubMed Google F. Kartner N. Lukacs G.L. Nat. Struct. Biol. 1998; 5: 180-183Crossref PubMed Scopus (131) Google Scholar). that of and membrane degradation S. Annu. Rev. Genet. 1993; PubMed Scopus Google Scholar), that the of the ΔF508 CFTR is than its wt of CFTR are not to the protease susceptibility of native and the thermoaggregation of CFTR of cell from rescued or wt CFTR, at between 37 and by and CFTR remaining in the was by The of CFTR was by the temperature at which 50% of CFTR is into SDS-resistant The of rescued ΔF508 CFTR was °C than its wt °C than the core-glycosylated ΔF508 CFTR The that the of the rescued and core-glycosylated ΔF508 CFTR is the of was wt and mutant CFTR not that is of could be in the thermoaggregation of the in cells, or in or rescued ΔF508 CFTR and ER to due to in in to membrane which to C. B. J. 1996; PubMed Scopus Google Scholar) and most between the thermoaggregation of rescued and wt CFTR was detected was in with a suggesting that of in C. B. J. 1996; PubMed Scopus Google Scholar), the in the of CFTR in with was an and more to that the rescued ΔF508 CFTR is structurally from its wt This was in the of the more than of the CFTR in the wt and the core-glycosylated ΔF508 CFTR and to the folding of wt CFTR F. Kartner N. Lukacs G.L. Nat. Struct. Biol. 1998; 5: 180-183Crossref PubMed Scopus (131) Google Scholar). with and the of and rescued mutant CFTR, with limited and with of the that the core-glycosylated CFTR was degraded to the of in the wt or rescued ΔF508 CFTR The in situ protease of rescued ΔF508 CFTR to and was than that of the wt CFTR, of the or the monoclonal and antibody was and and not in the between the rescued ΔF508 and wt CFTR could be which was more with the than with the This that the of the are not to the NBD1, the in the rescued ΔF508 CFTR and A more was in the protease and of the rescued and core-glycosylated ΔF508 CFTR and the the with membrane the protease susceptibility of the wt CFTR F. Kartner N. Lukacs G.L. Nat. Struct. Biol. 1998; 5: 180-183Crossref PubMed Scopus (131) Google Scholar), that the and/or conformational stability of the rescued ΔF508 CFTR between that of the core-glycosylated ΔF508 CFTR and the wt CFTR. of the ER be in ΔF508 CFTR, is not the in the homozygous ΔF508 CFTR its P.J. van Doorninck J.H. Peters R.H. Verbeek E. Ameen N.A. Marino C.R. de Jonge H.R. Bijman J. Scholte B.J. J. Clin. Invest. 1996; 98: 1304-1312Crossref PubMed Scopus (124) Google Scholar, Drumm M.L. 1999; Full Text Full Text PDF PubMed Scopus Google Scholar). measurements have that at of the wt CFTR activity is in the apical membrane of the and in of the homozygous ΔF508 which could be by at the reduced temperature P.J. van Doorninck J.H. Peters R.H. Verbeek E. Ameen N.A. Marino C.R. de Jonge H.R. Bijman J. Scholte B.J. J. Clin. Invest. 1996; 98: 1304-1312Crossref PubMed Scopus (124) Google Scholar). from stabilization of the ΔF508 CFTR could be for a of the plasma membrane chloride whereas the of the could be to folding at the of the spontaneously or pharmacologically rescued ΔF508 CFTR its to accumulate at the plasma membrane, for the in spontaneously ΔF508 CFTR in native tissues and in (9Cheng S.H. Gregory R.J. Marshall J. Paul S. Souza D.W. White G.A. O'Riordan C.R. Smith A.E. Cell. 1990; 63: 827-834Abstract Full Text PDF PubMed Scopus (1427) Google G.M. Ostedgaard L.S. Welsh M.J. J. Cell Biol. 1992; 118: 551-559Crossref PubMed Scopus (158) Google Scholar, 16Kartner N. Augustinas O. Jensen T.J. Naismith A.L. Riordan J.R. Nat. Genet. 1992; 1: 321-327Crossref PubMed Scopus (328) Google Scholar). A of or the could the of the ΔF508 CFTR. may the and the of the ΔF508 CFTR from the to the cell the degradation in the of the mutant from post-ER to the may to the of in the N. J. Cell Biol. PubMed Scopus Google Scholar). to ΔF508 CFTR in the and the cell surface of the mutant was with and functional is to be the deletion of Phe-508 may the of a degradation comprised of and in the rescued to in the R.G. EMBO J. 1999; PubMed Scopus Google Scholar) and conceivably in the structurally J.A. R. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar), can be by the and are for the a the and the transmembrane is for the accelerated degradation of the ΔF508 CFTR or are remains to be the that of CFTR and with accelerated and in the of the mutant CFTR the activity of the ubiquitin-proteasome degradation M. and G. L. the The in vivo and in susceptibility of the core-glycosylated ΔF508 and the folding intermediate of wt CFTR, with the of ΔF508 CFTR in the suggested that ΔF508 the of folding intermediate(s) F. Kartner N. Lukacs G.L. Nat. Struct. Biol. 1998; 5: 180-183Crossref PubMed Scopus (131) Google Scholar). This may occur by a the folding of CFTR, by the native or a of recombinant domain are with a P.J. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar, P.J. J. Biol. Chem. 1996; 271: Full Text Full Text PDF PubMed Scopus Google Scholar), the and P.J. P. J. J.M. P.L. J. Biol. Chem. 1992; Full Text PDF PubMed Google Scholar) that deletion of Phe-508 has The mutation not with the posttranslational folding in a by a of (4Lukacs G.L. Mohamed A. Kartner N. Chang X.-B. Riordan J.R. Grinstein S. EMBO J. 1994; 13: 6076-6086Crossref PubMed Scopus (342) Google Scholar, 5Ward C.L. Kopito R.R. J. Biol. Chem. 1994; 269: 25710-25718Abstract Full Text PDF PubMed Google Scholar, 10Denning G.M. Anderson M.P. Amara J.F. Marshall J. Smith A.E. Welsh M.J. Nature. 1992; 350: 761-764Crossref Scopus (1063) Google Scholar, 11Zhang F. Kartner N. Lukacs G.L. Nat. Struct. Biol. 1998; 5: 180-183Crossref PubMed Scopus (131) Google Scholar, 21French P.J. van Doorninck J.H. Peters R.H. Verbeek E. Ameen N.A. Marino C.R. de Jonge H.R. Bijman J. Scholte B.J. J. Clin. Invest. 1996; 98: 1304-1312Crossref PubMed Scopus (124) Google Scholar), renders temperature-sensitive stability defect to the ΔF508 CFTR. to the the of the temperature-sensitive folding is the of in the at the temperature J. PubMed Scopus Google Scholar). ΔF508 CFTR appears not to to of could partially the folding defect of the ΔF508 CFTR at 37 was to the impaired stability of the rescued and G. L. that are for the folding and the stability the in vivo measurements with the protease susceptibility and thermoaggregation that whereas the folding defect can be partially the and of the rescued ΔF508 CFTR, at from its wt of to the native ΔF508 CFTR the therapeutic to correct the folding defect at the ER, in (10Denning G.M. Anderson M.P. Amara J.F. Marshall J. Smith A.E. Welsh M.J. Nature. 1992; 350: 761-764Crossref Scopus (1063) Google Scholar, 22Egan M.E. Schweibert E.M. Guggino W.B. Am. J. Physiol. 1995; 268: C243-C251Crossref PubMed Google Scholar, C.R. Hong-Brown L.Q. Biwersi J. Verkman A.S. Cell Stress Chaperones. 1996; 1: 117-125Crossref PubMed Scopus (362) Google Scholar, 24Sato S. Ward C.L. Krouse M.E. Wine J.J. Kopito R.R. J. Biol. Chem. 1996; 271: 635-638Abstract Full Text Full Text PDF PubMed Scopus (468) Google Scholar, 25Bebok Z. Venglarik C.J. Panczel Z. Jilling T. Kirk K.L. Sorcher E.J. Am. J. Physiol. 1998; 275: C599-C607Crossref PubMed Google Scholar, R.C. M.E. Zeitlin P.L. J. Clin. Invest. PubMed Scopus Google Scholar), in CF mice P.J. van Doorninck J.H. Peters R.H. Verbeek E. Ameen N.A. Marino C.R. de Jonge H.R. Bijman J. Scholte B.J. J. Clin. Invest. 1996; 98: 1304-1312Crossref PubMed Scopus (124) Google Scholar), and in R.C. Zeitlin P.L. Am. J. 1998; PubMed Scopus Google Scholar, P.L. Guggino W.B. L. 1998; Scholar). We are to N. Kartner for the and We A. and R. for of the C. for and S. for

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