Pivotal Role of Calnexin and Mannose Trimming in Regulating the Endoplasmic Reticulum-associated Degradation of Major Histocompatibility Complex Class I Heavy Chain

We have established a mammalian semipermeabilized cell system that faithfully reconstitutes the proteasome-mediated degradation of major histocompatibility complex Class I heavy chain. We show that degradation required unfolding of the protein and was cytosol- and ATP-dependent and that dislocation and degradation required proteasome activity. When the interaction of heavy chain with calnexin was prevented, the rate of degradation was accelerated, suggesting that an interaction with calnexin stabilized heavy chain. Stabilization of heavy chain to degradation was also achieved either by preventing mannose trimming or by removal of theN-linked glycosylation site. This demonstrates that glycosylation and mannose trimming are required to ensure degradation of heavy chain. When degradation or mannose trimming was inhibited, heavy chain formed a prolonged interaction with immunoglobulin heavy chain binding protein, ERp57, and protein disulfide isomerase. Taken together, these results indicate that calnexin association and mannose trimming provide a mechanism to regulate the folding, assembly, and degradation of glycoproteins entering the secretory pathway. We have established a mammalian semipermeabilized cell system that faithfully reconstitutes the proteasome-mediated degradation of major histocompatibility complex Class I heavy chain. We show that degradation required unfolding of the protein and was cytosol- and ATP-dependent and that dislocation and degradation required proteasome activity. When the interaction of heavy chain with calnexin was prevented, the rate of degradation was accelerated, suggesting that an interaction with calnexin stabilized heavy chain. Stabilization of heavy chain to degradation was also achieved either by preventing mannose trimming or by removal of theN-linked glycosylation site. This demonstrates that glycosylation and mannose trimming are required to ensure degradation of heavy chain. When degradation or mannose trimming was inhibited, heavy chain formed a prolonged interaction with immunoglobulin heavy chain binding protein, ERp57, and protein disulfide isomerase. Taken together, these results indicate that calnexin association and mannose trimming provide a mechanism to regulate the folding, assembly, and degradation of glycoproteins entering the secretory pathway. endoplasmic reticulum binding protein major histocompatibility complex ER-associated degradation ER protein of 57 kDa protein disulfide isomerase transporter associated with antigen processing 1-deoxymannorijimycin n-acetyl-l-leucyl-l-leucinyl-norleucinal dithiothreitol castanospermine semipermeabilized polyacrylamide gel electrophoresis The ability of the endoplasmic reticulum (ER)1 to recognize and retain unassembled and malfolded proteins has been identified as an important regulatory mechanism to ensure that only correctly folded and assembled proteins are transported through the secretory pathway (1.Hammond C. Helenius A. Curr. Opin. Cell Biol. 1995; 7: 523-529Crossref PubMed Scopus (591) Google Scholar). One consequence of this “quality control” is that retained proteins are selectively degraded by a mechanism that is distinct from lysosomal degradation and has previously been suggested to take place within the ER (2.Klausner R.D. Sitia R. Cell. 1990; 62: 611-614Abstract Full Text PDF PubMed Scopus (493) Google Scholar). It is now clear from a number of studies that this degradation takes place within the cytosol, is mediated by the proteasome, and requires dislocation of the polypeptide targeted for degradation from the ER to the cytosol (reviewed in Refs. 3.Kopito R. Cell. 1997; 88: 427-430Abstract Full Text Full Text PDF PubMed Scopus (484) Google Scholar and 4.Brodsky J.L. McCracken A.A. Trends Cell Biol. 1997; 7: 151-156Abstract Full Text PDF PubMed Scopus (185) Google Scholar). However, it is not clear how the cell regulates the ER-associated degradation (ERAD) process or how proteins destined for degradation are transported from the ER to the proteasome. Several workers have tried to address these questions, and a consensus is emerging as to the sequence of events that results in the degradation of proteins originally located within the ER. Once translocated into the ER, newly synthesized polypeptides will normally fold and assemble to form native protein molecules that can be further transported to the Golgi apparatus. However, during conditions of stress (5.Knittler M.R. Dirks S. Haas I.G. Proc. Natl. Acad. Sci. U. S. A. 1995; 92: 1764-1768Crossref PubMed Scopus (132) Google Scholar), in the absence of interacting subunits (6.Yu H. Kaung G. Kobayashi S. Kopito R. J. Biol. Chem. 1997; 272: 20800-20804Abstract Full Text Full Text PDF PubMed Scopus (204) Google Scholar), or after viral infection (7.Wiertz E.J. Jones T.R. Sun L. Bogyo M. Geuze H.J. Ploegh H.L. Cell. 1996; 84: 769-779Abstract Full Text Full Text PDF PubMed Scopus (921) Google Scholar, 8.Wiertz E.J. Tortorella D. Bogyo M., Yu, J. Mothes W. Jones T.R. Rapoport T.A. Ploegh H.L. Nature. 1996; 384: 432-438Crossref PubMed Scopus (962) Google Scholar), newly synthesized proteins are unable to adopt the correct native structure and either form prolonged interactions with ER molecular chaperones or are rapidly degraded. A similar process occurs when mutant proteins that are unable to fold correctly are expressed, either in yeast (9.Biederer T. Volkwein C. Sommer T. EMBO J. 1996; 15: 2069-2076Crossref PubMed Scopus (242) Google Scholar, 10.Werner E.D. Brodsky J.L. McCracken A.A. Proc. Natl. Acad. Sci. U. S. A. 1996; 93: 13797-13801Crossref PubMed Scopus (396) Google Scholar) or in mammalian cells (11.Qu D. Teckman J.H. Omura S. Perlmutter D.H. J. Biol. Chem. 1996; 271: 22791-22795Abstract Full Text Full Text PDF PubMed Scopus (311) Google Scholar, 12.Liu Y. Choudhury P. Cabral C.M. Sifers R.N. J. Biol. Chem. 1997; 272: 7946-7951Abstract Full Text Full Text PDF PubMed Scopus (116) Google Scholar). Degradation is coupled to dislocation, probably via the Sec61 complex, in a process believed to be the reverse of translocation into the ER. Evidence for the role of Sec61 complex in dislocation comes from both genetic (13.Plemper R.K. Bohmler S. Bordallo J. Sommer T. Wolf D.H. Nature. 1997; 388: 891-895Crossref PubMed Scopus (477) Google Scholar,14.Pilon M. Schekman R. Romisch K. EMBO J. 1997; 16: 4540-4548Crossref PubMed Scopus (349) Google Scholar), and biochemical approaches (8.Wiertz E.J. Tortorella D. Bogyo M., Yu, J. Mothes W. Jones T.R. Rapoport T.A. Ploegh H.L. Nature. 1996; 384: 432-438Crossref PubMed Scopus (962) Google Scholar, 15.Bebok Z. Mazzochi C. King S.A. Hong J.S. Sorscher E.J. J. Biol. Chem. 1998; 273: 29873-29878Abstract Full Text Full Text PDF PubMed Scopus (101) Google Scholar, 16.de Virgilio M. Weninger H. Ivessa N.E. J. Biol. Chem. 1998; 273: 9734-9743Abstract Full Text Full Text PDF PubMed Scopus (146) Google Scholar) that have a in degradation of in Sec61 mutant yeast and an interaction of proteins targeted for degradation with of the Sec61 Once to the cytosol, the polypeptide can be degraded by the proteasome (6.Yu H. Kaung G. Kobayashi S. Kopito R. J. Biol. Chem. 1997; 272: 20800-20804Abstract Full Text Full Text PDF PubMed Scopus (204) Google Scholar) or can be (7.Wiertz E.J. Jones T.R. Sun L. Bogyo M. Geuze H.J. Ploegh H.L. Cell. 1996; 84: 769-779Abstract Full Text Full Text PDF PubMed Scopus (921) Google Scholar) or Virgilio M. Weninger H. Ivessa N.E. J. Biol. Chem. 1998; 273: 9734-9743Abstract Full Text Full Text PDF PubMed Scopus (146) Google Scholar) to the sequence of events it is clear that a and the and for degradation the It has been suggested that mechanism proteins can be targeted for degradation is by the trimming of P. J. M. J. Cell Biol. 1998; PubMed Scopus Google Scholar, Y. Choudhury P. Cabral C.M. Sifers R.N. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar). to folding, glycoproteins with either or the interaction with these proteins is by the of the chain A. Trends Cell Biol. 1997; 7: Full Text PDF PubMed Scopus Google Scholar). the is from calnexin to correct folding, the chain can be by of binding to This binding to calnexin glycoproteins within the ER fold correctly or assemble with subunits to form The chain can also be within the ER by the of I that mannose of the mannose trimming either by in ER yeast P. J. M. J. Cell Biol. 1998; PubMed Scopus Google Scholar) or by of mammalian cells in the of Y. Choudhury P. Cabral C.M. Sifers R.N. J. Biol. Chem. 1997; 272: 7946-7951Abstract Full Text Full Text PDF PubMed Scopus (116) Google Scholar, Y. Choudhury P. Cabral C.M. Sifers R.N. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar, Virgilio M. C. M. G. Ivessa N.E. Biol. Cell. PubMed Scopus Google Scholar, M. Omura S. J.S. J. 1998; PubMed Scopus (204) Google Scholar) results in of polypeptide that normally be degraded. results have to the that mannose trimming a mechanism the of a protein within the ER to be the molecular interactions during the process of have a system to the degradation of Class I heavy chain. have been in the to that the degradation process is requires can of the and occurs after dislocation through the Sec61 complex E.D. Brodsky J.L. McCracken A.A. Proc. Natl. Acad. Sci. U. S. A. 1996; 93: 13797-13801Crossref PubMed Scopus (396) Google Scholar,14.Pilon M. Schekman R. Romisch K. EMBO J. 1997; 16: 4540-4548Crossref PubMed Scopus (349) Google Scholar, A.A. Brodsky J.L. J. Cell Biol. 1996; PubMed Scopus (349) Google Scholar). results the role by calnexin in and was a from Ploegh and a from was from to was from to was to was from to was from to protein disulfide isomerase and calnexin as previously EMBO J. PubMed Scopus Google Scholar, S. J. 1997; PubMed Scopus Google and to was to a the of the was and 1-deoxymannorijimycin from and from for degradation was as was not with and not J. 1996; Scopus Google Scholar). was from from and from and by was from The cell was from the of Cell and cells a from The was by by of the of Class I heavy chain to the and reverse and after the of the was with as previously EMBO J. PubMed Scopus Google Scholar). in for not the of disulfide in newly synthesized polypeptides R. J. J.L. S. J. 1995; PubMed Scopus Google Scholar). A of of of of of of and of or semipermeabilized cells as previously R. J. J.L. S. J. 1995; PubMed Scopus Google Scholar). cells to with either castanospermine or for with for was with for for by the of of heavy chain was in the of cells for as these the of was the protein was by the of to The cells by for and in of The cells into of and by for cells in either of or of and of system Degradation of heavy chain was for to proteasome or cells for with either or cells and of of by with and for the of chain cells as with and and with the of for cells in and and to cells from in of was and for for and was to a of ERp57, and of and and of with protein and for for to protein and was with and in of to by of cells in from the degradation of of and and for in to a and The proteins from the by the of and and for proteins by for with of for and in The cells in in of and a to a that was of in a cells and for in of The was for and the was a through the by for We have previously that when Class I heavy chain was in the of cells from the cell a heavy be as by with the R. J. J.L. S. J. 1995; PubMed Scopus Google Scholar). This was probably to a of or in this cell of assembly, the cell is in and has been previously to Class I A. P. Biol. 1995; PubMed Scopus Google Scholar). these studies have the of Class I heavy chain. When was in the of the polypeptide chain as by a in with the of in the absence of cells The was also translocated and into the of the ER, as by from with The in when with the of to is to the removal of the The of was degraded when the cells with to that the was not to It is that the during not and the of disulfide as previously R. J. J.L. S. J. 1995; PubMed Scopus Google Scholar). the heavy chain folded correctly and formed a with was by the with to this protein This was as was with into a complex also be by with the G. G. C. A. Cell. Full Text PDF PubMed Scopus Google Scholar) of heavy chain with the unassembled H. Ploegh H.L. J. Google Scholar) that the newly synthesized heavy chain is in the of complex by the of The for the of heavy chain a that has been previously to Proc. Natl. Acad. Sci. U. S. A. PubMed Scopus Google Scholar). to be to cells from the after further protein by the of The cells either in or in the of an that not J. 1996; Scopus Google Scholar). When the cells in the of this an and a in the heavy chain was When the cells in It is important to that of was this is to degradation of the The degradation of heavy chain in the of was to be ATP-dependent However, to ER chaperones as be during the degradation This for the in heavy chain during the Degradation of heavy chain in the of was by the of is a that will the disulfide within the heavy the of the degradation process is to be to the unfolding of unassembled heavy chain and of Class I molecules formed during When heavy chain is in the absence of it will not fold and when this was to the degradation a process of degradation was This degradation be to either the heavy chain or the that was for from the ER. results that the heavy chain can be degraded from cells by a process that is an this an and and can be by that the unfolding of this degradation was to ensure unfolding of the heavy chain. the degradation process that in cells was mediated by the proteasome, the ability of and to this and are of the proteasome, is an within the G. S. E.J. 1995; PubMed Scopus Google Scholar). The the degradation of heavy chain from cells when in the of and was to degradation by when a of not degradation was when cells in the of a not further that the degradation was the proteasome was in the cells or the either the cells or with the proteasome G. S. E.J. 1995; PubMed Scopus Google Scholar). of cells not of degradation This further that the is in the and is mediated by the proteasome. studies into the mechanism of degradation of heavy chain in the of the protein or has that in the of proteasome heavy chain can be into the cytosol, it as a protein (7.Wiertz E.J. Jones T.R. Sun L. Bogyo M. Geuze H.J. Ploegh H.L. Cell. 1996; 84: 769-779Abstract Full Text Full Text PDF PubMed Scopus (921) Google Scholar, 8.Wiertz E.J. Tortorella D. Bogyo M., Yu, J. Mothes W. Jones T.R. Rapoport T.A. Ploegh H.L. Nature. 1996; 384: 432-438Crossref PubMed Scopus (962) Google Scholar). We heavy chain be in the of proteasome within cell We cells after a in the of the and the heavy chain within the of the cells or was these heavy chain from the cells This not the that heavy chain was associated with the of the ER of the cells during the of this a to the heavy chain was in the ER The results that the heavy chain within the is from the to degradation by that the protein is in the and that the is in the absence of proteasome the degradation of heavy chain is inhibited, and the dislocation from the ER is also This that in the dislocation of Class I heavy chain is coupled to When Class I heavy chain is in cell in can and a assembled complex can be the cell A. P. Biol. 1995; PubMed Scopus Google Scholar). when heavy chain is in cells in the of of calnexin heavy chain degradation and the of Class I molecules A. M.R. EMBO J. 1996; 15: PubMed Scopus Google Scholar). is not an for be required to the degradation of heavy chain. the role of calnexin in heavy chain in cells from the cell chain these conditions assembled to form a Class I that be with to and with the not When the cells from the and to the degradation was This degradation was the of the and was both by and suggesting that it was mediated by the proteasome not an the interaction of calnexin and with heavy chain by in the of and with of trimming with as castanospermine interactions with these proteins A. Trends Cell Biol. 1997; 7: Full Text PDF PubMed Scopus Google Scholar). We also that castanospermine an interaction with calnexin and during of heavy chain in cells not When the cells from these and to in the of a degradation of heavy chain was that with synthesized in the absence of castanospermine This degradation of heavy chain in the of castanospermine was with not results that preventing an interaction with calnexin or the rate of that this degradation occurs in cells that this degradation is probably to a of an interaction with It has been previously that calnexin with heavy chain an of Class I to association with M. R.D. J. Biol. Chem. 1995; Full Text Full Text PDF PubMed Scopus Google Scholar). are and heavy chain to and after of the heavy chain will have from this is the and calnexin with heavy chain during the of after the of the calnexin not with heavy after a clear in calnexin association was and the of with calnexin with heavy chain degradation and A similar interaction of heavy chain with calnexin was when degradation was both with and The was with the of the of degradation in the of was associated with heavy with in a complex P. Curr. Biol. 1998; Full Text Full Text PDF PubMed Google Scholar) of the heavy chain from This of from heavy chain the of the heavy and also of heavy chain from the complex the of the heavy chain is in a complex with and not the of degradation that takes place in the of the heavy chain the complex and with results and the that preventing calnexin association degradation that calnexin association the degradation of heavy chain by heavy chain from the dislocation and degradation pathway. degradation to an of heavy chain within the ER that and associated with preventing the degradation of heavy have that dislocation can be to a of heavy chain in the ER. We the that conditions in degradation is inhibited, heavy chain with ER this of during the in the of with to ERp57, and the interaction of heavy chain with after M.R. Haas I.G. EMBO J. Scopus Google Scholar). A in the of heavy chain with and was the and Class I not with as a to the A in the heavy chain with to was This interaction was to be as of the with to of the complex results show that ER molecular as as with heavy chain when degradation is further the role of glycosylation during the of heavy a mutant form of heavy chain that the glycosylation site. a protein has been previously and has been to be of into Class I molecules M. R.D. J. Biol. Chem. 1995; Full Text Full Text PDF PubMed Scopus Google Scholar). the glycosylation also results in a of binding to suggesting that calnexin association is not an for When the cells from the and in the of degradation of the mutant was This is in to and that either the chain is required to the protein for or the unassembled heavy chain with ER the mutant form an preventing dislocation and However, of the a that the protein and not form molecular heavy chain requires an chain to ensure and binding to calnexin the regulates binding to It is established that of binding to removal of mannose has also been to either to calnexin P. J. M. J. Cell Biol. 1998; PubMed Scopus Google Scholar) interactions to a of Y. Choudhury P. Cabral C.M. Sifers R.N. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar). the role of mannose trimming in heavy chain in the of an of ER cells and in the of cytosol and The of in the of the heavy chain to degradation This studies the of proteins in both yeast P. J. M. J. Cell Biol. 1998; PubMed Scopus Google Scholar) and mammalian cells Virgilio M. C. M. G. Ivessa N.E. Biol. Cell. PubMed Scopus Google Scholar, 12.Liu Y. Choudhury P. Cabral C.M. Sifers R.N. J. Biol. Chem. 1997; 272: 7946-7951Abstract Full Text Full Text PDF PubMed Scopus (116) Google Scholar, Y. Choudhury P. Cabral C.M. Sifers R.N. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar) and the role of mannose trimming in proteins for is in the degradation heavy chain not only is stabilized also a prolonged interaction with calnexin when either mannose trimming or degradation is inhibited, the or heavy chain associated with We have that when in the of the rate of degradation was castanospermine this mannose trimming and We preventing mannose trimming the degradation of heavy chain after in the of When in to was during the degradation degradation This that preventing not the protein for degradation that trimming of the mannose is also results that the of a is an for degradation to It has now established that of the for degradation of proteins from the ER dislocation of the from the of this to the proteasome within the cytosol R. Cell. 1997; 88: 427-430Abstract Full Text Full Text PDF PubMed Scopus (484) Google Scholar, 4.Brodsky J.L. McCracken A.A. Trends Cell Biol. 1997; 7: 151-156Abstract Full Text PDF PubMed Scopus (185) Google Scholar). the is now the molecular in the process are not have the proteins in the process been identified or The process can be as either proteins or unassembled subunits are as and targeted for to the of polypeptides are from the ER to the cytosol through the Sec61 translocation the polypeptides either are degraded or are or to degradation by the proteasome. a number of are to be this in how the process is to degradation of polypeptides during pathway and the role of proteins in proteins to the for The results to the of these questions, studies P. J. M. J. Cell Biol. 1998; PubMed Scopus Google Scholar, Y. Choudhury P. Cabral C.M. Sifers R.N. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar) that suggested a role by calnexin and mannose trimming in the and degradation of The of this process are in a of a previously Y. Choudhury P. Cabral C.M. Sifers R.N. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar) and after the newly synthesized heavy chain with an interaction that is by the of the chain. Once from either the heavy chain is by as and to or it with to form a interacting with or the of ER I in the removal of a mannose The polypeptide assemble with be and with or be degraded by the proteasome. It has been suggested Y. Choudhury P. Cabral C.M. Sifers R.N. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar) that the association with calnexin the protein for this the of the chain from to a from the pathway. However, have that of an interaction with calnexin by of the polypeptide in the of to the of the or by in to an degradation can in the absence of an interaction with calnexin or either of these molecular chaperones the protein for the newly synthesized heavy chain can from degradation by with or by interacting with This interaction with calnexin provide for the polypeptide to fold correctly or assemble with this interaction is degradation is We and Y. Choudhury P. Cabral C.M. Sifers R.N. J. Biol. Chem. 1997; 272: 7946-7951Abstract Full Text Full Text PDF PubMed Scopus (116) Google Scholar, Virgilio M. C. M. G. Ivessa N.E. Biol. Cell. PubMed Scopus Google Scholar, M. Omura S. J.S. J. 1998; PubMed Scopus (204) Google Scholar) have also that of mannose trimming also show that when calnexin association and mannose trimming are degradation is This demonstrates that the of the chain is to ensure degradation of This a further as to how ER glycoproteins are from entering the degradation as will It be that a of mannose trimming and unfolding is required to ensure degradation with the protein interacting with ER as (5.Knittler M.R. Dirks S. Haas I.G. Proc. Natl. Acad. Sci. U. S. A. 1995; 92: 1764-1768Crossref PubMed Scopus (132) Google R.K. Bohmler S. Bordallo J. Sommer T. Wolf D.H. Nature. 1997; 388: 891-895Crossref PubMed Scopus (477) Google Scholar). The with a that the protein to the as has been suggested previously P. J. M. J. Cell Biol. 1998; PubMed Scopus Google Scholar, M. Omura S. J.S. J. 1998; PubMed Scopus (204) Google Scholar). the of calnexin binding and mannose trimming provide a mechanism to and this not the protein for a of molecules during the with heavy chain either assembled with or unassembled We show that the of this is binding of either form to an that is with results that calnexin only during the of Class I M. R.D. J. Biol. Chem. 1995; Full Text Full Text PDF PubMed Scopus Google Scholar). When a and the degradation a with calnexin to This that of heavy chain occurs and that interaction with calnexin with when degradation an interaction with ER as ERp57, and these the mannose the chain be calnexin is not for correct and of the Class I it provide a mechanism unassembled heavy chain can to fold and with a for heavy chain to to calnexin be targeted for degradation also the of dislocation of heavy chain conditions in degradation has been workers have the degradation and dislocation and suggested that the proteasome the dislocation process T. S. EMBO J. 1998; PubMed Scopus Google Scholar). One that the dislocation of polypeptides from the ER to the cytosol of the polypeptide chain from interactions within the ER. This with either or to the polypeptide chain within the cytosol, as ensure It has also been that the of heavy chain is required for dislocation, that a association the and is required to this process C.M. Ploegh H.L. Proc. Natl. Acad. Sci. U. S. A. PubMed Scopus Google Scholar). chain not to to dislocation C.M. Ploegh H.L. Proc. Natl. Acad. Sci. U. S. A. PubMed Scopus Google Scholar, C.M. Rapoport T.A. Ploegh H.L. J. Cell Biol. PubMed Scopus Google Scholar), the dislocation be from and activity. in a of coupled to a interaction of heavy chain with calnexin to a in dislocation and an of polypeptide interacting with calnexin and ER This is not the in in the and dislocation of heavy chain into the cytosol, in the absence of (7.Wiertz E.J. Jones T.R. Sun L. Bogyo M. Geuze H.J. Ploegh H.L. Cell. 1996; 84: 769-779Abstract Full Text Full Text PDF PubMed Scopus (921) Google Scholar, 8.Wiertz E.J. Tortorella D. Bogyo M., Yu, J. Mothes W. Jones T.R. Rapoport T.A. Ploegh H.L. Nature. 1996; 384: 432-438Crossref PubMed Scopus (962) Google Scholar). when heavy are in cells in or the of heavy chain synthesized the ability of ER chaperones to dislocation, to an of polypeptides in the cytosol C. P. Proc. Natl. Acad. Sci. U. S. A. 1996; Scopus Google Scholar). The results in this that of Class I heavy chain requires both glycosylation and of the in cell have similar for of proteins within both yeast (9.Biederer T. Volkwein C. Sommer T. EMBO J. 1996; 15: 2069-2076Crossref PubMed Scopus (242) Google Scholar, P. J. M. J. Cell Biol. 1998; PubMed Scopus Google Scholar) and mammalian cells (11.Qu D. Teckman J.H. Omura S. Perlmutter D.H. J. Biol. Chem. 1996; 271: 22791-22795Abstract Full Text Full Text PDF PubMed Scopus (311) Google Scholar, 12.Liu Y. Choudhury P. Cabral C.M. Sifers R.N. J. Biol. Chem. 1997; 272: 7946-7951Abstract Full Text Full Text PDF PubMed Scopus (116) Google Scholar, Y. Choudhury P. Cabral C.M. Sifers R.N. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar), that are this process in cell the role of that both the protein for dislocation and the of the polypeptide to the This will the of these proteins and will a of the dislocation in proteins are targeted for from the ER to the