Endoplasmic Reticulum Chaperones Stabilize Nicotinic Receptor Subunits and Regulate Receptor Assembly

Abstract

We examined interactions between the endoplasmic reticulum (ER) chaperones calnexin (CN), ERp57, and immunological heavy chain-binding protein (BiP) and nicotinic acetylcholine receptor (nAChR) subunits. The three chaperones rapidly associate with newly synthesized nAChR subunits. Interactions between nAChR subunits and ERp57 occur via transient intermolecular disulfide bonds and do not require subunit N-linked glycosylation. The associations of ERp57 or CN with AChR subunits are long lived and prolong subunit lifetime ∼10-fold. Coexpression of CN or ERp57 alone does not affect nAChR assembly or trafficking, but together they cause a significant decrease in nAChR expression and assembly. In contrast, associations with BiP are shorter lived and do not alter nAChR expression and assembly. However, a mutated BiP that slows its dissociation significantly increases its associations and decreases nAChR expression and assembly. Our results suggest that interactions with the chaperones regulate the levels of nAChRs assembled in the ER by stabilizing and sequestering subunits during assembly. We examined interactions between the endoplasmic reticulum (ER) chaperones calnexin (CN), ERp57, and immunological heavy chain-binding protein (BiP) and nicotinic acetylcholine receptor (nAChR) subunits. The three chaperones rapidly associate with newly synthesized nAChR subunits. Interactions between nAChR subunits and ERp57 occur via transient intermolecular disulfide bonds and do not require subunit N-linked glycosylation. The associations of ERp57 or CN with AChR subunits are long lived and prolong subunit lifetime ∼10-fold. Coexpression of CN or ERp57 alone does not affect nAChR assembly or trafficking, but together they cause a significant decrease in nAChR expression and assembly. In contrast, associations with BiP are shorter lived and do not alter nAChR expression and assembly. However, a mutated BiP that slows its dissociation significantly increases its associations and decreases nAChR expression and assembly. Our results suggest that interactions with the chaperones regulate the levels of nAChRs assembled in the ER by stabilizing and sequestering subunits during assembly. The endoplasmic reticulum (ER) 2The abbreviations used are: ER, endoplasmic reticulum; AChR, acetylcholine receptor; nAChR, nicotinic acetylcholine receptor; Ab, antibody; Bgt, bungarotoxin; BiP, immunological heavy chain binding protein; CN, calnexin; DTT, dithiothreitol; ERAD, endoplasmic reticulum-associated degradation; HA, hemagglutinin; DMEM, Dulbecco's modified Eagle's medium; PBS, phosphate-buffered saline.2The abbreviations used are: ER, endoplasmic reticulum; AChR, acetylcholine receptor; nAChR, nicotinic acetylcholine receptor; Ab, antibody; Bgt, bungarotoxin; BiP, immunological heavy chain binding protein; CN, calnexin; DTT, dithiothreitol; ERAD, endoplasmic reticulum-associated degradation; HA, hemagglutinin; DMEM, Dulbecco's modified Eagle's medium; PBS, phosphate-buffered saline. is highly specialized to promote and regulate the synthesis and maturation of membrane and secreted proteins. These processes are dependent on quality ER control mechanisms that identify and degrade misfolded proteins. Components of the ER quality control machinery are molecular chaperones (for reviews, see Refs. 1Gething M.J. Kim J. Nature. 1992; 355: 33-45Crossref PubMed Scopus (3590) Google Scholar and 2Gething M.J. Guidebook to Molecular Chaperones and Protein-folding Catalysts. Oxford University Press, Oxford1997Google Scholar). Recent studies have identified protein domains and processing events that mediate interactions between ER chaperones and their substrates (3Molinari M. Kim A. Science. 2000; 288: 331-333Crossref PubMed Scopus (287) Google Scholar). Less is known about chaperone-substrate dynamics after the interaction is established. Here, we examine interactions between nicotinic acetylcholine receptor (nAChR) subunits and the ER chaperones, calnexin (CN), ERp57, and immunological heavy chain-binding protein (BiP). CN is a type I membrane protein that interacts directly with N-linked glycan (4Helenius A. Kim M. Annu. Rev. Biochem. 2004; 73: 1019-1049Crossref PubMed Scopus (1604) Google Scholar) and/or with polypeptide domains (5Danilczyk U.G. Kim D.B. J. Biol. Chem. 2001; 276: 25532-25540Abstract Full Text Full Text PDF PubMed Scopus (89) Google Scholar, 6Leach M.R. Kim D.B. J. Biol. Chem. 2004; 279: 9072-9079Abstract Full Text Full Text PDF PubMed Scopus (58) Google Scholar, 7Wanamaker C.P. Kim W.N. J. Biol. Chem. 2005; 280: 33800-33810Abstract Full Text Full Text PDF PubMed Scopus (33) Google Scholar). CN is found in a complex with ERp57, a member of the protein-disulfide isomerase superfamily that catalyzes the formation of intramolecular disulfide bonds in nascent proteins. ERp57 has been shown to specifically recognize only trimmed glycoproteins (8Oliver J.D. Kim F.J. Bulleid N.J. High S. Science. 1997; 275: 86-88Crossref PubMed Scopus (346) Google Scholar, 9Zapun A. Kim N.J. Tessier D.C. Michalak M. Bergeron J.J. Thomas D.Y. J. Biol. Chem. 1998; 273: 6009-6012Abstract Full Text Full Text PDF PubMed Scopus (303) Google Scholar, 10Molinari M. Kim A. Nature. 1999; 402: 90-93Crossref PubMed Scopus (272) Google Scholar). However, the lectin specificity of ERp57 may be a result of its association with CN rather than an intrinsic ERp57 lectin domain (11Leach M.R. Kim M.F. Thomas D.Y. Williams D.B. J. Biol. Chem. 2002; 277: 29686-29697Abstract Full Text Full Text PDF PubMed Scopus (180) Google Scholar, 12Schrag J.D. Kim J.J. Li Y. Borisova S. Hahn M. Thomas D.Y. Cygler M. Mol. Cell. 2001; 8: 633-644Abstract Full Text Full Text PDF PubMed Scopus (315) Google Scholar, 13Frickel E.M. Kim R. Jelesarov I. Helenius A. Wuthrich K. Ellgaard L. Proc. Natl. Acad. Sci. U. S. A. 2002; 99: 1954-1959Crossref PubMed Scopus (238) Google Scholar), which suggests cooperativity between CN and ERp57. Meanwhile, BiP function occurs via ATP-driven cycles of binding to and release of substrate proteins, which appears to promote protein folding by maintaining a state competent for folding by preventing aggregation (14Gething M.J. Semin. Cell Dev. Biol. 1999; 10: 465-472Crossref PubMed Scopus (434) Google Scholar). Release of substrate requires ATP hydrolysis by the BiP ATPase (15Wei J. Kim J.R. Hendershot L.M. J. Biol. Chem. 1995; 270: 26677-26682Abstract Full Text Full Text PDF PubMed Scopus (124) Google Scholar), and ATPase mutations block substrate release (16Hendershot L. Kim J. Gaut J. Melnick J. Aviel S. Argon Y. Proc. Natl. Acad. Sci. U. S. A. 1996; 93: 5269-5274Crossref PubMed Scopus (131) Google Scholar). BiP plays a variety of additional roles in the ER, including the translocation of newly synthesized proteins across the ER membrane (17Corsi A.K. Kim R. J. Cell Biol. 1997; 137: 1483-1493Crossref PubMed Scopus (124) Google Scholar, 18McClellan A.J. Kim J.B. Vogel J.P. Palazzi D. Rose M.D. Brodsky J.L. Mol. Biol. Cell. 1998; 9: 3533-3545Crossref PubMed Scopus (73) Google Scholar) and a role in proteasome degradation (19Plemper R.K. Kim S. Bordallo J. Sommer T. Wolf D.H. Nature. 1997; 388: 891-895Crossref PubMed Scopus (469) Google Scholar, 20Brodsky J.L. Kim E.D. Dubas M.E. J.L. J. Biol. Chem. 1999; Full Text Full Text PDF PubMed Scopus Google Scholar). In we have interactions between the ER chaperones and nAChR subunits affect subunit and nAChR assembly and The nAChR is of and that nAChR assembly occurs in the ER Kim J. J.P. J. Biol. Chem. Full Text PDF PubMed Google Scholar, Y. Kim J. Cell Biol. PubMed Scopus Google Scholar, Kim W.N. T. J. Cell Biol. PubMed Scopus Google Scholar, Kim Thomas D.Y. Bergeron J.J. J. Biol. Chem. 1995; 270: Full Text Full Text PDF PubMed Scopus Google Scholar) and is and with only of the synthesized subunits assembled J.P. Kim J. Cell. Full Text PDF PubMed Scopus Google Scholar, Kim W.N. J. 2004; PubMed Scopus Google Scholar). three chaperones have been found to associate with subunits. CN with subunits C.P. Kim W.N. J. Biol. Chem. 2005; 280: 33800-33810Abstract Full Text Full Text PDF PubMed Scopus (33) Google Scholar, Kim Thomas D.Y. Bergeron J.J. J. Biol. Chem. 1995; 270: Full Text Full Text PDF PubMed Scopus Google Scholar, Kim J. J. Biol. Chem. 1996; Full Text Full Text PDF PubMed Scopus Google Scholar), ERp57 with subunits Kim J. J. Biol. Chem. 1998; 273: Full Text Full Text PDF PubMed Scopus Google Scholar), and BiP with subunits Kim J.P. J. Cell Biol. PubMed Scopus Google Scholar, Kim W.N. T. J. Cell Biol. PubMed Scopus Google Scholar, J.R. Kim Y. J. Cell Biol. 1992; PubMed Scopus Google Scholar). examine the role of CN, ERp57, and BiP during the assembly of the AChR we used the AChR subunits The of the subunits are that are that recognize the subunits are to after and their assembly is which slows the assembly significantly and to be W.N. Kim T. Cell. Full Text PDF PubMed Scopus Google Scholar). nAChR we found that ERp57 and BiP, CN, associate with newly synthesized AChR subunits but not with subunits during the of assembly. an association with ERp57 or CN significantly subunit in the levels of nAChRs assembled or the However, CN and ERp57 together or a BiP ATPase nAChR assembly and Our suggest that chaperones regulate levels of by stabilizing and sequestering subunits during the of assembly in the used for transient the a Kim R. Google Scholar). with in Dulbecco's modified Eagle's with of ERp57 and to ERp57 of the The of the by of Kim PubMed Scopus Google Scholar). the of a or and a that of the and of ERp57. The ERp57 used a and a the the The and of the by to control for in the the to CN by the The the the The of the for the of used a or with a that a of the and a of CN or to the The used are a and a the are the chaperones, and AChR subunit of Kim R. Google Scholar) a Kim W.N. 1998; PubMed Scopus Google Scholar). for associations between AChR subunits and chaperones, of used of with or for the of a on or expression AChR we with and subunit with of or and for the of a on AChR we the with of and of and subunit with of or or of and and AChR subunits and chaperones, in W.N. Kim T. Cell. Full Text PDF PubMed Scopus Google Scholar, W.N. Kim C.P. J. Biol. Chem. 1997; Full Text Full Text PDF PubMed Scopus Google Scholar, W.N. Kim C.P. J. 1998; PubMed Google Scholar). the in for The for in with of a The with the of and not with phosphate-buffered The in of and and to the the to formation of disulfide bonds after the the three with and with the the the and for and in with and in DMEM, and in the three with PBS, and with The and with a or an that not The three on DTT, for with to the on a and to with an The of a subunit a to a and the The the by the We a and the to of the to that the the of the the in expression of transient of or with of The to the transient of and to for with or and and the by by to the of the the subunits associate with and with an or an and and a AChR subunit three and with a and an and or an and binding of to after the for after for AChR binding of for after the The with and in for which results in of in PBS, and the by binding by the in to binding after with with to the and to binding by AChR with interactions between CN and nAChR subunits and found that CN rapidly with nAChR but the interactions not on subunit N-linked C.P. Kim W.N. J. Biol. Chem. 2005; 280: 33800-33810Abstract Full Text Full Text PDF PubMed Scopus (33) Google Scholar). In we a of to interactions between ERp57 and nAChR subunits. to the of ERp57, which of ERp57, which not with not expression of in a of ERp57 in and nAChR subunit with the and with the of nAChR subunit with the of subunit with an and with the of subunit synthesis by with and nAChR subunit found to with to we subunit we that of of of and of for subunits associate with the during the ERp57, rapidly with of the nAChR to we with CN C.P. Kim W.N. J. Biol. Chem. 2005; 280: 33800-33810Abstract Full Text Full Text PDF PubMed Scopus (33) Google Scholar). ERp57 is a disulfide isomerase that associate with nascent proteins via transient intermolecular disulfide bonds M. Kim A. Nature. 1999; 402: 90-93Crossref PubMed Scopus (272) Google Scholar). with transient disulfide bonds between the nAChR subunits and ERp57, we that the nAChR subunit and ERp57 on with with that and on and after on the but their of proteins that with ERp57. with and the of subunits with not with has been used a rapidly of I. Kim J. Helenius A. J. 1992; PubMed Scopus Google Scholar) that of the disulfide bonds after we in or the of We in the subunit after with ERp57 the or not not a or subunits and the and nAChR subunits in we not subunit with or These results that association and disulfide bonds between ERp57 and nAChR subunits occur only the of the ER and are the result of intermolecular bonds between ERp57 and a between ERp57 and substrate proteins be dependent on the of N-linked to is with may result an association with CN rather than a lectin domain on ERp57 (11Leach M.R. Kim M.F. Thomas D.Y. Williams D.B. J. Biol. Chem. 2002; 277: 29686-29697Abstract Full Text Full Text PDF PubMed Scopus (180) Google Scholar). we found that subunit interactions are of the N-linked C.P. Kim W.N. J. Biol. Chem. 2005; 280: 33800-33810Abstract Full Text Full Text PDF PubMed Scopus (33) Google Scholar), we examined ERp57 with AChR subunits mutated to the the on and the mutated nAChR we found a of association between and the subunits However, the subunit with than the type subunit in and with and The that subunit associate with is with results that AChR subunits associate with CN in a C.P. Kim W.N. J. Biol. Chem. 2005; 280: 33800-33810Abstract Full Text Full Text PDF PubMed Scopus (33) Google Scholar). We the that ERp57 is with the and subunit do not AChR assembly C.P. Kim W.N. J. Biol. Chem. 2005; 280: 33800-33810Abstract Full Text Full Text PDF PubMed Scopus (33) Google Scholar, Kim T. K. Mol. 1997; PubMed Scopus Google Scholar). AChR with ERp57 and CN are found a complex S. Kim M.F. D. Bergeron J.J. K. I. Thomas D.Y. J. 2004; PubMed Scopus Google Scholar), we of the is with the subunits We the on CN with a in to with and we found that the chaperones and that ERp57 and CN are of the and with a AChR subunit and of the subunits with and and and of subunit are with In the of and significantly the association between and ERp57 to the association between and to The of ERp57 and CN the associations between the subunit and CN or ERp57 to for These results suggest that ERp57 and CN are of the and they in with AChR subunits. AChR with studies have found that the subunit with BiP Kim J.P. J. Cell Biol. PubMed Scopus Google Scholar, Kim W.N. T. J. Cell Biol. PubMed Scopus Google Scholar, J.R. Kim Y. J. Cell Biol. 1992; PubMed Scopus Google Scholar). the subunits associate with BiP, we a of the association of BiP with AChR subunits with ERp57 we used an Kim Biochem. 1995; PubMed Scopus Google Scholar) to associations between BiP and AChR subunits of BiP not In to of in only a of BiP expression We to synthesis of and BiP, the of the the of BiP on with the of BiP of to BiP and the levels of BiP only examine which AChR subunits associate with BiP and levels of of the subunits the the and subunits with BiP but levels than for CN C.P. Kim W.N. J. Biol. Chem. 2005; 280: 33800-33810Abstract Full Text Full Text PDF PubMed Scopus (33) Google Scholar) and ERp57 on of we that of and of subunits with the The BiP association with and subunits not occur subunit and BiP in and after Our to for the of and subunits with BiP by the of proteins that with and and subunit on but the of and subunits than that with the CN C.P. Kim W.N. J. Biol. Chem. 2005; 280: 33800-33810Abstract Full Text Full Text PDF PubMed Scopus (33) Google Scholar) and the of subunits with ERp57 an we used to for of or subunits with and subunits on and to for used to for and BiP with and subunits. to BiP with the nAChR subunits than CN or ERp57. be that the interactions occur during the assembly than CN or ERp57. not significantly of or subunits with BiP not subunits not with BiP the levels of associations by shorter lived we the ATPase of BiP, a to a The release of substrate BiP is an and the ATPase slows the release of the subunits by BiP J.R. Kim L.M. J. Biol. Chem. Full Text PDF PubMed Google Scholar). of with in a of BiP the of the BiP ATPase on associations with AChR of the subunits in of and of subunits with the which is a significant with with the binding to but the subunits we the interaction between nAChR subunits and after of the with we of the AChR subunits with the subunits and We used to interacts with and and we that of the and of the subunit with with and These results that the ATPase increases the of nAChR subunit with results suggest that BiP interacts with nAChR subunits by a that of CN and ERp57, which to the nAChR subunits. BiP binding appears to be shorter lived and may cycles of binding and release for its interaction with substrates (16Hendershot L. Kim J. Gaut J. Melnick J. Aviel S. Argon Y. Proc. Natl. Acad. Sci. U. S. A. 1996; 93: 5269-5274Crossref PubMed Scopus (131) Google Scholar). CN and ERp57 the interactions of CN and ERp57 affect nAChR subunits in the ER, we to subunit nAChR subunits are they to are not to the and are rapidly by Kim W.N. J. 2004; PubMed Scopus Google Scholar). with subunits for and for the shown in in the are the subunit a function of after the and the of the is a of of subunits is by shown by the the and in has a of is with of the of nAChR the by with of T. Kim W.N. J. Cell Biol. PubMed Scopus Google Scholar, W.N. Kim T. Proc. Natl. Acad. Sci. U. S. A. PubMed Scopus Google Scholar, Kim J.P. J. Cell Biol. PubMed Scopus Google Scholar). results for the degradation of and subunits and the subunit by long lived and in a of CN, ERp57, or the together with subunits the of subunit and the are by and the of CN to subunit we of subunit a that is with that in the of CN and a about the of ERp57 of CN, we of subunit a and a The with ERp57 appears to be significantly than the in the of of CN and ERp57, the of degradation to with ERp57 the to with CN examine the of interactions on subunit we the degradation of the subunits with on nAChR subunits with CN the the of subunits and after or CN and for the subunits that with the interactions between CN and subunits are that the is not a of subunits CN but rather that the the degradation of the subunits to The interaction between CN and the subunits with that CN slows the degradation of the subunits. the subunits in the complex only a the degradation of the subunits be is not and of the subunits to CN are with a the of with the subunit which that the does not result subunit dissociation The of for subunits is than the in the of the subunit interactions with CN the degradation for a of the subunits the degradation for the of the subunits. appears the for the CN with subunits and ERp57 with subunits. ERp57 and CN or of nAChR with CN and ERp57 to the lifetime of nAChR subunits nAChR subunit in the ER to subunit assembly with of CN with the nAChR subunits to nAChR assembly and expression Kim J. Biol. Chem. 1997; Full Text Full Text PDF PubMed Scopus Google Scholar). in we the of CN on nAChR for nAChRs used by Kim J. Biol. Chem. 1997; Full Text Full Text PDF PubMed Scopus Google Scholar) and nAChR subunit together with the CN In with Kim J. Biol. Chem. 1997; Full Text Full Text PDF PubMed Scopus Google Scholar), CN nAChR expression by binding with in which only nAChR subunit binding to the and the However, we the increases in binding to receptor for in which the of with the of expression the CN These control not in the of Kim J. Biol. Chem. 1997; Full Text Full Text PDF PubMed Scopus Google Scholar), and we that CN has affect on nAChR to the or the formation of nAChRs in the of with nAChR subunits binding to receptor about of the increases not with the nAChR We the results we or BiP with nAChR subunits. CN, ERp57, or BiP alone with the nAChR subunits not alter levels of nAChR In a that nAChRs and we found in nAChR with with nAChRs alone not in nAChR expression and together or the mutated BiP with and the of and nAChR levels by by with and nAChR levels by to interactions be we the chaperones associate with the subunits after we binding with the chaperones significant of binding with or and formation occurs during nAChR assembly in the ER J.P. Kim J. Cell. Full Text PDF PubMed Scopus Google Scholar, W.N. Kim T. Cell. Full Text PDF PubMed Scopus Google Scholar). These that the chaperones during assembly to and the of and together and that block formation we of and together the which binding during nAChR assembly and the levels of binding but not have significant on the of binding appears that CN and ERp57 nAChR subunits to formation of binding on receptor subunits. In we have and the ER chaperones, CN, ERp57, and BiP, with nAChR subunits and affect their assembly. significant between BiP and the chaperones that the levels of nAChR subunit association by for BiP than for CN and ERp57. The levels of subunit association by a of the ATPase that slows substrate The that results is that interactions are is with studies of BiP interactions with which that BiP cycles of BiP to and substrates rather than of BiP to substrate (16Hendershot L. Kim J. Gaut J. Melnick J. Aviel S. Argon Y. Proc. Natl. Acad. Sci. U. S. A. 1996; 93: 5269-5274Crossref PubMed Scopus (131) Google Scholar). The lived of the associations between BiP and the nAChR subunits the of that we with the proteins that we In to the interaction of BiP with the the interactions of ERp57 and CN long of the for the and long lived interaction between ERp57 and the nAChR subunits is that intermolecular disulfide bonds to between nAChR subunits and ERp57 The transient disulfide bonds are with ERp57 a disulfide and the formation of transient disulfide bonds between ERp57 and folding proteins has been for glycoproteins M. Kim A. Nature. 1999; 402: 90-93Crossref PubMed Scopus (272) Google Scholar). However, a is that ERp57 associations with nAChR subunits with the nAChR subunit subunits that that ERp57 directly with the of the to we have with CN C.P. Kim W.N. J. Biol. Chem. 2005; 280: 33800-33810Abstract Full Text Full Text PDF PubMed Scopus (33) Google Scholar). of the long lived association of ERp57 and CN with the subunits that by than of We to directly association of the chaperones with the subunits by the of degradation of subunits that with CN we that the lifetime of associations with subunits in the of In the of subunit with a lifetime in the of the subunits are to ER In a nAChR subunit levels Kim W.N. J. 2004; PubMed Scopus Google Scholar), subunit only be by with subunits with and subunits with The of ERp57 on subunit degradation to that of CN and that its associations with the subunits of a In a Kim W.N. J. 2004; PubMed Scopus Google Scholar), we found that nAChR subunit which we subunit degradation by the nAChR subunits in the ER in nAChR assembly. the subunit which with of CN or ERp57, in in nAChR assembly. We to the results of a that of CN with the nAChR subunits nAChR assembly and expression Kim J. Biol. Chem. 1997; Full Text Full Text PDF PubMed Scopus Google Scholar) subunits that result by CN and ERp57 not to be the of and that subunits are by the CN and ERp57 In to stabilizing nAChR of the subunit associations with CN and ERp57 the and the of subunit for CN associations with subunits and ERp57 associations with subunits. In the association than of the subunits to degrade a These of CN and ERp57 in which the of subunit is have been for proteins, D. Kim S. D.H. J. Biol. Chem. 1996; Full Text Full Text PDF PubMed Scopus Google Scholar). studies that CN via to proteins, to M. Kim Science. PubMed Scopus Google Scholar, Y. Kim I. K. Science. PubMed Scopus Google Scholar), the studies that CN and via the N-linked on the which does not to be the for the nAChR subunits. we that CN and ERp57 of the ER quality control of folding proteins and the of misfolded proteins. of results that ERp57 and CN in and together have not with is that ERp57 and CN with that the proteins are found in a complex the levels of nAChR subunits that with the proteins ERp57 and CN with we found that the levels of and by by the of ERp57 or CN However, nAChR and to ERp57 and CN together with the subunits. binding on nAChR subunits during their the of ERp57 and CN together to occur during nAChR with the result that nAChR assembly is highly binding do not with ERp57 and CN the of ERp57 and CN occur in an of nAChR assembly to binding do ERp57 and CN alone nAChR assembly but together to block is that the only associate with subunits they have assembled The associations of ERp57 or CN alone the nAChR subunits but the subunits to associate with the their to with subunits is in The is that the associate with the assembly and to assembly. We that CN interacts with nAChR subunits in the ER C.P. Kim W.N. J. Biol. Chem. 2005; 280: 33800-33810Abstract Full Text Full Text PDF PubMed Scopus (33) Google Scholar). In we that newly synthesized nAChR subunits with proteins or nAChR subunits in to subunit and or the of subunits are rapidly by ERAD, is with subunits alone and is in which with ERp57 and CN together with ERp57. are additional ER chaperones be binding to assembly the the of and chaperones on interactions that of the they are of the nAChR In proteins a role for subunits during the assembly ER proteins associate with assembled is to or subunit folding and processing events that occur after the subunits have We have that subunit folding after formation of and results in formation of binding and binding on and W.N. Kim T. Cell. Full Text PDF PubMed Scopus Google Scholar, W.N. Kim C.P. J. Biol. Chem. 1997; Full Text Full Text PDF PubMed Scopus Google Scholar, W.N. Kim C.P. J. 1998; PubMed Google Scholar). The formation of binding and the assembly of the and subunits are events In we that are by the to ER chaperones the subunits. shown in the of CN or ERp57 the and subunits to be the occurs during ER assembly. The association between the complex and block in shown in is that proteins, in ERp57, mediate subunit processing events that occur the subunits are assembled disulfide binding W.N. Kim C.P. J. Biol. Chem. 1997; Full Text Full Text PDF PubMed Scopus Google Scholar) and N-linked glycan C.P. Kim W.N. J. Biol. Chem. 2005; 280: 33800-33810Abstract Full Text Full Text PDF PubMed Scopus (33) Google Scholar, M. Kim C.P. W.N. J. 2001; PubMed Google Scholar) of the subunits occur they in the ER chaperones block subunit folding and/or mediate processing that a function of ER chaperones is to control the which nAChRs and We I. for the S. High for the ERp57 and for the We J. for the and and for AChR that to the of