Role of calnexin in the glycan-independent quality control of proteolipid protein.
The endoplasmic (ER) quality control apparatus ensures that misfolded or unassembled proteins are not deployed within the cell, but are retained in the ER and degraded. A glycoprotein-specific system involving the ER lectins calnexin and calreticulin is well documented, but very little is known about mechanisms that may operate for non-glycosylated proteins. We have used a folding mutant of a non- glycosylated membrane protein, proteolipid protein (PLP), to examine the quality control of this class of polypeptide. We find that calnexin associates with newly synthesized PLP molecules, binding stably to misfolded PLP. Calnexin also binds stably to an isolated transmembrane domain of PLP, suggesting that this chaperone is able to monitor the folding and assembly of domains within the ER membrane. Notably, this glycan-independent interaction with calnexin significantly retards the degradation of misfolded PLP. We propose that calnexin contributes to the quality control of non-glycosylated polytopic membrane proteins by binding to misfolded or unassembled transmembrane domains, and discuss our findings in relation to the role of calnexin in the degradation of misfolded proteins.
Swanton
Eileithyia
E
School of Biological Sciences, University of Manchester, 2.205 Stopford Building, Oxford Road, Manchester M13 9PT, UK. lisa.swanton@man.ac.uk
High
Stephen
S
Woodman
Philip
P
D000818:Animals; D019556:COS Cells; D037281:Calnexin / Q000378:metabolism*; D037282:Calreticulin / Q000378:metabolism; D002352:Carrier Proteins / Q000378:metabolism; D002460:Cell Line; D004721:Endoplasmic Reticulum / Q000378:metabolism; D000091342:Endoplasmic Reticulum Chaperone BiP; D006031:Glycosylation; D006360:Heat-Shock Proteins; D006801:Humans; D008565:Membrane Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D051379:Mice; D018832:Molecular Chaperones / Q000378:metabolism; D018991:Myelin Proteolipid Protein / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D011134:Polysaccharides / Q000378:metabolism*; D011388:Prolactin / Q000235:genetics / Q000378:metabolism; D011485:Protein Binding; D011487:Protein Conformation; D017510:Protein Folding; D011498:Protein Precursors / Q000235:genetics / Q000378:metabolism; D034741:RNA, Small Interfering / Q000378:metabolism
D037282:Calreticulin; D002352:Carrier Proteins; D000091342:Endoplasmic Reticulum Chaperone BiP; D006360:Heat-Shock Proteins; D008565:Membrane Proteins; D018832:Molecular Chaperones; D018991:Myelin Proteolipid Protein; D011134:Polysaccharides; D011498:Protein Precursors; D034741:RNA, Small Interfering; D037281:Calnexin; C017671:preprolactin; D011388:Prolactin