title: Persistent glycoprotein misfolding activates the glucosidase II/UGT1-driven calnexin cycle to delay aggregation and loss of folding competence.
abstract: The UDP-glucose:glycoprotein glucosyltransferase (UGT) is a central player of glycoprotein quality control in the endoplasmic reticulum (ER). UGT reglucosylation of nonnative glycopolypeptides prevents their release from the calnexin cycle and secretion. Here, we compared the fate of a glycoprotein with a reversible, temperature-dependent folding defect in cells with and without UGT1. Upon persistent misfolding, tsO45 G was slowly released from calnexin and entered a second level of retention-based ER quality control by forming BiP/GRP78-associated disulfide-bonded aggregates. This correlated with loss in the ability to correct misfolding. Deletion of UGT1 did not affect the stringency of ER quality control. Rather, it accelerated release from calnexin and transfer to the second ER quality control level, but it did so after an unexpectedly long lag, showing that cycling in the calnexin chaperone system is not frenetic, as claimed by existing models, and is fully activated only upon persistent glycoprotein misfolding.
authors.affiliation: Institute for Research in Biomedicine, CH-6500 Bellinzona, Switzerland. maurizio.molinari@irb.unisi.ch
authors.forename: Maurizio
authors.initials: M
authors.lastname: Molinari
authors.forename: Carmela
authors.initials: C
authors.lastname: Galli
authors.forename: Omar
authors.initials: O
authors.lastname: Vanoni
authors.forename: Stacey M
authors.initials: SM
authors.lastname: Arnold
authors.forename: Randal J
authors.initials: RJ
authors.lastname: Kaufman
mesh_terms: D000818:Animals; D037281:Calnexin / Q000378:metabolism*; D002460:Cell Line; D003553:Cystine / Q000378:metabolism; D004721:Endoplasmic Reticulum / Q000201:enzymology / Q000378:metabolism; D000091342:Endoplasmic Reticulum Chaperone BiP; D017353:Gene Deletion; D005964:Glucosyltransferases / Q000235:genetics / Q000502:physiology*; D006023:Glycoproteins / Q000737:chemistry* / Q000378:metabolism* / Q000502:physiology; D006031:Glycosylation; D006358:Hot Temperature; D051379:Mice; D011489:Protein Denaturation; D017510:Protein Folding; D013234:Stem Cells / Q000201:enzymology / Q000378:metabolism; D000520:alpha-Glucosidases / Q000235:genetics / Q000502:physiology*
chemical_list: D000091342:Endoplasmic Reticulum Chaperone BiP; D006023:Glycoproteins; C000717248:Hspa5 protein, mouse; D037281:Calnexin; D003553:Cystine; D005964:Glucosyltransferases; C072423:mannosylglycoprotein 1,3-glucosyltransferase; C034251:4-nitrophenyl-alpha-glucosidase; D000520:alpha-Glucosidases