title: A complex of Pdi1p and the mannosidase Htm1p initiates clearance of unfolded glycoproteins from the endoplasmic reticulum.
abstract: Endoplasmic reticulum (ER)-resident mannosidases generate asparagine-linked oligosaccharide signals that trigger ER-associated protein degradation (ERAD) of unfolded glycoproteins. In this study, we provide in vitro evidence that a complex of the yeast protein disulfide isomerase Pdi1p and the mannosidase Htm1p processes Man(8)GlcNAc(2) carbohydrates bound to unfolded proteins, yielding Man(7)GlcNAc(2). This glycan serves as a signal for HRD ligase-mediated glycoprotein disposal. We identified a point mutation in PDI1 that prevents complex formation of the oxidoreductase with Htm1p, diminishes mannosidase activity, and delays degradation of unfolded glycoproteins in vivo. Our results show that Pdi1p is engaged in both recognition and glycan signal processing of ERAD substrates and suggest that protein folding and breakdown are not separated but interconnected processes. We propose a stochastic model for how a given glycoprotein is partitioned into folding or degradation pathways and how the flux through these pathways is adjusted to stress conditions.
authors.affiliation: Institute of Microbiology, Department of Biology, Swiss Federal Institute of Technology Zurich, 8093 Zurich, Switzerland.
authors.forename: Robert
authors.initials: R
authors.lastname: Gauss
authors.forename: Kazue
authors.initials: K
authors.lastname: Kanehara
authors.forename: Pedro
authors.initials: P
authors.lastname: Carvalho
authors.forename: Davis T W
authors.initials: DT
authors.lastname: Ng
authors.forename: Markus
authors.initials: M
authors.lastname: Aebi
mesh_terms: D004721:Endoplasmic Reticulum / Q000737:chemistry / Q000378:metabolism*; D006023:Glycoproteins / Q000737:chemistry / Q000378:metabolism*; D008361:Mannosidases / Q000737:chemistry / Q000378:metabolism*; D017354:Point Mutation; D019704:Protein Disulfide-Isomerases / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D058767:Protein Unfolding; D012441:Saccharomyces cerevisiae / Q000166:cytology / Q000201:enzymology / Q000378:metabolism*; D029701:Saccharomyces cerevisiae Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*
chemical_list: D006023:Glycoproteins; C080576:PDI1 protein, S cerevisiae; D029701:Saccharomyces cerevisiae Proteins; C434527:MNL1 protein, S cerevisiae; D008361:Mannosidases; D019704:Protein Disulfide-Isomerases