A Complex of Htm1 and the Oxidoreductase Pdi1 Accelerates Degradation of Misfolded Glycoproteins

• Published in: The Journal of biological chemistry 2016-06, Vol.291 (23), p.12195-12207
• Main Authors: Pfeiffer, Anett, Stephanowitz, Heike, Krause, Eberhard, Volkwein, Corinna, Hirsch, Christian, Jarosch, Ernst, Sommer, Thomas
• Format: Article
• Language: English
• Subjects: Endoplasmic Reticulum - metabolism; Endoplasmic Reticulum-Associated Degradation; endoplasmic-reticulum-associated protein degradation (ERAD); ER quality control; glycan processing; glycoprotein; Glycoproteins - chemistry; Glycoproteins - genetics; Glycoproteins - metabolism; Immunoblotting; Mannosidases - chemistry; Mannosidases - genetics; Mannosidases - metabolism; Multiprotein Complexes - chemistry; Multiprotein Complexes - genetics; Multiprotein Complexes - metabolism; Mutation; Polysaccharides - chemistry; Polysaccharides - metabolism; Protein Binding; protein degradation; protein disulfide isomerase; Protein Disulfide-Isomerases - chemistry; Protein Disulfide-Isomerases - genetics; Protein Disulfide-Isomerases - metabolism; Protein Folding; Protein Synthesis and Degradation; Saccharomyces cerevisiae - genetics; Saccharomyces cerevisiae - metabolism; Saccharomyces cerevisiae Proteins - chemistry; Saccharomyces cerevisiae Proteins - genetics; Saccharomyces cerevisiae Proteins - metabolism
• ISSN: 0021-9258; 1083-351X
• DOI: 10.1074/jbc.M115.703256

A quality control system in the endoplasmic reticulum (ER) efficiently discriminates polypeptides that are in the process of productive folding from conformers that are trapped in an aberrant state. Only the latter are transported into the cytoplasm and degraded in a process termed ER-associated protein degradation (ERAD). In the ER, an enzymatic cascade generates a specific N-glycan structure of seven mannosyl and two N-acetylglucosamine residues (Man7GlcNAc2) on misfolded glycoproteins to facilitate their disposal. We show that a complex encompassing the yeast lectin-like protein Htm1 and the oxidoreductase Pdi1 converts Man8GlcNAc2 on glycoproteins into the Man7GlcNAc2 signal. In vitro the Htm1-Pdi1 complex processes both unfolded and native proteins albeit with a preference for the former. In vivo, elevated expression of HTM1 causes glycan trimming on misfolded and folded proteins, but only degradation of the non-native species is accelerated. Thus, modification with a Man7GlcNAc2 structure does not inevitably commit a protein for ER-associated protein degradation. The function of Htm1 in ERAD relies on its association with Pdi1, which appears to regulate the access to substrates. Our data support a model in which the balanced activities of Pdi1 and Htm1 are crucial determinants for the efficient removal of misfolded secretory glycoproteins.