title: Endoplasmic reticulum (ER) mannosidase I is compartmentalized and required for N-glycan trimming to Man5-6GlcNAc2 in glycoprotein ER-associated degradation.
abstract: We had previously shown that endoplasmic reticulum (ER)-associated degradation (ERAD) of glycoproteins in mammalian cells involves trimming of three to four mannose residues from the N-linked oligosaccharide Man(9)GlcNAc(2). A possible candidate for this activity, ER mannosidase I (ERManI), accelerates the degradation of ERAD substrates when overexpressed. Although in vitro, at low concentrations, ERManI removes only one specific mannose residue, at very high concentrations it can excise up to four alpha1,2-linked mannose residues. Using small interfering RNA knockdown of ERManI, we show that this enzyme is required for trimming to Man(5-6)GlcNAc(2) and for ERAD in cells in vivo, leading to the accumulation of Man(9)GlcNAc(2) and Glc(1)Man(9)GlcNAc(2) on a model substrate. Thus, trimming by ERManI to the smaller oligosaccharides would remove the glycoprotein from reglucosylation and calnexin binding cycles. ERManI is strikingly concentrated together with the ERAD substrate in the pericentriolar ER-derived quality control compartment (ERQC) that we had described previously. ERManI knockdown prevents substrate accumulation in the ERQC. We suggest that the ERQC provides a high local concentration of ERManI, and passage through this compartment would allow timing of ERAD, possibly through a cycling mechanism. When newly made glycoproteins cannot fold properly, transport through the ERQC leads to trimming of a critical number of mannose residues, triggering a signal for degradation.
authors.affiliation: Department of Cell Research and Immunology, George Wise Faculty of Life Sciences, Tel Aviv University, Tel Aviv 69978, Israel.
authors.forename: Edward
authors.initials: E
authors.lastname: Avezov
authors.forename: Zehavit
authors.initials: Z
authors.lastname: Frenkel
authors.forename: Marcelo
authors.initials: M
authors.lastname: Ehrlich
authors.forename: Annette
authors.initials: A
authors.lastname: Herscovics
authors.forename: Gerardo Z
authors.initials: GZ
authors.lastname: Lederkremer
mesh_terms: D000818:Animals; D037263:Asialoglycoprotein Receptor / Q000378:metabolism*; D002451:Cell Compartmentation; D002460:Cell Line; D004721:Endoplasmic Reticulum / Q000201:enzymology*; D006801:Humans; D008361:Mannosidases / Q000378:metabolism*; D051379:Mice; D008954:Models, Biological; D009844:Oligosaccharides / Q000378:metabolism*; D011134:Polysaccharides / Q000378:metabolism*; D011499:Protein Processing, Post-Translational; D013379:Substrate Specificity
chemical_list: D037263:Asialoglycoprotein Receptor; D009844:Oligosaccharides; D011134:Polysaccharides; C477742:mannosyl(6)-N-acetyl(2)glucose; C058642:mannosyl(5)-N-acetyl(2)-glucose; D008361:Mannosidases; C046090:mannosyl-oligosaccharide 1,2-alpha-mannosidase