title: Cytosolic-free oligosaccharides are predominantly generated by the degradation of dolichol-linked oligosaccharides in mammalian cells.
abstract: During asparagine (N)-linked protein glycosylation, eukaryotic cells generate considerable amounts of free oligosaccharides (fOSs) in the cytosol. It is generally assumed that such fOSs are produced by the deglycosylation of misfolded N-glycoproteins that are destined for proteasomal degradation or as the result of the degradation of dolichol-linked oligosaccharides (DLOs), which serve as glycan donor substrates in N-glycosylation reactions. The findings reported herein show that the majority of cytosolic fOSs are generated by a peptide:N-glycanase (PNGase) and an endo-β-N-acetylglucosaminidase (ENGase)-independent pathway in mammalian cells. The ablation of the cytosolic deglycosylating enzymes, PNGase and ENGase, in mouse embryonic fibroblasts had little effect on the amount of cytosolic fOSs generated. Quantitative analyses of fOSs using digitonin-permeabilized cells revealed that they are generated by the degradation of fully assembled Glc3Man9GlcNAc2-pyrophosphate-dolichol (PP-Dol) in the lumen of the endoplasmic reticulum. Because the degradation of Glc3Man9GlcNAc2-PP-Dol is greatly inhibited in the presence of an N-glycosylation acceptor peptide that is recognized by the oligosaccharyltransferase (OST), the OST-mediated hydrolysis of DLO is the most likely mechanism responsible for the production of a large fraction of the cytosolic fOSs.
authors.affiliation: Glycometabolome Team, Systems Glycobiology Research Group, RIKEN-Max Planck Joint Research Center, Global Research Cluster, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan.
authors.forename: Yoichiro
authors.initials: Y
authors.lastname: Harada
authors.affiliation: Glycometabolome Team, Systems Glycobiology Research Group, RIKEN-Max Planck Joint Research Center, Global Research Cluster, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan.
authors.forename: Yuki
authors.initials: Y
authors.lastname: Masahara-Negishi
authors.affiliation: Glycometabolome Team, Systems Glycobiology Research Group, RIKEN-Max Planck Joint Research Center, Global Research Cluster, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan tsuzuki_gm@riken.jp.
authors.forename: Tadashi
authors.initials: T
authors.lastname: Suzuki
mesh_terms: D000118:Acetylglucosaminidase / Q000378:metabolism; D000818:Animals; D002478:Cells, Cultured; D003600:Cytosol / Q000378:metabolism*; D004286:Dolichols / Q000378:metabolism*; D004721:Endoplasmic Reticulum / Q000378:metabolism; D005347:Fibroblasts / Q000378:metabolism; D006023:Glycoproteins / Q000378:metabolism; D006026:Glycoside Hydrolases / Q000378:metabolism; D006602:Hexosyltransferases / Q000378:metabolism; D006868:Hydrolysis; D008565:Membrane Proteins / Q000378:metabolism; D051379:Mice; D009844:Oligosaccharides / Q000378:metabolism*
chemical_list: D004286:Dolichols; D006023:Glycoproteins; D008565:Membrane Proteins; D009844:Oligosaccharides; D006602:Hexosyltransferases; C025620:dolichyl-diphosphooligosaccharide - protein glycotransferase; D006026:Glycoside Hydrolases; C020668:glycanase; D000118:Acetylglucosaminidase
keywords: dolichol-linked oligosaccharides; endo-β-N-acetylglucosaminidase; free oligosaccharides; oligosaccharyltransferase; peptide:N-glycanase