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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., (© The Author 2015. Published by Oxford University Press. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.)

Peptide:N-glycanase (PNGase) is a deglycosylating enzyme that acts on N-glycoproteins. A growing evidence exists to indicate that the cytosolic form of PNGase, which is ubiquitously distributed throughout eukaryotes, is not only implicated in the efficient degradation of misfolded glycoproteins destined for the proteasomal degradation but also in the generation of free oligosaccharides as the initial step in the non-lysosomal catabolism of N-glycans. This article summarizes the current state of our knowledge of the physiological and molecular functions of the cytosolic PNGase in a model organism, Saccharomyces cerevisiae, and also discusses the functional/structural diversities of this molecule within eukaryotes., (Copyright © 2014 Elsevier Ltd. All rights reserved.)

Peptide:N-glycanase (PNGase) is the enzyme responsible for de-N-glycosylation of misfolded glycoproteins in the cytosol. Here, we report the molecular identification and characterization of PNGase (png-1, F56G4.5) from Caenorhabditis elegans. This enzyme released both high mannose- and complex-type N-glycans from glycopeptides and denatured glycoproteins. Deglycosylation activity was inhibited by Zn2+ and z-VAD-fmk, but not by EDTA. PNG-1 has a thioredoxin-like domain in addition to a transglutaminase domain, the core domain of PNGases, and exhibited protein disulphide reductase activity in vitro. Our biochemical studies revealed that PNG-1 is a unique bifunctional protein possessing two enzyme activities. [ABSTRACT FROM PUBLISHER]