Specifically deglycosylates the denatured form of N-linked glycoproteins in the cytoplasm and assists their proteasome-mediated degradation (PubMed:17509531, PubMed:17522090). Cleaves the beta-aspartyl-glucosamine (GlcNAc) of the glycan and the amide side chain of Asn, converting Asn to Asp (PubMed:17522090). Prefers proteins containing high-mannose over those bearing complex type oligosaccharides (PubMed:17522090). Can recognize misfolded proteins in the endoplasmic reticulum that are exported to the cytosol to be destroyed and deglycosylate them, while it has no activity toward native proteins (PubMed:17509531). Deglycosylation is a prerequisite for subsequent proteasome-mediated degradation of some, but not all, misfolded glycoproteins (PubMed:17509531). Also displays oxidoreductase (thioredoxin) activity (PubMed:17509531, PubMed:17522090). Involved in regulating the expression of proteasomal subunits such as rpt-3 in order to confer resistance to proteasomal dysfunction (PubMed:27528192). Hydrolysis of an N(4)-(acetyl-beta-D-glucosaminyl)asparagine residue in which the glucosamine residue may be further glycosylated, to yield a (substituted) N-acetyl-beta-D-glucosaminylamine and a peptide containing an aspartate residue. Binds 1 zinc ion per subunit. Inhibited by Zn(2+) and z-VAD-fmk (caspase inhibitor) but unaffected by EDTA. ER localization inferred from partial colocalization with an ER marker, membrane protein PIG-X. Double knockout with rpt-5 RNAi results in failed expression of the proteasomal subunit rpt-3. Belongs to the transglutaminase-like superfamily. PNGase family.