Cytoplasmic peptide:: N-glycanase (PNGase) in eukaryotic cells:: occurrence, primary structure, and potential functions

A cytoplasmic peptide: N-glycanase has been implicated in the proteasomal degradation of newly synthesized misfolded glycoproteins exported from the endoplasmic reticulum. The gene encoding this enzyme (Png1p) has been identified in yeast. Based on sequence analysis, Png1p was classified as a member of the 'transglutaminase-like superfamily' that contains a putative catalytic triad of amino acids (cysteine, histidine, and aspartic acid). More recent studies in yeast indicate that Png1p can bind to the 26S proteasome through its interaction with the DNA repair protein Rad23p. A mouse homologue of Png1p (mPng1p) bound not only to the Rad23 protein, but also to various proteins related to ubiquitin and/or the proteasome through an extended amino-terminal domain. This NH2 terminus of mPng1p, which is not found in yeast, contains a PUB domain predicted to be involved in the ubiquitin-related pathway. This review will focus on the primary structure and potential functions of the cytoplasmic PNGases.