Occurrence and biological roles of 'proximal glycanases' in animal cells.

Glycosylation of particular proteins and lipids has become generally acknowledged as being important for these molecules to express their functions in various biological events. However, much less attention has been paid to the biological significance of deglycosylation of such once-glycosylated molecules in the context other than catabolism and recycling in the lysosome. Recently, in various kinds of animal cells and tissues we found non-lysosomal peptide: N-glycanase (PNGase) activities. Before these findings, PNGase was only known in plants and bacteria, and our findings indicated that de-N-glycosylation reaction catalysed by PNGase occurred universally in bioorganisms, and might function as a certain biologically important modification, not as a degradative pathway. Now, we put forward and extend the concept to all the glycoconjugates that deglycosylation as well as glycosylation occur as a universal cellular system to modulate the function of the present molecules, and postulate 'proximal glycanases' (PROXIases) as enzymes that are responsible for the detachment of intact glycan from glycoconjugates and form free glycan and apo-glycoconjugates. In this article, we review the occurrence and possible function of proximal glycanases in animal cells.