Site-specific de-N-glycosylation of diglycosylated ovalbumin in hen oviduct by endogenous peptide: N-glycanase as a quality control system for newly synthesized proteins

Hen ovalbumin (OVA) is known to exist as a singly N-glycosylated form with a glycan chain on Asn-292 in egg white. Previous studies showed that di-N-glycosylated form of OVA [Di-OVA; CHO-Asn-292/CHO-Asn-311 (CHO, N-glycan chain)], which has two N-glycan chains on Asn-292 and Asn-311, was expressed only transiently in hen oviduct. Di-OVA was not found in egg white, suggesting that this form cannot be secreted normally and may possibly be converted to mono-N-glycosylated OVA (CHO-Asn-292/Asp-311) by the action of peptide:N-glycanase (PNGase) during synthesis and secretion. In this study, we have identified the putative PNGase activity in the homogenate of hen oviduct, purified 1,000-fold, and designated as PNGase HO. We examined the reactivity of Di-OVA to PNGase HO and found that this enzyme site-specifically cleaved off the glycan chain at Asn-311 to convert Di-OVA into the mono-N-glycosylated form (CHO-Asn-292/Asp-311). In contrast, this enzyme was found not to act on the mono-N-glycosylated OVA (CHO-Asn-292/Asn-311) found in egg white when it was tested as a substrate. The present findings support our view that de-N-glycosylation catalyzed by PNGase may be involved in quality control of newly synthesized proteins by converting its diglycosylated form into the mono-N-glycosylated form that can be secreted. However, the alternative possibility that de-N-glycosylation may trigger cytosolic degradation of the aberrantly glycosylated glycoprotein cannot be ruled out.