Glucose trimming and reglucosylation determine glycoprotein association with calnexin in the endoplasmic reticulum

TLDR

The study aimed to determine how N‑linked oligosaccharides influence glycoprotein folding by analyzing in vitro translated influenza hemagglutinin in dog pancreas microsomes. The authors used dog pancreas microsomes to study HA processing, where monoglucosylated glycans were produced by glucosidases I and II trimming or by reglucosylation of high‑mannose glycans. Binding of HA to calnexin occurs only when the core glycan is monoglucosylated, and release of fully folded HA requires glucosidase II removal of the last glucose, linking glycosylation state to folding and demonstrating the roles of trimming and reglucosylation in the ER.

Abstract

To determine the role of N-linked oligosaccharides in the folding of glycoproteins, we analyzed the processing of in vitro translated influenza hemagglutinin (HA) in dog pancreas microsomes. We found that binding to calnexin, a membrane-bound molecular chaperone, was specific to molecules that possessed monoglucosylated core glycans. In the microsomes, these were generated either by glucose removal from the original triglucosylated core oligosaccharide by glucosidases I and II or by reglucosylation of already unglucosylated high mannose glycans. Release of fully folded HA from calnexin required the removal of the remaining glucose by glucosidase II. The results provided an explanation for trimming and reglucosylation activities in the endoplasmic reticulum and established a direct correlation between glycosylation and folding.