Protein Glucosylation and Its Role in Protein Folding

An unconventional mechanism for retaining improperly folded glycoproteins and facilitating acquisition of their native tertiary and quaternary structures operates in the endoplasmic reticulum. Recognition of folding glycoproteins by two resident lectins, membrane-bound calnexin and its soluble homolog, calreticulin, is mediated by protein-linked monoglucosylated oligosaccharides. These oligosaccharides contain glucose (Glc), mannose (Man), and N-acetylglucosamine (GlcNAc) in the general form Glc1Man7-9GlcNAc2. They are formed by glucosidase I- and II-catalyzed partial deglucosylation of the oligosaccharide transferred from dolichol diphosphate derivatives to Asn residues in nascent polypeptide chains (Glc3Man9GlcNAc2). Further deglucosylation of the oligosaccharides by glucosidase II liberates glycoproteins from their calnexin/calreticulin anchors. Monoglucosylated glycans are then recreated by the UDP-Glc:glycoprotein glucosyltransferase (GT), and thus recognized again by the lectins, only when linked to improperly folded protein moieties, as GT behaves as a sensor of glycoprotein conformations. The deglucosylation-reglucosylation cycle continues until proper folding is achieved. The lectin-monoglucosylated oligosaccharide interaction is one of the alternative ways by which cells retain improperly folded glycoproteins in the endoplasmic reticulum. Although it decreases the folding rate, it increases folding efficiency, prevents premature glycoprotein oligomerization and degradation, and suppresses formation of non-native disulfide bonds by hindering aggregation and thus allowing interaction of protein moieties of folding glycoproteins with classical chaperones and other proteins that assist in folding.