Biosynthesis of N-acetylglucosamine-P-P-dolichol, the committed step of asparagine-linked oligosaccharide assembly

Asparagine-linked glycosylation is initiated by the synthesis of N-acetylglucosaminylpyrophosphoryl dolichol (GlcNAc2-P-P-dolichol), which is extended by a series of glycosyltrans-ferases to yield Glc₃Man₉GIcNAc₂-P-P-dolichol (where Glc is glucose and Man is mannose). The oligosaccharide unit is then transferred en bloc to asparagine residues of nascent polypeptides in the lumen of the rough endoplasmic reticu-lum. The question here is whether GlcNAc-P-P-dolichol biosynthesis is a fixed process unaffected by cellular events, or a regulated reaction responsive to cellular requirements for glycoprotein biosynthesis. Several lines of evidence indicate that the latter is the case and that GlcNAc-P-P-dolichol biosynthesis may be subject to multiple forms of regulation. Recent information about the N-acetylglucosamine-1-P transferase (GPT) responsible for this reaction and the cloning of cDNA candidates for this enzyme have provided further insight into these mechanisms. This review will examine current hypotheses dealing with GPT and its role in the committed step of asparagine-linked glycosylation.