Studies on the synthesis and processing of the asparagine-linked carbohydrate units of glycoproteins

Abstract

It has become apparent in recent years from the work of a number of laboratories that the N -glycosylation of both membrane and secretory glycoproteins is effected by the transfer en bloc to nascent polypeptides of a glucose-containing oligosaccharide (Glc ₃ Man ₉ GlcNAc ₂ ) from a dolichyl pyrophosphoryl carrier; this is followed by a series of modifying reactions to yield the mature polymannose and complex asparaginelinked carbohydrate units. The enzymic steps involved in the assembly of the precursor oligosaccharide, its transfer to protein and its subsequent processing represent potential sites for the regulation of glycoprotein synthesis. Studies performed in our laboratory have dealt primarily with thyroid slices and particulate enzymes with special regard to the role of glucose in these events. Thyroglobulin, the major secretory glycoprotein of this tissue, has well defined complex and polymannose saccharide units, and indeed the most complete form of the latter (Man ₉ GlcNAc ₂ ) has the same structure as the lipidlinked oligosaccharide without the glucose. Our studies indicate that effective N -glycosylation requires a complete glucose chain (Glc ₃ ) and that the glucose sequence is assembled from dolichol-P-glucose in a stepwise manner through the concerted action of at least two transferases in a fashion complementary to the subsequent excision of this sugar by glucosidases. Pulse-chase studies indicate that, after the transfer to protein, the removal of all three glucose residues as well as of the first mannose takes place in the endoplasmic reticulum and three additional mannoses are excised in the Golgi complex, because in the presence of an inhibitor of intracellular transport, carbonyl cyanide m -chlorophenylhydrazone (CCGP), there is a pronounced accumulation of protein-linked Man ₈ GlcNAc ₂ . Studies with metabolic inhibitors (GCGP, antimycin, N ₂ ) indicate that, under conditions of energy depletion, glucosylation of oligosaccharide-lipid is selectively impaired, resulting in an accumulation, as measured chemically or metabolically, of high-mannose-containing (Man ₉ GlcNAc ₂ and Man ₈ GlcNAc ₂ ) lipid-linked saccharides. Further evidence that the glucosylation reaction is very sensitive to the metabolic state is suggested by the observation that tissues not rapidly frozen after removal from the animal show a similar depletion of the glucose-containing oligosaccharide lipids. Another important aspect for the regulation of N -glycosylation of proteins is the availability of dolichyl phosphate for the formation of the lipid-linked mono- and oligosaccharides. Our studies with puromycin suggest that there is a limited supply of the lipid carrier, because in the presence of this inhibitor there is no accumulation of any of the oligosaccharide-lipid species.