Co-operative nature of N-glycosylation of proteins at multiple sites: evidence from studies with tunicamycin.

The pattern of glycosylation of newly synthesized human and murine immunoglobulin mu-chains varies according to the degree of inhibition of N-glycosylation effected by using the antibiotic tunicamycin. Tunicamycin, at high concentrations, can apparently block N-glycosylation of mu-chains completely as judged by SDS-PAGE analysis of the biosynthetically labelled products. At lower concentrations of tunicamycin, fully glycosylated and totally non-glycosylated chains were the predominant molecular species. The paucity of the predicted partially glycosylated mu-chains leads us to suggest that the addition of oligosaccharides to the appropriate acceptor sites is a cooperative process. Long exposure of fluorographs reveals each of the predicted intermediately glycosylated forms of the mu-chain, and counting the number of bands on such fluorographs may prove useful in the preliminary determination of the number of N-linked oligosaccharides in a given glycoprotein.