Influence of quaternary structure on glycosylation. Differential subunit association affects the site-specific glycosylation of the common beta-chain from Mac-1 and LFA-1.

The influence of quaternary structure on glycosylation was evaluated in a macrophage-like cell line, P388D1. This cell line simultaneously synthesizes two structurally related glycoproteins, Mac-1 and LFA-1. Mac-1 and LFA-1 each contain two subunits in noncovalent association in an alpha 1 beta 1 structure. The beta-chain polypeptides of these two glycoproteins have identical primary structures while their alpha-chain polypeptides are distinct. For both Mac-1 and LFA-1, the association of the alpha- and beta-chains occurs prior to any Golgi-mediated processing of the oligosaccharide moieties on either one of the subunits. To evaluate the effects of differential subunit association on the site-specific glycosylation of the beta-chain, [3H]glucosamine-labeled oligosaccharides were isolated from the beta-chain of Mac-1 and LFA-1 and were compared by a variety of enzymatic and chromatographic techniques. Reverse-phase high performance liquid chromatography analyses of tryptic-chymotryptic glycopeptides suggest that each beta-chain has at least five glycosylation sites. Structural analysis of oligosaccharides from each corresponding glycopeptide fraction of the beta-chains of Mac-1 or LFA-1 (comparing their glycosidase sensitivities, behavior on serial lectin affinity chromatography, size heterogeneity, extent of sialylation, and branching) indicates that the LFA-1 beta-chain is glycosylated substantially differently on at least four of its sites, compared to the corresponding sites of the Mac-1 beta-chain, even though they are simultaneously synthesized in the same cells. Thus, these data demonstrate that quaternary structure can influence the site-specific glycosylation of a protein, even when the polypeptide structure and the cellular glycosylation machinery remain constant.