Carbohydrate moeity of HLA antigens. Antigenic properties and amino acid sequences around the site of glycosylation

The carbohydrate moiety of HLA antigens was radioactively labeled in galactose residues by treatment of membranes with neuraminidase, galactose oxidase, and NaB³H₄. A single N-linked oligosaccharide side chain M(r) 2700 to 3300 was labeled by this treatment. Unique pronase and tryptic ³H-glycopeptides were purified, using lentil-lectin affinity chromatography. They contained all the radioactivity and all the carbohydrate of the HLA heavy chain. The properties on gel filtration and high voltage electrophoresis of the pronase ³H-glycopeptides from HLA-A2 and HLA-B7 were identical, whereas the tryptic ³H-glycopeptides showed significant differences in both size and charge. The tryptic glycopeptide preparation had less than 0.1% of the HLA antigenic activity of the intact molecule. Removal by glycosidases of all the carbohydrate, except for single residues of N-acetylglucosamine and fucose, from the intact molecule was accompanied by no change in HLA antigenic activity. These data show that the carbohydrate plays no direct role in determining the antigenic specificity of HLA antigens. The single site of glycosylation in the HLA heavy chain was localized to a position about 100 residues from the NH₂-terminal end. Amino acid sequences of tryptic ³H-glycopeptides from HLA-A2 and HLA-B7 were highly homologous. These sequences also showed homology with variable regions of human immunoglobulin heavy chains and with NH₂-terminal sequences of HLA heavy chains. This suggests that HLA heavy chains are structurally related to immunoglobulins and made up of structural homology units.