Glycopeptides from variant surface glycoproteins of Trypanosoma Brucei. C-terminal location of antigenically cross-reacting carbohydrate moieties.

Glycopeptides have been purified after trypsin or pronase cleavage of variant surface glycoproteins from five antigenic variants of one clone of Trypanosoma brucei. Each peptide has been characterised by its amino acid and sugar composition and partial or complete amino acid sequence. The peptides were assayed in an indirect precipitation radioimmunoassay for inhibition of precipitation of one variant surface glycoprotein by a heterologous antiserum raised against another variant. Two classes of glycopeptide were obtained, those which showed complete inhibition and those which had no inhibitory activity. The non-cross-reacting glycopeptides were all derived from internal sites in the polypeptide chain and had only mannose and glucosamine as the constituents of the glycosyl group with one exception which also contained galactose. The carbohydrate moieties involved in the immunological cross-reaction were found to be attached at or very close to the C-terminus of the protein and contained galactose, mannose and glucosamine. Considerable variation in the size and composition of these glycosyl moieties occurred between the variant glycoproteins and, in two cases, within individual glycoproteins. The immunological cross-reactivity was constant despite this oligosaccharide heterogeneity. There was significant amino acid sequence homology between glycopeptides from certain variants in contrast to the absence of homology observed previously for the N-terminal sequence of intact surface glycoproteins. Some C-terminal glycopeptide sequences suggest the occurrence of proteolytic processing subsequent to glycosylation and prior to variant surface glycoprotein isolation. The terminal amino acid (Asx or Ser) of all isolated variant surface glycoproteins was glycosylated.