Biosynthesis of Trypanosoma brucei variant surface glycoproteins - analysis of carbohydrate heterogeneity and timing of post-translational modifications.

The variant surface glycoproteins from two cloned populations of Trypanosoma brucei brucei which were known to migrate as multiple bands on SDS gels have been studied. The heterogeneity present was located in those oligosaccharide side chains the addition of which is tunicamycin-sensitive. The time required for the trypanosome to synthesize and express a variant surface glycoprotein molecule in vitro was found, from pulse-chase and limited trypsinisation experiments, to be approximately 40 min. In the light of these data, pulse-chase experiments on the two antigens known to have heterogeneity in their oligosaccharide side chains demonstrated that the heterogeneity probably arose by two different mechanisms. Pulse-chase experiments on three different clones of trypanosomes have also been used to investigate the timing of cleavage of the carboxyl-terminal extension, known to be encoded on variant surface glycoprotein mRNA. Similar pulse-chase experiments followed by immunoprecipitation using affinity purified antiserum have been used to investigate the addition of the cross-reacting determinant. The timing of both these events has been discussed in relation to the time necessary for the synthesis and expression of the variant surface glycoprotein on the surface of the trypanosome.