Role of carbohydrates within variant surface glycoprotein of Trypanosoma congolense. Protection against proteolytic attack.

The effects of tunicamycin on different aspects of structure and biosynthesis of variant surface glycoprotein from Trypanosoma congolense have been studied. Deglycosylated variant antigen becomes synthesized in vitro, is transported through the cell, and is deposited on the cell surface in equivalent amounts compared to the glycosylated species. In contrast to the glycosylated molecule only marginal amounts of high-molecular-mass fragments can be removed from the parasitic cell by externally added proteases in the case of tunicamycin-treated cells. Most of the material removed by proteases from the cell surface of tunicamycin-treated cells has a molecular mass lower than 2 kDa. Many additional proteolytic cleavage sites become accessible after removal of the glycan chains. There is no indication that in the deglycosylated molecule the same preferential protease-sensitive site exists as is found in the glycosylated species. These results suggest that glycosylation of variant surface glycoprotein could be important for the survival of the parasite within the host organism.