Physical studies of Trypanosoma brucei variant surface glycoproteins and their antigenic determinants

Secondary structure determinations have been carried out on two antigenically related variant surface glycoproteins (VSG's) from Trypanosoma brucei, WaTat 1.1 and WaTat 1.12. The two molecules, which bear highly homologous amino-terminal sequences, showed subtle differences in their circular dichroism (CD). Computer analysis revealed that the contribution of alpha helix to the secondary structure of the VSG's was 49% for WaTat 1.1 and 52% for WaTat 1.12. Unfolding studies using guanidine hydrochloride suggested that the WaTat 1.12 VSG was slightly more resistant than WaTat 1.1 VSG to the effect of this reagent. The membrane form of WaTat 1.1 VSG purified by reverse-phase high-performance liquid chromatography gave CD and fluorescence spectra indicative of a partially unfolded or denatured molecule. It was also shown that the antigenic differences between the VSG's were due to surface-oriented topographically assembled epitopes which were highly sensitive to structural perturbations. Monoclonal antibodies specific for these epitopes bound to four discreet determinants on WaTat 1.1, one of which was absent from WaTat 1.12.