Trypanosoma Brucei Telomere Functions in Antigenic Variation

Trypanosoma brucei is a protozoan parasite that causes sleeping sickness in humans and Nagana in cattle. They evade the host\u27s immune defense by periodically switching their major surface antigen, variant surface glycoprotein (VSG), a phenomenon termed antigenic variation. Inside its mammalian host, bloodstream form (BF) T. brucei monoallelically expresses its major surface molecule VSG from the VSG Expression Sites (ESs) located at subtelomeric loci. Monoallelic VSG expression ensures effective antigenic variation and maximizes the efficiency of T. brucei pathogenesis. In the mid-gut of its insect host (tsetse), procyclic form (PF) T. brucei expresses procyclins as the major surface molecules and all VSGs are silent. After the migration to the salivary glands of the tsetse fly, T. brucei cells differentiate into metacyclic forms and express metacyclic VSGs (mVSGs). Therefore, VSG silencing is important for the normal development of T. brucei. Telomeres are important for the regulation of antigenic variation. TbRAP1 was previously identified as an intrinsic component of the T. brucei telomere complex and was shown to be important for ES-linked VSG silencing in BF cells. Our studies further established that TbRAP1 is essential for cell proliferation and required for VSG silencing in PF cells. Apart form ES-linked VSGs, TbRAP1 also regulates the silencing of mVSGs in both BF and PF cells. The strength of TbRAP1 mediated VSG silencing is stronger in PF cells compared to that in the BF cells. In addition, the TbRAP1-mediated VSG silencing in PF cells involves chromatin remodeling. TbTRF, a duplex telomere DNA binding protein that interacts with TbRAP1, does not affect VSG silencing but regulates telomere structure. TbTRFH (TRF homology domain) is required for the homodimerization of TbTRF as well as for interaction with TbRAP1. We established several TbTRFH mutants and determined the critical regions required for homodimerization of TbTRF by performing yeast two-hybrid analysis. TERRA (telomeric repeat- c