Trypanosoma brucei Tin2 suppresses VSG switching rates.

Trypanosoma brucei, the causative protozoan parasite for Human African Trypanosomiasis, evades the host immune response by regularly switching its surface antigen - Variant Surface Glycoproteins (VSG). VSGs are exclusively expressed in a monoallelic manner from VSG expression sites (ESs) located at subtelomeric loci. Telomeres are nucleoprotein complexes located at the ends of linear chromosomes. They often form a heterochromatin structure that affects subtelomeric gene expression. In fact, we have found that telomeres are important for subtelomeric VSG silencing, as depletion of a telomere protein RAP1 led to derepression of ES-linked VSG genes. In addition, telomeres protect the chromosome ends from illegitimate DNA processes including degradation, repair and recombination. Because homologous recombination-mediated gene conversion is one of several important mechanisms for VSG switching, it is possible that the telomere structure also influences VSG switching. We have now identified another telomere protein, the T. brucei Tin2 homolog, which interacts with the duplex TTAGGG repeat binding factor, TRF, directly. Depletion of Tin2 led to growth arrest but does not seem to affect the silencing of subtelomeric VSG genes. However, using a cell line carrying both a positive and a negative selective marker in the active ES, we found that a temporary depletion of Tin2 led to an increased VSG switching rate, indicating for the first time that the telomere structure is also important for the regulation of VSG switching. In addition, we also found that depletion of Tin2 caused a decrease in TRF protein level without affecting its mRNA amount, indicating that Tin2 is important for TRF protein stability.