Discussion

For successful immune evasion the mammalian infectious bloodstream form (BSF) of Trypanosoma brucei relies on antigenic variation. This process is based on monoallelic expression of variant surface glycoprotein (VSG) genes and their periodic switching. The active VSG is transcribed from one of 15 subtelomeric expression sites (ES), while the remaining ES are silenced. All ES are inactive in the insect vector stage procyclic form (PCF). Previous reports have shown that the telomere complex components TRF, RAP1 and TIF2 are involved in ES regulation. However, the precise nature of their contribution remains unclear. To determine how telomeres influence the transcriptional control of VSG expression, it is first essential to identify the complete composition of the telomere-binding protein complexes. We used two approaches to find novel telomeric factors in T. brucei - a pull-down assay with telomeric repeat oligonucleotides, and co-immunoprecipitation (CoIP) to find TRF-interacting proteins. Here, we describe a new telomere-binding protein, TelBP1, which was found in both experiments. Indirect immunofluorescence analysis and reciprocal CoIP verified TelBP1 as a telomeric component. Mass spectrometry and immunoblot analyses showed an upregulation of TelBP1 in BSF cells. Interestingly, VSG silencing was faster in TelBP1-depleted BSF cells during their differentiation to PCF cells. Our results suggest that TelBP1 influences ES silencing kinetics.