Distinct roles for two RAD51-related genes in Trypanosoma brucei antigenic variation

Abstract

In Trypanosoma brucei, DNA recombination is crucial in antigenic variation, a strategy for evading the mammalian host immune system found in a wide variety of pathogens. T.brucei has the capacity to encode >1000 antigenically distinct variant surface glycoproteins (VSGs). By ensuring that only one VSG is expressed on the cell surface at one time, and by periodically switching the VSG gene that is expressed, T.brucei can evade immune killing for prolonged periods. Much of VSG switching appears to rely on a widely conserved DNA repair pathway called homologous recombination, driven by RAD51. Here, we demonstrate that T.brucei encodes a further five RAD51-related proteins, more than has been identified in other single-celled eukaryotes to date. We have investigated the roles of two of the RAD51-related proteins in T.brucei, and show that they contribute to DNA repair, homologous recombination and RAD51 function in the cell. Surprisingly, however, only one of the two proteins contributes to VSG switching, suggesting that the family of diverged RAD51 proteins present in T.brucei have assumed specialized functions in homologous recombination, analogous to related proteins in metazoan eukaryotes.

Trypanosoma brucei TIF2 suppresses VSG switching by maintaining subtelomere integrity

“…VSG switching is mostly dependent on TbRAD51 [39], its paralogue TbRAD51-3 [38], and its interacting factor BRCA2 [40]. In addition, HR-mediated GC events are the preferred mechanism for VSG switching in our lab strain [39,42].…”

“…TbRAD51 and some of its paralogues are required for most VSG switching events [38,39]. Since TbTIF2 depletion elevated the DSB amount at BESs, we hypothesized that more TbRAD51 might associate with BESs.…”

“…RAD51 binds to the single-stranded 3′ overhang at DNA DSBs following 5′ resection and promotes strand invasion during HR [37]. Deletion of TbRAD51 or TbRAD51-3 leads to decreased VSG switching frequencies [38,39]. TbBRCA2 is similarly required for efficient VSG switching [40].…”

Telomere as an Important Player in Regulation of Microbial Pathogen Virulence

“…In T. brucei, six RAD51 related proteins have been identified: RAD51, DMC1, RAD51-3, RAD51-4, RAD51-5, and Rad51-6 (Proudfoot & McCulloch 2005). Among these, deletion of TbRAD51 and TbRAD51-3 led to a decrease in VSG switching rate while deletion of TbRAD51-5 did not have any effect (Proudfoot & McCulloch 2005). In addition, deletion of TbBRCA2 also led to a similar decreased VSG switching rate (Hartley & McCulloch 2008).…”

DNA Recombination Strategies During Antigenic Variation in the African Trypanosome

“…The numbers of Rad51 paralogs vary widely in eukaryotes, with five in mammals but only one in Caenorhabditis elegans. Among single-celled eukaryotes, trypanosomes appear to possess a relatively large repertoire of four proteins (108,144). In T. brucei each RAD51 paralog acts in DNA recombination and repair.…”