College of Medical Veterinary and Life Sciences, University of Glasgow, Institute of Infection, Immunity and Inflammation, The Wellcome Trust Centre for Molecular Parasitology, Sir Graeme Davis Building, 120 University Place, Glasgow G128TA, UK.
Mol Microbiol. 2011 Jul;81(2):434-56. doi: 10.1111/j.1365-2958.2011.07703.x. Epub 2011 May 26.
Homologous recombination in Trypanosoma brucei is used for moving variant surface glycoprotein (VSG) genes into expression sites during immune evasion by antigenic variation. A major route for such VSG switching is gene conversion reactions in which RAD51, a universally conserved recombinase, catalyses homology-directed strand exchange. In any eukaryote, RAD51-directed strand exchange in vivo is mediated by further factors, including RAD51-related proteins termed Rad51 paralogues. These appear to be ubiquitously conserved, although their detailed roles in recombination remain unclear. In T. brucei, four putative RAD51 paralogue genes have been identified by sequence homology. Here we show that all four RAD51 paralogues act in DNA repair, recombination and RAD51 subnuclear dynamics, though not equivalently, while mutation of only one RAD51 paralogue gene significantly impedes VSG switching. We also show that the T. brucei RAD51 paralogues interact, and that the complexes they form may explain the distinct phenotypes of the mutants as well as observed expression interdependency. Finally, we document the Rad51 paralogues that are encoded by a wide range of protists, demonstrating that the Rad51 paralogue repertoire in T. brucei is unusually large among microbial eukaryotes and that one member of the protein family corresponds with a key, conserved eukaryotic Rad51 paralogue.
在逃避免疫过程中，布氏锥虫通过抗原变异将变异表面糖蛋白 (VSG) 基因转移到表达位点的过程中利用了同源重组。这种 VSG 转换的主要途径是基因转换反应，其中普遍保守的重组酶 RAD51 催化同源定向链交换。在任何真核生物中，RAD51 定向的体内链交换都由其他因素介导，包括RAD51 相关蛋白，称为 Rad51 同源物。尽管它们在重组中的详细作用尚不清楚，但这些同源物似乎普遍保守。在布氏锥虫中，通过序列同源性鉴定出了四个假定的 RAD51 同源物基因。在这里，我们表明所有四个 RAD51 同源物都参与 DNA 修复、重组和 RAD51 亚核动力学，但作用不等，而仅突变一个 RAD51 同源物基因就会显著阻碍 VSG 转换。我们还表明，布氏锥虫 RAD51 同源物相互作用，并且它们形成的复合物可能解释了突变体的不同表型以及观察到的表达相互依赖性。最后，我们记录了广泛的原生动物编码的 Rad51 同源物，证明了布氏锥虫中的 Rad51 同源物库在微生物真核生物中异常庞大，并且该蛋白家族的一个成员对应于关键的、保守的真核 Rad51 同源物。
