Jehi Sanaa E, Li Xiaohua, Sandhu Ranjodh, Ye Fei, Benmerzouga Imaan, Zhang Mingjie, Zhao Yanxiang, Li Bibo
Center for Gene Regulation in Health and Disease, Department of Biological, Geological, and Environmental Sciences, Cleveland State University, Cleveland, OH 44115, USA.
Department of Applied Biology and Chemical Technology, State Key Laboratory of Chirosciences, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, P. R. China The Hong Kong Polytechnic University Shenzhen Research Institute, Shenzhen, P.R. China.
Nucleic Acids Res. 2014 Nov 10;42(20):12899-911. doi: 10.1093/nar/gku942. Epub 2014 Oct 13.
Trypanosoma brucei causes human African trypanosomiasis and regularly switches its major surface antigen, VSG, in the bloodstream of its mammalian host to evade the host immune response. VSGs are expressed exclusively from subtelomeric loci, and we have previously shown that telomere proteins TbTIF2 and TbRAP1 play important roles in VSG switching and VSG silencing regulation, respectively. We now discover that the telomere duplex DNA-binding factor, TbTRF, also plays a critical role in VSG switching regulation, as a transient depletion of TbTRF leads to significantly more VSG switching events. We solved the NMR structure of the DNA-binding Myb domain of TbTRF, which folds into a canonical helix-loop-helix structure that is conserved to the Myb domains of mammalian TRF proteins. The TbTRF Myb domain tolerates well the bulky J base in T. brucei telomere DNA, and the DNA-binding affinity of TbTRF is not affected by the presence of J both in vitro and in vivo. In addition, we find that point mutations in TbTRF Myb that significantly reduced its in vivo telomere DNA-binding affinity also led to significantly increased VSG switching frequencies, indicating that the telomere DNA-binding activity is critical for TbTRF's role in VSG switching regulation.
布氏锥虫可引发人类非洲锥虫病，并会在其哺乳动物宿主的血液中定期更换其主要表面抗原——变异表面糖蛋白（VSG），以逃避宿主的免疫反应。VSG仅从亚端粒位点表达，我们之前已经表明端粒蛋白TbTIF2和TbRAP1分别在VSG转换和VSG沉默调控中发挥重要作用。我们现在发现，端粒双链DNA结合因子TbTRF在VSG转换调控中也起着关键作用，因为TbTRF的短暂缺失会导致显著更多的VSG转换事件。我们解析了TbTRF的DNA结合Myb结构域的核磁共振结构，其折叠成一个典型的螺旋-环-螺旋结构，与哺乳动物TRF蛋白的Myb结构域保守。TbTRF的Myb结构域能很好地耐受布氏锥虫端粒DNA中的大体积J碱基，并且在体外和体内，J碱基的存在均不影响TbTRF的DNA结合亲和力。此外，我们发现TbTRF Myb中的点突变显著降低了其体内端粒DNA结合亲和力，同时也导致VSG转换频率显著增加，这表明端粒DNA结合活性对于TbTRF在VSG转换调控中的作用至关重要。
