在 T. brucei RAP1 中，RRM 介导的 RNA 结合活性对于 VSG 单等位基因表达是必需的。
The RRM-mediated RNA binding activity in T. brucei RAP1 is essential for VSG monoallelic expression.
机构信息
Center for Gene Regulation in Health and Disease, Department of Biological, Geological, and Environmental Sciences, College of Arts and Sciences, Cleveland State University, 2121 Euclid Avenue, Cleveland, OH, 44115, USA.
The Wistar Institute, Philadelphia, PA, 19104, USA.
出版信息
Nat Commun. 2023 Mar 22;14(1):1576. doi: 10.1038/s41467-023-37307-0.
Trypanosoma brucei is a protozoan parasite that causes human African trypanosomiasis. Its major surface antigen VSG is expressed from subtelomeric loci in a strictly monoallelic manner. We previously showed that the telomere protein TbRAP1 binds dsDNA through its RKRRR patch to silence VSGs globally. How TbRAP1 permits expression of the single active VSG is unknown. Through NMR structural analysis, we unexpectedly identify an RNA Recognition Motif (RRM) in TbRAP1, which is unprecedented for RAP1 homologs. Assisted by the RKRRR patch, TbRAP1 RRM recognizes consensus sequences of VSG 3'UTRs in vitro and binds the active VSG RNA in vivo. Mutating conserved RRM residues abolishes the RNA binding activity, significantly decreases the active VSG RNA level, and derepresses silent VSGs. The competition between TbRAP1's RNA and dsDNA binding activities suggests a VSG monoallelic expression mechanism in which the active VSG's abundant RNA antagonizes TbRAP1's silencing effect, thereby sustaining its full-level expression.
布氏锥虫是一种原生动物寄生虫，会引起人类非洲锥虫病。它的主要表面抗原 VSG 是通过端粒位点以严格的单等位基因方式表达的。我们之前曾表明，端粒蛋白 TbRAP1 通过其 RKRRR 簇结合 dsDNA 来全局沉默 VSG。然而，TbRAP1 如何允许单个活性 VSG 的表达尚不清楚。通过 NMR 结构分析，我们出人意料地在 TbRAP1 中鉴定出一个 RNA 识别基序（RRM），这在 RAP1 同源物中是前所未有的。在 RKRRR 簇的辅助下，TbRAP1 RRM 在体外识别 VSG 3'UTR 的共有序列，并在体内结合活性 VSG RNA。突变保守的 RRM 残基会使 RNA 结合活性丧失，显著降低活性 VSG RNA 水平，并解除沉默 VSG 的抑制。TbRAP1 的 RNA 和 dsDNA 结合活性之间的竞争表明了 VSG 单等位基因表达机制，其中活性 VSG 的丰富 RNA 拮抗 TbRAP1 的沉默作用，从而维持其全水平表达。
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