The structure of the C-terminal domain of the largest editosome interaction protein and its role in promoting RNA binding by RNA-editing ligase L2.

Trypanosomatids, such as the sleeping sickness parasite Trypanosoma brucei, contain a ∼20S RNA-editing complex, also called the editosome, which is required for U-insertion/deletion editing of mitochondrial mRNAs. The editosome contains a core of 12 proteins including the large interaction protein A1, the small interaction protein A6, and the editing RNA ligase L2. Using biochemical and structural data, we identified distinct domains of T. brucei A1 which specifically recognize A6 and L2. We provide evidence that an N-terminal domain of A1 interacts with the C-terminal domain of L2. The C-terminal domain of A1 appears to be required for the interaction with A6 and also plays a key role in RNA binding by the RNA-editing ligase L2 in trans. Three crystal structures of the C-terminal domain of A1 have been elucidated, each in complex with a nanobody as a crystallization chaperone. These structures permitted the identification of putative dsRNA recognition sites. Mutational analysis of conserved residues of the C-terminal domain identified Arg703, Arg731 and Arg734 as key requirements for RNA binding. The data show that the editing RNA ligase activity is modulated by a novel mechanism, i.e. by the trans-acting RNA binding C-terminal domain of A1.

锥虫，例如昏睡病寄生虫布氏锥虫，含有一个约20S的RNA编辑复合物，也称为编辑体，它是线粒体mRNA的尿嘧啶插入/缺失编辑所必需的。编辑体包含12种蛋白质的核心，包括大相互作用蛋白A1、小相互作用蛋白A6以及编辑RNA连接酶L2。利用生化和结构数据，我们确定了布氏锥虫A1的不同结构域，它们特异性识别A6和L2。我们提供的证据表明，A1的N末端结构域与L2的C末端结构域相互作用。A1的C末端结构域似乎是与A6相互作用所必需的，并且在反式作用中对RNA编辑连接酶L2的RNA结合也起着关键作用。A1的C末端结构域的三个晶体结构已被阐明，每个结构都与一个纳米抗体作为结晶伴侣形成复合物。这些结构使得能够识别假定的双链RNA识别位点。对C末端结构域保守残基的突变分析确定了Arg703、Arg731和Arg734是RNA结合的关键要素。数据表明，编辑RNA连接酶的活性是通过一种新的机制来调节的，即通过A1的反式作用的RNA结合C末端结构域。