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Structure of the 30S translation initiation complex

机译：30S翻译起始复合物的结构

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Translation initiation, the rate-limiting step of the universal process of protein synthesis, proceeds through sequential, tightly regulated steps. In bacteria, the correct messenger RNA start site and the reading frame are selected when, with the help of initiation factors IF1, IF2 and IF3, the initiation codon is decoded in the peptidyl site of the 30S ribosomal subunit by the fMet-tRNA~(fMet) anticodon. This yields a 30S initiation complex (30SIC) that is an intermediate in the formation of the 70S initiation complex (70SIC) that occurs on joining of the 50S ribosomal subunit to the 30SIC and release of the initiation factors. The localization of IF2 in the 30SIC has proved to be difficult so far using biochemical approaches, but could now be addressed using cryo-electron microscopy and advanced particle separation techniques on the basis of three-dimensional statistical analysis. Here we report the direct visualization of a 30SIC containing mRNA, fMet-tRNA~(fMet) and initiation factors IF1 and GTP-bound IF2. We demonstrate that the fMet-tRNA~(fMet) is held in a characteristic and precise position and conformation by two interactions that contribute to the formation of a stable complex: one involves the transfer RNA decoding stem which is buried in the 30S peptidyl site, and the other occurs between the carboxy-terminal domain of IF2 and the tRNA acceptor end. The structure provides insights into the mechanism of 70SIC assembly and rationalizes the rapid activation of GTP hydrolysis triggered on 30SIC-50S joining by showing that the GTP-binding domain of IF2 would directly face the GTPase-activated centre of the 50S subunit.
机译：翻译起始是蛋白质合成普遍过程中的限速步骤，它是通过顺序，严格调控的步骤进行的。在细菌中，当借助起始因子IF1，IF2和IF3通过fMet-tRNA〜（30S核糖体亚基的肽基位点解码起始密码子时，选择正确的信使RNA起始位点和阅读框。 fMet）反密码子。这产生了30S起始复合物（30SIC），其是70S起始复合物（70SIC）形成中的中间体，其在50S核糖体亚基与30SIC结合和起始因子释放时发生。迄今为止，已经证明使用生化方法很难将IF2定位在30SIC中，但是现在可以在三维统计分析的基础上使用冷冻电子显微镜和先进的颗粒分离技术来解决这一问题。在这里，我们报告了包含mRNA，fMet-tRNA〜（fMet）和起始因子IF1和GTP结合的IF2的30SIC的直接可视化。我们证明了fMet-tRNA〜（fMet）通过两个相互作用保持在特征性和精确的位置和构象中，这两个相互作用有助于形成稳定的复合物：一个涉及埋在30S肽基位点中的转移RNA解码茎，另一个发生在IF2的羧基末端结构域和tRNA受体末端之间。通过显示IF2的GTP结合域将直接面对50S亚基的GTPase激活的中心，该结构提供了对70SIC组装机理的见解，并合理化了在30SIC-50S加入时触发的GTP水解的快速激活。

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