Department of Biology, Institute of Molecular Biology and Biophysics, ETH Zurich, Zurich, Switzerland.
Harry Perkins Institute of Medical Research, Centre for Medical Research, QEII Medical Centre and School of Molecular Sciences, The University of Western Australia, Nedlands, Western Australia, Australia.
Nature. 2018 Aug;560(7717):263-267. doi: 10.1038/s41586-018-0373-y. Epub 2018 Aug 8.
Mitochondria maintain their own specialized protein synthesis machinery, which in mammals is used exclusively for the synthesis of the membrane proteins responsible for oxidative phosphorylation. The initiation of protein synthesis in mitochondria differs substantially from bacterial or cytosolic translation systems. Mitochondrial translation initiation lacks initiation factor 1, which is essential in all other translation systems from bacteria to mammals. Furthermore, only one type of methionyl transfer RNA (tRNA) is used for both initiation and elongation, necessitating that the initiation factor specifically recognizes the formylated version of tRNA (fMet-tRNA). Lastly, most mitochondrial mRNAs do not possess 5' leader sequences to promote mRNA binding to the ribosome. There is currently little mechanistic insight into mammalian mitochondrial translation initiation, and it is not clear how mRNA engagement, initiator-tRNA recruitment and start-codon selection occur. Here we determine the cryo-EM structure of the complete translation initiation complex from mammalian mitochondria at 3.2 Å. We describe the function of an additional domain insertion that is present in the mammalian mitochondrial initiation factor 2 (mtIF2). By closing the decoding centre, this insertion stabilizes the binding of leaderless mRNAs and induces conformational changes in the rRNA nucleotides involved in decoding. We identify unique features of mtIF2 that are required for specific recognition of fMet-tRNA and regulation of its GTPase activity. Finally, we observe that the ribosomal tunnel in the initiating ribosome is blocked by insertion of the N-terminal portion of mitochondrial protein mL45, which becomes exposed as the ribosome switches to elongation mode and may have an additional role in targeting of mitochondrial ribosomes to the protein-conducting pore in the inner mitochondrial membrane.
线粒体维持着自己专门的蛋白质合成机制，该机制在哺乳动物中专门用于合成负责氧化磷酸化的膜蛋白。线粒体蛋白质合成的起始与细菌或细胞质翻译系统有很大的不同。线粒体翻译起始缺乏起始因子 1，而起始因子 1 在从细菌到哺乳动物的所有其他翻译系统中都是必不可少的。此外，只有一种甲硫氨酰转移 RNA（tRNA）用于起始和延伸，这需要起始因子专门识别甲酰化的 tRNA（fMet-tRNA）。最后，大多数线粒体 mRNA 没有 5' 前导序列来促进 mRNA 与核糖体结合。目前，人们对哺乳动物线粒体翻译起始的机制知之甚少，也不清楚 mRNA 结合、起始 tRNA 募集和起始密码子选择是如何发生的。在这里，我们以 3.2Å 的分辨率确定了来自哺乳动物线粒体的完整翻译起始复合物的冷冻电镜结构。我们描述了存在于哺乳动物线粒体起始因子 2（mtIF2）中的一个额外结构域插入的功能。通过关闭解码中心，该插入稳定了无领 mRNA 的结合，并诱导参与解码的 rRNA 核苷酸发生构象变化。我们确定了 mtIF2 的独特特征，这些特征对于特异识别 fMet-tRNA 和调节其 GTP 酶活性是必需的。最后，我们观察到起始核糖体的核糖体隧道被线粒体蛋白 mL45 的 N 端部分插入所阻塞，当核糖体切换到延伸模式时，该部分插入被暴露，并且可能在靶向线粒体核糖体到线粒体内膜的蛋白通道中具有额外的作用。
