Department of Biochemistry and Molecular Biophysics, Columbia University, New York, NY 10032, USA.
Cell. 2013 May 23;153(5):1108-19. doi: 10.1016/j.cell.2013.04.036.
Eukaryotic translation initiation begins with assembly of a 43S preinitiation complex. First, methionylated initiator methionine transfer RNA (Met-tRNAi(Met)), eukaryotic initiation factor (eIF) 2, and guanosine triphosphate form a ternary complex (TC). The TC, eIF3, eIF1, and eIF1A cooperatively bind to the 40S subunit, yielding the 43S preinitiation complex, which is ready to attach to messenger RNA (mRNA) and start scanning to the initiation codon. Scanning on structured mRNAs additionally requires DHX29, a DExH-box protein that also binds directly to the 40S subunit. Here, we present a cryo-electron microscopy structure of the mammalian DHX29-bound 43S complex at 11.6 Å resolution. It reveals that eIF2 interacts with the 40S subunit via its α subunit and supports Met-tRNAi(Met) in an unexpected P/I orientation (eP/I). The structural core of eIF3 resides on the back of the 40S subunit, establishing two principal points of contact, whereas DHX29 binds around helix 16. The structure provides insights into eukaryote-specific aspects of translation, including the mechanism of action of DHX29.
真核生物翻译起始始于 43S 起始前复合物的组装。首先，甲硫氨酸化起始甲硫氨酸转移 RNA（Met-tRNAi(Met)）、真核起始因子（eIF）2 和鸟苷三磷酸形成三元复合物（TC）。TC、eIF3、eIF1 和 eIF1A 协同结合到 40S 亚基上，生成 43S 起始前复合物，该复合物准备与信使 RNA（mRNA）结合并开始扫描起始密码子。在结构复杂的 mRNA 上进行扫描还需要 DHX29，这是一种 DExH 盒蛋白，也直接与 40S 亚基结合。在这里，我们呈现了分辨率为 11.6 Å 的哺乳动物 DHX29 结合的 43S 复合物的冷冻电镜结构。它揭示了 eIF2 通过其α亚基与 40S 亚基相互作用，并以出人意料的 P/I 取向（eP/I）支持 Met-tRNAi(Met)。eIF3 的结构核心位于 40S 亚基的背面，建立了两个主要接触点，而 DHX29 则围绕着 helix 16 结合。该结构提供了对真核生物翻译特异性方面的深入了解，包括 DHX29 的作用机制。
