In eukaryotic translation initiation, the 48S preinitiation complex (PIC) scans the 5′ untranslated region of mRNAs to search for the cognate start codon (AUG) with assistance from various eukaryotic initiation factors (eIFs). Cognate start codon recognition is precise, rejecting near-cognate codons with a single base difference. However, the structural basis of discrimination of near-cognate start codons was not known. We have captured multiple yeast 48S PICs with a near-cognate AUC codon at the P-site, revealing that the AUC codon induces instability in the codon-anticodon at the P-site, leading to a disordered N-terminal tail of eIF1A. Following eIF1 dissociation, the N-terminal domain of eIF5 fails to occupy the vacant eIF1 position, and eIF2β becomes flexible. Consequently, 48S with an AUC codon is less favourable for initiation. Furthermore, we observe hitherto unreported metastable states of the eIF2-GTP-Met-tRNAMet ternary complex, where the eIF2β helix-turn-helix domain may facilitate eIF5 association by preventing eIF1 rebinding to 48S PIC. Finally, a swivelled head conformation of 48S PIC appears crucial for discriminating incorrect and selection of the correct codon-anticodon pair during translation initiation.

中文翻译：

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酵母翻译起始过程中 AUC 密码子鉴别的结构基础


在真核翻译起始中，48S 前起始复合物 （PIC） 在各种真核起始因子 （eIF） 的帮助下扫描 mRNA 的 5' 非翻译区以寻找同源起始密码子 （AUG）。同源起始密码子识别是精确的，拒绝具有单个碱基差异的近同源密码子。然而，近同源起始密码子区分的结构基础尚不清楚。我们在 P 位点捕获了多个具有近同源 AUC 密码子的酵母 48S PIC，揭示了 AUC 密码子在 P 位点诱导密码子-反密码子的不稳定性，导致 eIF1A 的 N 末端尾部无序。eIF1 解离后，eIF5 的 N 端结构域未能占据空置的 eIF1 位置，eIF2β 变得灵活。因此，带有 AUC 密码子的 48S 不太有利于起始。此外，我们观察到迄今为止未报道的 eIF2-GTP-Met-tRNAMet 三元复合物的亚稳态，其中 eIF2β 螺旋转角螺旋结构域可能通过阻止 eIF1 与 48S PIC 重新结合来促进 eIF5 结合。最后，48S PIC 的旋转头构象对于在翻译起始过程中区分错误和选择正确的密码子-反密码子对似乎至关重要。