The formation of eukaryotic ribosomal subunits extends from the nucleolus to the cytoplasm and entails hundreds of assembly factors. Despite differences in the pathways of ribosome formation, high-resolution structural information has been available only from fungi. Here we present cryo-electron microscopy structures of late-stage human 40S assembly intermediates, representing one state reconstituted in vitro and five native states that range from nuclear to late cytoplasmic. The earliest particles reveal the position of the biogenesis factor RRP12 and distinct immature rRNA conformations that accompany the formation of the 40S subunit head. Molecular models of the late-acting assembly factors TSR1, RIOK1, RIOK2, ENP1, LTV1, PNO1 and NOB1 provide mechanistic details that underlie their contribution to a sequential 40S subunit assembly. The NOB1 architecture displays an inactive nuclease conformation that requires rearrangement of the PNO1-bound 3′ rRNA, thereby coordinating the final rRNA folding steps with site 3 cleavage.Cryo-EM structures of late intermediates in the assembly of human 40S ribosomal subunits help to define the principles by which immature rRNA conformations and ribosomal biogenesis factors shape the 40S maturation process.

中文翻译：

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可视化人类 40S 核糖体亚基成熟的晚期状态

真核生物核糖体亚基的形成从核仁延伸到细胞质，需要数百个组装因子。尽管核糖体形成的途径存在差异，但只能从真菌中获得高分辨率的结构信息。在这里，我们展示了晚期人类 40S 组装中间体的冷冻电子显微镜结构，代表了一种体外重组状态和五种天然状态，范围从核到晚期细胞质。最早的颗粒揭示了生物发生因子 RRP12 的位置和伴随 40S 亚基头形成的不同未成熟 rRNA 构象。后效组装因子 TSR1、RIOK1、RIOK2、ENP1、LTV1、PNO1 和 NOB1 的分子模型提供了它们对连续 40S 亚基组装的贡献的机制细节。