Ribosome assembly is driven by numerous assembly factors, including the Rix1 complex and the AAA ATPase Rea1. These two assembly factors catalyze 60S maturation at two distinct states, triggering poorly understood large-scale structural transitions that we analyzed by cryo-electron microscopy. Two nuclear pre-60S intermediates were discovered that represent previously unknown states after Rea1-mediated removal of the Ytm1-Erb1 complex and reveal how the L1 stalk develops from a pre-mature nucleolar to a mature-like nucleoplasmic state. A later pre-60S intermediate shows how the central protuberance arises, assisted by the nearby Rix1-Rea1 machinery, which was solved in its pre-ribosomal context to molecular resolution. This revealed a Rix1₂-Ipi3₂ tetramer anchored to the pre-60S via Ipi1, strategically positioned to monitor this decisive remodeling. These results are consistent with a general underlying principle that temporarily stabilized immature RNA domains are successively remodeled by assembly factors, thereby ensuring failsafe assembly progression.

核糖体的组装受众多组装因素的驱动，包括Rix1复合物和AAA ATPase Rea1。这两个组装因子在两个不同的状态下催化60S的成熟，从而触发了人们对低温结构的广泛理解，我们通过低温电子显微镜对其进行了分析。发现了两个60S核前中间体，它们代表Rea1介导的Ytm1-Erb1复合物去除后的先前未知状态，并揭示了L1茎如何从早熟的核仁发展为成熟的核质态。后来的60S之前的中间体显示了中心突起如何产生，并借助附近的Rix1-Rea1机器帮助解决了这一问题，该机器在其核糖体之前的背景下得以解决。这揭示了Rix1 ₂ -Ipi3 ₂四聚体通过Ipi1锚定在60S之前，在策略上定位于监测这种决定性的重塑。这些结果与总的基本原理是一致的，该基本原理是暂时稳定的未成熟RNA结构域被组装因子相继重塑，从而确保了故障安全组装的进行。