Architecture of the Rix1-Rea1 checkpoint machinery during pre-60S-ribosome remodeling

Ribosome synthesis is catalyzed by similar to 200 assembly factors, which facilitate efficient production of mature ribosomes. Here, we determined the cryo-EM structure of a Saccharomyces cerevisiae nucleoplasmic pre-60S particle containing the dynein-related 550-kDa Real AAA(+) ATPase and the Rixl subcomplex. This particle differs from its preceding state, the early Arxl particle, by two massive structural rearrangements: an similar to 180 degrees rotation of the 5S ribonucleoprotein complex and the central protuberance (CP) rRNA helices, and the removal of the 'foot' structure from the 3' end of the 5.8S rRNA. Progression from the Arxl to the Rixl particle was blocked by mutational perturbation of the Rixl-Real interaction but not by a dominant-lethal Real AAA(+) ATPase-ring mutant. After remodeling, the Rixl subcomplex and Real become suitably positioned to sense correct structural maturation of the CP, which allows unidirectional progression toward mature ribosomes.