The SPATA5-SPATA5L1-CINP-C1ORF109 complex is essential for cytoplasmic pre-60S ribosome maturation

The ribosome is a universally conserved and essential protein complex, but its biogenesis in mammals is more complex than in single-celled eukaryotes. To explore this added complexity, we conducted a protein-protein interaction screen in human cells. This led to the identification of the eumetazoan-specific SPATA5-SPATA5L1-CINP-C1ORF109 (55LCC) complex as a key regulator of ribosome biogenesis. Structural analyses using cryo-EM and X-ray crystallography defined the architecture of 55LCC. Functional studies following acute depletion revealed that each component is essential for pre-60S maturation. Swapping endogenous 55LCC components with mutant versions pinpointed critical functional interactions and showed that SPATA5's ATPase activity is more important than SPATA5L1's. Our findings support that SPATA5 evolved from the solitary yeast ATPase Drg1 into the multiprotein 55LCC complex in metazoans. This work provides insights into the complexity of ribosome biogenesis and lays the foundation for deeper exploration of 55LCC's role in pre-60S maturation.