ATPase-dependent role of the atypical kinase Rio2 on the evolving pre-40S ribosomal subunit

Rio2 is an atypical protein kinase required for pre-40S subunit maturation. The crystal structure of eukaryotic Rio2 with bound ATP and Mg 2+ reveals an unusual phosphoaspartate intermediate typically observed in P-type ATPases. Rio2 has in vitro ATPase activity, and its catalytic activity stimulates its own dissociation from the ribosome, which is required for pre-40S maturation. Ribosome synthesis involves dynamic association of ribosome-biogenesis factors with evolving preribosomal particles. Rio2 is an atypical protein kinase required for pre-40S subunit maturation. We report the crystal structure of eukaryotic Rio2–ATP–Mg 2+ complex. The active site contains ADP-Mg 2+ and a phosphoaspartate intermediate typically found in Na + , K + and Ca 2+ ATPases but not protein kinases. Consistent with this finding, ct Rio2 exhibits a robust ATPase activity in vitro . In vivo , Rio2 docks on the ribosome, with its active site occluded and its flexible loop positioned to interact with the pre-40S subunit. Moreover, Rio2 catalytic activity is required for its dissociation from the ribosome, a necessary step in pre-40S maturation. We propose that phosphoryl transfer from ATP to Asp257 in Rio2's active site and subsequent hydrolysis of the aspartylphosphate could be a trigger to power late cytoplasmic 40S subunit biogenesis.