Required for the final endonucleolytic cleavage at site D converting 20S pre-rRNA into the mature 18S rRNA. Required for the final steps of cytoplasmic maturation of the 40S ribosomal subunit. Despite the protein kinase domain is proposed to act predominantly as an ATPase (By similarity). ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Belongs to the protein kinase superfamily. RIO-type Ser/Thr kinase family.