Required for the final endonucleolytic cleavage of 20S pre-rRNA at site D in the cytoplasm, converting it into the mature 18S rRNA. Involved in normal export of the pre-40S particles from the nucleus to the cytoplasm (By similarity). ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Component of a late pre-40S ribosomal particle. Belongs to the protein kinase superfamily. RIO-type Ser/Thr kinase family.