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. No longer associates with pre-40S subunits in RPS19 disruptions, suggesting it acts after the ribosomal protein in 18S rRNA maturation. 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, composed of the 40S ribosomal proteins and the non-ribosomal factors DIM1, DIM2, ENP1, HRR25, LTV1, NOB1, RIO2 and TSR1. Predominantly cytoplasmic. Autophosphorylated. Present with 10300 molecules/cell in log phase SD medium. Belongs to the protein kinase superfamily. RIO-type Ser/Thr kinase family.