Serine/threonine-protein kinase involved in the final steps of cytoplasmic maturation of the 40S ribosomal subunit. Involved in export of the 40S pre-ribosome particles (pre-40S) from the nucleus to the cytoplasm. Its kinase activity is required for the release of NOB1, PNO1 and LTV1 from the late pre-40S and the processing of 18S-E pre-rRNA to the mature 18S rRNA. May regulate the timing of the metaphase-anaphase transition during mitotic progression, and its phosphorylation, may regulate this function. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Associated with late 40S pre-ribosomal particles. Interacts with PLK1 (via its N-terminus). Autophosphorylated (in vitro). Phosphorylation affects the timing of the metaphase-anaphase transition. Belongs to the protein kinase superfamily. RIO-type Ser/Thr kinase family.