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 (PubMed:19564402). Regulates the timing of the metaphase-anaphase transition during mitotic progression, and its phosphorylation, most likely by PLK1, regulates this function (PubMed:21880710). 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 (PubMed:16037817, PubMed:21081503). Interacts with PLK1 (via its N-terminus) (PubMed:21880710). Exported out of the nucleus via its NES in a XPO1-dependent manner. Autophosphorylated (in vitro) (PubMed:19564402, PubMed:21880710). Phosphorylation at Ser-335, Ser-380, Ser-548 by PLK1 affects the timing of the metaphase-anaphase transition (PubMed:21880710). Belongs to the protein kinase superfamily. RIO-type Ser/Thr kinase family.