Plays multiple essential roles during mitosis. Phosphorylates the N-terminal domain of cdc25, which leads to cyclin b-cdc2 activation and mitotic entry. Also required for organization of bipolar spindles, and for exit from mitosis. Phosphorylates tpx2 (By similarity). ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Interacts with plk1 and kif2a. Interacts with fbxo5. Localized at centrosomes at prophase, spindles at metaphase, and at the midbody during cytokinesis. Activated by phosphorylation on Thr-201 during M phase. Protein levels are down-regulated by proteasomal degradation in anaphase. Belongs to the protein kinase superfamily. Ser/Thr protein kinase family.