Acts as a negative regulator of entry into mitosis (G2 to M transition) by phosphorylation of the CDK1 kinase specifically when CDK1 is complexed to cyclins. Mediates phosphorylation of CDK1 predominantly on 'Thr-14'. Also involved in Golgi fragmentation. May be involved in phosphorylation of CDK1 on 'Tyr-15' to a lesser degree, however tyrosine kinase activity is unclear and may be indirect. May be a downstream target of Notch signaling pathway during eye development. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Negatively regulated by hyperphosphorylation during mitosis. The plk1/plk1 protein kinase may be required for mitotic phosphorylation. Inactivated during oocyte maturation by phosphorylation by RSK2 and Mos kinase. Interacts with CDC2-CCNB1 complex (By similarity). Interacts with Mos during oocyte maturation. Autophosphorylated. Phosphorylated on undefined residues by RSK2 and Mos kinases. Phosphorylation at Thr-478 by cdk1 creates a docking site for plk1/plx1, leading to subsequent phosphorylation by plk1/plk1 and inhibition of the protein kinase activity kinase activity. Belongs to the protein kinase superfamily. Ser/Thr protein kinase family. WEE1 subfamily.