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 hyperphosphorylated form does not associate with CCNB1-CDC2 complexes. The PLK1 protein kinase may be required for mitotic phosphorylation. Interacts with CDC2-CCNB1 complex. Can also interact with PIN1 when phosphorylated by CDC2-CCNB1. The membrane-association motif is essential for the localization to membrane of Golgi stack. According to some authors, it is a transmembrane domain; the existence of a transmembrane region is however unproven. Autophosphorylated. Phosphorylated by CDC2-CCNB1 complexes on undefined serine and threonine residues. The phosphorylation by CDC2-CCNB1 complexes may inhibit the catalytic activity. Belongs to the protein kinase superfamily. Ser/Thr protein kinase family. WEE1 subfamily. Chimeric cDNA.