Acts as a zygotic negative regulator of entry into mitosis (G2 to M transition) by protecting the nucleus from cytoplasmically activated cyclin B1-complexed cdk1 before the onset of mitosis by mediating phosphorylation of cdk1 on 'Tyr-15'. Specifically phosphorylates and inactivates cyclin B1-complexed cdk1 reaching a maximum during G2 phase and a minimum as cells enter M phase. Phosphorylation of cyclin B1-cdk1 occurs exclusively on 'Tyr-15' and phosphorylation of monomeric cdk1 does not occur. ATP + L-tyrosyl-[protein] = ADP + H(+) + O-phospho-L-tyrosyl-[protein] Interacts (when phosphorylated at Thr-186) with pin1. Zygotically expressed. Present in oocytes and postgastrula embryos (at least until the tailbud stage). Expression begins at the midblastula stage and increases after the early gastrula stage. Phosphorylation at Thr-186 during M-phase by cdk1 inhibits the kinase activity and leads to interaction with pin1. Belongs to the protein kinase superfamily. Ser/Thr protein kinase family. WEE1 subfamily. Truncated N-terminus.