Acts as a 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. Its activity increases during S and G2 phases and decreases at M phase when it is hyperphosphorylated. A correlated decrease in protein level occurs at M/G1 phase, probably due to its degradation (By similarity). ATP + L-tyrosyl-[protein] = ADP + H(+) + O-phospho-L-tyrosyl-[protein] Binds 2 magnesium ions per subunit. Synthesis is increased during S and G2 phases, presumably by an increase in transcription; activity is decreased by phosphorylation during m phase. Protein levels fall in M phase as a result of decreased synthesis combined with degradation. Activity seems to be negatively regulated by phosphorylation upon entry into mitosis, although N-terminal phosphorylation might also regulate the protein stability via protection from proteolysis or might regulate the subcellular location (By similarity). Binds to 14-3-3 protein zeta. Phosphorylated during M and G1 phases. Also autophosphorylated. Phosphorylation at Ser-642 by BRSK1 and BRSK2 in post-mitotic neurons, leads to down-regulate WEE1 activity in polarized neurons. Phosphorylated at Ser-52 and Ser-123 by PLK1 and CDK1, respectively, generating an signal for degradation that can be recognized by the SCF(BTRC) complex, leading to its ubiquitination and degradation at the onset of G2/M phase (By similarity). Dephosphorylated at Thr-239 by CTDP1 (By similarity). Dephosphorylated at Ser-52 and Ser-123 by the serine/threonine-protein phosphatase 2A preventing its ubiquitin-mediated degradation (By similarity). Ubiquitinated and degraded at the onset of G2/M phase. Belongs to the protein kinase superfamily. Ser/Thr protein kinase family. WEE1 subfamily.