Serine/threonine kinase that plays a key role in M phase by acting as a regulator of mitosis entry and maintenance. Acts by promoting the inactivation of protein phosphatase 2A (PP2A) during M phase: does not directly inhibit PP2A but acts by mediating phosphorylation and subsequent activation of arpp19 and ensa at 'Ser-62' and 'Ser-74', respectively. ARPP19 and ENSA are phosphatase inhibitors that specifically inhibit the ppp2r2d (PR55-delta) subunit of PP2A. Inactivation of PP2A during M phase is essential to keep cyclin-B1-CDK1 activity high. Following DNA damage, it is also involved in checkpoint recovery by being inhibited (By similarity). May be involved in megakaryocyte differentiation (PubMed:19460416). ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] During interphase is mainly nuclear, upon nuclear envelope breakdown localizes at the cytoplasm and during mitosis at the centrosomes (By similarity). Upon mitotic exit moves to the cleavage furrow. Phosphorylation at Thr-722 by CDK1 during M phase activates its kinase activity. Maximum phosphorylation occurs in prometaphase (By similarity). Morpholino knockdown of the protein results in reduced number of circulating thrombocytes, distinguishable at 3 days post fertilization. Belongs to the protein kinase superfamily. AGC Ser/Thr protein kinase family.