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-67', 2 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. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Interacts with arpp19 and ensa, leading to their phosphorylation. During interphase is mainly nuclear, upon nuclear envelope breakdown localizes at the cytoplasm and during mitosis at the centrosomes. Phosphorylation at Thr-748 by CDK1 during M phase activates its kinase activity. Not active during other phases of the cell cycle. Has the ability to autophosphorylate. Belongs to the protein kinase superfamily. AGC Ser/Thr protein kinase family.