Protein phosphatase inhibitor that specifically inhibits protein phosphatase 2A (PP2A) during mitosis. When phosphorylated at Ser-67 during mitosis, specifically interacts with ppp2r2d (PR55-delta) and inhibits its activity, leading to inactivation of PP2A, an essential condition to keep cyclin-B1-CDK1 activity high during M phase (By similarity). Interacts (when phosphorylated at Ser-67) with ppp2r2d. Phosphorylation at Ser-67 by gwl during mitosis is essential for interaction with ppp2r2d (PR55-delta) and subsequent inactivation of PP2A. Belongs to the endosulfine family. Extended N-terminus.