Inhibitor of protein-phosphatase 1. Heterodimer with PP1. Phosphorylation on Thr-73 by GSK3 activates PP1 by dissociating the PP1-PPP1R2 complex (By similarity). Phosphorylation on Ser-44 by ATM activates PP1 by dissociating the PP1-PPP1R2 complex. Belongs to the protein phosphatase inhibitor 2 family.