The MgATP‐Dependent Protein Phosphatase and Protein Phosphatase 1 Have Identical Substrate Specificities

The MgATP‐dependent phosphorylase phosphatase was found to have a broad substrate specificity. Its activity against all phosphoproteins tested was dependent upon preincubation with the activating factor F_A and MgATP. The enzyme dephosphorylated and inactivated phosphorylase kinase and inhibitor 1, and dephosphorylated and activated glycogen synthase and acetyl‐CoA carboxylase. Glycogen synthase was dephosphorylated at similar rates whether it had been phosphorylated by cyclic‐AMP‐dependent protein kinase, phosphorylase kinase or glycogen synthase kinase 3. The enzyme also catalysed the dephosphorylation of ATP citrate lyase, initiation factor elF‐2, and troponin 1. The properties of the MgATP‐dependent protein phosphatase from either dog liver or rabbit skeletal muscle showed a remarkable similarity to highly purified preparations of protein phosphatase, l from rabbit skeletal muscle. The relative activities of the two enzymes against all phosphoproteins tested was very similar. Both enzymes dephosphorylated the β‐subunit of phosphorylase kinase 40‐fold faster than the α‐subunit, and both enzymes were inhibited by identical concentrations of the two proteins termed inhibitor 1 and inhibitor 2, which inhibit protein phosphatase 1 specifically. These results demonstrate that the MgATP‐dependent protein phosphatase is a type‐1 protein phosphatase, and is distinct from type‐2 protein phosphatases which dephosphorylate the α‐subunit of phosphorylase kinase and are unaffected by inhibitor 1 and inhibitor 2. The possibility that the MgATP‐dependent protein phosphatase is an inactive form of protein phosphatase 1 and that both proteins share the same catalytic subunit is discussed.