The Protein Phosphatases Invloved in Cellular Regulation. 4. Classification of Two Homogeneous Myosin Light Chain Phosphatases from Smoth Muscle as Protein Phosphatase-2A1 and 2C, and a Homogeneous Protein Phosphatase from Reticulocytes Active on Protein Synthesis Initiation Factor eIF-2 as Protein Phosphatase-2A2

Two homogeneous protein phosphatases, termed 'smooth muscle phosphatase-I' and 'smooth muscle phosphatase-II', isolated from turkey gizzard as enzymes active against the 20-kDa light chain of smooth muscle myosin, and a third homogeneous protein phosphatase from rabbit reticulocytes, purified as an enzyme active against protein synthesis initiation factor eIF-2, were classified using the criteria defined by Ingebritsen and Cohen [Eur. J. Biochem. (1983) 132, 255-261]. All three enzymes were type-2 protein phosphatases based on their specificity for the alpha-subunit of phosphorylase kinase and insensitivity to inhibitor-1 and inhibitor-2. The substrate specificities of smooth muscle phosphatase-I and the eIF-2 phosphatase were similar to the catalytic subunit of protein phosphatase-2A. Smooth muscle phosphatase-I could be designated as protein phosphatase-2A1 and eIF-2 phosphatase as protein phosphatase-2A2 on the basis of their subunit compositions. The substrate specificity, dependence of activity on Mg2+ and subunit composition of smooth muscle phosphatase-II allowed its assignment as protein phosphatase-2C.