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‐III’, 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, purfied 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 phopsphatases bawsed on their specificity for the α subunit of phosphorylase kinase and insensitivity to inhibitor‐1 and inhibitor‐2. The substrate specificities of smoth 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.