Three distinct forms of type 2A protein phosphatase in human erythrocyte cytosol

Two type 2A protein phosphatases, phosphatases I (Mr = 180,000) and III (Mr = 177,000), were purified to near homogeneity from human erythrocyte cytosol. Phosphatase I was composed of alpha (34 kDa), beta (63 kDa), and delta (74 kDa) subunits in a ratio of 1:1:1. Phosphatase III comprised alpha, beta, and gamma (53 kDa) subunits in the same ratio. Heparin-Sepharose column chromatography converted most of phosphatase I and 20% of phosphatase III into alpha 1 beta 1 which were indistinguishable from phosphatase IV (Usui, H., Kinohara, N., Yoshikawa, K., Imazu, M., Imaoka, T., and Takeda, M. (1983) J. Biol. Chem. 258, 10455-10463). The catalytic subunit alpha and the beta subunit of phosphatases I, III, and IV displayed identical V8 and papain peptide maps, respectively, while the peptide maps of the alpha, beta, gamma, and delta subunits were clearly distinct. The molar ratio of phosphatases I, III, and IV in erythrocyte cytosol was estimated to be 6:1:14. Comparison of molecular activities of alpha, alpha 1 beta 1, alpha 1 beta 1 delta 1, and alpha 1 beta 1 gamma 1 revealed that beta suppressed phosphorylase and P-H2B histone phosphatase activities of alpha but stimulated the P-H1 histone phosphatase activity, and delta suppressed all the phosphatase activities of alpha 1 beta 1. The gamma subunit stimulated the P-histone phosphatase activity of alpha 1 beta 1 but inhibited the phosphorylase and P-spectrin phosphatase activities. The beta subunit increased the Mg2+ or Mn2+ requirement for P-H2B histone phosphatase activity of alpha, an effect which was counteracted by delta. The effects of heparin, H1 histone, protamine, and polylysine on the phosphorylase phosphatase activity of phosphatases I, III, IV, and alpha were described and discussed in connection with the functions of the subunits.