Protein Phosphatase 2A Inhibitors, I1 PP2A and I2 PP2A, Associate with and Modify the Substrate Specificity of Protein Phosphatase 1*

Recombinant I 1 PP2A and I2 PP2A did not affect the activity of the catalytic subunit of protein phosphatase 1 (PP1C) with32P-labeled myelin basic protein, histone H1, and phosphorylase when assayed in the absence of divalent cations. However, in the presence of Mn2+, I1 PP2A and I2 PP2A stimulated PP1Cactivity by 15–20-fold with myelin basic protein and histone H1 but not phosphorylase. Half-maximal stimulation occurred at 2 and 4 nm I1 PP2A and I2 PP2A, respectively. Moreover, I1 PP2A and I2 PP2A reduced the Mn2+requirement by about 30-fold to 10 μm. In contrast, PP1C activity was unaffected by I1 PP2A and I2 PP2A in the presence of Co3+ (0.1 mm), Mg2+ (2 mm), Ca2+ (0.5 mm), and Zn2+ (0.1 mm). Following gel filtration chromatography on Sephacryl S-200 in the presence of Mn2+, PP1C coeluted with I1 PP2Aand I2 PP2A in the void volume. However, when I1 PP2A and I2 PP2A or Mn2+ were omitted, PP1C emerged with aV e/V 0 of ∼1.6. The results demonstrate that I1 PP2A and I2 PP2A associate with and modify the substrate specificity of PP1C in the presence of physiological concentrations of Mn2+. A novel role is suggested for I1 PP2A and I2 PP2A in the reciprocal regulation of two major mammalian serine/threonine phosphatases, PP1 and PP2A .