Identification of inhibitor-2 as the ATP-mg-dependent protein phosphatase modulator.

In previous reports (Yang, S. D., Vandenheede, J. R., Goris, J., and Merlevede, W. (1980) J. Biol. Chem. 255, 11759-11767; Vandenheede, J. R., Yang, S.-D., and Merlevede, W. (1981) J. Biol. Chem. 256, 5894-5900), we have described two slightly different purification procedures for the isolation of the inactive ATP-Mg-dependent protein phosphatase (FC) which could be activated by a protein activator (FA) in the presence of ATP-Mg. Although the two procedures consistently produced FC preparations that showed very similar polyacrylamide gel patterns, the specific activity of the purified enzymes varied considerably. The preparations characterized by a rather low specific activity could be stimulated up to 10-fold when a titrated amount of inhibitor-2 was included during the FA and ATP-Mg-mediated activation. Inhibitor-2, only recently implicated as an inactivating protein for activated FC (Vandenheede, J. R., Goris, J., Yang, S.-D., Camps, T., and Merlevede, W. (1981) FEBS Lett. 127, 1-3) has now been identified as a modulator protein, necessary for the reversible activation of the protein phosphatase.