Kinase FA-mediated regulation of rabbit skeletal muscle protein phosphatase. Reversible phosphorylation of the modulator subunit.

A mechanism of activation of the ATP.Mg-dependent protein phosphatase (FC.M) has been proposed (Jurgensen, S., Shacter, E., Huang, C. Y., Chock, P. B., Yang, S.-D., Vandenheede, J. R., and Merlevede, W. (1984) J. Biol. Chem. 259, 5864-5870) in which a transient phosphorylation by the kinase FA of the modulator subunit (M) is the driving force for the transition of the inactive catalytic subunit (FC) into its active conformation. Incubation of FC.M with kinase FA and Mg2+ and adenosine 5'-(gamma-thio)triphosphate results in thiophosphorylation of M and also a conformational change in the phosphatase catalytic subunit; however, the enzyme remains inactive. Proteolysis of this inactive, thiophosphorylated complex causes proteolytic destruction of the modulator subunit and yields an active phosphorylase phosphatase species. Similar treatment of the native inactive enzyme does not yield active phosphatase. Evidence is presented, suggesting that a molecule of modulator is bound at an "inhibitory site" on the native enzyme. This modulator does not prevent the conformational change in the phosphatase catalytic subunit upon incubation with kinase FA and ATP.Mg but does partially inhibit the expression of the phosphorylase phosphatase activity.