Characterization of the Interaction of a Protein Inhibitor with Adenosine 3′,5′-Monophosphate-dependent Protein Kinases

The interaction of a protein inhibitor with the catalytic subunit derived from adenosine 3′,5′-monophosphate-dependent protein kinase is described. A kinetic analysis of casein phosphorylation in the presence of inhibitor reveals a noncompetitive interaction between the inhibitor and the catalytic subunit substrates, ATP and casein. The inhibitor does not function by destroying ATP nor by acting as a phosphoprotein phosphatase. The inhibitor is equally effective when protein kinase is assayed with various protein substrates. Catalytic subunit activity is inhibited to the same plateau level by the inhibitor and by regulatory subunit derived from adenosine 3′,5′-monophosphate-dependent protein kinase. The inhibitor interaction with the catalytic subunit prevents the catalytic subunit-induced release of adenosine 3′,5′-monophosphate from the regulatory subunit.