The function of Mg-ATP in interactions between the regulatory and catalytic subunits of type I cAMP-dependent protein kinase from rabbit skeletal muscle

1. 1. The regulatory subunit of Type I cAMP-dependent protein kinase from rabbit skeletal muscle can bind [ 3 H]cAMP to form the R-[ 3 H]cAMP complex, and the slow phase of the enhanced exchange of free cAMP with [ 3 H]cAMP from the R-[ 3 H]cAMP complexes was studied under various conditions using the equilibrium isotope exchange technique. 2. 2. Results indicate that Mg-ATP and the catalytic subunit are absolutely required for the enhanced exchange reaction to occur, but phosphorylation of the regulatory subunit by Mg-ATP does not play a determining role in the slow rate of the dissociation/association of the Type I protein-kinase in the presence of cAMP and the catalytic subunit. 3. 3. We interpret the role of Mg-ATP as being one in which it may provide the structural attributes required for formation of a stabilized transient state of the cAMP-regulatory subunit-catalytic subunit ternary complex, an obligatory intermediate involved in the dissociation/association of Type I cAMP-dependent protein kinase.