Interaction of the subunits of adenosine 3':5'-cyclic monophosphate-dependent protein kinase of muscle.

Two cAMP-dependent protein kinases purified from rabbit skeletal muscle were shown to bind the same amount of cAMP per unit of enzyme activity at several concentrations of this nucleotide. A preparation containing both of these kinases was separated into catalytic (C) and regulatory (R) subunit fractions in the presence of cAMP, the regulatory subunit being obtained as an R.cAMP complex. Addition of increasing amounts of the R.cAMP complex to the holoenzyme (RC) increased the concentration of cAMP required for half-maximal activity of the enzyme. cAMP was liberated from the R.cAMP complex in the presence of added catalytic subunit in a reaction that was facilitated by Mg(2+), ATP, and warming. These findings are presented in support of a model for activation of the protein kinase by cAMP. The possibility that excess regulatory subunit may serve as a sink for intracellular cAMP is also discussed. It is shown that cAMP bound to the R subunit is not a substrate for the cAMP phosphodiesterase.