Activation of type I cyclic AMP-dependent protein kinases is impaired by a point mutation in cyclic AMP binding sites

The type I regulatory (R-I) subunit of cyclic AMP-dependent protein kinase (A-kinase) was expressed in E. coli, and a single amino acid substitution in cyclic AMP binding sites A or B was introduced by site-directed mutagenesis. The cyclic AMP binding activity and cyclic AMP-stimulated phosphotransferase activity of the holoenzymes formed by wild-type or mutant R-Is and the purified bovine catalytic subunit of A-kinase were then examined. The wild-type holoenzyme was activated by low concentrations of cyclic AMP, a finding in accord with its high-affinity binding to cyclic AMP. In contrast, although the two mutant holoenzymes showed high-affinity cyclic AMP binding at their non-mutated sites, both holoenzymes were resistant to activation by cyclic AMP. Thus, binding of cyclic AMP to the non-mutated cyclic AMP binding site is not sufficient to dissociate the catalytic subunit from the mutant R-Is upon cyclic AMP binding. These results suggest that both A and B cyclic AMP binding sites are required for efficient coupling between cyclic AMP binding and activation of the enzyme.