Mutations that prevent cyclic nucleotide binding to binding sites A or B of type I cyclic AMP-dependent protein kinase

Cyclic nucleotide binding and activation properties of cAMP-dependent protein kinases from five independent mutants of S49 mouse lymphoma cells were studied. These mutants were all hemizygous for expression of mutant regulatory (R) subunits of the type I kinase with lesions that altered the electrostatic charge of R subunit: lesions in three of the mutants mapped to cAMP-binding site A, and those in two of the mutants mapped to cAMP-binding site B. A nucleotide mismatch assay using 32P-labeled cRNA and ribonuclease A confirmed and refined localization of the mutations to single amino acid residues implicated in cAMP binding. R subunits from all mutants retained the ability to bind cAMP, but binding behaved as if it were entirely to nonmutated sites: 1) relative affinities of 11 adenine-modified derivatives of cAMP for mutant enzymes were identical to their relative affinities for the site of wild-type kinase that corresponded to the nonmutated site of the mutant; 2) the potencies of these analogs as activators of mutant kinases were strictly correlated with their binding affinities (for wild-type enzyme activation potencies were correlated with mean affinities of the analogs for cAMP-binding sites A and B); 3) combinations of analogs with strong preferences for opposite cAMP-binding sites in wild-type kinase showed no synergism in activating mutant kinases; 4) dissociation of cAMP from mutant kinases was monophasic; and 5) high salt accelerated dissociation of cAMP from kinases with site B lesions but retarded dissociation from those with site A lesions.