Differential responses of cyclic GMP-dependent and cyclic AMP-dependent protein kinases to synthetic peptide inhibitors.

The peptide Arg-Lys-Arg-Ala-Arg-Lys-Glu was synthesized and tested as an inhibitor of cyclic GMP-dependent protein kinase. This synthetic peptide is a non-phosphorylatable analogue of a substrate peptide corresponding to a phosphorylation site (serine-32) in histone H2B. The peptide was a competitive inhibitor of cyclic GMP-dependent protein kinase with respect to synthetic peptide substrates, with a Ki value of 86 microM. However, it did not inhibit phosphorylation of intact histones by cyclic GMP-dependent protein kinase under any conditions tested. Arg-Lys-Arg-Ala-Arg-Lys-Glu competitively inhibited the phosphorylation of either peptides or histones by the catalytic subunit of cyclic AMP-dependent protein kinase, with similar Ki values (550 microM) for both of these substrates. The peptide Leu-Arg-Arg-Ala-Ala-Leu-Gly, which was previously reported to be a selective inhibitor of both peptide and histone phosphorylation by cyclic AMP-dependent protein kinase, was a poor inhibitor of cyclic GMP-dependent protein kinase acting on peptide substrates (Ki = 800 microM), but did not inhibit phosphorylation of histones by cyclic GMP-dependent protein kinase. The selectivity of these synthetic peptide inhibitors toward either cyclic GMP-dependent or cyclic AMP-dependent protein kinases is probably based on differences in the determinants of substrate specificity recognized by these two enzymes. It is concluded that histones interact differently with cyclic GMP-dependent protein kinase from the way they do with the catalytic subunit of cyclic AMP-dependent protein kinase.