An active twenty-amino-acid-residue peptide derived from the inhibitor protein of the cyclic AMP-dependent protein kinase.

Digestion with Staphylococcus aureus V8 proteinase of the inhibitor protein of the cyclic AMP-dependent protein kinase results in the sequential formation of three active inhibitory peptides. The smallest active peptide has the sequence Thr-Thr-Tyr-Ala-Asp-Phe-Ile-Ala-Ser-Gly-Arg-Thr-Gly-Arg-Arg-Asn-Ala-Ile- His-Asp . This 20-amino-acid-residue peptide has 20-40% of the activity of the native molecule and a Ki of 0.2 nM. Inhibition, as a minimum, appears to be based upon the inhibitor protein containing the recognition sequences that dictate protein-substrate-specificity. This inhibitory peptide also has sequence homology with the phosphorylation site for a protein kinase other than the cyclic AMP-dependent enzyme.