The protein inhibitor of adenosine 3':5'-monophosphate-dependent protein kinases. Isolation and characterization of three isoinhibitors.

The protein inhibitor of adenosine 3':5 monophosphate-dependent protein-kinases has been purified from rabbit skeletal muscle by affinity chromatography on Sepharose-bound catalytic subunit of the kinase (Demaille et al. (1977) Biochemistry 16, 3080-3086). The inhibitory material can be separated into three species A, B and C either by polyacrylamide gel electrophoresis or by anion-exchange on DEAE-cellulose. However, the isoinhibitors display the same molecular weight and isoelectric point, and the same absence of acid-stable covalently bound phosphate. Their amino acid compositions are strikingly similar and their NH2-terminus are blocked. Also, their COOH-terminus appear to be very close to one another when studied by carboxypeptidase Y digestion. Their major difference lies in their inhibitory properties which are significantly weaker in inhibitor C (Ki = 0.87 nM) than in A and B (Ki = 0.18 and 0.23 nM, respectively). This is the first report on the existence of various forms of the protein-kinase inhibitor that exhibit different inhibitory properties and may play a role in the regulation of the protein-kinase system.