PRIMARY-STRUCTURE REQUIREMENTS FOR INHIBITION BY THE HEAT-STABLE INHIBITOR OF THE CAMP-DEPENDENT PROTEIN-KINASE

The amino acid sequence of the heat-stable inhibitor of the cAMP-dependent protein kinase (PKI) was determined recently [Scott, J. D., Fischer, E.H., Takio, K., Demaille, J.G. and Krebs, E.G. (1985) Proc, Natl. Acad. Sci. USA 82, 5732-5736]. An earlier report [Scott, J. D., Fischer, E. H., Demaille, J. G. and Krebs, E.G. (1985) Proc. Natl. Acad. Sci. USA 82, 4379-4383] showed that at least part of the inhibitory domain of PKI is located in a 20-residue segment extending from residue 11 to residue 30: Ile-Ala-Ser-Gly-Arg-Thr-Gly-Arg-Arg-Asn-Ala-Ile-His-Asp-Ile-Leu-Val-Ser-Ser-Ala. In the present study, we further mapped the inhibitory region of PKI by addition or deletion of residues at both ends of this peptide and by substitutions for specific amino acids. The result show that (i) deletion of residues 25-30 did not change inhibitory activity but addition of residues toward the amino terminus increased the inhibitory potency up to 150-fold (Ki 4.8 nM), to a level approaching that of PKI; (ii) replacement of alanine-21 by serine converted the inhibitor into a substrate having a relatively low affinity (Km 280 .mu.M) for the enzyme; (iii) replacement of alanine-21 by phosphoserine or .alpha.-aminobutyric acid decreased inhibitory activity by a factor of 120 and 20, respectively; (iv) replacement of serine-13 had essentially no effect, whereas substitution of threonine-16 decreased inhibitory activity. The greatest decreases of inhibitory potency occurred with replacements of the arginines in positions 18 and 19.