| Abstract: | The substrate specificity of cGMP-dependent protein kinase has been investigated by examining the ability of the enzyme to phosphorylate a series of synthetic peptides that correspond to the amino acid sequence at its site of autophosphorylation. The undecapeptide Ile53-Gly-Pro-Arg-Thr-Thr58-Arg-Ala-Gln-Gly-Ile63 which corresponds to the sequence around threonine-58 in cGMP-dependent protein kinase (Takio, K., Smith, S.B., Walsh, K.A., Krebs, E.G., and Titani, K. (1983) J. Biol. Chem. 258, 5531-5536) was synthesized and tested as a substrate for that enzyme. It was phosphorylated to the extent of 1.0 mol of phosphate/mol of peptide. Analysis of the products of Edman degradation of the phosphopeptide indicated that only threonine-58 was phosphorylated, as is the case for the autophosphorylation reaction in the native enzyme. The peptide was phosphorylated by cGMP-dependent protein kinase with a Km value of 578 +/- 25 microM and a Vmax of 0.069 +/- 0.003 mumol/min/mg of enzyme. This low Vmax value is consistent with the relatively slow rate of the autophosphorylation reaction. An analog peptide that contained serine in place of threonine-58 was also phosphorylated to 1.0 mol of phosphate/mol of peptide. That phosphopeptide contained only phosphoserine. The serine-containing analog peptide had a Km value similar to that of the parent peptide but was phosphorylated with a 70-fold higher Vmax value. Substitution of arginine-56 in the parent peptide by an alanine residue resulted in a peptide that was essentially not a substrate. Substitution of arginine-59, COOH-terminal to the phosphorylatable threonine, yielded a peptide with a Vmax similar to that of the parent peptide but a Km value of almost 22,000 microM. These results indicate that serine is a better phosphate-accepting residue than is threonine and that both arginine residues around the site of autophosphorylation are important specificity determinants for the cGMP-dependent protein kinase. |
