Substrate specificity of a multifunctional calmodulin-dependent protein kinase

The substrate specificity of the multifunctional calmodulin-dependent protein kinase from skeletal muscle has been studied using a series of synthetic peptide corresponding to the NH₂-terminal 10 residues of glycogne synthase, Pro-Leu-Ser-Arg-Thr-Leu-Ser-Val-Ser-Ser-NH₂, stoichiometrically at Ser-7, the same residue phosphorylated in the parent protein. The synthetic peptide was phosphorylatd with a V(max) of 12.5 μmol·min^-1·mg^-1 and an apparent K(m) of 7.5 μM compared to values of 1.2 μmol·min^-1·mg^-1 and 3.1 μM, respectively, for glycogen synthase. Similarly, a synthetic peptide corresponding to the NH₂-terminal 23 residues of smooth muscle myosin light chain was readily phosphorylated on Ser-19 with a K(M) of 4 μM and a V(max) of 5.4 μmol·min^-1·mg^-1. The importance of the arginine 3 redisues NH₂-terminal to the phosphorylated serine in each of these peptides was evident from experiments in which this arginine was substituted by either leucine or alanine, as well as from experiments in which its position in the myosin light chain sequence was varied. Positioning arginine 16 at residues 14 or 17 abolished phosphorylation, while location at residue 15 not only decreased V(max) 14-fold but switched the major site of phosphorylation from Ser-19 to Thr-18. It is concluded that the sequence Arg-X-Y-Ser(Thr) represents the minimum specificity determinant for the multifunctional calmodulin-dependent protein kinases. Studies with various synthetic peptide substratesa and their analogs revealed that the specificity determinants of the multifunctional calmodulin-dependent protein kinase were distinct from several other 'arginine-requiring' protein kinases.