Calcium-dependent phosphorylation of glycogen synthase by phosphorylase kinase.

Purified preparations of rabbit skeletal muscle glycogen synthase (specific activity 6 to 42 units/mg of protein) were found to be contaminated with calciumdependent kinase(s) which catalyzed the phosphorylation of glycogen phosphorylase b and glycogen synthase. In a series of glycogen synthase preparations there was a correlation between the phosphorylation rates of glycogen synthase and of added phosphorylase b. Both reactions were stimulated by calcium and, to varying degree, by the calcium-dependent regulatory protein in the presence of calcium. When approximately 0.5 mol of phosphate was introduced per mol of glycogen synthase subunit, there was a small decrease in glycogen synthase activity measured in the absence of glucose B-phosphate. Phosphorylase b inhibited the inactivation of glycogen synthase and purified phosphorylase kinase increased the rates of phosphorylation of glycogen synthase. DEAE-cellulose chromatography of purified phosphorylase kinase failed to separate calcium-dependent glycogen synthase kinase and phosphorylase kinase activities. Activation of phosphorylase kinase by cyclic AMP-dependent protein kinase increased the rate of phosphorylation of both glycogen synthase and phosphorylase b. For nonactivated phosphorylase kinase and for phosphorylase kinase activated by cyclic AMP-dependent protein kinase, the rate of glycogen synthase phosphorylation was approximately one-fifth the rate of phosphorylase b phosphorylation. These observations suggest that glycogen synthase is a potential substrate for phosphorylase kinase in vivo.