CA2+-STIMULATED PHOSPHORYLATION OF MUSCLE GLYCOGEN SYNTHASE BY PHOSPHORYLASE-B KINASE

Abstract

Phosphorylase b kinase from rabbit muscle phosphorylates glycogen synthase purified from the same tissue. The reaction is markedly stimulated by Ca2+ and results in a decrease in the synthase %I activity [the ratio of the activity in the absence of G-6-P to that in the presence of 7.2 mM sugar phosphate, expressed as a percentage]. Phosphorylase .beta. kinase action leads to the incorporation of phosphate (0.6-0.8 mol/mol of subunit), preferentially into a single cyanogen bromide fragment of synthase (fragment III). Cyclic[c]AMP-independent synthase kinase also shows a specificity for the site(s) contained in fragment III whereas the cAMP-dependent protein kinase exerts a preference for the site(s) located in a distinct cyanogen bromide fragment (fragment II). A Ca2+-stimulated endogenous kinase also results in the phosphorylation of fragment III, and can be attributed to the presence of phosphorylase .beta. kinase. The finding of a Ca2+-stimulated phosphorylation of glycogen synthase has important implications for the regulation of glycogen metabolism, and particularly those processes thought to be controlled by cytoplasmic Ca2+ concentrations.