Dephosphorylation of rabbit skeletal muscle glycogen synthase (phosphorylated by cyclic AMP-independent synthase kinase 1) by phosphatases.

Phosphorylation of rabbit skeletal muscle glycogen synthase by cyclic AMP-independent synthase kinase 1 results in the incorporation of 4 mol of PO4/subunit. Incubation of the phosphorylated synthase with rabbit muscle phosphoprotein phosphatase brings about the hydrolysis of phosphates from all four major tryptic peptides and an increase in the synthase activity ratio from 0.01 to 0.85. Incubation of the phosphorylated synthase with calf intestinal alkaline phosphatase brings about the preferential hydrolysis of phosphates from three of the four major tryptic peptides and a slight increase in the four major tryptic peptides and a slight increase in the synthase activity ratio from 0.01 to 0.1. The phosphorylation site which is resistant to hydrolysis by calf intestinal alkaline phosphatase can be dephosphorylated by subsequent incubation with rabbit muscle phosphoprotein phosphatase. This dephosphorylation is accompanied by an increase in the synthase activity ratio to approximately 0.9. Measurements of the changes in the kinetic properties of the synthase samples dephosphorylated by alkaline phosphatase reveal that the phosphorylation sites susceptible to hydrolysis by alkaline phosphatase mainly affect the binding of glucose-6-P to the synthase. Comparison of the kinetic properties of the synthase samples dephosphorylated by alkaline phosphatase and by phosphoprotein phosphatase we find that the phosphorylation site resistant to hydrolysis by alkaline phosphatase affects both the binding of UDP-glucose and glucose-6-P to the synthase.