A Reinvestigation of the Phosphorylation of Rabbit Skeletal‐Muscle Glycogen Synthase by Cyclic‐AMP‐Dependent Protein Kinase: Identification of the Third Site of Phosphorylation as Serine‐7

Glycogen synthase preparations which were free of endogenous phosphorylase kinase were phosphorylated to different extents with cyclic‐AMP‐dependent protein kinase and the phosphopeptides obtained by tryptic digestion were analysed. Limited tryptic digestion of the native enzyme quantitatively released two phosphopeptides (site 1 a and site 1 b) which were soluble in trichloroacetic acid. The trichloroacetic‐arid pellet contained a distinct phosphorylation site, which was shown to be identical to the serine residue which is phosphorylated by phosphorylase kinase (site 2). Sites 1 a, 1 b and 2 accounted for virtually all of the phosphorylation of glycogen synthase up to 2.5 molecules phosphate incorporated per subunit. Site 1 a was initially phosphorylated 7–10‐fold faster than site 2, and 15–20‐fold faster than site 1 b. The phosphorylation of site 1 a became much slower after about 0.5 molecules of phosphate had been incorporated per subunit. Cyclic‐GMP‐dependent protein kinase also phosphorylated glycogen synthase at sites 1 a, 1 b and 2. The order of phosphorylation was the same as that observed with cyclic‐AMP‐dependent protein kinase (site 1 a > site 2 > site 1 b). All three sites phosphorylated by cyclic‐AMP‐dependent protein kinase could be dephosphorylated by protein phosphatase 1. Site 2 was initially dephosphorylated 5–10‐fold faster than site la and more than 100‐fold faster than site 1 b. The MgATP‐dependent protein phosphatase dephosphorylated site 2, site 1a and site 1 b to a very similar manner to protein phosphatase 1. The different rates of phosphorylation and dephosphorylation of the three sites allowed the demonstration to be made that the activity of glycogen synthase depended on the state of phosphorylation of site 2 and site 1 a. The phosphorylation of site 1 b did not influence the activity under the conditions used. Incubation of glycogen synthase with high levels of phosphorylase kinase did not lead to the phosphorylation of either site 1 a or 1 b, or of the three sites phosphorylated by glycogen synthase kinase 3 (sites 3a, 3b and 3c). Similarly no phosphorylation of sites 3a, 3b or 3c by cyclic‐AMP‐dependent protein kinase was detected. It is concluded that site 2 is the only site at which overlapping substrate specificity occurs between these three glycogen synthase kinases.