Multiple phosphorylation of rat-liver glycogen synthase by protein kinases

The phosphorylation sites in liver synthase were studied using gel filtration and high performance liquid chromatography of 32P-labeled tryptic peptides. Phosphorylase b kinase, calmodulin-dependent glycogen synthase kinase and glycogen synthase kinase 4 from liver phosphorylated the same low Mr tryptic peptide. cAMP-dependent protein kinase mainly phosphorylated the low Mr tryptic peptide, but also incorporated phosphate into two other peptides. Glycogen synthase kinase 5 phosphorylated a single tryptic peptide, whereas glycogen synthase kinase 3 phosphorylated several tryptic peptides. Calcium-phospholipid-dependent protein kinase phosphorylated two tryptic peptides, the major one of which had the same chromatographic properties as the low Mr peptide described above. These findings confirm that liver glycogen synthase undergoes multi-site phosphorylation and suggest that the topography of the sites is generally similar to that in muscle glycogen synthase.