Glycogen synthase in rat adipocytes and skeletal muscle is phosphorylated on both serine and threonine

Glycogen synthase is phosphorylated both in vivo and in vitro on multiple sites per subunit. All phosphorylations of the enzyme thus far identified occur on serines which are found in two cyanogen bromide fragments, denoted CB-1 and CB-2. We have immunoprecipitated [32P]glycogen synthase from rat adipocytes and epitrochlearis muscles incubated with [32P]phosphate. Phosphoamino acid analyses by two-dimensional electrophoresis after acid hydrolysis revealed no [32P]phosphotyrosine, but significant levels of [32P]phosphothreonine (6-14% of the [32P](phosphoserine). The [32P]phosphothreonine was recovered in the large CNBr-fragment (CB-2), indicative of a hitherto unknown phosphorylation site(s).