The Phosphorylation of Rabbit Skeletal Muscle Glycogen Synthase by Glycogen Synthase Kinase‐2 and Adenosine‐3′: 5′‐Monophosphate‐Dependent Protein Kinase

Purified glycogen synthase is contaminated with traces of two protein kinases that can phosphorylate the enzyme. One is protein kinase dependent on adenosine 3′: 5′‐monophosphate (cyclic AMP) and the second is an activity termed glycogen synthase kinase‐2 [Nimmo, H. G. and Cohen, P. (1974)]. Glycogen synthase kinase‐2 has been found to be localized relatively specifically in the proteinglycogen complex. It has been purified 4000‐fold by two procedures, both of which involve disruption of the complex, followed by DEAE‐cellulose and phosphocellulose chromatographies. However the salt concentration at which glycogen synthase kinase‐2 is eluted from DEAE‐cellulose depends on the method that is used to disrupt the complex. The results indicate that glycogen synthase kinase‐2 is firmly attached to a protein component of the complex. The isolation procedures separate glycogen synthase kinase‐2 from phosphorylase kinase, cyclic‐AMP‐dependent protein kinase and other glycogen‐metabolising enzymes. Glycogen synthase kinase‐2 is the major phosvitin kinase in skeletal muscle, although glycogen synthase is a six to eight‐fold better substrate than phosvitin under the standard assay conditions. Phosphorylase kinase and phosphorylase b are not substrates for glycogen synthase kinase‐2.