Rabbit Liver Glycogen Synthase Kinases

A rabbit liver protein kinase (PCO.,), able to phosphorylate glycogen synthase and phosvitin, has been extensively purified. The enzyme had apparent M, = 170,000-190,000 as judged by gel filtration and was associated with two major polypeptide species, a (Mr = 43,000) and lp (Mr = 25,000). Two other polypeptides, Mr = 38,000 and M, = 35,000, were also detected. Treatment with trypsin led to an enzyme composed only of polypeptides of M, = 35,000 and M, = 25,000. The @polypeptide underwent autophosphorylation when incubated with M&+ and ATP or GTP. The protein kinase was effective in utilizing both ATP and GTP as the phosphoryl donor (apparent K,,, values 5-11 PM and 9-19 PM, respectively). The enzyme phosphorylated phosvitin, casein, and glycogen synthase but not histone or phosphorylase and was inhibited by heparin. Phosphorylation of glycogen synthase proceeded to approximately 0.5 phosphate/subunit with little inactivation of the glycogen synthase. The phosphorylation occurred predominantly in a 21,000-dalton CNBr fragment of glycogen synthase that had been previously shown to reside toward the COOH terminus of the molecule. The liver PCo., appeared very similar to an analogous enzyme isolated from rabbit muscle (DePaoli-Roach, A. A., Ahmad, Z., and Roach, P. J. (1981) J. BioL Chem 256,8955-8962). The present work, therefore, provides a point of contact between the Ca2+ and cyclic nucleotide-independent glycogen synthase kinases of rabbit liver and muscle.