Phosphorylation of glycogen synthase by cyclic AMP-independent casein kinase-2 from rabbit skeletal muscle.

Cyclic AMP-independent casein kinase-2 from rabbit skeletal muscle has been extensively purified by procedures including phosphocellulose, Bio-Gel A-1.5m, casein-Sepharose, and DEAE-cellulose column chromatographies. The casein kinase and glycogen synthase kinase activities are co-purified throughout the purification. The casein kinase-2 has Mr approximately equal to 135,000 as determined by glycerol density gradient centrifugation and by gel filtration. Analysis of the purified kinase by gel electrophoresis in the presence of sodium dodecyl sulfate reveals the presence of two major protein bands having Mr = 42,000 and 27,000 in a ratio of 0.85:1. The kinase phosphorylates glycogen synthase, casein, and phosvitin either in the presence of ATP or GTP; however, the Km values for GTP are slightly higher than those of ATP. The activity of this kinase is inhibited by heparin but is not affected by the addition of cyclic AMP, heat-stable inhibitor of cyclic AMP-dependent protein kinase, Ca2+, ethylene glycol bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid, or the combination of Ca2+ and calmodulin. The phosphorylation of the synthase by the kinase results in the incorporation of approximately 0.8 mol of PO4/subunit and a reduction in the synthase activity ratio from 0.82 to 0.6. The extent of phosphorylation of the synthase catalyzed by casein kinase-2 is similar to that by phosphorylase kinase; however, the sites phosphorylated by these two kinases are different. The relationships of casein kinase-2 to the cyclic AMP-independent synthase kinases that have been described by other laboratories and to the casein kinases isolated from other tissues are discussed.