Phosphorylation and activation of rabbit skeletal muscle phosphorylase kinase by a cyclic nucleotide- and Ca2+-independent protein kinase.

Phosphorylase kinase from rabbit skeletal muscle can be phosphorylated and activated by a cyclic nucleotide- and Ca2+-independent protein kinase previously identified as a glycogen synthase kinase (Itarte, E., and Huang, K.-P. (1979) J. Biol. Chem. 254, 4052-4057). This independent kinase phosphorylates the beta subunit of phosphorylase kinase approximately 15 times faster than it does the alpha subunit. The cAMP-dependent and -independent kinases separately catalyze the incorporation of 1 mol of phosphate into the beta subunit. Analyses of the tryptic peptides from the beta subunit phosphorylated with either kinase by isoelectric focusing and peptide mapping indicate that both kinases phosphorylate the same site on the beta subunit. Activation of phosphorylase kinase catalyzed by the independent kinase is only 60% of that observed with cAMP-dependent kinase. If phosphorylase kinase is first incubated with the independent kinase to phosphorylate the beta subunit, subsequent addition of cAMP-dependent kinase results in a predominant phosphorylation of the alpha subunit. This additional phosphorylation of the alpha subunit is accompanied by a further activation of the alpha subunit is accompanied by a further activation of phosphorylase kinase to the same extent as that achieved by cAMP-dependent kinase alone. Hence, the phosphorylation of the alpha subunit is clearly required for full activation of phosphorylase kinase, even at low [Mg2+].