High and intermediate affinity calmodulin binding domains of the alpha and beta subunits of phosphorylase kinase and their potential role in phosphorylation-dependent activation of the holoenzyme.

Abstract

Phosphorylase kinase is a calcium-regulated multimeric enzyme of composition (alpha beta gamma delta)4, which contains calmodulin as the integral delta subunit and also is activated further by addition of extrinsic calmodulin. Previous studies by Dasgupta, M., Honeycutt, T., and Blumenthal, D.K. ((1989) J. Biol. Chem. 264, 17156-17163) have identified gamma 302-326 and gamma 342-366 as two calmodulin binding regions. Using peptides that were synthesized based on alpha and beta primary structure and that were predicted to contain the basic amphiphilic alpha-helix motif thought important for calmodulin binding, four additional potential calmodulin binding domains have now been identified: one of high affinity, beta 770-794; two of intermediate affinity, beta 5-28 and beta 920-946; and one with marginally low affinity, alpha 1070-1093. Peptide beta 770-794 was of higher calmodulin affinity than either gamma 302-326 or gamma 342-366; it was of higher affinity than the model synthetic peptide IV defined by O'Neil, K.T., and DeGrado, W.F. ((1990) Trends Biochem. Sci. 15, 59-64); and it is currently the most potent calmodulin-binding peptide so far described. Correlated with their affinity for calmodulin, all six phosphorylase kinase-derived peptides and several other established calmodulin-binding peptides inhibited phosphorylase kinase previously activated by cAMP-dependent phosphorylation, reducing its activity to the level of the nonactivated enzyme. However, these peptides did not inhibit (and some peptides slightly activated) the nonphosphorylated enzyme. Even in the presence of these peptides both activated and nonactivated enzyme remained fully Ca(2+)-dependent. The beta 770-794 peptide has at least a 5-fold greater calmodulin binding affinity than the holo-phosphorylase kinase. This, and its higher affinity for calmodulin than either of the sites on the gamma subunit, raises the possibility that in the native enzyme it may be involved in binding the intrinsic delta subunit. Further, inhibition of activated but not nonactivated enzyme by calmodulin-binding peptides would suggest that the phosphorylation-dependent activation of phosphorylase kinase may be mediated by changes in the binding interactions of the intrinsic calmodulin delta subunit.