Inhibition by calmodulin of the cAMP-dependent protein kinase activation of phosphorylase kinase.

Calmodulin is shown to inhibit both the activation and phosphorylation of phosphorylase kinase by cAMP-dependent protein kinase. Maximal inhibition of both processes was approximately 66% at the highest calmodulin concentration tested (5.5 microM). It was found that the inhibition of phosphorylation was calcium-dependent, reversible by trifluoperazine, and specific for the beta subunit of phosphorylase kinase with no significant inhibition of phosphorylation of the alpha subunit. This inhibitory activity of calmodulin appears to be due to an interaction between calmodulin and the substrate, phosphorylase kinase. This finding implies either that the site of exogenous calmodulin interaction with phosphorylase kinase is at the beta subunit or that this interaction results in a conformational change of phosphorylase kinase that inhibits the interaction between cAMP-dependent protein kinase and the beta subunit of phosphorylase kinase. The beta subunit may contain a regulatory site that is recognized by either protein kinase or calmodulin. These findings further substantiate the role of the beta subunits in the activation of phosphorylase kinase and provide an additional example of substrate-directed control of phosphorylation.