Inhibitory effect of calmodulin on phosphorylation of NAP-22 with protein kinase C.

NAP-22, a recently identified neural tissue-enriched acidic protein, was shown to be a substrate of protein kinase C in vitro. Its phosphorylation site was assigned as Ser6 using deleted mutants expressed in Escherichia coli. Calmodulin inhibited this phosphorylation reaction. This inhibitory effect of calmodulin was dose-dependent and much stronger than its inhibitory effect to the phosphorylation of neuromodulin (GAP-43) with protein kinase C. The dissociation constant of NAP-22 and calmodulin obtained using the fluorescence change of dansyl-labeled calmodulin was much lower than that of neuromodulin and calmodulin. The phosphorylation of NAP-22 inhibited the association with calmodulin.