Regulation of the AKAP79-Protein Kinase C Interaction by Ca/Calmodulin*

The A kinase-anchoring protein AKAP79 coordinates the location of the cAMP-dependent protein kinase (protein kinase A), calcineurin, and protein kinase C (PKC) at the postsynaptic densities in neurons. Individual enzymes in the AKAP79 signaling complex are regulated by distinct second messenger signals; however, both PKC and calcineurin are inhibited when associated with the anchoring protein, suggesting that additional regulatory signals must be required to release active enzyme. This report focuses on the regulation of AKAP79PKC interaction by calmodulin. AKAP79 binds calmodulin with high affinity (KD of 28 6 4 nM (n 5 3)) in a Ca-dependent manner. Immunofluorescence staining shows that both proteins exhibit overlapping staining patterns in cultured hippocampal neurons. Calmodulin reversed the inhibition of PKCbII by the AKAP79(31–52) peptide and reduced inhibition by the full-length AKAP79 protein. The effect of calmodulin on inhibition of a constitutively active PKC fragment by the AKAP79(31–52) peptide was shown to be partially dependent on Ca. Ca/calmodulin reduced PKC coimmunoprecipitated with AKAP79 and resulted in a 2.6 6 0.5-fold (n 5 6) increase in PKC activity in a preparation of postsynaptic densities. Collectively, these findings suggest that Ca/calmodulin competes with PKC for binding to AKAP79, releasing the inhibited kinase from its association with the anchoring protein.