Phosphorylation by cAMP-dependent protein kinase (PKA) increases the activity of class C L-type Ca2+ channels which are clustered at postsynaptic sites and are important regulators of neuronal functions. We investigated a possible mechanism that could ensure rapid and efficient phosphorylation of these channels by PKA upon stimulation of cAMP-mediated signaling pathways. A kinase anchor proteins (AKAPs) bind to the regulatory R subunits of PKA and target the holoenzyme to defined subcellular compartments and substrates. Class C channels isolated from rat brain extracts by immunoprecipitation contain an endogenous kinase that phosphorylates kemptide, a classic PKA substrate peptide, and also the main phosphorylation site for PKA in the pore-forming α1 subunit of the class C channel complex, serine 1928. The kinase activity is inhibited by the PKA inhibitory peptide PKI(5-24) and stimulated by cAMP. Physical association of the catalytic C subunit of PKA with the immunoisolated class C channel complex was confirmed by immunoblotting. A direct protein overlay binding assay performed with 32p-labeled RIIβ revealed a prominent AKAP with an M(r) of 280,000 in class C channel complexes. The protein was identified by immunoblotting as the microtubule-associated protein MAP2B, a well established AKAP. Class C channels did not contain tubulin and MAP2B association was not disrupted by dilution or addition of nocodazole, two treatments that cause dissociation of microtubules. In vitro experiments show that MAP2B can directly bind to the % subunit of the class C channel. Our findings indicate that PKA is an integral part of neuronal class C L-type Ca2+ channels and suggest that the AKAP MAP2B may mediate this interaction. Neither PKA nor MAP2B were detected in immunoprecipitates of α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid- type glutamate receptors or class B N-type Ca2+ channels. Accordingly, MAP2B docked at class C Ca2+ channels may be important for recruiting PKA to postsynaptic sites.
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