Abstract
Phosphorylation by cAMP-dependent protein kinase (PKA) increases the activity of class C L-type Ca(2+) 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 alpha(1) 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 (32)P-labeled RIIbeta 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 alpha(1) subunit of the class C channel. Our findings indicate that PKA is an integral part of neuronal class C L-type Ca(2+) channels and suggest that the AKAP MAP2B may mediate this interaction. Neither PKA nor MAP2B were detected in immunoprecipitates of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid-type glutamate receptors or class B N-type Ca(2+) channels. Accordingly, MAP2B docked at class C Ca(2+) channels may be important for recruiting PKA to postsynaptic sites.
Highlights
Ca2ϩ influx through voltage-gated L-type Ca2ϩ channels controls a variety of neuronal functions including synaptic plasticity, membrane excitability, and gene expression [1,2,3,4,5]
A kinase anchor proteins (AKAPs)-mediated targeting of protein kinase (PKA) appears to be essential for efficient phosphorylation and regulation of ligand- and voltagegated ion channels, including AMPA receptors and L-type channels [19, 23, 24]
Our results suggest that the class C L-type Ca2ϩ channel complex contains a docking site for an AKAP1⁄7PKA complex
Summary
Ca2ϩ influx through voltage-gated L-type Ca2ϩ channels controls a variety of neuronal functions including synaptic plasticity, membrane excitability, and gene expression [1,2,3,4,5]. Incubation of immunoprecipitated class C channel complexes with purified PKC and ATP resulted in phosphorylation of serine 1928; the phosphorylation was blocked by the PKI peptide (Fig. 1B, lanes 3 and 4). The specificity of the co-immunoprecipitation of PKA activity with class C channels was further confirmed by a peptide phosphorylation assay (Fig. 4).
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