Ca2+/calmodulin-dependent protein kinase II (CaMKII) phosphorylates the β2a subunit of voltage-gated Ca2+ channels at Thr498 to facilitate cardiac L-type Ca2+ channels. CaMKII colocalizes with β2a in cardiomyocytes and also binds to a domain in β2a that contains Thr498 and exhibits an amino acid sequence similarity to the CaMKII autoinhibitory domain and to a CaMKII binding domain in the NMDA receptor NR2B subunit (Grueter, C. E. et al. (2006) Mol. Cell 23, 641). Here, we explore the selectivity of the actions of CaMKII among Ca2+ channel β subunit isoforms. CaMKII phosphorylates the β1b, β2a, β3, and β4 isoforms with similar initial rates and final stoichiometrics of 6-12 mol of phosphate per mol of protein. However, activated/ autophosphorylated CaMKII binds to β1b and β2a with a similar apparent affinity but does not bind to β3 or β4. Prephosphorylation β1b, and β2a by CaMKII substantially reduces the binding of autophosphorylated CaMKII. Residues surrounding Thr498 in β2a are highly conserved in β1b but are different in β3 and β4. Site-directed mutagenesis of this domain in β2a showed that Thr498 phosphorylation promotes dissociation of CaMKII-β2a complexes in vitro and reduces interactions of CaMKII with β2a in cells. Mutagenesis of Leu493 to Ala substantially reduces CaMKII binding in vitro and in intact cells but does not interfere with β2a phosphorylation at Thr498. In combination, these data show that phosphorylation dynamically regulates the interactions of specific isoforms of the Ca2+ channel β subunits with CaMKII.
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