Functional interactions between distinct sodium channel cytoplasmic domains through the action of calmodulin

Sodium channels are fundamental signaling molecules in excitable cells, and are molecular targets for local anesthetic agents and intracellular free Ca²⁺ ([Ca²⁺]_i). Two regions of Na_V1.5 have been identified previously as [Ca²⁺]i-sensitive modulators of channel inactivation. These include a C-terminal IQ motif that binds calmodulin (CaM) in different modes depending on Ca²⁺ levels, and an immediately adjacent C-terminal EF-hand domain that directly binds Ca²⁺. Here we show that a mutation of the IQ domain (A1924T; Brugada Syndrome) that reduces CaM binding stabilizes Na_V1.5 inactivation, similarly and more extensively than even reducing [Ca²⁺]_i. Because the DIII-DIV linker is an essential structure in Na_V1.5 inactivation, we evaluated this domain for a potential CaM binding interaction. We identified a novel CaM binding site within the linker, validated its interaction with CaM by NMR spectroscopy, and revealed its micromolar affinity by isothermal titration calorimetry. Mutation of three consecutive hydrophobic residues (Phe¹⁵²⁰-Ile¹⁵²¹-Phe¹⁵²²) to alanines in this CaM-binding domain recapitulated the electrophysiology phenotype observed with mutation of the C-terminal IQ domain: NaV1.5 inactivation was stabilized; moreover, mutations of either CaM binding domain abolish the well described stabilization of inactivation by lidocaine. The direct physical interaction of CaM with the C-terminal IQ domain and the DIII-DIV linker, combined with the similarity in phenotypes when CaM-binding sites in either domain are mutated, suggests these cytoplasmic structures could be functionally coupled through the action of CaM. These findings have bearing upon Na⁺ channel function in genetically altered channels and under pathophysiologic conditions where [Ca²⁺]i impacts cardiac conduction.