Carboxyl Terminal Domain Missense Mutations Alter Distinct Gating Properties of the Cardiac Sodium Channel

Voltage-gated sodium channels undergo reversible voltage/time-dependent transitions from closed to open and inactivated states. The voltage setpoints and efficiency of cardiac sodium channel Na _v 1.5 state transitions are crucial for tuning the initiation and conduction of myocardial action potentials. The channel’s cytoplasmic carboxyl-terminal domain (CTD) regulates gating by intramolecular interactions and by serving as a hub for the binding of accessory proteins. We have investigated the roles of the CTD in intrinsic and FGF homologous factor (FHF)-modulated Na _v 1.5 gating through structure-guided CTD subdomain mutagenesis. The EF-hand module within the CTD was found to exert the most profound effects on channel gating, strongly influencing voltage-dependence of inactivation and activation, accelerating inactivation from the closed state, decelerating inactivation from the open state, minimizing persistent sodium current, and serving as the binding domain for FHF proteins. Na _v 1.5 ^D1788K bearing a missense mutation in the EF-hand motif displayed a depolarizing shift in voltage dependence of activation and generated greatly enhanced persistent sodium current without altering the voltage dependence of channel inactivation. Reciprocally, Na _v 1.5 ^L1861A bearing a different missense mutation in the EF-hand underwent closed-state inactivation at more negative membrane potential and at an accelerated rate, but did not display other phenotypes associated with CTD deletion. Na _v 1.5 ^{V1776A/T1778A} bearing mutations in the juxtamembrane region between the EF-hand and the channel pore helices displayed wild-type intrinsic gating properties, while FHF modulation of inactivation gating was impaired. Our channel physiology studies together with prior structural data suggest that the voltage and rate of channel inactivation from the closed state are governed by an intramolecular hydrophobic interaction of the CTD EF-hand with the cytoplasmic inactivation loop helix and the extension of this binding interface upon FHF-induced restructuring of the juxtamembrane region, while a distinct CTD intramolecular electrostatic interaction modulates voltage-dependent activation and minimizes persistent sodium current.