Role for Fast Inactivation in Domain I of Voltage Gated Sodium Channels

In sodium channels, the voltage sensing S4 segments move outward in response to membrane depolarization to promote activation and fast inactivation of the channel. Domain-specific roles for S4 segments include activation and fast inactivation. In the present work we investigated the role of domain I in activation, deactivation and fast inactivation of the skeletal muscle sodium channel hNaV1.4. Charge reversal at R1 (R219D) produced a significant hyperpolarizing shift of the activation curve, an effect confirmed in an inactivation deficient background (R219D/QQQ) and with gating currents. Charge reversal at residues lower in the S4 segment (R225D, K228D) produced a depolarizing shift of the activation curve. R219D selectively enhanced fast inactivation from closed states (hyperpolarizing shift of the steady-state fast inactivation curve, and accelerated kinetics of closed-state fast inactivation. Fast inactivation from the open state was not affected by R219D. Lesser effects on closed-state fast inactivation were observed for charge reversal at DIS4 residues lower in that voltage sensor. Charge reversal at inner negative countercharges (E171R, S2; D197R, S3) also produced a hyperpolarizing shift of the steady-state fast inactivation curve and accelerated kinetics of closed-state fast inactivation. These results suggest that in the wild type NaV1.4 channel, interactions of inner negative charges with the outer charge in DIS4 restrict outward movement of that voltage sensor in response to weak depolarization, and limit fast inactivation elicited from closed states. This work was supported by NIH 1R15NS093579-01A1 to JRG and NIH P20GM103408 to ISU.