Measuring The Contribution Of S4 Charges On Gating Currents Of A Sodium Channel

Abstract

Upon membrane depolarization, conformational changes in the S4 voltage sensors results in the opening of the voltage-gated sodium channel. The movement of the positively charged residues on the four distinct voltage-sensors in the membrane electric field generates a measurable transient current referred to as the gating current. The ON gating currents of the Na+ channel has two kinetic components and the fast component correlates well with the kinetics of fluorescence changes of probes attached to the voltage sensors of domains I, II, and III. The slow component of the gating current, however, matches with the fluorescence kinetics of probes on domain IV voltage-sensor (Chanda and Bezanilla JGP 2002 120; 629-45). In an attempt to specify the molecular origin of the slow and fast components of the gating current, we neutralized the first three charged residues (Q3 mutants) in each of the four voltage-sensing segments of the sodium channel. Our data supports the hypothesis that the voltage-sensor of domain IV primarily contributes to the slow component of the gating current and provides support to the notion that the movement of this voltage sensor is slower than the opening of the pore.