Slow Inactivation Does Not Block the Aqueous Accessibility to the Outer Pore of Voltage-gated Na Channels

Slow inactivation of voltage-gated Na channels is kinetically and structurally distinct from fast inactivation. Whereas structures that participate in fast inactivation are well described and include the cytoplasmic III-IV linker, the nature and location of the slow inactivation gating mechanism remains poorly understood. Several lines of evidence suggest that the pore regions (P-regions) are important contributors to slow inactivation gating. This has led to the proposal that a collapse of the pore impedes Na current during slow inactivation. We sought to determine whether such a slow inactivation-coupled conformational change could be detected in the outer pore. To accomplish this, we used a rapid perfusion technique to measure reaction rates between cysteine-substituted side chains lining the aqueous pore and the charged sulfhydryl-modifying reagent MTS-ET. A pattern of incrementally slower reaction rates was observed at substituted sites at increasing depth in the pore. We found no state-dependent change in modification rates of P-region residues located in all four domains, and thus no change in aqueous accessibility, between slow- and nonslow-inactivated states. In domains I and IV, it was possible to measure modification rates at residues adjacent to the narrow DEKA selectivity filter (Y401C and G1530C), and yet no change was observed in accessibility in either slow- or nonslow-inactivated states. We interpret these results as evidence that the outer mouth of the Na pore remains open while the channel is slow inactivated.