A mutation in segment IVS6 disrupts fast inactivation of sodium channels.

Na(+)-channel inactivation is proposed to occur by binding of an intracellular inactivation gate to a hydrophobic inactivation gate receptor in the intracellular mouth of the pore. Amino acid residues in transmembrane segment S6 of domain IV (IVS6) that are critical for fast inactivation were identified by alanine-scanning mutagenesis. Mutant VIL1774-6AAA, in which three adjacent residues (Val-Ile-Leu) at the intracellular end of segment IVS6 were converted to alanine, had substantial (> 85%) sustained Na+ currents remaining 15 ms after depolarization, while a nearby mutation of three residues to alanine had no effect. Single-channel analysis revealed continued reopenings late in 40-ms depolarizing pulses indicating that inactivation was substantially impaired compared to wild type. The mean open time for VIL1774-6AAA was longer than wild type, suggesting that this mutation also decreases the rate of entry into the fast inactivated state. These results suggest that residues near the intracellular end of segment IVS6 are critical for fast Na(+)-channel inactivation and may form part of the hydrophobic receptor site for the fast inactivation gate.