Inactivation Voltage Sensor S4 in Domain IV of Nav1.2 Controls Immobilization of S4 in Domain III as Shown by Omega Currents

Abstract

The role of S4DIV for inactivation of skeletal muscle Na channel Nav1.4 was recognized after deciphering the channelopathia Paramyotonia congenita. We showed with point mutations in the rat brain sodium channel Nav1.2 the central role of S4DIV for inactivation (Kühn and Greeff, 1999): The single mutation R4H in S4DIV slowed the recovery from inactivation about 20 times in parallel for ionic current and immobilized gating charge. Immobilization concerns about 50 % of total gating charge returning slowly to the resting state during recovery while the other half of gating charge returns very quickly. Clearly, the amount of immobilized charge is more than just the one from S4DIV. So we speculated that S4DIV would control S4s in other domains. Now, we are able to monitor the return of S4 into the resting position for each domain separately by recording the leak current of resting-state omega pore mutants (this Meeting). We find that S4DIV with the omega mutation RR12QQ shows a fast onset of omega leak current for channels at rest; however, after an inactivating prepulse, the leak current grows with the time course of recovery as expected, since this voltage sensor controls the recovery and returns into resting position accordingly. Checking the return of S4 in the other domains, we find a fast return in DI and DII while in DIII the return follows DIV. Combining these mutations with R4H in DIV, the return in both domains III and IV is about 20 times more slowly. This suggests that immobilization of gating charge across the domains is most likely achieved by the cytoplasmic loop between DIII and DIV which under control of S4DIV closes the alpha-pore and immobilizes S4DIII.