Voltage-gated sodium channels (NaVs) are critical for initiating action potentials, with NaV1.4 serving as the major isoform in skeletal muscle. Mutations in the arginines of the S4 segment of voltage-sensing domains (VSDs) can generate aberrant ion permeation pathways (gating pore currents) in closed channels that underlie hypokalaemic periodic paralysis (HypoPP). While cryoEM structures have provided high-resolution models of NaV1.4 in open or inactivated states, structural details of the closed, resting state remain limited. Here, we model the full-length NaV1.4 using the cryoEM structure (PDB ID: 6AGF) as a starting point, incorporating the β1 subunit, and ascertain the closed state using molecular dynamics (MD) simulations of channel deactivation. The channel was embedded in a fully hydrated POPC bilayer and simulated under a hyperpolarizing membrane potential implemented as a uniform electric field. Long-timescale molecular dynamics simulations, extending to ∼150 μs, allowed us to probe slow conformational dynamics within the VSDs. Under moderate negative potentials, we observed multiple downward transitions of S4 helices consistent with voltage-sensor deactivation. To accelerate these transitions, we employed modified force fields that enhanced the movement of the S4 helices. These results provide new insights into the conformational landscape of NaV1.4 in the closed state and establish conditions under which voltage-sensor transitions can be captured in full-channel protein models. This framework sets the stage for identifying gating-pore conformations and potential drug-binding sites to inhibit anomalous permeation of the VSD in HypoPP-associated mutant channels.
Saha et al. (Sun,) studied this question.