Multiscale modeling shows that dielectric differences make NaV channels faster than KV channels.

Abstract

The generation of action potentials in excitable cells requires different activation kinetics of voltage-gated Na (NaV) and K (KV) channels. NaV channels activate much faster and allow the initial Na+ influx that generates the depolarizing phase and propagates the signal. Recent experimental results suggest that the molecular basis for this kinetic difference is an amino acid side chain located in the gating pore of the voltage sensor domain, which is a highly conserved isoleucine in KV channels but an equally highly conserved threonine in NaV channels. Mutagenesis suggests that the hydrophobicity of this side chain in Shaker KV channels regulates the energetic barrier that gating charges cross as they move through the gating pore and control the rate of channel opening. We use a multiscale modeling approach to test this hypothesis. We use high-resolution molecular dynamics to study the effect of the mutation on polarization charge within the gating pore. We then incorporate these results in a lower-resolution model of voltage gating to predict the effect of the mutation on the movement of gating charges. The predictions of our hierarchical model are fully consistent with the tested hypothesis, thus suggesting that the faster activation kinetics of NaV channels comes from a stronger dielectric polarization by threonine (NaV channel) produced as the first gating charge enters the gating pore compared with isoleucine (KV channel).