Roles for Countercharge in the Voltage Sensor Domain of Ion Channels

Abstract

Voltage-gated ion channels share a common structure typified by peripheral, voltage sensor domains. Their S4 segments respond to alteration in membrane potential with translocation coupled to ion permeation through a central pore domain. The mechanisms of gating in these channels have been intensely studied using pioneering methods such as measurement of charge displacement across a membrane, sequencing of genes coding for voltage-gated ion channels, and the development of all-atom molecular dynamics simulations using structural information from prokaryotic and eukaryotic channel proteins. One aspect of this work has been the description of the role of conserved negative countercharges in S1, S2, and S3 transmembrane segments to promote sequential salt-bridge formation with positively charged residues in S4 segments. These interactions facilitate S4 translocation through the lipid bilayer. In this review, we describe functional and computational work investigating the role of these countercharges in S4 translocation, voltage sensor domain hydration, and in diseases resulting from countercharge mutations.

Keywords: channelopathy; countercharge; crystallography; electrostatic; ion channel; molecular dynamics; sliding helix model; voltage sensor domain.

Figure 1

Multiple sequence alignments of voltage sensor domains (VSDs) (S1–S4) from a sample of voltage-gated ion channel (VGIC) alpha subunit domains. Prokaryotic sodium channels are Na_VAb from Arcobacter butzleri (Payandeh et al., 2011), and the NaChBac orthologue Na_VRh from alpha proteobacterium (Zhang et al., 2012). The remaining sequences are human. For each alignment, putative consensus negative charge regions are highlighted with boxes colored according to frequently observed residues (glutamate, yellow; asparagine, red; aspartate, blue). The first five positively charged residues in S4 are shown in green.

Figure 2

(A) Conservation of amino acid residues in voltage sensor domains (VSDs) across taxa, highlighting the evolutionary conservation of polar or acidic residues in S1–S3 (putative countercharges), positively charged arginine or lysine residues in S4, and conserved aromatic residues. With permission from J. Gen. Physiol. and first author (Palovcak et al., 2014). (B) Homology model of hNa_V1.4 domain IV VSD based on prokaryotic structural information [3RVY.pdb, (Payandeh et al., 2011)] showing locations of consensus S1–S3 countercharges in the extracellular negatively charged (ENC) and intracellular negatively charged (INC) regions. A conserved aromatic in S2 (yellow) is also shown as part of the gating charge transfer center (GCTC). (C) Top view of the VSD showing side chains of S1–S3 countercharges facing the S4 arginine guanidyl groups.

Figure 3

Scanning mutagenesis of putative countercharges in domains II and IV of the skeletal muscle sodium channel Na_V1.4. Charge-reversing mutations in domain II decrease activation probability (indicated by shift in V_0.5), while those in domain IV slow the entry of channels into a fast-inactivated state (indicated by the ratio of time constant (tau) of mutation with respect to that of wild type, at 20 mV). These effects are consistent with the hypothesis that countercharges facilitate certain domain-specific functions highlighted by green arrows (DII activation, DIV fast inactivation). Adapted from Groome and Winston (2013). Colors indicate countercharge loci as extracellular negatively charged (ENC) (green, red), or intracellular negatively charged (INC) (blue).

Figure 4

Electrostatic interactions between countercharges (aspartate, blue; glutamate, green; asparagine, purple; serine, yellow, cysteine, cyan) and each of four to five positively charged residues in S4 segments (arginine or lysine, red) in domains I–IV of the skeletal muscle sodium channel Na_V1.4. Nodes indicate charged and polar residues within the voltage sensor, while edge width indicates strength of interaction. The profile of interactions is given for a presumptive resting state, intermediate closed state, and activated state. Reproduced with permission from Idaho State University Libraries (Bayless-Edwards, 2019).