NaChBac: the long lost sodium channel ancestor

Abstract

In excitable cells, the main mediators of sodium conductance across membranes are voltage-gated sodium channels (Na(V)s). Eukaryotic Na(V)s are essential elements in neuronal signaling and muscular contraction and in humans have been causally related to a variety of neurological and cardiovascular channelopathies. They are complex heavily glycosylated intrinsic membrane proteins present in only trace quantities that have proven to be challenging objects of study. However, in recent years, a number of simpler prokaryotic sodium channels have been identified, with NaChBac from Bacillus halodurans being the most well-characterized to date. The availability of a bacterial Na(V) that is amenable to heterologous expression and functional characterization in both bacterial and mammalian systems has provided new opportunities for structure--function studies. This review describes features of NaChBac as an exemplar of this class of bacterial channels, compares prokaryotic and eukaryotic Na(V)s with respect to their structural organization, pharmacological profiling, and functional kinetics, and discusses how voltage-gated ion channels may have evolved to deal with the complex functional demands of higher organisms.

Figure 1

Schematic diagram comparing eukaryotic and prokaryotic voltage-gated sodium channel structures. The four homologous domains in the eukaryotic channel are designated DI–DIV. Transmembrane helices comprising the voltage sensor subdomains (S1–S4) are colored light blue, and the helices (S5 and S6) that form the pore subdomains are colored dark blue. Selectivity filter regions are colored red and interconnecting loops black, and the inactivation gate between DIII and DIV found in eukaryotic channels is colored green. N and C denote the N- and C-termini, respectively.

Figure 2

Sequence alignments of the DIII domains of human isoforms Na_V1.1 (P35498), Na_V1.2 (Q99250), Na_V1.3 (Q9NY46), Na_V1.4 (P35499), Na_V1.5 (Q14524), Na_V1.7 (NP002967.2), Na_V1.8 (NP_055006.1), and Na_V1.9 (Q15858.2) and NaChBac (NP242367). Identical residues are colored black and conserved residues dark gray. The highly conserved arginine residues in the S4 helix are shaded, as is the proposed glycine hinge in S6. The LESWAS filter residues in NaChBac are shown in italics. The transmembrane regions of NaChBac are indicated by the boxes. The alignments were created with CLUSTALW(81) with default parameters, and the locations of the transmembrane regions were estimated using TMHMM.(82)

Figure 3

Scheme of the proposed evolutionary relationships among members of the voltage-gated ion channel family.