Figure 1.

High Field Strength (HFS) site EEEE formed by selectivity filter glutamates (E177) in the homotetrameric bacterial sodium channel NavAb is equivalent to the EEEE site in of Ca_v1 and Ca_v2 calcium channels and the DEKA site of Na_v1 sodium channels, which are contributed by the four homologous repeat domains. (A) Residues to the EEEE HFS site in bacterial sodium channels are contributed by four identical gene products. (B) Residues to asymmetric selectivity filters in eukaryotic calcium and sodium channels are contributed by four homologous repeat domains. (C) Structural contours of the NavAb selectivity filter and positions of corresponding residues of bacterial NavAb and human Ca_v1.2 and Na_v1.2. The narrowest pore level is the selectivity filter HFS site, which can accommodate a hydrated Na⁺ ion, but not a hydrated K⁺ ion. Panel(C) is adapted from Payandeh et al. (2011), with permission.