Important sites in the S4-S5 linker and S6 end region, which are putatively crucial for electromechanical coupling. (A) The S4-S5 linker and the S6 tail regions of five different voltage-gated ion channels are aligned. The demarcation of the helical regions is based on the structure of the KV1.2/2.1 chimera. In blue are the regions of the protein that have been analyzed through scanning mutagenesis, and the residues marked in red are the ones which have been proposed to be involved in coupling voltage sensor and gate motions (Shaker: Ledwell and Aldrich, 1999; Soler-Llavina et al., 2006; Batulan et al., 2010, Haddad and Blunck, 2011; NaV1.4-Domain III: Muroi et al., 2010; KCNQ1: Labro et al., 2008, 2011; hERG: Sanguinetti and Xu, 1999; Tristani-Firouzi et al., 2002, Van Slyke et al., 2010; HCN2: Chen et al., 2001; Decher et al., 2004). For the Shaker channel, the regions marked by the magenta bars are the complementary regions posited to be crucial for voltage sensor pore coupling in Lu et al. (2002). In the NaV sequence, the brown residues are sites that might be involved in coupling but lack the distinct phenotypic response of opposite shifts in the FV and GV curves (unlike the positions in red). (B) An orthogonal assay of coupling performed in the NaV1.4 channel (Domain III) using the lock-in strategy (see main text) identified sites (marked in magenta) within the same regions that are for the voltage sensor pore coupling (Arcisio-Miranda et al., 2010). Many of these residues coincide with or are close to sites that perturb coupling, as shown in Muroi et al. (2010; compare with residues marked in the NaV sequence in A).
