Reduced model of the S2 site in the selectivity filter of the KcsA channel used by Yu et al. (2010b). (A) The full-length KcsA channel is shown with its four subunits. (B) The selectivity filter is represented with three K⁺ ions occupying the sites S0, S2, and S4. (C) The reduced model of the site S2 based on four diglycines is shown. This is adapted from Yu et al. (2010b).