Different views of Ba²⁺ binding in potassium channels. (Left) Results from X-ray analysis of KcsA channels. The figure shows two of the four KcsA subunits and the superposition of electron densities due to Ba²⁺ binding (as green mesh) and Rb⁺ binding (as red mesh). (Center) The positions of the ion peaks at higher resolution. Ba²⁺ binds at the inner of the two inner Rb⁺ (or K⁺) sites, and the Rb⁺ density in the cavity is comparable to the densities in the selectivity filter. (Right) The ion positions as deduced by Neyton and Miller (J. Gen. Physiol. 92:549 and 569, 1998). The external lock-in, enhancement and internal lock-in positions (○) denote the monovalent cation positions. Figure is reproduced from Y. Jian and R. MacKinnon (J. Gen. Physiol. 115:269, 2000).