Open and Closed Structures of a Barium-Blocked Potassium Channel

Barium (Ba²⁺) is a classic permeant blocker of potassium (K⁺) channels. The "external lock-in effect" in barium block experiments, whereby the binding of external K⁺ impedes the forward translocation of the blocker, provides a powerful avenue to investigate the selectivity of the binding sites along the pore of potassium channels. Barium block experiments show that the external lock-in site is highly selective for K⁺ over Na⁺. Wild-type KcsA was crystallized in low K⁺ conditions, and the crystals were soaked in solutions containing various concentrations of barium. Structural analysis reveals open and closed gate conformations of the KcsA channel. Anomalous diffraction experiments show that Ba²⁺ primarily binds to the innermost site S4 of the selectivity filter of the open-gate conformation and also the site S2, but no binding is detected with the closed-gate conformation. Alchemical free-energy perturbation calculations indicate that the presence of a Ba²⁺ ion in the selectivity filter boosts the specificity of K⁺ binding relative to Na⁺ in the external sites S0-S2.