The barium site in a potassium channel by X-ray crystallography

X-ray diffraction data were collected from frozen crystals (100°K) of the KcsA K⁺ channel equilibrated with solutions containing barium chloride. Difference electron density maps (F(barium) - F(native), 5.0 Å resolution) show that Ba²⁺ resides at a single location within the selectivity filter. The Ba²⁺ blocking site corresponds to the internal aspect (adjacent to the central cavity) of the 'inner ion' position where an alkali metal cation is found in the absence of the blocking Ba²⁺ ion. The location of Ba²⁺ with respect to Rb⁺ ions in the pore is in good agreement with the findings on the functional interaction of Ba²⁺ with K⁺ (and Rb⁺) in Ca²⁺-activated K⁺ channels (Neyton, J., and C. Miller. 1988. J. Gen. Physiol. 92:549-567). Taken together, these structural and functional data imply that at physiological ion concentrations a third ion may interact with two ions in the selectivity filter, perhaps by entering from one side and displacing an ion on the opposite side.