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

X-ray diffraction data were collected from frozen crystals (100 degrees K) of the KcsA K+ channel equilibrated with solutions containing barium chlori de. Difference electron density maps (F-barium - F-native, 5.0 Angstrom res olution) show that Ba2+ resides at a single location within the selectivity filter. The Ba2+ blocking site corresponds to the internal aspect (adjacen t 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 Ba 2+ with respect to Rb+ ions in the pore is in good agreement with the findi ngs on the functional interaction of Ba2+ with K+ (and Rb+) in Ca2+-activat ed K+ channels (Neyton, J., and C. Miller. 1988. J. Gen. Physiol. 92:549-56 7). Taken together, these structural and functional data imply that at phys iological 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.