FIGURE 1. Binding sites for K⁺ ions in the KcsA K⁺ channel.

a, Ribbon representation of the KcsA channel with the subunit closest to the viewer removed. Potassium ions (green spheres) bind at four locations in the selectivity filter (yellow) and in the water-filled cavity at the membrane centre (bottom ion). b, Close-up view of the selectivity filter in ball-and-stick representation, with the front and back subunits removed. The four K⁺ ions are numbered to indicate the location of binding sites in the filter; position 1 is closest to the extracellular solution and position 4 is closest to the cavity. Key amino acids forming the selectivity filter are shown. Pictures were prepared with GL-Render^26,27.