Structure of KcsA. (A) Overall structure of KcsA, with the front subunit removed for clarity. Selectivity filter (boxed) is colored yellow. Ions in the selectivity filter and cavity are shown as green spheres. M1, outer helix; M2, inner helix; P, pore helix. (B) Zoom-in view of the selectivity filter. The K⁺ ion in the cavity is surrounded by eight water molecules (red spheres). The four ion-binding sites within the filter are labeled 1–4 from top to bottom.