Overall structure of the NaK2K–K⁺ complex in open conformation with the front and back subunits removed for clarity. A magnified view of its selectivity filter (boxed) is shown. Electron density (blue mesh) from the F_o–F_c ion omit map (4σ) shows K⁺ ions (modeled as green spheres) in the filter and water molecules (modeled as red spheres) at site 0. The four ion-binding sites within the filter are labeled 1–4 from top to bottom.