Structure of NaK. (A) Overall structure of NaK, with the front subunit removed for clarity. Selectivity filter is colored yellow. Ions in the selectivity filter are shown as green spheres. (B) Zoom-in view of the selectivity filter of NaK in K (green spheres). The water molecules in the vestibule and at the external entrance are shown as red spheres. The two ion-binding sites within the filter are labeled 3 and 4. (C) Zoom-in view of the selectivity filter of NaK in Na (green spheres).