Selectivity filter structures of NaK mutants. (A) Partial sequence alignment between NaK, K⁺ channels (MthK, KcsA, and Shaker), and human CNG channel α subunits (A1–A3). The sequences of NaK2CNG-D and NaK2K mutants are also shown in which the mutated residues are boxed. Residues forming the selectivity filter are shaded orange. (B) Structure of NaK2CNG chimera. The three ion-binding sites within the filter are labeled 2–4 from top to bottom. (C) Structure of the K⁺-selective NaK2K mutant.