Structure of NaK2CNG-Dm. (A) Overall structure of NaK2CNG-Dm, with the front and back subunit removed for clarity. (B) A stereo view of the selectivity filter region boxed in A. 2F_o-Fc (2σ) electron density map is shown in mesh. (C; left), Superimposition of NaK2CNG-Dm (blue), NaK2CNG-E (orange), wild-type NaK (gray), and potassium channel NaK2K (yellow) shows that the four channels differ mainly at the filter region (boxed). (Right) The selectivity filters of NaK2CNG-Dm, NaK2CNG-E, NaK, and NaK2K adopt different conformations at the external half and consequently have varying numbers of ion-binding sites.