The selectivity filter of the nonselective NaK and NaK2CNG-D channels. An F_o–F_c ion omit map (5σ) shows the electron density in the filter. (A) The WT NaK channel in the presence of K⁺ ions (PDB accession no. 3E8H). (B) Comparison of electron density in nonselective NaK2CNG-D channel. (Left) The selectivity filter of NaK2CNG-D with bound K⁺ ions (PDB accession no. 3K03). (Right) The selectivity filter of NaK2CNG-D with bound Na⁺ ions (PDB accession no. 3K04). Structures are from Alam and Jiang (2009b) and Derebe et al. (2011a).