KcsA selectivity filter conformations from x-ray crystallography experiments in the following conditions: (A) high K⁺ (Zhou et al., 2001) (deposited in the Protein Data Bank under accession no. 1K4C); (B) high Na⁺ and no K⁺ (Zhou et al., 2001) (Protein Data Bank accession no. 2ITC); (C) high Li⁺ and no K⁺ (Thompson et al., 2009) (Protein Data Bank accession no. 3GB7). (Inset) An increased representation of the S3 and S4 sites at the bottom of the selectivity filter where Li⁺ is proposed to bind, with the coordination distances indicated. Purple, K⁺ ions; red, waters; blue, Na⁺ ions; green, Li⁺ ions.