Fig. 1.

(A) Selectivity filter of a representative K channel, KcsA (7). The orange spheres denote K⁺ ions bound to two of their four preferred binding sites, S2 and S4 (Protein Data Bank ID code 1K4C). Threonine (Thr) side chains that make up the S4 site are highlighted in green. (B) Sequence alignment of the selectivity filter region of K channels that have serine residues in their S4 sites. The K channels whose names are appended with an asterisk are engineered via site-directed mutagenesis and were functionally characterized recently by Chatelain et al. (16). (C) Representative configurations of threonine and serine amino acids.