Fig. 1. Structure of the MthK pore domain.

Only two subunits of the tetramer are shown for clarity. The four potassium-binding sites (pink dots) in the selectivity filter are labeled as S1–S4. The distance between Oγ atoms, the oxygen atoms of the Thr59 side chains, is depicted as the dashed line. Other residues that play a critical role in gating and ion permeation are labeled accordingly.