Ion permeation pathway and selectivity filter. (a) Ribbon representation of one KtrB protomer (4J7C) highlighting the permeation pathway with the selectivity filter (purple box) and the intramembrane loop (yellow). Domains D1 and D4 in the front and the back were removed for clarity. (b) Selectivity filter signature sequence of all four KtrB domains from V. alginolyticus in comparison with the signature sequence of KcsA. (c) Stick representations of the selectivity filters of KtrB (4J7C), TrkH (4J9U), KdpA (5MRW), and KcsA (1BL8) shown with the so far confirmed K⁺ binding sites as purple spheres. Domains D2 and D4 of KtrB, TrkH, and KdpA, and two subunits of KcsA, respectively, were removed for clarity.