(A) Section view of the ion-binding site and interface region of PaNhaP from the cytoplasmic side. Interface helices of the two protomers are shown in blue and beige, respectively. The acidic side chains of Glu73, Asp159, a water molecule held by Asp130, the hydroxyl group of Ser155 and the main-chain carbonyl of Thr129 coordinate the substrate ion. The anomalous density for the Tl+ ion (grey sphere) in the substrate-binding site between helix H3, H6 and the unwound stretch of H5 is shown in magenta at 4σ. The 3σ omit map for the H2O molecule next to Tl+ is green. The water molecule near Glu154 and Asn158 is not directly involved in ion coordination. (B, C) Detailed views of the substrate-coordinating residues from the extracellular and cytoplasmic side, respectively. (D) Side view of core helices and substrate-binding residues in the 6-helix bundle.