Fig. 3: Ions and water molecules are resolved in human SLC26A9 structures.

a Three water molecules are observed in the traditional substrate-binding pocket. Residues participating in coordination are labeled. b A chloride ion is identified near the N-terminus of the TM3 α-helix. c A sodium ion is coordinated through the hydroxyl groups of S107, T127, and Q85. d A chloride ion is bound near the extracellular side of SLC26A9. Residues from the TM1–TM2 loop and TM4 participate in coordination. e Electrophysiology of mutants that are involved in ion coordination. The currents recorded were –0.79 ± 0.06, –0.37 ± 0.03, –0.31 ± 0.02, –0.30 ± 0.02, and –0.26 ± 0.03 pA, and the cell numbers patched for each group were n = 19, 10, 10, 6, and 6, respectively, ****P < 0.0001. f Molecular dynamics simulation analysis of the sodium and chloride ion-binding sites. The TM10, TM1 and TM3, TM8 of the modeled structure are colored in pink and cyan, respectively. Modeled densities of Na⁺ and Cl^– are shown in pink and yellow dots, respectively. The positions of the ions in the cryo-EM structure are illustrated as magenta and green spheres. No apparent disagreement is observed between the modeled and actual positions of the ions. g The distance to site is plotted against time for Cl^– ions. h The distance to site is plotted against time for Na⁺ ions.