Fig. 3: Two anion binding sites.

a–d Anion binding sites of ssPendrin in the presence of Cl⁻ a, I⁻ b, HCO₃⁻ c, and apo d, respectively. Residues are shown as sticks, anions as spheres, and densities as cyan surface. e Left panel, two anion binding sites (violet spheres) in a pendrin monomer with the transport and scaffold domains colored in green and cyan, respectively. Right panel, anion binding sites and the transport domain. f Zoom-in views of the two anion binding sites. Direct interactions are marked with dashed lines. g The stoichiometry of ssPendrin anion binding. ¹²⁵I⁻ binding by ssPendrin-wild-type (WT), -S408A, and -Q101L was assayed with the SPA. Data (n = 3), expressed as mol-to-mol binding ratios, were subjected to the Hill equation in Prism 8, yielding the following EC₅₀, Hill coefficient, and molar I⁻-to-protein binding ratios: WT – 9.25 ± 0.49 mM, 2.69 ± 0.34, 1.78 ± 0.06 l; S408A – 10.87 ± 2.66 mM, 1.34 ± 0.31, 0.95 ± 0.12; Q101L – 11.54 ± 3.73, 1.18 ± 0.29, 1.12 ± 0.17. h, i 10-min uptake of 100 µM ¹⁴C-HCO₃⁻ and 10 mM I⁻ by ssPendrin-WT or variants with mutations of anion binding site residues. Data points are the mean ± s.e.m. (n ≥ 3). Two-tailed Student’s t tests were applied for comparison. **** indicates p < 0.0001. j Cutaway surface representation of pendrin.