CLDN15 D55 is the principal cation binding site, and this is abrogated in the D55N mutation. (A) Plots show contact time data as the average number of Na⁺ ions that are found within 4.0 Å of CLDN15 amino acid residues in voltage simulations (V = ±800, 1,200 mV) for CLDN15^WT, CLDN15^D55E, and CLDN15^D55N. (B) Average number of cations (Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺) within each pore for CLDN15^WT, CLDN15^D55E, CLDN15^D55N, calculated from voltage trajectories. (C) Position of key cation-interacting sites on CLDN15: I39, D55, S56, N61, D64, E157.