The difference in Ca ²⁺ binding at the SF of TRPV channels leads to different ion permeation patterns. (A) The contour plot of Ca²⁺ density on the R-z plane of the SF region of TRPV6. Binding sites are indicated by arrows and labeled. The Cα positions of key residues are indicated by dashed lines. The boundaries of the SF are indicated with solid lines. N is the average number of Ca²⁺ in the SF region. (B) The representative snapshot of Ca²⁺ binding at the SF region. The average ion density map is shown as a meshed isosurface. Binding sites are indicated by arrows and labeled. Residues forming the binding sites are shown as licorice. The Ca²⁺ is shown as a pink sphere. The backbone of protein is shown as a transparent cartoon. Only two chains are shown for clarity. The ion density isosurface is plotted using a threshold of 0.1 Å⁻³. (C and D) Similar to A and B, respectively, but for TRPV1.