Fig. 4.

Calcium binding and modulation in TRIC channels. (A) Ribbon diagram of Ca²⁺ binding in the structure of GgTRIC-A, as viewed from membrane. To have a better view for the Ca²⁺ binding, a section of the front protomer (inside the red box) is removed. The bound Ca²⁺ ion (dark green) and water molecules (red) are shown as spheres. 2Fo-Fc electron density contours are shown for both water molecules and Ca²⁺ ion, contoured at 1.5σ and colored in blue. The anomalous density is also shown for Ca²⁺ ion, colored in purple and contoured at 5.0σ. (B) A close-up view of the Ca²⁺ binding to the backbone carbonyl oxygen atom of G74. Water molecules and Ca²⁺ ion are shown, as in A. G74 O to Ca²⁺, distance = 2.4 Å; Ca²⁺ to water molecules, distance = 2.5 Å (lower) or 2.6 Å (upper). (C) GgTRIC-A channel modulation by luminal Ca²⁺. Relative channel open probabilities (P_O) were determined from planar lipid bilayer recordings at +50 mV voltages (200 mM KCl in both trans and cis solutions) in the presence of varied Ca²⁺ concentrations in the cis side (n = 3). (D) Superimposition of GgTRIC-A, the Ca²⁺-bound structure vs. the Ca²⁺-free structure. The residues involved in hydrogen bonding interactions are shown as sticks. The hydrogen bonds (distance <3.2 Å) are indicated as a black dashed line. Water molecules involved in interaction are shown as red spheres. Ca²⁺ ion is shown as a sphere colored dark green. (E and F) Close-up views of hydrogen bonding interactions in Ca²⁺-bound GgTRIC-A (E) and Ca²⁺-free GgTRIC-A (F).