Figure 4. Ca²⁺ binding sites in Ca²⁺-bound hIP₃R3.

a, Monomeric structure of Ca²⁺-bound hIP₃R3 colored by domain viewed in the plane of the membrane. Ca²⁺ and Zn²⁺ ions are shown as spheres. b, CD Ca²⁺-binding site at the CLD-ARM2 interface. Residues coordinating the Ca²⁺ are shown as sticks and the CD Ca²⁺ ion is shown as a magenta sphere. c, Superposition of the CLD and ARM2 of Ca²⁺-bound (colored by domain) and apo (grey, left) or Ca²⁺-bound (colored by domain) and IP₃ class 2 (grey, right) aligned by the CLD. d, JD Ca²⁺-binding site at the ARM3-JD interface. Residues coordinating the Ca²⁺ are shown as sticks and the Ca²⁺ ion is shown as a green sphere. e, Superposition of ARM3 and the JD of Ca²⁺-bound (colored by domain) and apo (grey) aligned by the JD.