Three-dimensional Rearrangements within Inositol 1,4,5-Trisphosphate Receptor by Calcium

Allosteric binding of calcium ion (Ca²⁺) to inositol 1,4,5-trisphosphate (IP₃) receptor (IP₃R) controls channel gating within IP₃R. Here, we present biochemical and electron microscopic evidence of Ca²⁺-sensitive structural changes in the three-dimensional structure of type 1 IP₃R (IP₃R1). Low concentrations of Ca²⁺ and high concentrations of Sr ²⁺ and Ba²⁺ were shown to be effective for the limited proteolysis of IP₃R1, but Mg²⁺ had no effect on the proteolysis. The electron microscopy and the limited proteolysis consistently demonstrated that the effective concentration of Ca²⁺ for conformational changes in IP₃R1 was <10^-7 M and that the IP₃ scarcely affected the conformational states. The structure of IP₃R1 without Ca²⁺, as reconstructed by three-dimensional electron microscopy, had a "mushroom-like" appearance consisting of a large square-shaped head and a small channel domain linked by four thin bridges. The projection image of the " head-to-head" assembly comprising two particles confirmed the mushroom-like side view. The "windmill-like" form of IP₃R1 with Ca²⁺ also contains the four bridges connecting from the Ip ₃-binding domain toward the channel domain. These data suggest that the Ca²⁺-specific conformational change structurally regulates the IP₃-triggered channel opening within Ip₃R1.