Two-state conformational changes in inositol 1,4,5-trisphosphate receptor regulated by calcium

Inositol 1,4,5-trisphosphate receptor (IP₃R) is a highly controlled calcium (Ca²⁺) channel gated by inositol 1,4,5-trisphosphate (IP₃). Multiple regulators modulate IP₃-triggered pore opening by binding to discrete allosteric sites within IP₃R. Accordingly we have postulated that these regulators structurally control ligand gating behavior; however, no structural evidence has been available. Here we show that Ca²⁺, the most pivotal regulator, induced marked structural changes in the tetrameric IP₃R purified from mouse cerebella. Electron microscopy of the IP₃R particles revealed two distinct structures with 4-fold symmetry: a windmill structure and a square structure. Ca²⁺ reversibly promoted a transition from the square to the windmill with relocations of four peripheral IP₃-binding domains, assigned by binding to heparin-gold. Ca²⁺-dependent susceptibilities to limited digestion strongly support the notion that these alterations exist. Thus, Ca²⁺ appeared to regulate IP₃ gating activity through the rearrangement of functional domains.