IP₃ receptors - lessons from analyses ex cellula

Inositol 1,4,5-trisphosphate receptors (IP₃Rs) are widely expressed intracellular channels that release Ca²⁺ from the endoplasmic reticulum (ER). We review how studies of IP₃Rs removed from their intracellular environment ('ex cellula'), alongside similar analyses of ryanodine receptors, have contributed to understanding IP₃R behaviour. Analyses of permeabilized cells have demonstrated that the ER is the major intracellular Ca²⁺ store, and that IP₃ stimulates Ca²⁺ release from this store. Radioligand binding confirmed that the 4,5-phosphates of IP₃ are essential for activating IP₃Rs, and facilitated IP₃R purification and cloning, which paved the way for structural analyses. Reconstitution of IP₃Rs into lipid bilayers and patch-clamp recording from the nuclear envelope have established that IP₃Rs have a large conductance and select weakly between Ca²⁺ and other cations. Structural analyses are now revealing how IP₃ binding to the N-terminus of the tetrameric IP₃R opens the pore ∼7 nm away from the IP₃-binding core (IBC). Communication between the IBC and pore passes through a nexus of interleaved domains contributed by structures associated with the pore and cytosolic domains, which together contribute to a Ca²⁺-binding site. These structural analyses provide evidence to support the suggestion that IP₃ gates IP₃Rs by first stimulating Ca²⁺ binding, which leads to pore opening and Ca²⁺ release.