IP receptors - lessons from analyses .

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