IP 3 receptors – Lessons from analyses ex cellula

Abstract

Inositol 1,4,5-trisphosphate receptors (IP 3 Rs) are widely expressed intracellular channels that release Ca 2+ from the endoplasmic reticulum (ER). We review how studies of IP 3 Rs removed from their intracellular environment (‘ex cellula’), alongside similar analyses of ryanodine receptors, have contributed to understanding IP 3 R behaviour. Analyses of permeabilized cells have demonstrated that the ER is the major intracellular Ca 2+ store, and that IP 3 stimulates Ca 2+ release from this store. Radioligand binding confirmed that the 4,5-phosphates of IP 3 are essential for activating IP 3 Rs, and facilitated IP 3 R purification and cloning, which paved the way for structural analyses. Reconstitution of IP 3 Rs into lipid bilayers and patch-clamp recording from the nuclear envelope have established that IP 3 Rs have a large conductance and select weakly between Ca 2+ and other cations. Structural analyses are now revealing how IP 3 binding to the N-terminus of the tetrameric IP 3 R opens the pore ∼7 nm away from the IP 3 -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 2+ -binding site. These structural analyses provide evidence to support the suggestion that IP 3 gates IP 3 Rs by first stimulating Ca 2+ binding, which leads to pore opening and Ca 2+ release.