The discovery and structural investigation of the IP₃ receptor and the associated IRBIT protein

The IP₃ receptor (IP₃R) is a Ca(2+) channel that releases Ca(2+) from the endoplasmic reticulum (ER) and plays a variety of roles in cell functions. This receptor was discovered as a developmentally regulated glyco-phosphoprotein, known as P400, which was absent in cerebellar mutant mice. The IP₃R has three different isoforms in vertebrates, and each IP₃R is composed of different subdomains. The affinities of the IP₃-binding core of the three isoforms of the IP₃R for IP₃ are similar. The N-terminal IP₃-binding suppressor region of each isoform is responsible for its isoform-specific IP₃-binding affinity. IP₃ binding to the IP₃-binding core leads to a conformational change, resulting in direct interactions of tyrosine-168 (in IP₃R1)/tryptophane-168 (in IP₃R2 and 3) in the N-terminal suppressor region with the loop region of transmembrane 4-5. The suppressor region and C-terminal -portion which associate with nearly 20 signaling molecules are located at the areas near the channel pore. The area including suppressor region and C-terminal portion are regarded as hot spots for the regulating opening and closing of the channel pore. A pseudo-ligand of the IP₃R, known as IRBIT (IP₃R binding protein released with inositol 1,4,5-trisphosphate), that interacts with the IP₃-binding core domain of the IP₃R was discovered. IRBIT not only regulates Ca(2+) release by binding to the IP₃-binding core domain but also regulates the acid-base balance by binding to various ion transporters, such as pancreas-type NBC1 (pNBC1) and CFTR. Most of the associated proteins bind to these areas and regulate IP₃R channel gating. Cryo-electron microscopy shows a balloon-like structure, which has vacancy inside the IP₃R with multi-porous surface area. The unique 3-dimensional structure of the IP₃R is convenient for associating with many IP₃R-associated proteins. Therefore, the IP₃R serves as a signaling hub, which forms macromolecular complex with various molecules.