W0447

Three-dimensional Rearrangement for the Gating of an Inositol-1,4,5-trisphosphate Receptor. Qiu-Xing Jiang¹, Chi-un Choe², Edwin C. Thrower², David W. Chester¹, Barbara E. Ehrlich^1,2, and Fred J. Sigworth¹, Yale Univ. School of Medicine, Depts. of Cellular and Molecular Physiology¹, and Pharmacology³, 333 Cedar Street, New Haven, CT 06520.

The Inositol-1,4,5-trisphosphate receptor (IP3R) is a ligand-gated calcium release channel. Its ligand-binding triggers a local conformational change in the cytoplasmic domain, which propagates to the transmembrane domain, and leads to the transition of the channel pore to its open conformation. The structural underpinnings of such ligand-gating have eluded various studies for a long time. Here we provide structural evidence for the allosteric conformational rearrangement underlying the gating transitions of Type 1 IP3Rs. By using cryo-electron microscopy and single particle reconstruction, we obtained a structure of the open-state IP3R complexed with chromogranin B and the highly potent agonist adenophostin A. When reconstituted into planar lipid bilayers, this complex shows a channel open probability greater than 90%. Comparison of this structure with the closed conformation revealed a global change of the receptor. The four laterally-protruding arms relaxed into the main body. The whole cytoplasmic domain huddles into a more compact shape with central openings near the cytosolic surface of the membrane, which connects to the central pore in the transmembrane domain, and delineates an ion flow pathway. Binding of chromogranin B from the luminal side, whose position was confirmed with the localization of succinylated Concanavalin A molecules bound to the receptor structure, helps stabilize the open conformation. These findings provide a mechanistic understanding of the coupling between the ligand-binding and channel opening of IP3Rs.