Inositol-1,4,5-trisphosphate receptors (InsP 3Rs) and ryanodine receptors (RyRs) are tetrameric intracellular Ca 2+ channels. In each of these receptor families, the pore, which is formed by carboxy-terminal transmembrane domains, is regulated by signals that are detected by large cytosolic structures. InsP 3R gating is initiated by InsP 3binding to the InsP 3-binding core (IBC, residues 224-604 of InsP 3R1) and it requires the suppressor domain (SD, residues 1-223 of InsP 3R1). Here we present structures of the amino-terminal region (NT, residues 1-604) of rat InsP 3R1 with (3.6Å) and without (3.0Å) InsP 3 bound. The arrangement of the three NT domains, SD, IBC-β and IBC-α, identifies two discrete interfaces (α and β) between the IBC and SD. Similar interfaces occur between equivalent domains (A, B and C) in RyR1 (ref. 9). The orientations of the three domains when docked into a tetrameric structure of InsP 3R and of the ABC domains docked into RyR are remarkably similar. The importance of the α-interface for activation of InsP 3R and RyR is confirmed by mutagenesis and, for RyR, by disease-causing mutations. Binding of InsP 3 causes partial closure of the clam-like IBC, disrupting the β-interface and pulling the SD towards the IBC. This reorients an exposed SD loop ('hotspot' (HS) loop) that is essential for InsP 3R activation. The loop is conserved in RyR and includes mutations that are associated with malignant hyperthermia and central core disease. The HS loop interacts with an adjacent NT, suggesting that activation re-arranges inter-subunit interactions. The A domain of RyR functionally replaced the SD in full-length InsP 3R, and an InsP 3R in which its C-terminal transmembrane region was replaced by that from RyR1 was gated by InsP 3 and blocked by ryanodine. Activation mechanisms are conserved between InsP 3R and RyR. Allosteric modulation of two similar domain interfaces within an N-terminal subunit reorients the first domain (SD or A domain), allowing it, through interactions of the second domain of an adjacent subunit (IBC-β or B domain), to gate the pore. © 2012 Macmillan Publishers Limited. All rights reserved.
