Structural and functional conservation of key domains in InsP(3) and ryanodine receptors

Inositol 1,4,5-trisphosphate receptors (InsP(3)R) and ryanodine receptors (RyR) are tetrameric intracellular Ca(2+) channels(1). For each, the pore is formed by C-terminal transmembrane domains and regulated by signals detected by the large cytosolic structures. InsP(3)R gating is initiated by InsP(3) binding to the InsP(3)-binding core (IBC, residues 224-604 of InsP(3)R1)(2) and it requires the suppressor domain (SD, residues 1-223)(2-8). We present structures of the N-terminal region (NT) of InsP(3)R1 with (3.6 Å) and without (3.0 Å) InsP(3) bound. The arrangement of the three NT domains, the 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(9). The orientations of the three domains docked into a tetrameric structure of InsP(3)R(10) and of the ABC domains in RyR(9) are remarkably similar. The importance of the α-interface for activation of InsP(3)R and RyR is confirmed by mutagenesis and, for RyR, by disease-causing mutations(9,11,12). 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 (HS-loop) that is essential for InsP(3)R activation(7). The loop is conserved in RyR and includes mutations associated with malignant hyperthermia and central core disease(9,11,12). 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 a full-length InsP(3)R, and an InsP(3)R 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(3)R and RyR. Allosteric modulation of two similar domain interfaces within an N-terminal subunit re-orients the first domain (SD or A-domain), allowing it, via interactions of the second domain of an adjacent subunit (IBC-β or B-domain), to gate the pore.