Structural and functional conservation of key domains in Ins[P.sub.3] and ryanodine receptors

Inositol-1,4,5-trisphosphate receptors (Ins[P.sub.3]Rs) and ryanodine receptors (RyRs) are tetrameric intracellular [Ca.sup.2+] channels (1). 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. Ins[P.sub.3]R gating is initiated by Ins[P.sub.3] binding to the Ins[P.sub.3]-binding core (IBC, residues 224-604 of Ins[P.sub.3]R1) (2) and it requires the suppressor domain (SD, residues 1-223 of Ins[P.sub.3]R1) (2-8). Here we present structures of the amino-terminal region (NT, residues 1-604) of rat Ins[P.sub.3]R1 with (3.6 Å) and without (3.0 [angstrom])Ins[P.sub.3] bound. The arrangement of the three NT domains, SD, IBC-β and IBC-α, identifies two discrete interfaces (a and b) 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 Ins[P.sub.3]R (10) and of the ABC domains docked into RyR (9) are remarkably similar. The importance of the α-interface for activation of Ins[P.sub.3]R and RyR is confirmed by mutagenesis and, for RyR, by disease-causing mutations (9,11,12). Binding of Ins[P.sub.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 Ins[P.sub.3]R activation (7). The loop is conserved in RyR and includes mutations that are 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 full-length Ins[P.sub.3]R, and an Ins[P.sub.3]R in which its C-terminal transmembrane region was replaced by that from RyR1 was gated by Ins[P.sub.3] and blocked by ryanodine. Activation mechanisms are conserved between Ins[P.sub.3]R 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.
The essential role of the SD in linking Ins[P.sub.3] binding to Ins[P.sub.3]R- gating highlights the need to define the structural consequences of Ins[P.sub.3] binding to the N-terminal region (NT, residues [...]