Critical regions for activation gating of the inositol 1,4,5-trisphosphate receptor

To understand the molecular mechanism of ligand-induced gating of the inositol 1,4,5-trisphosphate (IP₃) receptor (IP₃R)/Ca²⁺ release channel, we analyzed the channel properties of deletion mutants retaining both the IP₃-binding and channel-forming domains of IP₃R1. Using intrinsically IP₃R-deficient cells as the host cells for receptor expression, we determined that six of the mutants, those lacking residues 1-223, 651-1130, 1267-2110, 1845-2042, 1845-2216, and 2610-2748, did not exhibit any measurable Ca²⁺ release activity, whereas the mutants lacking residues 1131-1379 and 2736-2749 retained the activity. Limited trypsin digestion showed that not only the IP₃-gated Ca²⁺-permeable mutants lacking residues 1131-1379 and 2736-2749, but also two nonfunctional mutants lacking residues 1-223 and 651-1130, retained the normal folding structure of at least the C-terminal channel-forming domain. These results indicate that two regions of IP₃R1, viz. residues 1-223 and 651-1130, are critical for IP₃-induced gating. We also identified a highly conserved cysteine residue at position 2613, which is located within the C-terminal tail, as being essential for channel opening. Based on these results, we propose a novel five-domain structure model in which both N-terminal and internal coupling domains transduce ligand-binding signals to the C-terminal tail, which acts as a gatekeeper that triggers opening of the activation gate of IP₃R1 following IP₃ binding.