Long-range gating regulation by leaflet and autoinhibitory domains in mouse type 1 IP₃ receptors

The inositol 1,4,5-trisphosphate receptor (IP₃R) is a calcium channel that mediates Ca²⁺ release from the endoplasmic reticulum in response to IP₃. Structural studies have revealed that the IP₃-binding sites are located approximately 90 Å from the Ca²⁺-conducting pore within the transmembrane domain, suggesting a long-range force transmission mechanism between ligand binding and channel gating. However, the molecular basis of this mechanism remains poorly understood. We hypothesized that a unique leaflet domain mediates this force transmission from the cytosolic region to the channel pore. Supporting this, site-directed mutagenesis of three conserved residues— isoleucine, glutamate, and isoleucine (IEI)—within the leaflet domain to glycine abolished channel function. Moreover, deletion of a 31-amino acid segment at the C-terminus significantly enhanced IP₃-induced Ca²⁺ release, indicating that the C-terminal domain acts as an autoinhibitory domain (AID) rather than participating directly in gating. These findings suggest that the conserved IEI motif in the leaflet domain is critical for conveying IP₃-induced conformational changes to channel opening, whereas the C-terminal AID modulates gating allosterically from a distance. Together, these opposing regulatory elements may act in concert to fine-tune channel gating and maintain cellular homeostasis in health and disease.