Inter-domain Associations and Gating of the Inositol 1,4,5-trisphosphate Receptor
IP3 receptors (IP3Rs) are key players involved in the release of the crucial second messenger Ca2+ into the cytosol of a cell. Ca2+ is a diverse signaling molecule used by nearly every mammalian cell type and participates in a variety of physiological processes. The activation of IP3R requires both IP 3 and Ca2+. Upon activation Ca2+ is released into the cytosol from the lumen of the ER to induce many different signaling pathways. The gating mechanism of the IP3R involves the highly sensitive detection of IP3 and Ca2+ and subsequent conformational changes which lead to channel opening. The work presented in this thesis examined the mechanism of how the IP3R is activated to release Ca2+ by IP3. We discovered that the S4-S5 linker of the C-terminal channel-forming domain specifically associates with the suppressor domain of the N-terminal ligand-binding domain of the IP 3R. We propose that the interaction between the N- and C-termini (C-N interaction) plays a significant role in the gating mechanism of the IP 3R. In our gating model the binding of IP3 at the N-terminus gates the opening of the channel pore by inducing conformational changes in the suppressor domain which cause a mechanical pulling on the S4-S5 linker of the channel domain. The displacement of the S4-S5 linker permits the S6 helix bundle to relax and the permeation pathway for Ca2+ to be opened. We studied the functional effects of mutating specific residues in the putative pore helix, selectivity filter and S6 helix of the IP 3R. Our results suggested that the IP3R may contain structures homologous to the pore-helix, selectivity filter, hinge and activation gate noted in some K+ channels. However, we could not conclusively identify a hinge or gating residue along the S6 helix. We constructed a homology model of the IP3R pore region which will be used as a template for future studies on the key structural components of the IP3R pore. Altogether, these studies have led to the formulation of a mechanistic model for IP3R gating which identifies how IP3 binding leads to channel opening.
Schug, Zachary Tiberius, "Inter-domain Associations and Gating of the Inositol 1,4,5-trisphosphate Receptor" (2007). ETD Collection for Thomas Jefferson University. AAI3472218.