Structure-function Relationships in the Inositol 1,4,5-Trisphosphate Receptor

The divalent Ca2+ metal ion acts as a universal second messenger in virtually all eukaryotic cells from fungi to plants to mammals. In mammals, Ca2+ signaling is vital to a variety of physiological processes including fertilization, cell proliferation, secretion, and muscular contraction. In electrochemically non-excitable tissues, the release of Ca2+ from intracellular stores such as the endoplasmic reticulum is tightly regulated by the inositol 1,4,5-trisphosphate receptor (IP3R). The IP3R Ca2+ release channel is activated by the binding of the small molecule inositol 1,4,5-trisphosphate (IP3) in response to extracellular stimuli such as hormones, growth factors, and neurotransmitters. The conformational changes accompanying IP3 binding were investigated using a biophysical approach. A specific focus of this work is to decipher how signals of ligand binding are transmitted from the N-terminal IP3-binding core to the C-terminal channel domain. To such end, biophysical studies of the ligand-induced conformational changes within the N-terminal domain of IP3R (a.a. 1 – 604) were performed. The results implicated the presence of two flexible linkers which join stably folded domains. This prompted the proposal of a model in which an equilibrium mixture of conformational substrates containing compact and more extended structures co-exist. Determinants within the N- and C-terminal regions of IP3R have previously been reported to be critical to channel function. Employing nuclear magnetic resonance (NMR) as well as biochemical methods, an intermolecular interaction between the S4-S5 linker, the cytoplasmic loop between the fourth and fifth transmembrane helices of IP3R, and the suppressor domain was identified. The determination of the crystal structure of the suppressor domain from isoform type 3 IP3R (IP3R3SUP) allowed us to map the residues involved in this interaction to one face of the molecule. The characterization of this interaction provides insight into the N- and C-terminal determinants essential to the IP3R channel gating mechanism.Ph