Structural Insights into the Assembly and Function of the IP3 Receptor Ligand-Binding Module

Abstract

Inositol 1,4,5-trisphosphate receptors (IP3Rs) are calcium-releasing channels that are involved in numerous physiological functions such as learning and memory, muscle contraction and cell proliferation. Dysfunction of IP3Rs and IP3R-interacting proteins has been implicated in several diseases including neurodegenerative diseases and arrhythmia. The activation of IP3Rs requires the binding of both IP3 and Ca2+. The IP3-binding module is composed of two sub-regions: the suppressor domain and the IP3-binding core. Previous studies showed that the suppressor domain reduces the affinity of the IP3-binding core for IP3 and that the isoform-specific IP3-binding affinity of IP3R is due to the different interactions between these two domains.We crystallized and solved the structure of the whole N-terminal ligand-binding module of IP3R isoform 3 by X-ray crystallography to 3.4 A. This structure, which represents the closed state of the channel, shows a clear conformational change in the IP3-binding core compared to previous open state structures. The suppressor domain interacts with IP3-binding core mainly through a hydrophilic interface. Comparing this interface with the one from IP3R isoform 1, we identified the structural element that is crucial for the isoform-specific IP3-binding affinity. Docking into the IP3R electron microscopy map places the module on the cytoplasmic part of the channel around the four-fold symmetry axis. Four ligand-binding modules form a cluster at the apex of the mushroom-shaped IP3R protein. The crystal structure and the localization of IP3R ligand-binding module resemble another type of intracellular Ca2+-release channel, ryanodine receptors. We propose a model showing that these two channels share a common allosteric gating mechanism.