Structural insights into IP3R function

Abstract

Inositol 1,4,5-trisphosphate receptors (IP3Rs) are ubiquitously expressed intracellular ligand-gated Ca2+ channels present on the endoplasmic reticulum of virtually all eukaryotic cells. These channels mediate the Ca2+ release from intracellular stores in response to activation by the signaling molecule IP3, which functions to transmit diverse signals received by the cell, e.g. from hormones, neurotransmitters, growth factors and hypertrophic stimuli, to various signaling pathways within the cell. Thus, IP3R channels can be conceptualized as highly dynamic scaffold membrane protein complexes, where binding of ligands can change the scaffold structure leading to cellular Ca2+ signals that direct markedly different cellular actions. Although extensively characterized in physiological and biochemical studies, the detailed mechanisms of how IP3Rs produce highly controlled Ca2+ signals in response to diversified extra- and intracellular stimuli remains unknown and requires high-resolution knowledge of channel molecular architecture. Recently, single-particle electron cryomicroscopy (cryo-EM) has yielded a long-awaited near-atomic resolution structure of the entire full-length type 1 IP3R. This structure provides important insights into the molecular underpinnings of ligand-mediated activation and regulation of IP3R. In this chapter, we evaluate available information and research progress on the structure of IP3R channel in an attempt to shed light on its function.