Pore architecture of TRIC channels and insights into their gating mechanism

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Abstract

Intracellular Ca2+ signalling processes are fundamental to muscle contraction, neurotransmitter release, cell growth and apoptosis1,2. Release of Ca2+ from the intracellular stores is supported by a series of ion channels in sarcoplasmic or endoplasmic reticulum (SR/ER)3,4. Among them, two isoforms of the trimeric intracellular cation (TRIC) channel family, named TRIC-A and TRIC-B, modulate the release of Ca2+ through the ryanodine receptor or inositol triphosphate receptor, and maintain the homeostasis of ions within SR/ER lumen5,6. Genetic ablations or mutations of TRIC channels are associated with hypertension, heart disease, respiratory defects and brittle bone disease7,8,9,10,11,12. Despite the pivotal function of TRIC channels in Ca2+ signalling5,13,14, their pore architectures and gating mechanisms remain unknown. Here we present the structures of TRIC-B1 and TRIC-B2 channels fromCaenorhabditis elegansin complex with endogenous phosphatidylinositol-4,5-biphosphate (PtdIns(4,5)P2, also known as PIP2) lipid molecules. The TRIC-B1/B2 proteins and PIP2 assemble into a symmetrical homotrimeric complex. Each monomer contains an hourglass-shaped hydrophilic pore contained within a seven-transmembrane-helix domain. Structural and functional analyses unravel the central role of PIP2 in stabilizing the cytoplasmic gate of the ion permeation pathway and reveal a marked Ca2+-induced conformational change in a cytoplasmic loop above the gate. A mechanistic model has been proposed to account for the complex gating mechanism of TRIC channels.

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