Molecular Biophysics of Orai Store-Operated Ca2+ Channels

Abstract

Upon endoplasmic reticulum Ca 2þ store depletion, Orai channels in the plasma membrane are activated directly by endoplasmic reticulum-resident STIM proteins to generate the Ca 2þ -selective, Ca 2þ release-activated Ca 2þ (CRAC) current. After the molecular identification of Orai, a plethora of functional and biochemical studies sought to compare Orai homologs, determine their stoichiometry, identify structural domains responsible for the biophysical fingerprint of the CRAC current, identify the physiological functions, and investigate Orai homologs as potential therapeutic targets. Subsequently, the solved crystal structure of Drosophila Orai (dOrai) substantiated many findings from structure-function studies, but also revealed an unexpected hexameric structure. In this review, we explore Orai channels as elucidated by functional and biochemical studies, analyze the dOrai crystal structure and its implications for Orai channel function, and present newly available information from molecular dynamics simulations that shed light on Orai channel gating and permeation.

The Orai1 Store-operated Calcium Channel Functions as a Hexamer

“…In particular, the multimeric assembly of the Orai1 channel, the most commonly expressed of the three-member mammalian Orai channel family, has remained a contentious issue (1,2,5,6,22,23). Recent crystallographic evidence reveals that Drosophila Orai has a hexameric subunit structure, a result reinforced by cross-linking and chromatographic evidence (22).…”

“…Orai channels are highly Ca 2ϩ -selective plasma membrane (PM) 4 channels activated through an elaborate intermembrane coupling mechanism by the Ca 2ϩ -sensing STIM proteins of the endoplasmic reticulum (ER) (1)(2)(3)(4)(5)(6). Alterations in the function of Orai channels and STIM proteins are implicated in a large number of immunological, muscular, and inflammatory disease states (1,(7)(8)(9).…”

“…The members of the Orai family of ion channels are ubiquitously expressed among cell types and mediate "store-operated" Ca 2ϩ entry signals, crucial in the control of many responses including gene expression, cell growth, secretory events, and cell motility (1)(2)(3). Orai channels are highly Ca 2ϩ -selective plasma membrane (PM) 4 channels activated through an elaborate intermembrane coupling mechanism by the Ca 2ϩ -sensing STIM proteins of the endoplasmic reticulum (ER) (1)(2)(3)(4)(5)(6).…”

Purinergic and Store-Operated Ca2+ Signaling Mechanisms in Mesenchymal Stem Cells and Their Roles in ATP-Induced Stimulation of Cell Migration

“…ATP can also elevate the [Ca 21 ] c via G a,q/11 -protein-coupled P2Y receptors, and more specifically, activation of the P2Y 1 , P2Y 2 , and P2Y 11 receptors in human cells stimulates phospholipase C-b (PLC-b) to generate inositol-1,4,5-triphosphate (IP 3 ), which in turn activates the IP 3 receptor and induces Ca 21 release from the endoplasmic reticulum (ER). Reduction in the ER Ca 21 level can further induce store-operated Ca 21 entry through the Ca 21 release-activated Ca 21 (CRAC) a channels [7,8]. Mesenchymal stem cells (MSCs) exhibit an ability to differentiate into osteoblasts, adipocytes, and chondrocytes [9][10][11].…”

“…3F, 3I), providing strong evidence to support that store-operated Ca 21 entry as a downstream mechanism contributes to ATP-induced Ca 21 signaling. Recent studies have established that the store-operated Ca 21 entry in a variety of non-excitable cells is primarily mediated by the CRAC channel composed of plasma membrane pore-forming Orai1 and ER-localized Ca 21 sensor stromal interaction molecule 1 (Stim1) [7,8]. There is evidence to suggest a role for the Stim1 homolog, Stim2, in the regulation of the ER Ca 21 level [40].…”