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The CRAC channel protein, Orai1 (CRACM1) (Prakriya et al. 2006), complexed with the STIM1 or STIM2 protein (Feske et al., 2006). Replacement of the conserved glutamate in the first TMS  with glutamine (E106Q) acts as a dominant-negative protein, and substitution with aspartate (E106D) enhances Na+, Ba2+, and Sr2+ permeation relative to Ca2+. Mutating E190Q in TMS3 also affects channel selectivity, suggesting that glutamate residues in both TMS1 and TMS3 face the lumen of the pore (Vig et al. 2006). The Orai1:Stim stoichiometry = 4:2 (Ji et al., 2008). Human Orai1 and Orai3 channels are dimeric in the closed resting state and open states. They are tetrameric when complexed with STIM1 (Demuro et al., 2011). A dimeric form catalyzes nonselective cation conductance in the STIM1-independent mode.  STIM1 domains have been characterized (How et al. 2013). Alternative translation initiation of the Orai1 message produces long and short types of Ca2+ channels with distinct signaling and regulatory properties (Desai et al. 2015).  STIM2 plays roles similar to STIM1 in regulating basal cytosolic and endoplasmic reticulum Ca2+ concentrations by controling Orai1, 2 and 3.  STIM2 may inhibit STIM1-mediated Ca2+ influx.  It also regulates protein kinase A-dependent phosphorylation and trafficking of AMPA receptors (TC# 1.A.10) (Garcia-Alvarez et al. 2015). A mechanistic model for ROS (H2O2)-mediated inhibition of Orai1 has been determined (Alansary et al. 2016).

Orai1/STIM1 complex of Homo sapiens
Orai1 (Q96D31)
STIM1 (Q13586)

The ARC (Arachidonate-regulated Ca2+-selective) channel, a complex of STIM1, Orai1 and Orai3 (Mignen et al., 2008). It is a heteropentameric assembly of three Orai1 subunits and two Orai3 subunits (Mignen et al., 2009). (But see Demuro et al., 2011; 1.A.52.1.1). Molecular determinants within the N-terminus control channel activation and gating (Bergsmann et al., 2011).  Specifically activated by high concentrations (>50 microM) of 2-aminoethyl diphenylborinate (2-APB) (Amcheslavsky et al. 2014).

Orai3 of Homo sapiens (Q9BRQ5)

The CRAC channel Orai2 (DUF 1650) (264 aas) (Gross et al., 2007).

Orai2 of Mus musculus (Q8BH10)

Insect STIM1/Orai1 (Hull et al., 2010). Influences sex pheromone production in moths. 

Stim1/Orai1A or B of Bombyx mori 
Stim1 (B5BRC2)
Orai1, splice form A (B5BRC5)
Orai1, splice form B (B5BRC4) 

Ca2+ release-activated Ca2+ (CRAC) channel subunit, Orai, which mediates Ca2+ influx following depletion of intracellular Ca2+ stores.  In Greek mythology, the 'Orai' are the keepers of the gates of heaven.  The crystal structure (3.35 Å), revealed a hexameric assembly of Orai subunits arranged around a central ion pore which traverses the membrane and extends into the cytosol. A ring of glutamate residues on its extracellular side forms the selectivity filter. A basic region near the intracellular side can bind anions that may stabilize the closed state. The architecture of the channel differs from those of other solved ion channels (Hou et al. 2012).

Orai (Olf186-F) of Drosophila melanogaster

Orai homologue (494aas; 4 or 5 TMSs)

Orai homologue in Ostreococcus tauri (Q012G5)

Orai homologue (244aas; 4 TMSs)

Orai homologue in Phytophthora infestans T30-4 (D0NKP9)