Fig 1. Molecular basis of STIM-induced Orai channel activation.

(A) STIM proteins in the ER membrane are activated by luminal Ca²⁺ depletion and dissociation of Ca²⁺ from their N-terminal Ca²⁺-sensing EF-hand/SAM domains. Unfolding of the cytoplasmic domain allows the lysine-rich (K-rich) flexible C terminus attach to the PM to trap STIM proteins in ER-PM junctions. Exposed SOAR tether and activate Orai channels, mediating Ca²⁺ entry signals crucial in controlling gene expression, growth, secretory, and motile responses. (B) Crystal-derived structure of the dOrai channel comprising a hexameric arrangement of Orai channel subunits, each with 4 TM helices. One dOrai subunit dimer is highlighted in color. The TM1 central pore includes the outer Ca²⁺ selectivity filter, E178 (E106). The TM2 and TM3 helices assemble closely around the pore. The peripheral TM4 helix includes an outer TM4 extension (TM4_e) that may pair asymmetrically with an adjacent subunit TM4_e, each pair comprising an antiparallel association between TM4_e helices in a “bent” and “straight” configuration. dOrai mutations at 2 critical residues, H206 and P288, give rise to constitutively active channels. Residue numbers are for dOrai (hOrai1 in parentheses). (C, left) Model of the gating interaction between the active dimeric SOAR unit in STIM and the hexameric Orai channel (only 2 subunits shown) in which each SOAR unit interacts across the N-terminal TM1 and C-terminal TM4_e of Orai to directly gate the pore. (C, right) Alternative model in which SOAR binds only to the TM4_e helix and remotely induces pore gating through an allosteric conformational change propagated across the TM helices; each SOAR dimer can activate and cross-link 2 Orai subunits in separate channels. (D) The TM1 pore-lining residues in dOrai (hOrai1) that form the selectivity filter and hydrophobic and basic regions of the pore. dOrai, Drosophila Orai; ER, endoplasmic reticulum; hOrai, human Orai; PM, plasma membrane; SAM, sterile-alpha motif; SOAR, STIM-Orai Activating Region; STIM, Stromal Interaction Molecule; TM, transmembrane; TM4_e, TM4 extension.