Abstract
Store-operated Ca(2+) entry fulfills a crucial role in controlling Ca(2+) signals in almost all cells. The Ca(2+)-sensing stromal interaction molecule (STIM) proteins in the endoplasmic reticulum (ER) undergo complex conformational changes in response to depleted ER luminal Ca(2+), allowing them to unfold and become trapped in ER-plasma membrane (PM) junctions. Dimers of STIM proteins trap and gate the plasma membrane Orai Ca(2+) channels within these junctions to generate discrete zones of high Ca(2+) and regulate sensitive Ca(2+)-dependent intracellular signaling pathways. The STIM-Orai activating region (SOAR) of STIM1 becomes exposed upon store depletion and promotes trapping of Orai1 at the PM. Residue Phe-394 within SOAR forms an integral part of the high-affinity Orai1-interacting site. Our results demonstrate that only a single active site within the dimeric SOAR domain of STIM1 is required for the activation of Orai1 channel activity. This unimolecular model is strongly supported by evidence of variable STIM1:Orai1 stoichiometry reported in many studies. We hypothesize that unimolecular coupling promotes cross-linking of channels, localizing Ca(2+) signals, and regulating channel activity. We have also identified a key "nexus" region in Orai1 near the C-terminal STIM1-binding site that can be mutated to constitutively activate Ca(2+) entry, mimicking STIM1 activated channels. This suggests that STIM1 mediates gating of Orai1 in an allosteric manner via interaction with the Orai1 C-terminus alone. This model suggests the dual role of STIM1 in regulating both localization and gating of Orai1 channels and has important implications for the regulation of SOCE-mediated downstream signaling and the kinetics of channel activation.