The STIM-Orai Pathway: Conformational Coupling Between STIM and Orai in the Activation of Store-Operated Ca2+ Entry

Abstract

Store-operated Ca2+ entry fulfills a crucial role in controlling Ca2+ signals in almost all cells. The Ca2+-sensing stromal interaction molecule (STIM) proteins in the endoplasmic reticulum (ER) undergo complex conformational changes in response to depleted ER luminal Ca2+, allowing them to unfold and become trapped in ER-plasma membrane (PM) junctions. Dimers of STIM proteins trap and gate the plasma membrane Orai Ca2+ channels within these junctions to generate discrete zones of high Ca2+ and regulate sensitive Ca2+-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 Ca2+ 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 Ca2+ 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.

Host Calcium Channels and Pumps in Viral Infections

“…The protein Orai1 on the plasma membrane and STIM1 (stromal interaction molecule) on ER are the molecular identities that are responsible for SOC channels activation. The depletion of ER Ca 2+ stores promotes STIM1 proteins aggregation and interaction with Orai1 to open the channel, mediating Ca 2+ entry [29,30].…”

Ca2+ and lipid signals hold hands at endoplasmic reticulum–plasma membrane contact sites

“…These include the identification of the part of the STIM1 molecule that interacts with the Orai1 channel (called SOAR, Yuan et al 2009, or CAD, Park et al 2009) and the characterization of an intramolecular interaction between the SOAR domain and the first membrane-adjacent coiled-coil domain of the cytoplasmic part of STIM1 that keeps the SOAR domain from activating Orai1 in the resting state (Muik et al 2011;Zhou et al 2013;Fahrner et al 2014;Ma et al 2015). Similar structural and mutational studies reveal the key interactive motifs that form the basis of interaction between SOAR and Orai1 and the mechanism of Orai1 channel activation (Derler et al 2013;Gudlur et al 2014;Zhou et al 2016;Nwokonko et al 2017;Palty et al 2017).…”

Alternative splicing switches STIM1 targeting to specialized membrane contact sites and modifies SOCE

“…Hippocampi were dispersed using a cell strainer in DMEM (Invitrogen). After centrifugation for 10 min at 1900 rpm at RT cells were resuspended and plated in collagen (0,5 mg/mL; Bioscience) coated culture flasks and cultured at 37 • C in culture medium (DMEM + 10 % FCS + 0,1 % P/S + 0,1 % Mito Serum Extender) at 8 % CO 2 until cultures became confluent (5)(6)(7). Cells were subsequently trypsinized, plated on collagen-coated coverslips and grown until confluent (5)(6)(7).…”

“…Especially relatively slow and long-lasting processes such as gene expression and cell proliferation cells rely on SOCE 1,2 . SOCE is triggered by receptor mediated release of Ca 2+ from the endoplasmic reticulum (ER), which is sensed by STIM1 and STIM2 proteins through their respective luminal EF hands which then cluster and trap ORAI channels located in the plasma membrane (PM) into ER-PM junctions (reviewed in [3][4][5][6] ). In patients and murine gene-modified models, loss-of-function mutations (LOF) of either STIM1 or ORAI1 lead to severe immune pathologies, myopathies, defects in enamel formation and anhidrosis (reviewed in [7][8][9] ).…”