Abstract
The stromal interaction molecule 1 (STIM1) is an endoplasmic reticulum (ER) Ca2+ sensor that regulates the activity of Orai plasma membrane Ca2+ channels to mediate the store-operated Ca2+ entry pathway essential for immunity. Uncoordinated 93 homolog B1 (UNC93B1) is a multiple membrane-spanning ER protein that acts as a trafficking chaperone by guiding nucleic-acid sensing toll-like receptors to their respective endosomal signaling compartments. We previously showed that UNC93B1 interacts with STIM1 to promote antigen cross-presentation in dendritic cells, but the STIM1 binding site(s) and activation step(s) impacted by this interaction remained unknown. In this study, we show that UNC93B1 interacts with STIM1 in the ER lumen by binding to residues in close proximity to the transmembrane domain. Cysteine crosslinking in vivo showed that UNC93B1 binding promotes the zipping of transmembrane and proximal cytosolic helices within resting STIM1 dimers, priming STIM1 for translocation. In addition, we show that UNC93B1 deficiency reduces store-operated Ca2+ entry and STIM1–Orai1 interactions and targets STIM1 to lighter ER domains, whereas UNC93B1 expression accelerates the recruitment of STIM1 to cortical ER domains. We conclude that UNC93B1 therefore acts as a trafficking chaperone by maintaining the pool of resting STIM1 proteins in a state primed for activation, enabling their rapid translocation in an extended conformation to cortical ER signaling compartments.
Highlights
The stromal interaction molecules (STIM) are endoplasmic reticulum (ER) localized, singlepass transmembrane proteins [1] that mediate the ubiquitous store-operated Ca2+ entry (SOCE) signaling pathway
Unc-93 homologue B1 (UNC93B1) is a protein chaperone for toll-like receptors (TLRs) that interacts with stromal interaction molecule 1 (STIM1) to positively regulate SOCE in dendritic cells [26]
Like STIM1, UNC93B1 is ubiquitously expressed according to the Human Protein Atlas [38], suggesting that its chaperone function might extend beyond immune cells
Summary
The stromal interaction molecules (STIM) are endoplasmic reticulum (ER) localized, singlepass transmembrane proteins [1] that mediate the ubiquitous store-operated Ca2+ entry (SOCE) signaling pathway. The elongation of STIM1 oligomers exposes a polybasic C-terminal tail that binds to negatively charged lipids, allowing STIM1 to accumulate in cortical ER domains tethered to the PM [5,8,9,10,11]. At these ER-PM junctions STIM1 can trap and gate Orai Ca2+ channels, eliciting an influx of Ca2+ from the extracellular milieu into the cytoplasm. The influx of Ca2+ sustains Ca2+-dependent signaling events and provides a source of Ca2+ for the replenishment of ER Ca2+ stores via the sarco/endoplasmic reticulum Ca2+-ATPase (SERCA) pump [12,13,14,15,16,17]
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