SUMMARY The stromal interaction molecule, STIM1 regulates Ca2+ entry via Orai1 channels in response to decreased concentration of ER luminal Ca2+. In search of a mechanism that switches the cytoplasmic aspect of STIM1 from an inactive to an active state, we identified an acidic motif within the STIM1 coiled-coil region that keeps the Ca2+ activation domain (CAD/SOAR) inactive. The STIM1 acidic motif shows significant homology to the C-terminal coiled-coil segment of Orai1, the postulated site of interaction with STIM1. Mutations within the acidic region make STIM1 constitutively active while those within a short basic segment of CAD/SOAR prevent Orai1 activation. We propose that during STIM1 activation, the CAD/SOAR domain is released from an intramolecular clamp allowing the basic segment to activate Orai1 channels. This evolutionary conserved activation mechanism of STIM1 resembles the regulation of protein kinases by intramolecular silencing with pseudosubstrate binding.

STIM1 and Orai1 are the main players in capacitative calcium entry (CCE). STIM1 senses [Ca(2+)] inside the endoplasmic reticulum (ER) and, when it decreases, opens Orai1, a store-operated calcium channel (SOC) in the plasma membrane that promotes Ca(2+) entry and increases cytosolic Ca(2+). The final destination of the entering Ca(2+) is the ER, which refills very efficiently (capacitatively) with it. We propose here that SERCA is the third element of CCE, to which is tightly coupled to favour rapid Ca(2+) pumping from the high Ca(2+) microdomains, generated at the SOC's mouth, to the ER. We find that, on depletion of the intracellular Ca(2+) stores, SERCA co-localizes with STIM1 at puncta. Adequate coupling of CCE and ER Ca(2+) pumping requires correct proportions of STIM1, Orai1 and SERCA. Overexpression of Orai1 decreased modestly Ca(2+) entry, but produced a dramatic fall of Ca(2+) uptake into ER, which was rescued by STIM1 co-expression or by increasing external Ca(2+). In permeabilized cells, Ca(2+) uptake into the ER was indistinguishable in the Orai1-expressing and in the control cells. We propose that excess Orai1 uncouples SERCA from Ca(2+) entry in the intact cell by disturbing the fine topology of Ca(2+) pumping complexes within the ER-plasma membrane junctions.

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