The sarco/endoplasmic reticulum Ca2+ ATPase (SERCA) is the third element in capacitative calcium entry

STIM1 and Orai1 are the main players in capacitative calcium entry (CCE). STIM1 senses [Ca2+] inside the endoplasmic reticulum (ER) and, when it decreases, opens Orai1, a store-operated calcium channel (SOC) in the plasma membrane that promotes Ca2+ entry and increases cytosolic Ca2+. The final destination of the entering Ca2+ 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 Ca2+ pumping from the high Ca2+ microdomains, generated at the SOC's mouth, to the ER. We find that, on depletion of the intracellular Ca2+ stores, SERCA co-localizes with STIM1 at puncta. Adequate coupling of CCE and ER Ca2+ pumping requires correct proportions of STIM1, Orai1 and SERCA. Overexpression of Orai1 decreased modestly Ca2+ entry, but produced a dramatic fall of Ca2+ uptake into ER, which was rescued by STIM1 co-expression or by increasing external Ca2+. In permeabilized cells, Ca2+ uptake into the ER was indistinguishable in the Orai1-expressing and in the control cells. We propose that excess Orai1 uncouples SERCA from Ca2+ entry in the intact cell by disturbing the fine topology of Ca2+ pumping complexes within the ER-plasma membrane junctions.