Abstract
Ca2+-release-activated Ca2+ (CRAC) channels generate sustained Ca2+ signals that are essential for a range of cell functions, including antigen-stimulated T lymphocyte activation and proliferation1,2. Recent studies3 have revealed that the depletion of Ca2+ from the endoplasmic reticulum (ER) triggers the oligomerization of stromal interaction molecule 1 (STIM1), the ER Ca2+ sensor, and its redistribution to ER–plasma membrane (ER–PM) junctions4,5,6,7,8 where the CRAC channel subunit ORAI1 accumulates in the plasma membrane and CRAC channels open9,10,11,12. However, how the loss of ER Ca2+ sets into motion these coordinated molecular rearrangements remains unclear. Here we define the relationships among [Ca2+]ER, STIM1 redistribution and CRAC channel activation and identify STIM1 oligomerization as the critical [Ca2+]ER-dependent event that drives store-operated Ca2+ entry. In human Jurkat leukaemic T cells expressing an ER-targeted Ca2+ indicator, CRAC channel activation and STIM1 redistribution follow the same function of [Ca2+]ER, reaching half-maximum at ∼200 µM with a Hill coefficient of ∼4. Because STIM1 binds only a single Ca2+ ion5, the high apparent cooperativity suggests that STIM1 must first oligomerize to enable its accumulation at ER–PM junctions. To assess directly the causal role of STIM1 oligomerization in store-operated Ca2+ entry, we replaced the luminal Ca2+-sensing domain of STIM1 with the 12-kDa FK506- and rapamycin-binding protein (FKBP12, also known as FKBP1A) or the FKBP-rapamycin binding (FRB) domain of the mammalian target of rapamycin (mTOR, also known as FRAP1). A rapamycin analogue oligomerizes the fusion proteins and causes them to accumulate at ER–PM junctions and activate CRAC channels without depleting Ca2+ from the ER. Thus, STIM1 oligomerization is the critical transduction event through which Ca2+ store depletion controls store-operated Ca2+ entry, acting as a switch that triggers the self-organization and activation of STIM1–ORAI1 clusters at ER–PM junctions.
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Acknowledgements
We thank N. Bhakta and D. Bautista for assistance and advice during the initial phase of these studies, R. Tsien for the gift of cameleon YC4er, P. Bacchawat for advice on BN–PAGE, and R. Dolmetsch for comments on the manuscript. This work was supported by a grant from the National Institutes of Health (NIH) and the Mathers Charitable Foundation.
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Luik, R., Wang, B., Prakriya, M. et al. Oligomerization of STIM1 couples ER calcium depletion to CRAC channel activation. Nature 454, 538–542 (2008). https://doi.org/10.1038/nature07065
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DOI: https://doi.org/10.1038/nature07065
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