Oligomerization of STIM1 couples ER calcium depletion to CRAC channel activation

Abstract

STIM1 is the messenger that signals endoplasmic reticulum Ca2+ store depletion and subsequently activates the store-operated channel ORAI. However, its precise mechanism of action is unclear. Here, Luik et al. show that STIM oligomerzation, not the concentration of Ca2+ in the endoplasmic reticulum, is the key event that triggers ORAI activation. 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.