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  • Title: Oligomerization of STIM1 couples ER calcium depletion to CRAC channel activation.
    Author: Luik RM, Wang B, Prakriya M, Wu MM, Lewis RS.
    Journal: Nature; 2008 Jul 24; 454(7203):538-42. PubMed ID: 18596693.
    Abstract:
    Ca(2+)-release-activated Ca(2+) (CRAC) channels generate sustained Ca(2+) signals that are essential for a range of cell functions, including antigen-stimulated T lymphocyte activation and proliferation. Recent studies have revealed that the depletion of Ca(2+) from the endoplasmic reticulum (ER) triggers the oligomerization of stromal interaction molecule 1 (STIM1), the ER Ca(2+) sensor, and its redistribution to ER-plasma membrane (ER-PM) junctions where the CRAC channel subunit ORAI1 accumulates in the plasma membrane and CRAC channels open. However, how the loss of ER Ca(2+) sets into motion these coordinated molecular rearrangements remains unclear. Here we define the relationships among [Ca(2+)](ER), STIM1 redistribution and CRAC channel activation and identify STIM1 oligomerization as the critical [Ca(2+)](ER)-dependent event that drives store-operated Ca(2+) entry. In human Jurkat leukaemic T cells expressing an ER-targeted Ca(2+) indicator, CRAC channel activation and STIM1 redistribution follow the same function of [Ca(2+)](ER), reaching half-maximum at approximately 200 microM with a Hill coefficient of approximately 4. Because STIM1 binds only a single Ca(2+) ion, 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 Ca(2+) entry, we replaced the luminal Ca(2+)-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 Ca(2+) from the ER. Thus, STIM1 oligomerization is the critical transduction event through which Ca(2+) store depletion controls store-operated Ca(2+) entry, acting as a switch that triggers the self-organization and activation of STIM1-ORAI1 clusters at ER-PM junctions.
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