Oligomerization of STIM1 couples ER calcium depletion to CRAC channel activation.
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.
Luik
Riina M
RM
Department of Molecular and Cellular Physiology, Stanford University School of Medicine, Stanford, California 94305, USA.
Wang
Bin
B
Prakriya
Murali
M
Wu
Minnie M
MM
Lewis
Richard S
RS
D002118:Calcium / Q000378:metabolism*; D015220:Calcium Channels / Q000378:metabolism*; D002462:Cell Membrane / Q000378:metabolism; D004721:Endoplasmic Reticulum / Q000378:metabolism*; D006801:Humans; D019169:Jurkat Cells; D008565:Membrane Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D009363:Neoplasm Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*
D015220:Calcium Channels; D008565:Membrane Proteins; D009363:Neoplasm Proteins; D002118:Calcium