The elementary unit of store-operated Ca2+ entry:: local activation of CRAC channels by STIM1 at ER-plasma membrane junctions

T he activation of store-operated Ca2+ entry by Ca2+ store depletion has long been hypothesized to occur via local interactions of the endoplasmic reticulum ( ER) and plasma membrane, but the structure involved has never been identified. Store depletion causes the ER Ca2+ sensor stromal interacting molecule 1 (STIM1) to form puncta by accumulating in junctional ER located 10-25 nm from the plasma membrane ( see Wu et al. on p. 803 of this issue). We have combined total internal reflection fluorescence (TIRF) microscopy and patch-clamp recording to localize STIM1 and sites of Ca2+ in flux through open Ca2+ release-activated Ca2+ ( CRAC) channels in Jurkat T cells after store depletion. CRAC channels open only in the immediate vicinity of STIM1 puncta, restricting Ca2+ entry to discrete sites comprising a small fraction of the cell surface. Orai1, an essential component of the CRAC channel, colocalizes with STIM1 after store depletion, providing a physical basis for the local activation of Ca2+ influx. These studies reveal for the first time that STIM1 and Orai1 move in a coordinated fashion to form closely apposed clusters in the ER and plasma membranes, thereby creating the elementary unit of store-operated Ca2+ entry.