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

The 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⁺ influx 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.