STIM is a Ca2+ sensor essential for Ca2+-store-depletion-triggered Ca2+ influx

CA(2+) signaling in nonexcitable cells is typically initiated by receptor-triggered production of inositol-1,4,5-trisphosphate and the release of CA(2+) from intracellular stores. An elusive signaling process senses the CA(2+) store depletion and triggers the opening of plasma membrane CA(2+) channels. The resulting sustained CA(2+) signals are required for many physiological responses, such as T cell activation and differentiation. Here, we monitored receptor-triggered CA(2+) signals in cells transfected with siRNAs against 2,304 human signaling proteins, and we identified two proteins required for CA(2+)-store-depletion-mediated CA(2+) influx, STIM1 and STIM2. These proteins have a single transmembrane region with a putative CA(2+) binding domain in the lumen of the endoplasmic reticulum. CA(2+) store depletion led to a rapid translocation of STIM1 into puncta that accumulated near the plasma membrane. Introducing a point mutation in the STIM1 CA(2+) binding domain resulted in prelocalization of the protein in puncta, and this mutant failed to respond to store depletion. Our study suggests that STIM proteins function as CA(2+) store sensors in the signaling pathway connecting CA(2+) store depletion to CA(2+) influx.