Plays a role in mediating store-operated Ca2+ entry (SOCE), a Ca2+ influx following depletion of intracellular Ca2+ stores. Functions as a highly sensitive Ca2+ sensor in the endoplasmic reticulum which activates both store-operated and store-independent Ca2+-influx. Regulates basal cytosolic and endoplasmic reticulum Ca2+ concentrations. Upon mild variations of the endoplasmic reticulum Ca2+ concentration, translocates from the endoplasmic reticulum to the plasma membrane where it probably activates the Ca2+ release-activated Ca2+ (CRAC) channels ORAI1, ORAI2 and ORAI3. May inhibit STIM1-mediated Ca2+ influx.