Domains of STIM proteins. Drosophila and human STIM proteins are situated in the ER membrane. Modules of STIM1, STIM2 and Stim include: the signal peptide, the predicted EF-hand and SAM domains, the transmembrane region and two regions predicted to form coiled-coil structures comprising the ERM domain. Proline-rich domains (P) and the lysine-rich C-terminal regions are unique to the mammalian STIM family members. Drosophila Stim contains an N-terminal sequence in the ER that is not present in either STIM1 or STIM2. The N-linked glycosylation sites at the SAM domain, experimentally verified for STIM1 (refs , ), are also indicated. Background colours represent basal Ca²⁺ concentrations of ~50 nM in the cytosol and > 400 µM in the ER lumen. Ca²⁺ ions are shown as red dots, including Ca²⁺ bound to the EF-hand domain.

STIM Ca²⁺ sensor, signal initiation and messenger functions. STIM molecules are shown in the basal state as dimers (left); the Ca²⁺ sensor EF-hand domain has bound Ca²⁺ (red dots) when the ER Ca²⁺ store is filled. Background colours represent basal Ca²⁺ concentrations of ~50 nM in the cytosol and > 400 µM in the ER lumen. ER Ca²⁺ store depletion causes Ca²⁺ to unbind from the low affinity EF-hand of STIM; this is the molecular switch that leads to STIM oligomerization and translocation (dashed arrows) to ER–plasma membrane junctions. Non-conducting Orai channel subunits are shown as dimers.

STIM-mediated organization of the elementary unit and activation of Orai channels. STIM accumulation induces Orai channels to cluster in the adjacent plasma membrane. The C-terminal effector domain of STIM induces Orai channels to open by direct binding of the distal coiled-coil domain, and Ca²⁺ enters the cell (red arrows). Two STIMs can activate a single CRAC channel consisting of an Orai tetramer; channel activation of Orai may involve a preliminary step of assembling Orai dimers into a functional tetramer. Background colours represent changes in cytosolic Ca²⁺ concentration (from that in the basal state as represented in and ) to > 1 µM due to Ca²⁺ entry, and ER luminal Ca²⁺ to < 300 µM.