Ca2+ ions represent versatile second messengers that regulate a huge diversity of processes throughout the cell's life. One prominent Ca2+ entry pathway into the cell is the Ca2+ release-activated Ca2+ (CRAC) ion channel. It is fully reconstituted by the two molecular key players: the stromal interaction molecule (STIM1) and Orai. STIM1 is a Ca2+ sensor located in the membrane of the endoplasmic reticulum, and Orai, a highly Ca2+ selective ion channel embedded in the plasma membrane. Ca2+ store-depletion leads initially to the activation of STIM1 which subsequently activates Orai channels via direct binding. Authentic CRAC channel hallmarks and biophysical characteristics include high Ca2+ selectivity with a reversal potential in the range of + 50 mV, small unitary conductance, fast Ca2+-dependent inactivation and enhancements in currents upon the switch from a Na+-containing divalent-free to a Ca2+-containing solution. This review provides an overview on the critical determinants and structures within the STIM1 and Orai proteins that establish these prominent CRAC channel characteristics.
Keywords: CRAC channel; Calcium; Electrophysiology; FRET; Gain-of-function mutants; Orai gating; Orai1; STIM1; STIM–Orai interaction; Structural resolution.