Mechanistic view on domains mediating STIM1-Orai coupling.

Calcium (Ca2+) entry into non-excitable cells is mainly carried by store-operated channels, which serve essential functions ranging from regulation of transcription to cell growth. The best-characterized store-operated current, initially discovered in T lymphocytes and mast cells, is the Ca2+ release-activated Ca2+ (CRAC) current. The search for the molecular components of the CRAC channel has recently identified stromal interaction molecule 1 (STIM1) as the Ca2+ sensor in the endoplasmic reticulum (ER) and Orai1 as the CRAC channel pore. ER store depletion results in formation of STIM1 puncta that trigger Ca2+ influx via Orai1 channels. This review covers the role of domains within STIM1 and Orai and enlightens their function in the STIM1/Orai coupling process. Moreover, a molecular interpretation focuses on interactions between cytosolic portions of STIM1 and Orai together with a mechanistic view on the loss of function of the SCID (severe combined immunodeficiency)-linked Orai1 R91W mutant channel. The architecture of the selectivity filter of Orai channels is finally elucidated based on permeation properties of Orai pore mutants.