Calcium sensing by the STIM1 ER-luminal domain

Stromal interaction molecule 1 (STIM1) monitors ER-luminal Ca(2+) levels to maintain cellular Ca(2+) balance and to support Ca(2+) signalling. The prevailing view has been that STIM1 senses reduced ER Ca(2+) through dissociation of bound Ca(2+) from a single EF-hand site, which triggers a dramatic loss of secondary structure and dimerization of the STIM1 luminal domain. Here we find that the STIM1 luminal domain has 5–6 Ca(2+)-binding sites, that binding at these sites is energetically coupled to binding at the EF-hand site, and that Ca(2+) dissociation controls a switch to a second structured conformation of the luminal domain rather than protein unfolding. Importantly, the other luminal-domain Ca(2+)-binding sites interact with the EF-hand site to control physiological activation of STIM1 in cells. These findings fundamentally revise our understanding of physiological Ca(2+) sensing by STIM1, and highlight molecular mechanisms that govern the Ca(2+) threshold for activation and the steep Ca(2+) concentration dependence.