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.