Figure 8

Cellular organization underlying STIM-ORAI Ca²⁺ signalling. (a) The basal condition in control cells expressing STIM1 and wildtype ORAI1. Upper, The ER maintains STIM1-competent junctions on the PM. Lower, Expanded view of a single junction after store depletion. ORAI1 has been recruited effectively and is engaged with STIM1. (b) siSeptin4/5-treated cells expressing STIM1 and wildtype ORAI1. The density of STIM1-competent ER-PM contacts is reduced. STIM1 still comes to the junctions, but ORAI1 is recruited less efficiently, and a lower fraction of junctional ORAI1 is engaged with STIM1. (c) Control cells expressing STIM1 and ORAI1-LSLD. STIM1 relocalizes to the junctions after store depletion, but ORAI1-LSLD is not recruited. This finding is the clearest evidence that septins and their associated proteins do not direct or confine ORAI1 to junctions, and implies the stronger conclusion that ORAI1 channels are recruited to ER-PM junctions solely by their interactions with STIM proteins.