Septins organize endoplasmic reticulum-plasma membrane junctions for STIM1-ORAI1 calcium signalling.

ORAI1 Ca channels in the plasma membrane (PM) are gated by STIM1 at endoplasmic reticulum (ER)-PM junctions to effect store-dependent Ca entry into cells, but little is known about how local STIM-ORAI signalling at junctions is coordinated with overall cellular architecture. Filamentous septins can specify cytoskeletal rearrangements and have been found recently to modulate STIM-ORAI signalling. Here we show by super-resolution imaging of ORAI1, STIM1, and septin 4 in living cells that septins facilitate Ca signalling indirectly. Septin 4 does not colocalize preferentially with ORAI1 in resting or stimulated cells, assemble stably at ER-PM junctions, or specify a boundary that directs or confines ORAI1 to junctions. Rather, ORAI1 is recruited to junctions solely through interaction with STIM proteins, while septins regulate the number of ER-PM junctions and enhance STIM1-ORAI1 interactions within junctions. Thus septins communicate with STIM1 and ORAI1 through protein or lipid intermediaries, and are favorably positioned to coordinate Ca signalling with rearrangements in cellular architecture.