An siRNA screen for NFAT activation identifies septins as coordinators of store-operated Ca2+ entry

The STIM1-ORAI1 pathway of store-operated Ca2+ entry is an essential component of cellular Ca2+ signalling(1). STIM1 senses depletion of intracellular Ca2+ stores in response to physiological stimuli, and relocalizes within the endoplasmic reticulum to plasma-membrane-apposed junctions, where it recruits and gates open plasma membrane ORAI1 Ca2+ channels. Here we use a genome-wide RNA interference screen in HeLa cells to identify filamentous septin proteins as crucial regulators of store-operated Ca2+ entry. Septin filaments and phosphatidylinositol-4,5-bisphosphate (also known as PtdIns(4,5)P-2) rearrange locally at endoplasmic reticulum-plasma membrane junctions before and during formation of STIM1-ORAI1 clusters, facilitating STIM1 targeting to these junctions and promoting the stable recruitment of ORAI1. Septin rearrangement at junctions is required for PtdIns(4,5) P2 reorganization and efficient STIM1-ORAI1 communication. Septins are known to demarcate specialized membrane regions such as dendritic spines, the yeast bud and the primary cilium, and to serve as membrane diffusion barriers and/or signalling hubs in cellular processes such as vesicle trafficking, cell polarity and cytokinesis2-4. Our data show that septins also organize the highly localized plasma membrane domains that are important in STIM1-ORAI1 signalling, and indicate that septins may organize membrane microdomains relevant to other signalling processes.