Atomic force microscopy (AFM) imaging suggests that stromal interaction molecule 1 (STIM1) binds to Orai1 with sixfold symmetry

Depletion of Ca2+ from the endoplasmic reticulum (ER) lumen triggers the opening of Ca2+ release-activated Ca2+ (CRAC) channels at the plasma membrane. CRAC channels are activated by stromal interaction molecule 1 (STIM1), an ER resident protein that senses Ca2+ store depletion and interacts with Orai1, the pore-forming subunit of the channel. The subunit stoichiometry of the CRAC channel is controversial. Here we provide evidence, using atomic force microscopy (AFM) imaging, that Orai1 assembles as a hexamer, and that STIM1 binds to Orai1 with sixfold symmetry. STIM1 associates with Orai1 in the form of monomers, dimers, and multimeric string-like structures that form links between the Orai1 hexamers. Our results provide new insights into the nature of the interactions between STIM1 and Orai1.