Junctate is a Ca2+-sensing structural component of Orai1 and stromal interaction molecule 1 (STIM1)

Orai1 and stromal interaction molecule (STIM)1 are critical components of Ca2+ release-activated Ca2+ (CRAC) channels. Orai1 is a pore subunit of CRAC channels, and STIM1 acts as an endoplasmic reticulum (ER) Ca2+ sensor that detects store depletion. Upon store depletion after T-cell receptor stimulation, STIM1 translocates and coclusters with Orai1 at sites of close apposition of the plasma membrane (PM) and the ER membrane. However, the molecular components of these ER-PM junctions remain poorly understood. Using affinity protein purification, we uncovered junctate as an interacting partner of Orai1-STIM1 complex. Furthermore, we identified a Ca2+-binding EF-hand motif in the ER-luminal region of junctate. Mutation of this EF-hand domain of junctate impaired its Ca2+ binding and resulted in partial activation of CRAC channels and clustering of STIM1 independently of store depletion. In addition to the known mechanisms of STIM1 clustering (i.e., phosphoinositide and Orai1 binding), our study identifies an alternate mechanism to recruit STIM1 into the ER-PM junctions via binding to junctate. We propose that junctate, a Ca2+-sensing ER protein, is a structural component of the ER-PM junctions where Orai1 and STIM1 cluster and interact in T cells.