A charge-sensing region in the stromal interaction molecule 1 luminal domain confers stabilization-mediated inhibition of SOCE in response to S-nitrosylation

Store-operated Ca2+ entry (SOCE) is a major Ca2+ signaling pathway facilitating extracellular Ca2+ influx in response to the initial release of intracellular endo/sarcoplasmic reticulum (ER/SR) Ca2+ stores. Stromal interaction molecule 1 (STIM1) is the Ca2+ sensor that activates SOCE following ER/SR Ca2+ depletion. The EF-hand and the adjacent sterile α-motif (EFSAM) domains of STIM1 are essential for detecting changes in luminal Ca2+ concentrations. Low ER Ca2+ levels trigger STIM1 destabilization and oligomerization, culminating in the opening of Orai1-composed Ca2+ channels on the plasma membrane. NO-mediated S-nitrosylation of cysteine thiols regulates myriad protein functions, but its effects on the structural mechanisms that regulate SOCE are unclear. Here, we demonstrate that S-nitrosylation of Cys49 and Cys56 in STIM1 enhances the thermodynamic stability of its luminal domain, resulting in suppressed hydrophobic exposure and diminished Ca2+ depletion–dependent oligomerization. Using solution NMR spectroscopy, we pinpointed a structural mechanism for STIM1 stabilization driven by complementary charge interactions between an electropositive patch on the core EFSAM domain and the S-nitrosylated nonconserved region of STIM1. Finally, using live cells, we found that the enhanced luminal domain stability conferred by either Cys49 and Cys56 S-nitrosylation or incorporation of negatively charged residues into the EFSAM electropositive patch in the full-length STIM1 context significantly suppresses SOCE. Collectively, our results suggest that S-nitrosylation of STIM1 inhibits SOCE by interacting with an electropositive patch on the EFSAM core, which modulates the thermodynamic stability of the STIM1 luminal domain.