Calcium influx through CRAC channels requires STIM1 binding to Orai1. Using crosslinking and mutagenesis, Söllner et al. show that widening the nexus-TM3 interface near the STIM1 coupling site is crucial for channel opening, while hydrophobicity and charges support signal transmission to the pore. Nexus-TM3 dynamics are vital for Orai1 function.