Store-operated Ca 2+ influx is mediated by store002Doperated Ca 2+ channels (SOCs) and is a central component of receptor-evoked Ca 2+ signals 1. The Orai channels mediate SOCs 2– 4 and STIM1 is the ER-resident Ca 2+ sensor that gates the channels 5, 6. How STIM1 gates and regulates the Orai channels is unknown. Here, we report the molecular basis for gating of Orais by STIM1. All Orai channels are fully activated by the conserved STIM1(344–442), which we termed SOAR (the STIM1 Orai Activating Region). SOAR acts in combination with STIM1(450–485) to regulate the strength of interaction with Orai1. Orai1 activated by SOAR recapitulates all the entire kinetic properties of Orai1 activated by STIM1. Mutations of STIM1 within SOAR prevent activation of Orai1 without preventing co-clustering of STIM1 and Orai1 in response to Ca 2+ store depletion, indicating that STIM1-Orai1 co-clustering is not sufficient for Orai1 activation. An intact C-terminus α-helicial region of Orai is required for activation by SOAR. Deleting most of Orai1 N terminus impaired Orai1 activation by STIM1, but (Δ1–73)Orai1 interacts with and is fully activated by SOAR. Accordingly, the characteristic inward rectification of Orai is mediated by an interaction between the polybasic STIM1(672–685) and a proline-rich region in the N terminus of Orai1. Hence, the essential properties of Orai1 function can be rationalized by interactions with discrete regions of STIM1.