A summary of functional roles for W76, Y80, and ID_STIM. Open-channel states are on the left, and inactivated states supported by the various combinations of functional elements tested are on the right. For convenience, we depict inactivation schematically as a conformational change in the inner pore, but other mechanisms are also possible (see Discussion). (A) Bulky hydrophobic side chains of W76 and Y80 in the inner pore are required for CDI. (B) The Orai1 Y80 side chain by itself is sufficient to support residual CDI (approximately one third of WT CDI) in the absence of a functional ID_STIM–W76 module. (C) In the absence of Y80 and ID_STIM, Orai1 W76 by itself can support an intermediate level of CDI (approximately two thirds of WT CDI), revealing an inhibitory role for Y80. (D) The ID_STIM–W76 module overcomes the Y80-dependent residual CDI depicted in B to enable full CDI. Y80 slows the kinetics of CDI, suggesting that it imposes an energy barrier on the path to full inactivation.