Bidirectional regulation of calcium release–activated calcium (CRAC) channel by SARAF

Store-operated calcium entry (SOCE) through the Ca²⁺ release–activated Ca²⁺ (CRAC) channel is a central mechanism by which cells generate Ca²⁺ signals and mediate Ca²⁺-dependent gene expression. The molecular basis for CRAC channel regulation by the SOCE-associated regulatory factor (SARAF) remained insufficiently understood. Here we found that following ER Ca²⁺ depletion, SARAF facilitates a conformational change in the ER Ca²⁺ sensor STIM1 that relieves an activation constraint enforced by the STIM1 inactivation domain (ID; aa 475–483) and promotes initial activation of STIM1, its translocation to ER–plasma membrane junctions, and coupling to Orai1 channels. Following intracellular Ca²⁺ rise, cooperation between SARAF and the STIM1 ID controls CRAC channel slow Ca²⁺-dependent inactivation. We further show that in T lymphocytes, SARAF is required for proper T cell receptor evoked transcription. Taking all these data together, we uncover a dual regulatory role for SARAF during both activation and inactivation of CRAC channels and show that SARAF fine-tunes intracellular Ca²⁺ responses and downstream gene expression in cells.