A Dual role for SARAF in regulation of Calcium-Release Activated Calcium (CRAC) channel activity

Calcium entry through the Ca²⁺ release activated Ca²⁺ (CRAC) channel is a central mechanism by which cells generate Ca²⁺ signals and mediate Ca²⁺ dependent gene expression. Previous work revealed that dimers of SARAF establish a critical element of slow Ca²⁺ dependent inactivation (SCDI) of CRAC channel, however the effect of SARAF on CRAC channel activation and downstream Ca²⁺ dependent gene expression in cells remained poorly understood. Here we show that T-cell receptor evoked transcription via the Ca²⁺-calcineurin-nuclear factor of activated T cells pathway is compromised in T-lymphocytes lacking SARAF expression and that both CRAC channel activation and SCDI are reduced in these cells. Mechanistically, we identify a group of acidic residues within the STIM1 inhibitory/modulatory domain (ID/CMD, amino acids 475-483) that controls the regulatory effects of SARAF and show that SARAF contributes to CRAC channel activation by relieving an activation constraint enforced by the STIM1 ID/CMD. We further show that a deletion mutation in the luminal domain of SARAF, which impairs SARAF dimerization and attenuates SCDI, facilitates channel activation suggesting that SARAF uses reversible oligomerization to switch between its positive and negative regulatory modes. Taken together, findings from this work show that by playing a dual role in regulation of CRAC channel activity SARAF fine-tunes intracellular Ca²⁺ responses and downstream gene expression in cells.