Fig. 4.

CRAC channel regulation by multiple Ca²⁺-sensing molecules in the ER and cytoplasm. (A) Schematic showing a possible mechanism of CRAC channel regulation. Under resting conditions, Orai1 and STIM1 are distributed at the PM and the ER membrane. Junctate is located at ER-PM junctions in a Ca²⁺-bound form via its ER-luminal EF hand domain (indicated in gray). Cytoplasmic Ca²⁺ sensors such as CRACR2A and calmodulin are not bound to Ca²⁺ in resting conditions. (B) Upon store depletion, STIM1 oligomerizes by sensing ER Ca²⁺ depletion with its ER-luminal EF-hand domain, and translocates to form clusters at the ER-PM junctions. By physical interactions with Orai1 through the CAD/SOAR domain (depicted in red), clustered STIM1 recruits and activates Orai1 in the ER-PM junctions. During the process, junctate loses bound Ca²⁺ and supports STIM1 recruitment into ER-PM junctions. CRACR2A is recruited into the Orai1-STIM1 complex to stabilize their interactions. (C) Following the increase of cytoplasmic [Ca²⁺], CRACR2A dissociates from the Orai1-STIM1 complex. Ca²⁺-bound calmodulin interacts with the N terminus of Orai1 and inactivates the channel via a mechanism called fast inactivation. The slow inactivation of CRAC channels depends on Ca²⁺ entry and interaction with SARAF. After channel inactivation, once the ER is refilled with Ca²⁺, Orai1 and STIM1 return to the resting status.