Calcium (Ca 2+) is an essential second messenger required for diverse signaling processes in immune cells. Ca 2+ release-activated Ca 2+ (CRAC) channels represent one main Ca 2+ entry pathway into the cell. They are fully reconstituted via two proteins, the stromal interaction molecule 1 (STIM1), a Ca 2+ sensor in the endoplasmic reticulum and the Ca 2+ ion channel Orai in the plasma membrane. After Ca 2+ store depletion, STIM1 proteins undergo a conformational rearrangement which leads to STIM1 oligomerization and their translocation into ER-PM junctions. Here, STIM1 proteins couple to Orai Ca2+ ion channels which results in Ca 2+ influx into the cell. STIM1 binding to and communication with Orai1 is predominantly achieved via a direct interaction of their C-terminal strands. Although binding of STIM1 to Orai1 C-terminus is an absolute premise, it is not sufficient for channel gating and still requires additional cytosolic domains of Orai1. Our studies uncovered a segment of the cytosolic Orai N-terminus, that is fully conserved among the Orai family and forms an elongated helix of the pore region, as indispensable for CRAC channel function. Several hot spots within this region control Orai channel activation, while other parts therein fine tune together with STIM1 the open Orai channel conformation. A communication of the N-terminus with a cytosolic loop region of Orai channels maintains their activation in an isoform-specific manner. Moreover, cholesterol has been identified to modulate Orai channel activation via its N-terminus. In summary, we present a multi-facetted role of the Orai N-terminus in CRAC channel activation.

Financing: FWF (P27641, P30567 to Isabella Derler, P28123 to Marc Fahrner, P28498 to Martin Muik, P28701 to Rainer Schindl, P27263, W 1250 to Christoph Romanin); program Inter-COST (project LTC17069 to Rudi Ettrich)