STIM1 Clusters and Activates CRAC Channels via Direct Binding of a Cytosolic Domain to Orai1

Abstract

SUMMARY Store-operated Ca2+ channels activated by the depletion of Ca2+ from the endoplasmic reticulum (ER) are a major Ca2+ entry pathway in non-excitable cells and are essential for T cell activation and adaptive immunity. Following store depletion, the ER Ca2+ sensor STIM1 and the CRAC channel protein Orai1 redistribute to ER-plasma membrane (PM) junctions, but the fundamental issue of how STIM1 activates the CRAC channel at these sites is unresolved. Here we identify a minimal, highly conserved 107-aa CRAC activation domain (CAD) of STIM1 that binds directly to the N- and C-termini of Orai1 to open the CRAC channel. Purified CAD forms a tetramer that clusters CRAC channels, but analysis of STIM1 mutants reveals that channel clustering is not sufficient for channel activation. These studies establish a molecular mechanism for store-operated Ca2+ entry in which the direct binding of STIM1 to Orai1 drives the accumulation and the activation of CRAC channels at ER-PM junctions.

Induction of cortical endoplasmic reticulum by dimerization of a coatomer-binding peptide anchored to endoplasmic reticulum membranes

“…They multimerize after calcium depletion before being transported to the cell periphery, where they activate a calcium channel (7,(27)(28)(29). These proteins harbor a lysine-rich cytosolic tail (29), highly similar to Ist2ct (13), which is required for their peripheral localization. STIM proteins also contain an EB1-interaction domain allowing their targeting to the microtubule plus-end (30).…”

Store-Operated Calcium Channels: New Perspectives on Mechanism and Function

“…To form puncta in the nominal absence of Orai, STIM1 requires an intact carboxyterminal polybasic domain, which has been proposed to form an amphipathic a-helix that interacts with phosphoinositides in the plasma membrane, in effect creating a diffusion trap (Huang et al 2006;Liou et al 2007;Park et al 2009). In line with this hypothesis, conditions that deplete phosphatidylinositol 4,5-bisphosphate (PIP 2 ) and phosphatidylinositol 3,4,5-trisphosphate (PIP 3 ) together strongly inhibit STIM1 puncta formation, and STIM1/2 polybasic domains bind to liposomes containing these lipids in vitro (Ercan et al 2009;Walsh et al 2010a).…”

Alternative splicing converts STIM2 from an activator to an inhibitor of store-operated calcium channels

“…Store depletion triggers oligomerization (Stathopulos et al, 2006;Covington et al, 2010) and conformational rearrangements of STIM proteins (Muik et al, 2011). These rearrangements expose the C-terminal polybasic domain, which interacts with phosphatidylinositol 4,5-bisphosphate in the plasma membrane (PM) and drives STIM accumulation at ER-PM junctions (Wu et al, 2006;Liou et al, 2007;Ercan et al, 2009;Park et al, 2009). Although STIM1 and STIM2 respond similarly to store depletion, STIM2 differs from STIM1 in being partially localized at ER-PM junctions even in store-replete cells, likely as a result of its lower affinity for ER Ca 2+ relative to STIM1 (Brandman et al, 2007;Zheng et al, 2008).…”