SOAR and the polybasic STIM1 domains gate and regulate Orai channels

Abstract

Store-operated Ca2+ influx is mediated by store002Doperated Ca2+ channels (SOCs) and is a central component of receptor-evoked Ca2+ signals1. The Orai channels mediate SOCs2–4 and STIM1 is the ER-resident Ca2+ sensor that gates the channels5, 6. How STIM1 gates and regulates the Orai channels is unknown. Here, we report the molecular basis for gating of Orais by STIM1. All Orai channels are fully activated by the conserved STIM1(344–442), which we termed SOAR (the STIM1 Orai Activating Region). SOAR acts in combination with STIM1(450–485) to regulate the strength of interaction with Orai1. Orai1 activated by SOAR recapitulates all the entire kinetic properties of Orai1 activated by STIM1. Mutations of STIM1 within SOAR prevent activation of Orai1 without preventing co-clustering of STIM1 and Orai1 in response to Ca2+ store depletion, indicating that STIM1-Orai1 co-clustering is not sufficient for Orai1 activation. An intact C-terminus α-helicial region of Orai is required for activation by SOAR. Deleting most of Orai1 N terminus impaired Orai1 activation by STIM1, but (Δ1–73)Orai1 interacts with and is fully activated by SOAR. Accordingly, the characteristic inward rectification of Orai is mediated by an interaction between the polybasic STIM1(672–685) and a proline-rich region in the N terminus of Orai1. Hence, the essential properties of Orai1 function can be rationalized by interactions with discrete regions of STIM1.

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

“…The CAD fragment forms a tetramer in solution ), whereas the longer STIM1 cytosolic fragments generally appear as dimers (Ji et al 2008;Penna et al 2008;Muik et al 2009;Yuan et al 2009;Zhou et al 2010a) that may need to unfold to expose the CAD and activate Orai1 (Korzeniowski et al 2010;Muik et al 2011). These findings raise the intriguing possibility that on store depletion, STIM1 undergoes a conformational change that results in formation of a tetramer through interactions among multiple CADs (Covington et al 2010), an idea that remains to be tested.…”

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

“…The CAD/SOAR region of STIM proteins is known to be necessary and sufficient to activate Orai channels (Park et al, 2009;Yuan et al, 2009). To confirm that the inability of STIM2β to activate Orai1 resulted from altered function of its CAD, we coexpressed Orai1 with STIM2α-or STIM2β-CAD.…”

“…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). At ER-PM junctions STIM proteins directly bind to and trap Orai channels (Park et al, 2009;Wu et al, 2014) through their CRAC activation domains (CADs; also known as SOAR [STIM1 Orai1 activation region] or CCb9; Kawasaki et al, 2009;Park et al, 2009;Yuan et al, 2009). STIM binding to Orai opens the channel by a nonlinear process that is highly sensitive to binding stoichiometry (Hoover and Lewis, 2011;Li et al, 2011).…”

STIM1/Orai1-mediated SOCE: current perspectives and potential roles in cardiac function and pathology

“…CC1 consists of three ␣-helices; CC2 and CC3a comprise the STIM1-Orai activating region (SOAR), which is the minimal domain required and sufficient for the activation of SOCE by Orai1 (144). Under resting conditions, interactions between the CC domains maintain STIM1 in a dimeric state with the SOAR domain bound to the C␣3 inhibitory helix within the CC1 region (67).…”