The STIM1 CTID Domain Regulates Orai1 Channel Function by Modulating SARAF Interaction

Abstract

Store operated Ca2+ entry (SOCE) is a key pathway for receptor evoked Ca2+ signaling. STIM1 and Orai1 are the two main molecular components of SOCE. Depletion of Ca2+ from endoplasmic reticulum (ER) clusters the ER calcium sensor STIM1 at ER/plasma membrane domains, which, in turn activates Orai1 channels in the plasma membrane. This results in Ca2+ influx to activate a plethora of cellular functions. The molecular mechanism underlying activation of Orai1 by STIM1 is still elusive. We modeled STIM1 amino acid sequence 234-530. The SOAR, (STIM1 Orai1-activating region (344-442) in our model exhibits high degree of homology with the recent crystal structure (Yang et al., PNAS 2012). Based on our model, we identified a critical domain in the C-terminus of STIM1, CTID (C Terminus Inhibitory Domain) that has a significant role in holding SOAR in its inactive state. This hypothesis is supported by immunolocalization, biochemical, imaging and electrophysiological data, which indicates that deletion of CTID or CTID sub-domains resulted in constitutive clustering of STIM1 and activation of Orai1. In addition, our data also suggests a mechanism of regulation of Orai1 gating by SARAF (SOCE-Associated Regulatory Factor), a negative regulator of SOCE. Both CC1 and CTID mutation/deletion affects SARAF binding to STIM1. Therefore, our study suggests a plausible conformational rearrangement in the cytoplasmic portion of STIM1 that is necessary to transform SOAR from an inactive to an active state which to activate Orai.