Distinct Roles of STIM1 and STIM2 C-Terminal Orai-Coupling Domains

Abstract

Store-operated Ca2+ entry (SOCe) is essential for Ca2+ homeostasis and signaling. SOCe is mediated by STIM proteins which function as ER Ca2+ store sensors, coupling with and activating PM Orai Ca2+ channels. While STIM1-Orai1 coupling is well characterized, the coupling between STIM2 and Orai1 shows some important functional differences from STIM1. The molecular basis of these differences remains uncharacterized. We examined the STIM2 C-terminal (S2-Ct) region that has considerable homology with the known functional coupling domains of S1-Ct. We examined the comparative functions of STIM1 and STIM2 fragments using a combination of Ca2+ imaging, patch-clamp current analysis, and analysis of the pharmacological modifier, 2-APB. Deletion of the “variable” C-terminal region (534-833) immediately downstream from the STIM-Orai activating region of STIM2 (SOAR2; 435-533) from either whole STIM2 or S2-Ct, had little effect on the activation of Orai1 channels. Similarly, deletion from S2-Ct of the N-terminal region (325-433) upstream from SOAR2, had little effect on Orai1-activation by S2-Ct. Thus the cytosolic regions outside SOAR2 seem to be less important for mediating STIM2 coupling to and activate Orai1. Interestingly, SOAR2 expression alone is sufficient to mimic some of the different coupling properties that distinguish full length STIM2 from STIM1, including the poor intrinsic coupling to activate Orai1 and the strong enhancement of Orai1 activation induced by 2-APB. To gain further insights on how the two SOAR domains couple and activate Orai1, we constructed a series of SOAR1 and SOAR2 chimeras. Using these chimeras, our results reveal that the Sα1-Sα3 helices in the SOAR molecules are important for defining the distinct Orai1 activating properties of STIM1 and STIM2.