Background: STIM/ORAI-mediated store-operated Ca2+ entry (SOCE)
mediates a myriad of Ca2+-dependent cellular activities in mammals. Genetic defects in
STIM1/ORAI1 lead to devastating severe combined immunodeficiency; whereas gain-offunction
mutations in STIM1/ORAI1 are intimately associated with tubular aggregate
myopathy. At molecular level, a decrease in the Ca2+ concentrations within the lumen of
endoplasmic reticulum (ER) initiates multimerization of the STIM1 luminal domain to
switch on the STIM1 cytoplasmic domain to engage and gate ORAI channels, thereby
leading to the ultimate Ca2+ influx from the extracellular space into the cytosol. Despite
tremendous progress made in dissecting functional STIM1-ORAI1 coupling, the
activation mechanism of SOCE remains to be fully characterized.
Objective and Methods: Building upon a robust fluorescence resonance energy
transfer assay designed to monitor STIM1 intramolecular autoinhibition, we aimed to
systematically dissect the molecular determinants required for the activation and
oligomerization of STIM1.
Results: Here we showed that truncation of the STIM1 luminal domain predisposes
STIM1 to adopt a more active conformation. Replacement of the single transmembrane
(TM) domain of STIM1 by a more rigid dimerized TM domain of glycophorin A abolished
STIM1 activation. But this adverse effect could be partially reversed by disrupting the TM
dimerization interface. Moreover, our study revealed regions that are important for the
optimal assembly of hetero-oligomers composed of full-length STIM1 with its minimal
STIM1-ORAI activating region, SOAR.
Conclusions: Our study clarifies the roles of major STIM1 functional domains in
maintaining a quiescent configuration of STIM1 to prevent preactivation of SOCE.