LAUSR

Stormorken syndrome is a multiorgan hereditary disease caused by dysfunction of the endoplasmic reticulum (ER) Ca2+ sensor protein STIM1, which forms the Ca2+ release–activated Ca2+ (CRAC) channel together with the plasma membrane channel Orai1. ER Ca2+ store depletion activates STIM1 by releasing the intramolecular “clamp” formed between the coiled coil 1 (CC1) and CC3 domains of the protein, enabling the C terminus to extend and interact with Orai1. The most frequently occurring mutation in patients with Stormorken syndrome is R304W, which destabilizes and extends the STIM1 C terminus independently of ER Ca2+ store depletion, causing constitutive binding to Orai1 and CRAC channel activation. We found that in cis deletion of one amino acid residue, Glu296 (which we called E296del) reversed the pathological effects of R304W. Homozygous Stim1 E296del+R304W mice were viable and phenotypically indistinguishable from wild-type mice. NMR spectroscopy, molecular dynamics simulations, and cellular experiments revealed that although the R304W mutation prevented CC1 from interacting with CC3, the additional deletion of Glu296 opposed this effect by enabling CC1-CC3 binding and restoring the CC domain interactions within STIM1 that are critical for proper CRAC channel function. Our results provide insight into the activation mechanism of STIM1 by clarifying the molecular basis of mutation-elicited protein dysfunction and pathophysiology. Description Disrupted intramolecular contacts underlie the effects of a disease-associated STIM1 mutant in cells and mice. Reversing STIM1 dysfunction in Stormorken syndrome Gain-of-function mutations in the ER Ca2+ sensor STIM1 result in constitutive activity of the plasma membrane Ca2+ channel Orai1 and underlie an inherited disease called Stormorken syndrome. Gamage and Grabmayr et al. found that deletion of Glu296 in STIM1 reversed the biophysical and pathophysiological effects of the most common mutation (R304W) associated with Stormorken syndrome. Mice expressing STIM1 with both the R304W mutation and Glu296 deletion were healthy and did not show the skeletal muscle, platelet, and bone phenotypes of mice expressing the R304W STIM1 mutant. Electrophysiological and biophysical analyses revealed that deletion of Glu296 in the R304W mutant restored the protein domain interactions that prevent STIM1 from associating with and activating Orai1 in the absence of stimuli. These results underscore the importance of intramolecular contacts in STIM1 in restraining its ability to stimulate Ca2+ influx through Orai1.—WW