Structural perturbations induced by Asn131 and Asn171 glycosylation converge within the EFSAM core and enhance stromal interaction molecule-1 mediated store operated calcium entry

A major intracellular calcium (Ca^2 +) uptake pathway in both excitable and non-excitable eukaryotic cells is store-operated Ca^2 + entry (SOCE). SOCE is the process by which endoplasmic reticulum (ER)-stored Ca^2 + depletion leads to activation of plasma membrane Ca^2 + channels to provide a sustained increase in cytosolic Ca^2 + levels that mediate a plethora of physiological processes ranging from the immune response to platelet aggregation. Stromal interaction molecule-1 (STIM1) is the principal regulator of SOCE and responds to changes in ER stored Ca^2 + through luminal sensing machinery composed of EF-hand and SAM domains (EFSAM). The EFSAM domain can undergo N-glycosylation at Asn131 and Asn171 sites; however, the precise role of EFSAM N-glycosylation in the Ca^2 + sensing mechanism of STIM1 is unclear. By establishing a site-specific chemical approach to covalently linking glucose to EFSAM and examining α-helicity, thermal stability, three dimensional atomic-resolution structure, Ca^2 + binding affinity and oligomerization, we show that N-glycosylation of the EFSAM domain enhances the properties that promote STIM1 activation. This augmentation occurs through changes in structure localized near the Asn131 and Asn171 sites that together permeate through the protein core and lead to decreased Ca^2 + binding affinity, reduced stability and enhanced oligomerization. Congruently, Ca^2 + influx via SOCE in HEK293 cells co-expressing Orai1 and STIM1 was diminished when N-glycosylation was blocked by introducing Asn131Gln and Asn171Gln mutations. Collectively, our data suggests that N-glycosylation enhances the EFSAM destabilization-coupled oligomerization in response to ER Ca^2 + depletion thereby augmenting the role of STIM1 as a robust ON/OFF regulator of SOCE. This article is part of a Special Issue entitled: ECS Meeting edited by Claus Heizmann, Joachim Krebs and Jacques Haiech.