Research ArticleCalcium signaling

Cell type–specific glycosylation of Orai1 modulates store-operated Ca2+ entry

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Sci. Signal.  08 Mar 2016:
Vol. 9, Issue 418, pp. ra25
DOI: 10.1126/scisignal.aaa9913

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Sugary coating on Orai1

Glycosylation is the addition of sugars to proteins and the chemical modification of these sugars to form highly complex structures. Glycosylation can affect protein stability, protein-protein interactions, and protein function. Human Orai1 is the channel-forming subunit of a complex that mediates calcium entry into cells when calcium in the endoplasmic reticulum becomes depleted (a process called SOCE). Orai1 has a single N-glycosylation site. Dörr et al. found that cells, including several types of immune cells, exposed to conditions that altered Orai1 glycosylation specifically or cellular glycosylation globally exhibited cell-specific effects on SOCE. In many cases, disruption of glycosylation enhanced SOCE without affecting Orai1 abundance or presence at the cell surface, suggesting that glycosylation limited Orai1 activity. Excess SOCE and altered glycosylation are associated with various immune diseases and differences in Orai1 function may be an important molecular connection between these two cellular events.

Abstract

N-glycosylation of cell surface proteins affects protein function, stability, and interaction with other proteins. Orai channels, which mediate store-operated Ca2+ entry (SOCE), are composed of N-glycosylated subunits. Upon activation by Ca2+ sensor proteins (stromal interaction molecules STIM1 or STIM2) in the endoplasmic reticulum, Orai Ca2+ channels in the plasma membrane mediate Ca2+ influx. Lectins are carbohydrate-binding proteins, and Siglecs are a family of sialic acid–binding lectins with immunoglobulin-like repeats. Using Western blot analysis and lectin-binding assays from various primary human cells and cancer cell lines, we found that glycosylation of Orai1 is cell type–specific. Ca2+ imaging experiments and patch-clamp experiments revealed that mutation of the only glycosylation site of Orai1 (Orai1N223A) enhanced SOCE in Jurkat T cells. Knockdown of the sialyltransferase ST6GAL1 reduced α-2,6–linked sialic acids in the glycan structure of Orai1 and was associated with increased Ca2+ entry in Jurkat T cells. In human mast cells, inhibition of sialyl sulfation altered the N-glycan of Orai1 (and other proteins) and increased SOCE. These data suggest that cell type–specific glycosylation influences the interaction of Orai1 with specific lectins, such as Siglecs, which then attenuates SOCE. In summary, the glycosylation state of Orai1 influences SOCE-mediated Ca2+ signaling and, thus, may contribute to pathophysiological Ca2+ signaling observed in immune disease and cancer.

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