N-glycans of sphingosine 1-phosphate receptor Edg-1 regulate ligand-induced receptor internalization

Endothelial differentiation gene-1 product (Edg-1) is a G-protein-coupled receptor (GPCR) for the platelet derived bioactive lipid mediator sphingosine 1-phosphate (Sph-1-P). Recent studies have shown that in response to Sph-1-P, Edg-1 mediates various signaling pathways through downstream signaling molecules, such as MAP kinase and calcium, via heterotrimeric G-proteins. We found for the first time that Edg-1 is glycosylated in its amino-terminal extracellular portion, and further identified the specific glycosylation site as asparagine 30 by creating a nonglycosylated mutant of Edg-1 (N30D-Edg-1) and transfecting it into cell lines. The nonglycosylated mutant receptors, resembling their wild-type controls, were predominantly expressed in the plasma membrane. Although there was no difference in ligand binding ability and ligand-induced MAP kinase activation in the wild-type and mutant receptors, nonglycosylated Edg-1 was much less responsive for ligand-induced internalization. Unlike the wild-type receptor, which was associated with the caveolae, nonglycosylated N30D-Edg-1 was dispersed broadly in the membrane fractions separated by sucrose density gradient centrifugation, suggesting that internalization and microdomain localization of N-glycosylated Edg-1 might be related. Although the precise molecular mechanism of the internalization of the N-glycosylated Edg-1 localized in the microdomain remains to be examined, the present study suggested that the presence of N-linked glycan in the receptor may play a regulatory role in the receptor dynamics in ligand-stimulated mammalian cells.