Truncation of the N-terminal ectodomain has implications in the N-glycosylation and transport to the cell surface of Edg-1/S1P1 receptor

The endothelial cell-expressed sphingosine 1-phosphate receptors Edg-1/S1P1 and Edg-3/S1P3 have been implicated in various physiological events such as the regulation of angiogenesis. Since there is an excess of a ligand constitutively in blood, these receptors may have some mechanism(s) avoiding overstimulation. In this study, we found that the N-terminal ectodomains of Edg-1/S1P1 and Edg-3/S1P3 were truncated in overexpressing cells. The truncated form of Edg-1/S1P1 expressed on the cell surface had undergone complex-type oligosaccharide modification at the Golgi. A deletion mutant lacking the N-terminal processing domain of Edg-1/S1P1 accumulated in the endoplasmic reticulum, and was not expressed on the cell surface. When a basic amino acid residue was introduced at the cleavage site of Edg-1/S1P1, the molecular weight of the glycosylated protein was greater in the mutant compared to the wild type, due to the bound oligosaccharide. These results demonstrated that the structure of the N-terminal ectodomain of Edg-1/S1P1 affects both its transport to the cell surface and the N-glycosylation process. Ectodomain shedding of many membrane proteins has been implicated in various diseases. Therefore, N-terminal processing of Edg-1/S1P1 and Edg-3/S1P3 might play roles in endothelial cell functions.