Biosynthesis of Sulfated L-Selectin Ligands in Human High Endothelial Venules (HEV)
High endothelial venules (HEVs) are specialized post-capillary venules found in lymphoid tissues, that support high levels of lymphocyte extravasation from the blood. Lymphocyte L-selectin plays a key role in the initial interaction of lymphocytes with HEVs by recognizing sulfated carbohydrate ligands on HEV mucin-like glycoproteins, GlyCAM-1, CD34 and MAdCAM-1. Sulfation is key to the uniqueness of the HEV ligands since 6 or 6'-sulfated-sLe(x) isoforms have recently been identified as major capping groups of GlyCAM-1 and sulfation of both GlyCAM1 and CD34 has been shown to be required for high-affinity L-selectin binding and recognition by the HEV-specific monoclonal antibody MECA-79. To characterize the molecular mechanisms involved in the biosynthesis of sulfated L-selectin ligands in HEVs, we have started to isolate genes that play a role in sulfate metabolism in HEVs. Studies with chlorate, a selective inhibitor of the synthesis of the high energy donor of sulfate, PAPS (3'-phosphoadénosine 5'-phosphosulfate), had previously revealed that PAPS synthesis is required for sulfation of HEV ligands and recognition by L-selectin. Therefore, we screened an HEV cDNA library in order to isolate cDNAs encoding enzymes involved in PAPS synthesis. This strategy allowed us to isolate a novel cDNA encoding the PAPS synthetase from human HEVs. The molecular characteristics of PAPS synthetase and its role in biosynthesis of sulfated L-selectin ligands in HEVs are discussed.