Many of the biological functions of heparan sulfate (HS) proteoglycans can be attributed to specialized structures within HS moieties, which are thought to modulate binding and function of various effector proteins. Cyclophilin B (CyPB), which was initially identified as a cyclosporin A-binding protein, triggers migration and integrin-mediated adhesion of peripheral blood T lymphocytes by a mechanism dependent on interaction with cell surface HS. Here we determined the structural features of HS that are responsible for the specific binding of CyPB. In addition to the involvement of 2-O, 6-O, and N-sulfate groups, we also demonstrated that binding of CyPB was dependent on the presence of N-unsubstituted glucosamine residues (GlcNH 2 ), which have been reported to be precursors for sulfation by 3-Osulfotransferases-3 (3-OST-3). Interestingly, 3-OST-3B isoform was found to be the main 3-OST isoenzyme expressed in peripheral blood T lymphocytes and Jurkat T cells. Moreover, downregulation of the expression of 3-OST-3 by RNA interference potently reduced CyPB binding and consequent activation of p44/42 mitogen-activated protein kinases. Altogether, our results strongly support the hypothesis that 3-O-sulfation of GlcNH 2 residues could be a key modification that provides specialized HS structures for CyPB binding to responsive cells. Given that 3-O-sulfation of GlcNH 2 -containing HS by 3-OST-3 also provides binding sites for glycoprotein gD of herpes simplex virus type I, these findings suggest an intriguing structural linkage between the HS sequences involved in CyPB binding and viral infection.
Heparan sulfate (HS)4 proteoglycans on the cell surface or in the extracellular matrix are involved in developmental, regeneratory, and inflammatory processes, as a consequence of their interactions with multiple proteins. These interactions are mediated mainly via the HS moieties of the proteoglycans, which bind to growth factors, cytokines, matrix components, enzymes, and enzyme inhibitors and thereby regulate tissue distribution, biological availability, and activity of the proteins. Characterization of heparin/HS oligosaccharides with high affinity to proteins such as antithrombin and growth factors has led to the identification of specialized protein binding domains with structural features exhibiting varying degrees of specificity (1-3). The structural distinctions in these functional heparin/HS domains are derived from enzymatic modifications in the Golgi apparatus of the nascent polymer composed of alternating D-glucuronic acid (GlcUA) and N-acetylated D-glucosamine (GlcNAc) units. The nonsulfated precursor is first subject to partial N-deacetylation/N-sulfation of GlcNAc residues, which leads to the occurrence of consecutively N-sulfated regions, regions that escape modification and remains N-acetylated, and regions of alternating N-acetylated and N-sulfated disaccharide units. Sometimes, the N-deacetylation/N-sulfation reaction is partial, giving rise to N-unsubstituted glucosamine units (GlcNH 2 ). The further mo...
