Biosynthesis of decorin and glypican glycosaminoglycan chains
Sammanfattning: Proteoglycans consist of core proteins substituted with glycosaminoglycan chains. The galactosaminoglycans (GlcUA/IdoUA-GalNAc) and the glucosaminoglycans (GlcUA/IdoUA-GlcNAc) are both initiated on the same tetrasaccharide linkage region GlcUA-Gal-Gal-Xyl-protein. The proteoglycan decorin is bound to collagen fibrils in the extracellular matrix and is substituted with a single galactosaminoglycan chain. We have studied early events in the assembly of the decorin galactosaminoglycan chain by expressing decorin with a C-terminal KDEL ER-retention signal KDEL in COS cells. The major decorin O-linked saccharide, isolated from transfected cells, was the linkage region tetrasaccharide substituted with the first GalNAc residue initiating the galactosaminoglycan synthesis. Retrieval of Golgi enzymes by addition of BFA to transfected cells, resulted in partial elongation of the galactosaminoglycan chain. These chains did not contain IdoUA or sulfate, but all carried Xyl-phosphate. Glypican is substituted with glucosaminoglycan chains and is anchored to the cell surface via a GPI-anchor. These anchors mediate sorting of membrane components to plasma membrane domains called caveolae. It has been demonstrated that glypican cycle between the plasma membrane and the Golgi apparatus. During this recycling of glypican the glucosaminoglycan chains are partially degraded and resynthesized on the remaining oligosaccharide stubs. Glypican recycling may be involved in uptake of HS-chain ligands and therefore influence how cells respond to various cytokines. We demonstrate that NO is essential for this process, probably by forming NO2-, that can cleave the chains at GlcNH2 residues formed during the biosynthesis. We determined that the GlcNH2 residues are located close to the core protein in the glucosaminoglycan chains.
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