BIOSYNTHESIS OF THE PROTEOGLYCAN DECORIN - TRANSIENT 2-PHOSPHORYLATION OF XYLOSE DURING FORMATION OF THE TRISACCHARIDE LINKAGE REGION

Phosphorylation of decorin was investigated by incubating a rat fibrob last cell line with radiolabelled phosphate and carbohydrate precursor s. There was a transient phosphorylation of the linkage-region sacchar ides in intracellular decorin prior to assembly of the galactosaminogl ycan chain. Phosphorylation gradually increased from xylosylated, gala ctosyl-xylosylated to galactosyl-galactosyl-xylosylated core protein w here all trisaccharide stubs were phosphorylated. Addition of the firs t glucuronate residue was accompanied by rapid dephosphorylation. Bref eldin A treatment resulted in segregation of galactosaminoglycan synth esis and dephosphorylation. Enzymatic degradation of brefeldin-A-arres ted immature proteoglycan with incomplete galactosaminoglycan chain [M oses, J., Oldberg, Angstrom., Eklund, E. & Fransson, L.-Angstrom. (199 7) fur. J. Biochem., in the press] by using chondroitin AC lyase and c hondro-glycuronidase, followed by beta-galactosidase treatment, demons trated the sequence galactosyl-galactosyl-phosphoxylose. The xylose wa s resistant to direct periodate oxidation, but sensitive after treatme nt with alkaline phosphatase, showing that the phosphate was located a t C2 of xylose. The transient 2-phosphorylation of xylose may be invol ved in intracellular transport and/or in the control of modifications of the glycan chain.