Biosynthesis of the proteoglycan decorin -- identification of intermediates in galactosaminoglycan assembly

Biosynthesis of decorin was investigated by incubating a rat fibroblast cell line with various radiolabelled protein and galactosaminoglycan precursors. The following cell-associated and distinct intermediates were isolated and identified: a pool of non-glycosylated core protein, two pools of decorin with incomplete chains, one with three sulphated disaccharide repeats and another with five or more sulphated disaccharide repeats, as well as decorin with mature chains. Results of pulse/chase experiments indicated that these pools represented discrete stages in chain growth. Treatment with brefeldin A, which blocks transport from the endoplasmic reticulum to the Golgi, resulted in accumulation of decorin with an incomplete chain containing six or seven largely unsulphated disaccharide repeats. During recovery from drug treatment, 4-sulfation reappeared earlier than 6-sulfation. The results suggest that the galactosaminoglycan assembly-line consists of separate multienzyme complexes that build only a limited section of the chain. Furthermore, brefeldin A causes segregation of compartments involved in separate stages of the assembly line. In an earlier report [Moses, J., Oldberg. A., Cheng, F. & Fransson, L.-A. (1997) Eur. J. Biochem. 248, 521-526] we took advantage of such segregation to identify and characterize a transient 2-phosphorylation of xylose in the linkage region.