BIOSYNTHESIS OF THE PROTEOGLYCAN DECORIN - IDENTIFICATION OF INTERMEDIATES IN GALACTOSAMINOGLYCAN ASSEMBLY

Biosynthesis of decorin was investigated by incubating a rat fibroblas t 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 disaccha ride repeats, as well as decorin with mature chains. Results of pulse/ chase experiments indicated that these pools represented discrete stag es 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 u nsulphated disaccharide repeats, During recovery from drug treatment, 4-sulfation reappeared earlier than 6-sulfation. The results suggest t hat the galactosaminoglycan assembly-line consists of separate multien zyme complexes that build only a limited section of the chain. Further more; brefeldin A causes segregation of compartments involved in separ ate stages of the assembly line. In an earlier report [Moses, J., Oldb erg, A., Cheng, F. & Fransson, L.-A. (1997) Eur: J. Biochem. 248, 521- 526] we took advantage of such segregation to identify and characteriz e a transient 2-phosphorylation of xylose in the linkage region.