A soluble
lectin, the
core-specific lectin (CSL), is synthesized and secreted by rat hepatocytes and the rat
hepatoma cell line, H-4-II-E. This
lectin binds
mannose and
N-acetylglucosamine residues in the "core" region of Asn-linked
oligosaccharides. Secretion of the CSL was found to occur over an extended period of time, greater than 4 h being required for secretion of 50% of the
lectin (Brownell, M. D., Colley, K. J., and Baenziger, J. U. (1984) J. Biol. Chem. 259, 3925-3932). We have determined that following synthesis in the endoplasmic reticulum, the CSL is rapidly transported to the Golgi where it is retained for an extended period of time prior to secretion. The
lectin undergoes two post-translational modifications within the Golgi: an increase from Mr 24,000 to 25,000 and a progressive decrease in pI with an accompanying increase in Mr to a final value of 26,000. The
lectin is also assembled into high molecular weight complexes of 150-260 X 10(3) and acquires the ability to bind
carbohydrate in the Golgi. In
hepatoma cells, the 24,000-25,000 modification is completed 20 min after initiation of synthesis. Assembly of the CSL subunits into high molecular weight complexes, acquisition of
carbohydrate binding activity, and the 25,000-26,000 modification occur between 20 and 80 min after initiation of synthesis. These events have slower kinetics in primary hepatocytes and this allowed us to determine that the sequence of these biosynthetic events is: the 24,000-25,000 modification, complex assembly, the 25,000-26,000 modification, and acquisition of
carbohydrate binding activity. The 24,000-25,000 modification occurs prior to complex assembly. Complex assembly may occur prior to, or concomitant with, the 25,000-26,000 modification. Assembly into the oligomeric form and the 25,000-26,000 modification correlate with the attainment of
carbohydrate binding activity. The kinetics of CSL modification and assembly cannot account for its retention within the Golgi. Interaction with Golgi components either through
carbohydrate binding or another interaction, may act to selectively retain the
lectin within the Golgi.