Synthesis, processing, and secretion of the core-specific lectin by rat hepatocytes and hepatoma cells

A lectin, previously designated the Man/GlcNAc-specific lectin or mannan-binding protein, is found in rat liver and plasma. Analysis of the structural requirements for oligosaccharide binding indicated that the specificity of this lectin is directed primarily at the "core" and peptide region of glycopeptides (Maynard, Y., and Baenziger, J. U. (1982) J. Biol. Chem. 257, 3788-3794). We have examined synthesis and secretion of the core-specific lectin by primary rat hepatocytes and a rat hepatoma, H-4-II-E, utilizing pulse labeling with [35S]methionine, immunoprecipitation with a monospecific rabbit antibody raised against the purified lectin, and analysis by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and fluorography. A post-translational modification occurs between 10 and 40 min of chase which results in an increase in the Mr from 24,000 to 26,000. This modification is not due to asparagine-linked glycosylation or oligosaccharide processing. The kinetics of secretion are unusual. Secretion begins at 1 h of chase and proceeds linearly for approximately 8 h until a maximum of 70% of the lectin has been secreted. Secretion, but not the post-translational modification is inhibited by monensin. The pattern of synthesis and secretion in conjunction with the presence of the lectin in plasma indicate that it is a plasma protein of hepatocyte origin. The slow kinetics of secretion compared to other secretory proteins indicate an unusual mechanism for the segregation of the lectin from other secretory proteins and/or a different intracellular pathway for secretion.