Post-translational Fate of a Mucin-like Leukocyte Sialoglycoprotein (CD43) Aberrantly Expressed in a Colon Carcinoma Cell Line

This paper describes the biosynthesis of L-CanAg, a mucin-like glycoprotein which carries the carcinoma-associated carbohydrate epitope sialyl-Lewis a and is secreted by the colon adenocarcinoma cell line COLO 205. Recently, it has been shown that L-CanAg is a novel glycoform of CD43, a surface sialoglycoprotein normally found only on hematopoietic cells. Immunoprecipitation with alpha-GPEP18, a novel antiserum against the cytoplasmic domain of CD43, detected a transmembrane form of L-CanAg carrying sialyl-Lewis a. Cell surface biotinylation experiments demonstrated the presence of transmembrane L-CanAg at the plasma membrane and that COLO 205, unlike the leukocyte cell line HL-60, contained significant amounts of glycosylated intracellular CD43. Immunoprecipitation of phosphate-labeled COLO 205 cells revealed that membrane-bound L-CanAg, like leukocyte CD43, is a phosphoprotein. Interestingly, both surface- and phosphate-labeled L-CanAg were eluted earlier from a gel filtration column than their unlabeled counterparts, indicating that this method could separate membrane-bound L-CanAg from its soluble form. Immunoprecipitations of pulse-chase-labeled COLO 205 lysates fractionated by gel filtration showed that decrease in membrane-bound L-CanAg was concomitant with an increase in the intracellular soluble form. Together, these data indicate that transmembrane L-CanAg is fully glycosylated and phosphorylated before the extracellular domain is cleaved off and stored inside the cell before exocytosis.