POSTTRANSLATIONAL 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 glycopr otein which carries the carcinoma-associated carbohydrate epitope sial yl-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 again st 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 membr ane and that COLO 205, unlike the leukocyte cell line HL-60, contained significant amounts of glycosylated intracellular CD43, Immunoprecipi tation of phosphate-labeled COLO 205 cells revealed that membrane boun d L-CanAg, like leukocyte CD43, is a phosphoprotein. Interestingly, bo th surface- and phosphate-labeled L-CanAg were eluted earlier from a g el filtration column than their unlabeled counterparts, indicating tha t this method could separate membrane-bound L-CanAg from its soluble f orm, Immunoprecipitations of pulse-chase-labeled COLO 205 lysates frac tionated by gel filtration showed that decrease in membrane-bound L-Ca nAg was concomitant with an increase in the intracellular soluble form , Together, these data indicate that transmembrane L-CanAg is fully gl ycosylated and phosphorylated before the extracellular domain is cleav ed off and stored inside the cell before exocytosis.