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.
Baeckström
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Department of Medical Biochemistry, University of Göteborg, Medicinaregatan 9, S-413 90 Göteborg, Sweden. d.baeckstrom@icrf.icnet.uk
D000230:Adenocarcinoma / Q000378:metabolism*; D015703:Antigens, CD / Q000096:biosynthesis* / Q000276:immunology; D015295:Antigens, Tumor-Associated, Carbohydrate / Q000096:biosynthesis* / Q000276:immunology; D018395:CA-19-9 Antigen; D002240:Carbohydrate Sequence; D003110:Colonic Neoplasms / Q000378:metabolism*; D005732:Gangliosides / Q000096:biosynthesis / Q000276:immunology; D006031:Glycosylation; D018922:HL-60 Cells; D006801:Humans; D007962:Leukocytes; D051917:Leukosialin; D008565:Membrane Proteins / Q000096:biosynthesis; D008969:Molecular Sequence Data; D009077:Mucins / Q000378:metabolism; D010750:Phosphoproteins / Q000096:biosynthesis / Q000276:immunology; D010766:Phosphorylation; D011499:Protein Processing, Post-Translational; D012795:Sialoglycoproteins / Q000096:biosynthesis* / Q000276:immunology; D014407:Tumor Cells, Cultured
D015703:Antigens, CD; D015295:Antigens, Tumor-Associated, Carbohydrate; D018395:CA-19-9 Antigen; D005732:Gangliosides; D051917:Leukosialin; D008565:Membrane Proteins; D009077:Mucins; D010750:Phosphoproteins; C465014:SPN protein, human; D012795:Sialoglycoproteins; C064564:sialyl Le(a) ganglioside