Biosynthesis, glycosylation and intracellular processing of the neuroglandular antigen, a human melanoma-associated antigen.

Neuroglandular antigen (NGA) was identified as a human melanoma-associated antigen by a panel of murine monoclonal antibodies of both IgG2a (LS62, LS76, LS159) and IgG1 (LS113, LS140, LS152) subclasses, developed in this laboratory (L. Sikora, A. Pinto, D. Demetrick, W. Dixon, S. Urbanski, and L. M. Jerry, Int. J. Cancer, 39: 138-145, 1987). Monoclonal antibody LS62 was used to immunoprecipitate NGA from radiolabeled cultured melanoma cells, and it behaved as a heterogeneous glycoprotein "smear" on sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis (Mr 29,000-70,000). Radioactive pulse-chase time course experiments using human melanoma cells cultured in the presence or absence of inhibitors of protein glycosylation showed that the antigen consisted of a core protein with a molecular weight of 22,000 on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. This molecule was modified by the addition of at least three N-linked oligosaccharide side chains (as revealed by limited N-glycanase digestion) to give a precursor form with a molecular weight of approximately 34,000. Subsequent processing steps yielded a heterogeneous family of glycoproteins with varying amounts of covalently attached carbohydrate. Much of this heterogeneity in both molecular weight and pI (as revealed by two-dimensional electrophoresis) could be removed by treatment of the antigen with neuraminidase, suggesting heavy sialylation of the glycoprotein. NGA could be detected on the surface of melanoma cells by fluorescence-activated cell sorter analysis, surface radioiodination, and, as previously shown, immunoperoxidase staining. However, there was a larger intracellular pool of the molecule and the antigen was rapidly released into the culture supernatant. The function of NGA remains unknown but its elevated expression in transformed melanocytes have prompted this characterization to understand its biochemical nature and relation to other melanoma-associated antigens.