Biosynthesis and intracellular processing of four human melanoma associated antigens.

By analyzing human melanoma cells with monoclonal antibodies (MoAb) we have identified four melanoma associated antigens with distinct tissue distribution and structural properties. They include the high molecular weight melanoma associated antigen (MAA), the Mr 120,000 MAA, the Mr 100,000 MAA, and the cytoplasmic MAA defined by MoAb 465.12. Previous studies have shown that these antigens may be useful markers to characterize the biology of melanoma cells and to develop immunodiagnostic and immunotherapeutic approaches to melanoma. In the present investigation pulse-chase intrinsic labeling studies combined with the biosynthetic inhibitor tunicamycin and with enzymatic degradations with endoglycosidase H have shown that the determinants recognized by the MoAb utilized are expressed on the core protein of the molecules. Furthermore the four MAAs are highly glycosylated with N-linked carbohydrate chains and are synthesized as precursors which bear endoglycosidase H-sensitive chains. The high molecular weight (Mr 500,000/280,000) MAA displays a major precursor with a molecular weight of 240,000 which expresses the epitopes recognized by the anti-high molecular weight MAA MoAbs 149.53, 225.28S, and 763.74T. This precursor has an apparent molecular weight of 220,000 when cells are grown in the presence of tunicamycin. The Mr 89,000 and Mr 36,000 subunits of the Mr 125,000 MAA have biosynthetic precursors with molecular weights of 76,000 and 25,000. Endoglycosidase H digestion of the Mr 76,000 precursor produces a Mr 46,000 polypeptide. The Mr 100,000 MAA has a Mr 87,000 biosynthetic precursor. The cytoplasmic MAA (Mr 100,000, 75,000, 72,000, and 25,000) has a single precursor with a molecular weight of 75,000 which appears as a Mr 60,000 polypeptide after endoglycosidase H digestion. Characterization of the biosynthesis of the four MAAs will contribute to the development of approaches to modulate their expression and shedding by melanoma cells.