CLONING AND EXPRESSION OF THE GENE FOR THE MELANOMA-ASSOCIATED ME20 ANTIGEN

Human melanoma cells, but not tumor cells of other histological origin , express a unique membrane-associated glycoprotein, designated ME20-M , and secrete a soluble glycoprotein, designated ME20-S, defined by mo noclonal antibody ME20. Here we report the isolation and characterizat ion of a cDNA clone that when transfected into COS cells directs the e xpression of ME20-M and ME20-S. This cDNA contains an open reading fra me which encodes a 661-amino-acid-long precursor that contains a 23-am ino-acid signal peptide and a 26-amino-acid transmembrane domain, sepa rated by a hydrophilic region containing 5 potential Asn-linked and 14 predicted Pro-associated, Thr-linked glycosylation sites. The transme mbrane domain is followed by a carboxy-terminal 45-amino-acid putative intracellular domain rich in Ser residues. Analysis of ME20-M by amin o acid sequencing identified the proteolytic processing site. Signal p eptide cleavage occurs at the Thr-24-Lys-25 peptide bond of the precur sor and results in the 637-amino-acid ME20-M with a calculated molecul ar weight of 67,782. ME20-M is derived from a single 3.3- to 3.4-kb mR NA transcript that is expressed at varying levels in melanoma cell lin es, correlating with immunofluorescence determination of protein expre ssion. The amino acid sequence of the ME20 antigen deduced from the cD NA differs from the human neonatal melanocyte-specific Pmel 17 gene pr oduct by a single amino acid substitution and deletion of 7 amino acid residues, and it is 80% homologous with the bovine retinal pigment RP E1 cDNA. The observed differences in the sequences, of the melanoma-de rived and normal melanocyte-derived gene products may suggest that ME2 0-M is a unique tumor antigen or, alternatively, an oncofetal self-ant igen, if the differences in the sequences are the result of a germ-lin e-encoded polymorphism.