MAGE-1 gene product is a cytoplasmic protein

MAGE-1 gene encodes a human melanoma antigen, recognized by syngeneic cytotoxic T lymphocytes (CTL). MAGE-1 transcripts are also detectable in breast cancers, in non-small-cell lung carcinomas and in central nervous system tumors. In order to identify, in cellular preparations, the protein encompassing the antigenic peptide, we generated a panel of monoclonal antibodies (MAbs) against the MAGE-1 gene product by using, as immunogen, a full-length recombinant preparation (rMAGE-1), obtained through expression cloning of the relevant gene in E. coli. Four reagents were obtained recognizing both rMAGE-1 and the 46-kDa native protein in cell lines expressing MAGE-I mRNA. No positivity could be detected in MAGE-1-mRNA-negative melanoma lines. No surface labelling of MAGE-1-positive cell lines could be observed. In contrast, on permeabilization of MZ2 melanoma cells, all 4 MAbs induced efficient staining, as detected by cytofluorography. Fluorescence microscopy shows that MAGE-I gene product is a cytoplasmic protein clustered in paranuclear organelle-like structures. Thus, MAGE-1 protein location closely resembles that of P91A and P198 murine-tumor antigens.