MAGE‐1 gene product is a cytoplasmic protein

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Abstract
MAGE‐I gene encodes a human melanoma antigen, recognized by syngeneic cytotoxic T lymphocytes (CTL). MAGE‐I transcripts are also detectable in breast cancers, in non‐smallcell 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‐I gene product by using, as immunogen, a full‐length recombinant preparation (rMAGE‐I), obtained through expression cloning of the relevant gene in E. coli. Four reagents were obtained recognizing both rMAGE‐I and the 46‐kDa native protein in cell lines expressing MAGE‐I mRNA. No positivity could be detected in MAGE‐I‐mRNA‐negative melanoma lines. No surface labelling of MAGE‐I‐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‐I protein location closely resembles that of P9IA and PI98 murine‐tumor antigens.