The mouse mAb 6C6, raised against a plasma-membrane preparation from human
breast-cancer cells, reacts with an
antigen that appears to be overexpressed in human breast
cancers and other human
tumors. Here we describe the
cDNA cloning and characterization of the
antigen recognized by the 6C6 mAb. The isolated
cDNA clone encodes a
protein of 246
amino acids, with a predicted molecular mass of 27 991 Da. The
protein contains three amino-terminal hydrophobic regions, which could represent transmembrane domains, and a hydrophilic carboxy-terminal region, which we show to be extracellular. The identity of the
protein encoded by the cloned
cDNA as the
6C6 antigen was confirmed by in vitro translation and immunoprecipitation experiments, and by transfection into cell lines that do not react with the 6C6 mAb, which resulted in the expression of a 28-kDa
surface protein that was recognized by the antibody. The
6C6 antigen appears to be a type II transmembrane
protein, with multiple membrane-spanning domains and a long extracellular non-glycosylated carboxy-terminal domain, to which the 6C6
epitope has been mapped. The overall structure of the
protein and weak
amino acid similarities with a family of multiple-transmembrane-spanning-domain
proteins that includes some
antigens (such as L6, CD63/ME491 and CO-029) that are overexpressed in
tumor cells, suggest that the
6C6 antigen may belong to this family of
proteins.