MOLECULAR-CLONING AND CHARACTERIZATION OF A TRANSMEMBRANE SURFACE-ANTIGEN IN HUMAN-CELLS

The mouse mAb 6C6, raised against a plasma-membrane preparation from h uman breast-cancer cells, reacts with an antigen that appears to be ov erexpressed in human breast cancers and other human tumors, Here we de scribe the cDNA cloning and characterization of the antigen recognized by the 6C6 mAb. The isolated cDNA clone encodes a protein of 246 amin o acids, with a predicted molecular mass of 27 991 Da. The protein con tains three amino-terminal hydrophobic regions, which could represent transmembrane domains, and a hydrophilic carboxy-terminal region, whic h we show to be extracellular. The identify of the protein encoded by the cloned cDNA as the 6C6 antigen was confirmed by in vitro translati on and immunoprecipitation experiments, and by transfection into cell lines that do not react with the 6C6 mAb, which resulted in the expres sion of a 28-kDa surface protein that was recognized by the antibody. The 6C6 antigen appears to be a type II transmembrane protein, with mu ltiple membrane-spanning domains and a long extracellular non-glycosyl ated carboxy-terminal domain, to which the 6C6 epitope has been mapped . The overall structure of the protein and weak amino acid similaritie s with a family of multiple transmembrane-spanning-domain proteins tha t 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.