MEMBRANE TOPOLOGY OF THE L6 ANTIGEN AND IDENTIFICATION OF THE PROTEINEPITOPE RECOGNIZED BY THE L6 MONOCLONAL-ANTIBODY

The murine monoclonal antibody (mAb) L6 recognizes an integral membran e glycoprotein that is highly expressed on lung, breast, colon, and ov arian carcinomas and is referred to as the L6 antigen. This antigen is an attractive target for therapeutic intervention due to its high lev el expression on malignant cells. We have previously reported the isol ation of a cDNA encoding the human L6 antigen (H-L6). Here, we report the isolation of a cDNA clone encoding the murine L6 antigen (M-L6). T his cDNA contains one long open reading frame, which encodes a 220-ami no acid polypeptide that is 78% homologous to H-L6. This protein conta ins short NH2- and COOH-terminal hydrophilic domains and four hydropho bic regions, each long enough to span the plasma membrane. Each of the se hydrophobic domains is separated by a hydrophilic domain, the longe st of which contains one possible N-linked glycosylation site and is l ocated between the third and fourth hydrophobic domains. We have previ ously demonstrated that the murine L6 mAb recognizes a protein epitope expressed on human tumor-derived cell lines. Now, using chimeric cDNA constructs encoding human-murine L6 antigen hybrids in conjunction wi th monoclonal antibody binding experiments, we show that the 42-residu e hydrophilic domain of the L6 antigen, located between the third and fourth hydrophobic domains, is outside the cell and that residues in t he NH2-terminal region of this domain are critical for the binding of the murine L6 mAb to H-L6.