Cluster 13 was defined by 2 independently derived murine monoclonal antibodies (MAbs), RS7 (IgG,) and MR54 (IgG2,), which were raised against human squamous-cell carcinoma of the lung and a human ovarian-carcinoma cell line, respectively. Immunologic and biochemical evidence demonstrated that RS7 and MR54, as well as 2 additional MAbs, MR6 (lgG2J and MR23 (IgG,), generated in the same fusion as MR54, recognize the same antigen, a 46-to 48-kDa glycoprotein. Evaluation of the expression of antigen on the surface of tumor cell lines, Western blotting analyses, competitive binding studies, and doubledeterminant ELISA assays, support this conclusion. Two distinct epitopes are defined by these MAbs.
In order to further characterize this antigen, amino-acidsequence analyses were performed on peptides derived from antigen purified by affinity chromatography with MAb RS7. The sequence data obtained from 2 peptides, which were independently generated by CNBr cleavage and tt-ypsin digestion respectively indicated identity to GA733-I. The GA733-I genomic DNA sequence predicted a type-I membrane protein of 35 kDa, with 4 potential N-linked glycosylation sites. The GA733-I protein product has not been identified previously, and MAbs to this tumor-associated antigen were not previously known.