A NOVEL MEMBER OF THE IMMUNOGLOBULIN GENE SUPERFAMILY EXPRESSED IN RAT CARCINOMA CELL-LINES

Defined by monoclonal antibody E4, the pE4 antigen is a 66,000-Da glycoprotein which is expressed at the cell surface of rat colon and mammary carcinomas, but only in trace amounts in normal adult rat tissues. To determine the structure of this tumor-associated antigen and to identify its functional domains, we have cloned a cDNA coding for this protein. It encodes a 416-amino acid protein with an expected molecular weight for the core protein of similar to 42,000. The predicted amino acid sequence reveals that pE4 contains the conserved amino acids and domain structures characteristic of members of the immunoglobulin gene superfamily. Comparison of this sequence with data banks revealed a significant homology with the human and mouse receptors for poliovirus. However, pE4 is not the rat receptor for poliovirus, as different patterns were obtained by hybridization of rat genomic DNA with both probes. A major similar to 2.2-kilobase transcript of the pE4 gene was detected in all the rat tumor cell lines tested. In contrast, barely detectable levels of pE4 mRNA were found in normal adult rat tissues.