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 glyc oprotein which is expressed at the cell surface of rat colon and mamma ry carcinomas, but only in trace amounts in normal adult rat tissues. To determine the structure of this tumor-associated antigen and to ide ntify its functional domains, we have cloned a cDNA coding for this pr otein. 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 s ignificant homology with the human and mouse receptors for poliovirus. However, pE4 is not the rat receptor for poliovirus, as different pat terns were obtained by hybridization of rat genomic DNA with both prob es. A major similar to 2.2-kilobase transcript of the pE4 gene was det ected in all the rat tumor cell lines tested. In contrast, barely dete ctable levels of pE4 mRNA were found in normal adult rat tissues.