Molecular cloning of yeast cytochrome c-like polypeptide expressed in human lung carcinoma: an antigen recognizable by lung cancer-specific human monoclonal antibody.

We previously determined the amino acid sequence to the epitope (ATLFKTR) of cytochrome c from Candida krusei, which is cross-reactive to the lung cancer-specific human monoclonal antibody HB4C5. Here we report that an antigen messenger RNA, which codes for a structure similar to the cytochrome c epitope, is expressed in the human lung adenocarcinoma A549. Sequencing analysis has revealed that this messenger RNA encodes a novel 190 amino acid polypeptide of 21-kDa containing an amino acid sequence (ALLFFT) similar to the cytochrome c epitope, although the total messenger RNA sequence is apparently different from the cytochrome c messenger RNA. Western analysis indicated that an antibody-recognizable 21-kDa antigen which has the same molecular weight as the predicted polypeptide is expressed in the A549 adenocarcinoma. The in vitro translated product of the antigen messenger RNA and synthesized ALLFFT peptide were both shown to be reactive with the monoclonal antibody, indicating that this protein contains the epitope which enables A549 cells to specifically react with the antibody. The antigen mRNA was not expressed in non-transformed fibroblasts, suggesting that the antigen mRNA expression was associated with cellular transformation. Also in part of the antigen nucleotide sequence, there was a segment that had about 90% homology to the long terminal repeat sequence (no. 297-475) of the human endogenous retrovirus HERV-K10, which was related to the mouse mammary tumor virus.