Molecular characterization of a tissue-polypeptide-specific-antigen epitope and its relationship to human cytokeratin 18

Tissue-polypeptide-specific antigen (TPS) from the human colon adenocarcinoma cell line WiDr was purified using the monoclonal antibody M3 as a probe. Upon SDS/polyacrylamide gradient gel electrophoresis, several TPS-positive bands were detected (corresponding to 13 kDa, 22 kDa and a doubler at 42 kDa). The 13-kDa moiety was purified about 30 000-fold by a 5-step protocol. The electro-phoretically homogeneous component was obtained in a 7% yield of the total TPS activity of the crude extract. N-terminal sequence analysis showed the presence of an N-terminally truncated molecule and identified the 13-kDa TPS component as a fragment of human cytokeratin 18, with a major form starting at position 284 of the parent molecule. Laser-desorption mass spectrometry showed the presence of one major component with a molecular mass corresponding to a C-terminal end close to position 396 (which gives 12 776 Da for the form with non-truncated N-terminus). The M3 antibody was also used to screen a human prostate cDNA lambda gt11 library. Four identical phage clones were detected, each producing a fusion protein with beta-galactosidase and the M3-positive component. PCR amplification showed the presence of an approximately 1200-bp insert, and sequence analysis revealed it to contain a 996-nucleotide fragment corresponding to residues 103-429 of human cytokeratin 18 (plus a non-coding human desmin artifact fragment). Smaller fragments, engineered by PCR and expressed as fusion proteins using the pET3xc vector in Escherichia coli, showed that the M3 epitope is localized to cytokeratin 18, residues 322-340. Two other TPS-active monoclonal antibodies were localized to cytokeratin 18 with similar techniques, ascribing an epitope (to M21) to residues 414-429 and another (to M24) to residues 139-297. Combined, the results demonstrate that TPS reactivity is derived from specific epitopes of human cytokeratin 18.