Molecular Identification of a Human Carcinoma‐associated Glycoprotein Antigen Recognized by Mouse Monoclonal Antibody FU‐MK‐1

Mouse monoclonal antibody FU‐MK‐1, raised against a human gastric adenocarcinoma, recognizes an antigen (termed MK‐1 antigen) present on the majority of carcinomas. The present study aimed to identify the MK‐1 molecule and to establish its relationship to other carcinoma antigens. Immunoprecipitation studies of human tumor cell lines revealed that FU‐MK‐1 recognizes a monomeric membrane glycoprotein with two forms, 40 kDa (major form) and 42 kDa (minor form), and with a molecular mass of 35 kDa following treatment with the N‐glycosylation inhibitor tunicamycin. The partial amino acid sequence of a main fragment of the MK‐1 molecule obtained by spontaneous cleavage under hypotonic conditions was examined, and the 17 contiguous NH(2)‐terminal amino acids were found to be identical with residues 81–97 of the 314‐residue GA733–2 protein [Szala et al.; Proc. Natl. Acad. Sci. USA, 87, 3542–3546 (1990)]. Hence, the GA733–2 cDNA was cloned and the specificity of FU‐MK‐1 was confirmed using four recombinant forms of the GA733–2 antigen expressed in COS‐1 cells. Immunoprecipitation with FU‐MK‐1 of the cell lysate transfected with the full‐length GA733–2 cDNA revealed two bands corresponding to those obtained from the tumor cell lines. FU‐MK‐1 also precipitated three other recombinant proteins consisting of amino acids 1–265, 1–201, and 1–139 of the GA733–2 protein, respectively. Furthermore, immunoblotting analysis indicated that FU‐MK‐1 binds to a small fragment (6 kDa) generated from a tumor cell line under hypotonic conditions, suggesting that the FU‐MK‐1 epitope exists on the distal 6‐kDa peptide of the extracellular domain of the GA733–2 molecule. We thus conclude that the MK‐1 antigen is the GA‐733–2 antigen, which is currently being used as a target in clinical trials with monoclonal antibodies.