Biochemical and immunological characterization of the human carcinoma-associated antigen MH 99/KS 1/4

We have characterized the 38-kDa transformation-associated membrane glycoprotein MH 99, whose expression is highly elevated in many epithelial malignancies. A spontaneous cleavage of MH 99 into a 32- and a 6-kDa chain in some carcinoma cell lines was recently shown. Sequence homologies to nidogen, a matrix-adhesion molecule, support the suggestion of a receptor-like function. In this study, we characterized biochemical and immunogenic aspects of MH 99. Transformed epithelial cell lines which do not spontaneously cleave MH 99 were exposed to 8 proteases with distinct specificities. Each of the enzymes produced similar specific fragmentation into chains of about 30 to 32 and 6 kDa, indicating a characteristic cleavage site of MH 99. The fragments were not distinguishable from those in carcinoma cells showing spontaneous cleavage of MH 99. The specific fragmentation depends on the localization in intact membranes and is not shared by other membrane proteins. N-glycosylation of MH 99 of about 4 kDa was exclusively found on the 32-kDa fragment. Characterization of antigenic epitopes was performed using 16 different monoclonal antibodies (MAbs). Only 2 independent determinants were found. One is located on the 32-kDa chain and is recognized only by the MM 104 MAb. The other 15 antibodies bind to a dominant epitope on the 6-kDa fragment which can be divided into 3 overlapping sub-epitopes. The unique features of MH 99 indicate that its immunogenic epitopes are mainly located at its 6-kDa chain, and support the suggestion of a transformation-associated cell-surface receptor which might be proteolytically regulated.