Identification of a 90 000-Dalton cell surface glycoprotein with elevated expression in human hepatoma cells

We describe a human-specific cell surface glycoprotein of molecular size 90 000-dalton (90K) and isoelectric point 5 defined by a monoclonal antibody prepared using human hepatoma-mouse hepatoma hybrid cells with a limited number of human chromosomes (6, 7, 14, 20, 21, and X) as immunogens in syngeneic mice. While detectable on cultured human cells of diverse origin, expression of the 90K protein is elevated in hepatoma cells. Moreover, a protein of identical molecular size and slightly more acidic isoelectric point is present in hepatoma culture supernatant. We sought to determine the identity of the 90K protein by comparing it to two hepatoma-expressed, major histocompatibility complex-linked proteins of similar molecular size, the alpha-chain of C4 and factor B; this comparison was also prompted by the presence of human chromosome 6 in the immunizing hybrids. We find no evidence, however, for these proteins being related. Melanoma-associated antigenic determinants carried by proteins of similar molecular size have been reported, and the possible relation of these proteins to the 90K protein is discussed.