Abstract
Mammalian cells transformed in tissue culture by SV40 were shown to contain, in addition to the SV40‐coded 94,000 d large T antigen and the 20, 000 d small t antigen, a ∼ 56,000 d cellular protein, which specifically precipitates with sera of animals bearing SV40‐induced tumor (s) (tumor or T serum). We investigated the presence of these three proteins at the surface of logarithmically growing SV40‐transformed cloned mouse cells, after metabolic labelling with [^35^S]‐methionine for 3 h. The 56,000 d protein was found to be susceptible to digestion by trypsin under conditions which did not disrupt the cells, while no small t antigen was found to be digested. Both the 56,000 d cellular protein and the SV40 large T antigen were susceptible to lactoperoxi‐dase‐catalyzed iodination from the outside of intact cells. Trypsin treatment removed both the iodinated 56,000 d protein and the iodinated SV40 large T antigen. These experiments indicated that (a certain amount of) the 56,000 d protein and a relatively small amount of the large T antigen (which is present mainly in the nucleus) are present on the cell surface. The results confirm and extend independent experiments using subcellular frac‐tionation techniques (Luborsky and Chandrasekaran, 1980; Soule and Butel, 1979). After heat treatment (at 50°C for 30 min) of the whole‐cell extract the 56,000 d cellular protein was precipitated by the tumor serum in the absence of precipitation of SV40 large T antigen. This result showed that the 56,000 d protein is more (thermostable (in the whole‐cell extract) than the SV40 large T antigen, and also indicated that the tumor serum employed had antibodies against the 56,000 d cellular antigen. The heat‐treated whole‐cell extract of SV40‐transformed mouse cells was able to immunize and fully protect mice against a lethal tumorigenic dose of SV40‐transformed cells. These results suggest the need for further experiments to characterize the chemical and immunologic properties of the 56,000 d protein.