The formation of variants with a reversion of properties of transformed cells. III. Reversion of the structure of the cell surface membrane

The cell surface structure of variants from polyoma‐transformed cells grown in animals and then cultured in vitro , which show a reversion of in vitro properties of transformation without losing the ability to synthesize the polyoma‐specific nuclear tumor (T) antigen, has been studied by using the carbohydrate‐binding protein concanavalin A (Con. A) which interacts with sites on the cell surface membrane. Transformed cells are agglutinated by this protein. Normal cells, whose sites for Con. A are mainly in a cryptic form, are agglutinated only after they have been treated with trypsin. It has been shown that some variants showed a partial, and others a complete, loss of agglutinability by Con. A, and that the agglutinability of these variants was restored by treatment with trypsin. Compared to the parental transformed cells, all the variants showed the same decrease in cloning efficiency in fluid medium and soft agar, and in saturation density. The results indicate that the surface structure of transformed cells can completely revert to the structure found in normal cells, as measured by reversion to the cryptic form of sites for Con. A; that the synthesis of T antigen, presumably due to integrated virus DNA, is by itself not sufficient to prevent this reversion; that variants with this reversion in cells grown in vitro can have a high degree of tumorigenicity in animals; and that a threshold for the number of exposed surface sites for Con. A has to be exceeded before the cells express the in vitro properties of transformation.