Transformation by polyoma virus middle T antigen.

The transforming protein of polyoma virus, middle T antigen, is a membrane-associated phosphoprotein. Middle T forms a complex with, and is phosphorylated by, a cellular tyrosine kinase pp60c-src. Mutant analysis suggests that formation of this complex is critical to transformation by polyoma. Middle T binding causes pp60c-src to autophosphorylate at novel sites in its amino terminus, and increases the specific activity of the enzyme. However, at least one non-transforming mutant of middle T, dl1015, can also activate pp60c-src in these ways. This suggests that properties of middle T other than the ability to activate pp60c-src are also necessary for transformation. These properties may include the ability to associate with a phosphatidylinositol kinase, and/or with a protein of 61 kDa. The mechanism by which middle T activates pp60c-src may involve its ability to alter the phosphorylation state of the enzyme, and thus interfere with the regulation of pp60c-src activity in vivo. Polyoma virus transformed cells might then be a good model system for investigating the control of pp60c-src activity as well as defining substrates of the enzyme.