Mechanisms of transformation by polyoma virus middle T antigen.

This review addresses a fundamental question of polyoma virus biology: What is the molecular mechanism by which the polyoma virus middle T antigen (MTAg) transforms cells in culture? Since MTAg has no known intrinsic biochemical activity, it is believed to act by modulating the properties of the host cell's proteins (see review by Courtneidge [26]). Experiments to date have largely focused on the interaction of MTAg with the cellular tyrosine kinase, pp60c-src. However, recent data from a number of laboratories have demonstrated the importance of other MTAg-associating cellular proteins in MTAg-mediated transformation, including pp62c-yes and a phosphatidylinositol kinase. In this review, we will summarize what is presently known about the proteins interacting with MTAg. The extent to which the currently known details of the biochemistry of MTAg and its associated proteins can explain the transforming properties of the various mutant alleles of MTAg will be assessed.