Molecular chaperone function of the SV40 large T antigen.

The SV40 large T antigen (T antigen) is a potent viral oncogene capable of inducing tumours in test animals and transforming cells in culture. T antigen possesses multiple transforming functions that act in a cell-type dependent manner. One of these transforming functions requires a physical association between T antigen and the cellular tumour suppressor p53, while another requires T antigen binding to the retinoblastoma family of tumour suppressors. A third transforming function, <<Activity X>>, maps to the amino terminus of large T antigen, sequences also present in small t antigen. Our recent studies have shown that Activity X: (i) is a J-domain and that T antigen possesses the biochemical properties of a DnaJ molecular chaperone; (ii) must act in cis with the T antigen Rb-family binding motif to transform; and, (iii) must act in cis with some factor through its carboxy terminal half of T antigen, possibly p53, to transform.