Inhibition of Rb and p53 is insufficient for SV40 T-antigen transformation.

The SV40 large T-antigen (TAg) has proven useful in studying pathways involved with cell division and tissue homeostasis. TAg disrupts the normal action of tumor suppressors pRb and p53. It is unclear whether T-antigen inhibition of p53 and pRb is sufficient for oncogenic transformation or if additional T-antigen activities are required. To pursue this question, cell lines were generated that coexpress an amino-terminal fragment of T-antigen (TAgN136), which has been shown to be sufficient to block pRb function, together with a dominant-negative p53. Neither focus formation nor saturation density was enhanced by coexpression of the dominant-negative p53 molecule, p53DD, along with TAgN136. Furthermore, a full-length TAg mutant incapable of binding p53 was capable of relieving contact inhibition, a hallmark of transformation. These results suggest the presence of a novel transforming activity in addition to the binding and inactivation of p53, requiring TAg amino acids 137 to 708.