Functional analysis of a simian virus 40 super T-antigen

The SV3T3 C120 line of simian virus 40-transformed mouse cells synthesizes no large T-antigen of molecular weight 94,000 but instead a super T-antigen of molecular weight 145,000. In the accompanying paper (Lovett et al., J. Virol. 44:963-973, 1982), we showed that the integrated viral DNA segment SV3T3-20-K contains a perfect, in-phase, tandem duplication of 1.212 kilobases within the large T-antigen coding sequences. Our data suggested that this integrated template encodes mRNAs of 3.9 and 3.6 kilobases, the smaller of which directs the synthesis of the super T-antigen of molecular weight 145,000. We transfected the DNA segment SV3T3-20-K into nonpermissive rat cells and into TK- mouse L cells and analyzed the T-antigens and viral mRNAs in the transfectants; these data prove directly the coding assignments suggested previously. The super T-antigen retained the ability to induce morphological transformation, and may even transform better than the wild-type protein. It also retained the ability to bind to the cell-coded p53 protein. Transfection into permissive CV-1 cells showed that the super T-antigen encoded by SV3T3-20-K was incapable of initiating DNA replication at the viral origin. The duplication in SV3T3-20-K thus defines a mutation which separates the transformation and DNA replication functions of large T-antigen. We discuss why such mutations may be selected in transformed cells.