RESUMO
Simian virus 40 large T antigen has been shown to inhibit p53-mediated transcription once tethered to p53-responsive promoters through interaction with p53. In this study we report that p53 stimulates transcription by enhancing the recruitment of the basal transcription factors, TFIIA and TFIID, on the promoter (the DA complex) and by inducing a conformational change in the DA complex. Significantly, we have demonstrated that T antigen inhibits p53-mediated transcription by blocking this ability of p53. We investigated the mechanism for this inhibition and found that DA complex formation was resistant to T-antigen repression when the TFIID-DNA complex was formed prior to addition of p53-T antigen complex, indicating that the T antigen, once tethered to the promoter by p53, targets TFIID. Further, we have shown that the p53-T antigen complex prevents the TATA binding protein from binding to the TATA box. Thus, these data suggest a detailed mechanism by which p53 activates transcription and by which T antigen inhibits p53-mediated transcription.
Assuntos
Antígenos Transformantes de Poliomavirus/metabolismo , Proteínas de Ligação a DNA/metabolismo , Regiões Promotoras Genéticas , TATA Box , Fatores de Transcrição TFII/genética , Fatores de Transcrição/genética , Fatores de Transcrição/metabolismo , Proteína Supressora de Tumor p53/metabolismo , Proteínas de Ligação a DNA/genética , Células HeLa , Humanos , Proteínas Recombinantes de Fusão/genética , Proteínas Recombinantes de Fusão/metabolismo , Proteína de Ligação a TATA-Box , Fator de Transcrição TFIIA , Fator de Transcrição TFIID , Ativação TranscricionalRESUMO
Simian virus 40 (SV40) large tumor antigen (T antigen) has been shown to inhibit p53-dependent transcription by preventing p53 from binding to its cognate cis element. Data presented in this report provide the first direct functional evidence that T antigen, under certain conditions, may also repress p53-dependent transcription by a mechanism in which the transactivation domain of p53 is abrogated while DNA binding is unaffected. Specifically, p53 purified as a complex with T antigen from mouse cells was found to bind DNA as a transcriptionally inactive intact complex, while that purified from human cells was found to bind DNA independently of T antigen and could activate p53-dependent transcription. This difference in activity may be dependent on a different interaction of T antigen with mouse and human p53 and, in addition, on the presence of super T, which is found only in transformed rodent cells. These results suggest that subtle yet important differences exist between the inhibition of p53 by T antigen in mouse and human cells. The implications of this finding with respect to SV40-associated malignancies are discussed.
