Modular organization of the E2F1 activation domain and its interaction with general transcription factors TBP and TFIIH.

The transcriptional activator E2F1 regulates the expression of genes at the G1/S boundary. We have characterized interactions of the E2F1 activation domain with two general transcription factors, the TATA-box binding protein (TBP) and TFIIH. Two distinct binding sites on E2F1 were identified for TBP (amino acids 386-417 and 415-437) each of which supported activation in mammalian cells when expressed as a fusion to a heterologous DNA-binding domain. Neither of these minimal activation domains independently bound TFIIH; rather, the TFIIH binding site of E2F1 overlaps both domains. Loss of TFIIH-binding by E2F1 resulted in a 60-65% reduction in transactivation, suggesting that the E2F1/TFIIH interaction is important, but not essential, for transactivation. The retinoblastoma protein (Rb) binds directly to E2F1 and represses E2F1-mediated transactivation. We have demonstrated that recombinant Rb can compete with TBP and the p62 subunit of TFIIH for binding to immobilized E2F1. A tumorigenic form of Rb deficient in repressing E2F1-mediated transactivation is likewise deficient in displacing TBP from E2F1. We propose that competition between Rb and both TBP and TFIIH for binding to E2F1 is a mechanism by which Rb inhibits transactivation by E2F1.