THE CBP COACTIVATOR STIMULATES E2F1 DP1 ACTIVITY/

The cell cycle-regulating transcription factors E2F1/DP1 activate gene s whose products are required for S phase progression. During most of the G1 phase, E2F1/DP1 activity is repressed by the retinoblastoma gen e product RB, which directly contacts the E2F1 activation domain and s ilences it. The E2F1 activation domain has sequence similarity to the N-terminal activation domain of E1A(12S), which contains binding sites for CBP as well as RB. Here, we present evidence that the CBP protein directly contacts E2F1/DP1 and stimulates its activation capacity. We show that CBP interacts with the activation domain of E2F1 both in vi tro and in vivo. Deletion of four residues from the E2F1 activation do main reduces CBP binding as well as transcriptional activation, but st ill allows the binding of RB and MDM2. This deletion removes residues which are conserved in the N-terminal activation domain of E1A and whi ch are required for the binding of CBP to E1A. When the E1A N-terminus is used as a competitor in squelshing experiments it abolishes CBP-in duced activation of E2F1/DP1, whereas an E1A mutant lacking CBP bindin g ability fails to do so. These results indicate that CBP can act as a coactivator for E2F1 and suggest that CBP recognises a similar motif within the E1A and E2F1 activation domains. The convergence of the RB and CBP pathways on the regulation of E2F1 activity may explain the co operativity displayed by these proteins in mediating the biological fu nctions of E1A. We propose a model in which E1A activates E2F not only by removing the RB repression but also by providing the CBP co-activa tor.