Relationship between E1A binding to cellular proteins, c-myc activation and S-phase induction.
We recently showed that p300/CREB-binding protein (CBP) plays an important role in maintaining cells in G0/G1 phase by keeping c-myc in a repressed state. Consistent with this, adenovirus E1A oncoprotein induces c-myc in a p300-dependent manner, and the c-myc induction is linked to S-phase induction. The induction of S phase by E1A is dependent on its binding to and inactivating several host proteins including p300/CBP. To determine whether there is a correlation between the host proteins binding to the N-terminal region of E1A, activation of c-myc and induction of S phase, we assayed the c-myc and S-phase induction in quiescent human cells by infecting them with Ad N-terminal E1A mutants with mutations that specifically affect binding to different chromatin-associated proteins including pRb, p300, p400 and p300/CBP-associated factor (PCAF). We show that the mutants that failed to bind to p300 or pRb were severely defective for c-myc and S-phase induction. The induction of c-myc and S phase was only moderately affected when E1A failed to bind to p400. Furthermore, analysis of the E1A mutants that fail to bind to p300, and both p300 and PCAF suggests that PCAF may also play a role in c-myc repression, and that the two chromatin-associated proteins may repress c-myc independently. In summary, these results suggest that c-myc deregulation by E1A through its interaction with these chromatin-associated proteins is an important step in the E1A-mediated cell cycle deregulation and possibly in cell transformation.