RE AND HBRM COOPERATE TO REPRESS THE ACTIVATION FUNCTIONS OF E2F1

Forced expression of the retinoblastoma (RB) gene product inhibits the proliferation of cells in culture, A major target of the RB protein i s the S-phase-inducing transcription factor E2F1. RB binds directly to the activation domain of E2F1 and silences it, thereby preventing cel ls from entering S phase, To induce complete G(1) arrest, RB requires the presence of the hbrm/BRG-1 proteins, which are components of the c oactivator SWI/SNF complex, This cooperation is mediated through a phy sical interaction between RB and hbrm/BRG-1. We show here that in tran sfected cells RB can contact both E2F1 and hbrm at the same time, ther eby targeting hbrm to E2F1, E2F1 and hbrm are indeed found within the same complex in vivo. Furthermore, RB and hbrm cooperate to repress E2 F1 activity in transient transfection assays, The ability of hbrm to c ooperate with RB to repress E2F1 is dependent upon several distinct do mains of hbrm, including the RB binding domain and the NTP binding sit e. However, the bromodomain seems dispensable for this activity, Taken together, our results point out an unexpected role of corepressor for the hbrm protein, The ability of hbrm and RB to cooperate in repressi ng E2F1 activity could be an underlying mechanism for the observed coo peration between hbrm and RB to induce G(1) arrest. Finally, we demons trate that the domain of hbrm that binds RB has transcriptional activa tion potential which RB can repress. This suggest that RB not only tar gets hbrm but also regulates its activity.