RB 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 is the S-phase-inducing transcription factor E2F1. RB binds directly to the activation domain of E2F1 and silences it, thereby preventing cells from entering S phase. To induce complete G1 arrest, RB requires the presence of the hbrm/BRG-1 proteins, which are components of the coactivator SWI/SNF complex. This cooperation is mediated through a physical interaction between RB and hbrm/BRG-1. We show here that in transfected cells RB can contact both E2F1 and hbrm at the same time, thereby targeting hbrm to E2F1. E2F1 and hbrm are indeed found within the same complex in vivo. Furthermore, RB and hbrm cooperate to repress E2F1 activity in transient transfection assays. The ability of hbrm to cooperate with RB to repress E2F1 is dependent upon several distinct domains of hbrm, including the RB binding domain and the NTP binding site. 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 repressing E2F1 activity could be an underlying mechanism for the observed cooperation between hbrm and RB to induce G1 arrest. Finally, we demonstrate that the domain of hbrm that binds RB has transcriptional activation potential which RB can repress. This suggest that RB not only targets hbrm but also regulates its activity.
Trouche
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D
Wellcome/Cancer Research Campaign Institute and Department of Pathology, University of Cambridge, Tennis Court Road, Cambridge CB2 1QR, United Kingdom.
Le Chalony
C
C
Muchardt
C
C
Yaniv
M
M
Kouzarides
T
T
D000595:Amino Acid Sequence; D002352:Carrier Proteins; D002453:Cell Cycle / Q000502:physiology*; D018797:Cell Cycle Proteins / Q000378:metabolism*; D002455:Cell Division; D002460:Cell Line; D002584:Cervix Uteri; D004268:DNA-Binding Proteins; D050684:E2F Transcription Factors; D050687:E2F1 Transcription Factor; D004847:Epithelial Cells; D005260:Female; D016193:G1 Phase; D006801:Humans; D008969:Molecular Sequence Data; D012516:Osteosarcoma; D011993:Recombinant Fusion Proteins / Q000737:chemistry / Q000378:metabolism; D016160:Retinoblastoma Protein / Q000378:metabolism*; D056465:Retinoblastoma-Binding Protein 1; D016196:S Phase; D016415:Sequence Alignment; D017386:Sequence Homology, Amino Acid; D050680:Transcription Factor DP1; D014157:Transcription Factors / Q000037:antagonists & inhibitors / Q000737:chemistry / Q000378:metabolism*; D014162:Transfection; D014407:Tumor Cells, Cultured
D002352:Carrier Proteins; D018797:Cell Cycle Proteins; D004268:DNA-Binding Proteins; D050684:E2F Transcription Factors; D050687:E2F1 Transcription Factor; C493753:E2F1 protein, human; D011993:Recombinant Fusion Proteins; D016160:Retinoblastoma Protein; D056465:Retinoblastoma-Binding Protein 1; C084011:SMARCA2 protein, human; D050680:Transcription Factor DP1; D014157:Transcription Factors