RB MAY ACT AS A TRANSCRIPTIONAL COACTIVATOR IN UNDIFFERENTIATED F9 CELLS

The reversible interaction of the retinoblastoma susceptibility gene p roduct (Rb) with the cellular transcription factor E2F has recently be en demonstrated. Activation of the adenovirus E2a promoter by the prod ucts of the viral E1a gene correlates with the ability of both early E 1a proteins to sequester Rb, thereby releasing E2F from inactive compl exes with this protein. The E2a promoter is also efficiently stimulate d by a product (17.5 kDa) of the viral E4 gene. The specific interacti on of this E4 protein with E2F results in the formation of complexes t hat bind cooperatively to the two neighboring E2F binding sites in the E2a promoter. We have previously shown that in undifferentiated F9 ce lls (F9EC) the E2a promoter is refractory to E2F-mediated activation b y E1a, but not by E4. Using both band-shift and transfection experimen ts, we now demonstrate (i) that in F9EC cells the E4 product, in combi nation with E2F, recruits Rb into a stable multiprotein complex and (i i) that in these undifferentiated cells, as opposed to their different iated counterpart, Rb is actively involved in the transcriptional stim ulation of the E2a promoter by E4. Our results suggest that, depending on the cell state, Rb may behave either as a transcriptional activato r (F9EC cells) or as a transcriptional inhibitor (differentiated F9 ce lls).