Association of Pur alpha and E2F-1 suppresses transcriptional activity of E2F-1

Protein-protein interaction can play an important role in the control of se veral biological events including gene transcription, replication and cell proliferation. E2F-1 is a DNA-binding transcription factor which, upon inte raction with its target DNA sequence, induces expression of several S phase specific genes allowing progression of the cell cycle. Evidently, the acti vity of this protein is modulated by its cellular partner, pRb, which in th e hypophosphorylated form, binds to E2F-1 and inactivates its transcription al ability. In this study, we have demonstrated that expression of a sequen ce-specific single-stranded DNA binding protein, Pur alpha, in cells decrea ses the ability of E2F-1 to exert its transcriptional activity upon the res ponsive promoter derived from DHFR, Results from band shift experiments rev ealed that while Pur alpha does not recognize the double-stranded DNA fragm ent containing the E2F-1 binding site, it has the ability to inhibit E2F-1 interaction with its target DNA sequence. Results from GST pull-down assays and the combined immunoprecipitation/Western blot analysis of nuclear extr acts revealed a direct association of E2F-1 with Pur alpha in the absence o f the DNA molecule containing the E2F-1 binding site, The association of Pu r alpha with E2F-1 may increase the stability of E2F-1, as a higher level o f E2F-1 was detected in cells coexpressing Pur alpha and E2F-1, The importa nce of these observations with respect to the role of Pur alpha in the cont rol of cell cycle progression is discussed.