The molecular and functional characterization of E2F-5 transcription factor.

The E2F activity plays a critical role in the control of cell cycle and action of tumor suppressor proteins and is also a target of the transforming proteins of small DNA tumor viruses. We describe here molecular cloning and functional characterization of a fifth member of the E2F family of transcription factors. E2F-5 protein is more homologous to E2F-4 (72% amino acid identity) than to E2F-1, E2F-2, and E2F-3 (35% amino acid identity). Based on structural and functional criteria, the E2F family appears to comprise two distinct sub-families, one composed of E2F-1, E2F-2, and E2F-3 and the other composed of E2F-4 and E2F-5, E2F-5 mRNA is expressed in a wide variety of human tissues. The protein is expressed as multiple species ranging in size from 46 to 54 kDa as a result of differential phosphorylation. The expression of a reporter gene containing E2F binding sites in the promoter is transcriptionally activated by E2F-5 in a cooperative manner with the DP-1 protein. The interaction between E2F-5 and DP-1 is demonstrated using a two-hybrid system in mammalian cells. We have also demonstrated the presence of a strong transactivation domain at the carboxy terminus (273-346 amino acid residues) of E2F-5 protein.