Molecular and functional characterisation of E2F-5, a new member of the E2F family.

The transcription factor DRTF1/E2F is implicated in the control of cellular proliferation due to its interaction with key regulators of cell cycle progression, such as the retinoblastoma tumour suppressor gene product and related pocket proteins, cyclins and cyclin-dependent kinases. DRTF1/E2F DNA binding activity arises when a member of two distinct families of proteins, DP and E2F, interact as DP/E2F heterodimers. Here, we report the isolation and characterisation of a new member of the E2F family of proteins, called E2F-5. E2F-5 was isolated through a yeast two hybrid assay in which a 14.5 d.p.c. mouse embryo library was screened for molecules capable of binding to murine DP-1, but also interacts with all known members of the DP family of proteins. E2F-5 exists as a physiological heterodimer with DP-1 in the generic DRTF1/E2F DNA binding activity present in mammalian cell extracts, an interaction which results in co-operative DNA binding activity and transcriptional activation through the E2F site. A potent transcriptional activation domain, which functions in both yeast and mammalian cells and resides in the C-terminal region of E2F-5, is specifically inactivated upon pocket protein binding. Comparison of the sequence with other members of the family indicates that E2F-5 shows a greater level of similarity with E2F-4 than to E2F-1, -2 and -3. The structural and functional similarity of E2F-5 and E2F-4 defines a subfamily of E2F proteins.