E2F-6 - A NOVEL MEMBER OF THE E2F FAMILY IS AN INHIBITOR OF E2F-DEPENDENT TRANSCRIPTION

The E2F family of transcription factors are essential for the regulati on of genes required for appropriate progression through the cell cycl e. Five members of the E2F family have been previously reported, namel y E2F1-5. All five are key elements in transcriptional regulation of e ssential genes, and they can be divided into two functional groups, th ose that induce S-phase progression when overexpressed in quiescent ce lls (E2Fs 1-3), and those that do not (E2Fs 4-5). Here, we describe th e identification of a novel member of this family, which we refer to a s E2F-6, E2F-6 shares significant homology with E2Fs 1-5, especially w ithin the DNA binding, heterodimerization and marked box domains. Unli ke E2Fs 1-5, E2F-6 lacks a transactivation and a pocket protein bindin g domain, hence, forms a unique third group within the E2F family, E2F -6 is a nuclear protein that can form heterodimers with the DP protein s (both DP-I and DP-2) in vitro and in vivo. Our results show that the complex formed between E2F-6 and the DP proteins, possesses high DNA binding activity, displaying a preference for a TTTCCCGC E2F recogniti on site, which is slightly different to the E2F consensus site derived from the E2 promoter (TTTCGCGC). In contrast to the other members of the E2F family, ectopic expression of E2F-6 inhibits transcription fro m promoters possessing E2F recognition sites rather than activating tr anscription. In addition, overexpression of E2F-6 suppresses the trans activational effects of coexpression of E2F-1 and DP-1. The inhibitory effect of E2F-6 is dependent on its DIVA binding activity and its abi lity to form heterodimers with the DPs, Interestingly, ectopic express ion of E2F-6 leads to accumulation of cells in S-phase, Our data sugge st that E2F-6 expression delays the exit from S-phase rather than indu cing S-phase, which further emphasizes the functional difference betwe en E2F-6 and the previously known E2F family members.