E2F transcription factors regulate genes involved in cell-cycle progression. In mammalian cells, physiological E2F exists as an E2F/DP heterodimer. Currently, eight E2F and two DP subunits have been characterized. We report here the characterization of a new member of the DP family, DP-4. While DP-4 exhibits certain similarities with members of the DP family, it also possesses a number of significant differences. Thus, DP-4 forms a heterodimer with E2F subunits, binds to the E2F site and associates with pocket proteins including pRb. In contrast to DP-1, however, DP-4/E2F-1 complexes exhibit reduced DNA binding activity. Furthermore, DP-4 interferes with E2F-1-dependent transcription and delays cell-cycle progression. These results highlight an emerging complexity in the DP family of E2F subunits, and suggest that DP-4 may endow E2F heterodimers with distinct transcription properties
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