Degradation of E2F by the ubiquitin-proteasome pathway: regulation by retinoblastoma family proteins and adenovirus transforming proteins.

E2F transcription factors are key regulators of transcription during the cell cycle. E2F activity is regulated at the level of transcription and DNA binding and by complex formation with the retinoblastoma pocket protein family. We show here that free E2F-1 and E2F-4 transcription factors are unstable and that their degradation is mediated by the ubiquitin-proteasome pathway. Both E2F-1 and E2F-4 are rendered unstable by an epitope in the carboxyl terminus of the proteins, in close proximity to their pocket protein interaction surface. We show that binding of E2F-1 to pRb or E2F-4 to p107 or p130 protects E2Fs from degradation, causing the complexes to be stable. The increased stability of E2F-4 pocket protein complexes may contribute to the maintenance of active transcriptional repression in quiescent cells. Surprisingly, adenovirus transforming proteins, which release pocket protein-E2F complexes, also inhibit breakdown of free E2F. These data reveal an additional level of regulation of E2F transcription factors by targeted proteolysis, which is inhibited by pocket protein binding and adenovirus early region 1 transforming proteins.