May function as a transcriptional repressor. May also function as a ubiquitin ligase mediating ubiquitination of chromatin-associated TP53. Functions in cell survival and proliferation through control of the cell cycle. Functions in the p53 and pRB tumor suppressor pathways and regulates the cyclin CCNA2 transcription. Identified as a cellular target of the adenoviral oncoprotein E1A, it is required for both transcriptional activation and repression of viral genes. S-ubiquitinyl-[E2 ubiquitin-conjugating enzyme]-L-cysteine + [acceptor protein]-L-lysine = [E2 ubiquitin-conjugating enzyme]-L-cysteine + N(6)-ubiquitinyl-[acceptor protein]-L-lysine. Protein modification; protein ubiquitination. Homodimer; binds DNA as a dimer. Forms a complex with CDKN2A and TP53. Interactions with TP53, RB1, ANP32A, BMI1 and FHL2 regulate E4F1 activity. Interacts with HDAC1, HMGA2 and RASSF1. (Microbial infection) Interacts with HBV protein X. A small fraction is detected in the cytoplasm. Excluded from the nucleolus where it is targeted upon CDKN2A overexpression. Localizes to the mitotic spindle during embryogenesis. Ubiquitously expressed. Expressed in a variety of fetal tissues. Up-regulated by estrogen. Proteolytic cleavage produces a 50 kDa N-terminal peptide (p50E4F) which has a DNA-binding activity and activates transcription in presence of the adenoviral E1A protein. The major full-length protein (p120E4F) functions as a repressor of transcription. Phosphorylated; p120E4F and p50E4F are both phosphorylated. Phosphorylation is cell cycle-dependent and differentially regulates DNA-binding activity and function of both forms. May be sumoylated by UBE2I upon interaction with CDKN2A.