Interacting domains of E2F1, DP1, and the adenovirus E4 protein.

Recent experiments demonstrate that a family of related proteins constitute the E2F transcription factor activity and that the interaction of two of these gene products, E2F1 and DP1, generates a heterodimer with DNA binding and transcriptional activating capacity. Previous experiments have shown that the adenovirus E4 19-kDa protein facilitates the formation of a stable E2F dimer on the adenovirus E2 promoter. We now show that coexpression of the E2F1 and DP1 products in transfected SAOS-2 cells, together with the E4 product, generates a multicomponent complex with specificity to the adenovirus E2 promoter. Using a yeast two-hybrid assay system, we find that the E2F1 hydrophobic heptad repeat (E2F1 amino acid residues 206 to 283) allows interaction with a corresponding domain of the DP1 protein (amino acids 196 to 245). We also find that the adenovirus E4 protein interacts with the DP1 hydrophobic heptad repeat domain, but we could not detect a direct interaction between E2F1 and E4. Additional assays demonstrate that the E4 protein can dimerize. Since our previous experiments have shown that mutations within the E2F1 hydrophobic heptad repeat element abolish the E4-mediated transcription enhancement in transfection assays, we conclude that the E4 protein likely interacts with the E2F1-DP1 heterodimer by directly binding to the DP1 product. As a consequence of the ability of E4 to dimerize, we propose that the stable complex formed on the two E2F sites within the E2 promoter is composed of two E2F1-DP1 heterodimers held together by an E4 dimer.