Role of p300, a transcriptional coactivator, in signalling of TGF-β

Background: Smad proteins are novel transcriptional regulators mediating the signalling of the transforming growth factor-beta (TGF-beta) superfamily, Coactivators such as p300/CBP promote transactivation by various transcription factors through a direct interaction with them. Adenoviral oncoprotein E1A, which binds p300, was shown to inhibit the signalling of TGF-beta, These findings raise the possibility that p300 may be involved in TGF-beta signalling. Results: We investigated whether p300 is involved in transactivation by Smads, p300 enhanced the Smad-induced transactivation of p3TP-Lux, a TGF-beta responsive reporter, E1A inhibited this enhancement, and the inhibition required its ability to bind p300/CBP. p300 and Smad3, as well as Smad2, interacted in vivo in a Ligand-dependent manner. The binding region in Smad3 was its C-terminal half that was previously shown to possess an intrinsic transactivation activity. The binding region in p300 was mapped to its C-terminal 678 amino acids. The minimal Smad2/3-interacting region, as well as the rest of the p300, inhibited the transactivation of p3TP-Lux in a dominant-negative fashion. Conclusion: p300 interacted with Smad2 and Smad3 in a ligand-dependent manner, and enhanced the transactivation by Smads, Our results present the molecular basis of the transactivation by Smad proteins.