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

Smad proteins are novel transcriptional regulators mediating the signalling of the transforming growth factor‐β (TGF‐β) 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‐β. These findings raise the possibility that p300 may be involved in TGF‐β signalling.

We investigated whether p300 is involved in transactivation by Smads. p300 enhanced the Smad‐induced transactivation of p3TP‐Lux, a TGF‐β 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.

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.