The Smad3 linker region contains a transcriptional activation domain

Transforming growth factor-beta (TGF-beta)/Smads regulate a wide variety of biological responses through transcriptional regulation of target genes. SMAD3 plays a key role in TGF-beta/Smad-mediated transcriptional responses. Here, we show that the proline-rich linker region of SMAD3 contains a transcriptional activation domain. When the linker region is fused to a heterologous DNA-binding domain, it activates transcription. We show that the linker region physically interacts with p300. The adenovirus E1a protein, which binds to p300, inhibits the transcriptional activity of the linker region, and overexpression of p300 can rescue the linker-mediated transcriptional activation. In contrast, an adenovirus E1a mutant, which cannot bind to p300, does not inhibit the linker-mediated transcription. The native SMAD3 protein lacking the linker region is unable to mediate TGF-beta transcriptional activation responses, although it can be phosphorylated by the TGF-beta Receptor at the C-terminal tail and has a significantly increased ability to form a heteromeric complex with SMAD4. We show further that the linker region and the C-terminal domain of SMAD3 synergize for transcriptional activation in the presence of TGF-beta. Thus our findings uncover an important function of the SMAD3 linker region in Smad-mediated transcriptional control.