Transforming growth factor-β-mediated signaling via the p38 MAP kinase pathway activates Smad-dependent transcription through SUMO-1 modification of Smad4

Post- translational modifications such as ubiquitination, phosphorylation, and acetylation play important roles in the regulation of Smad- mediated functions. Here, we demonstrate that Smad4 is covalently modified by SUMO- 1, which was characterized recently as a key modulator of many transcription factors. Sumoylation of Smad4 mainly occurs at lysine 159, located in the linker region, and facilitates Smad- dependent transcriptional activation. Furthermore, we show that the PIAS family proteins, PIAS1 and PIASxbeta, function as E3 ligase factors for Smad4. Intriguingly, sumoylation of Smad4 was strongly enhanced by TGF-beta- induced activation of the p38 MAP kinase pathway but not the Smad pathway. Activation of p38 not only stabilized PIASxbeta protein but also enhanced PIASxbeta gene expression, suggesting that PIAS- mediated sumoylation of Smad4 is regulated by the p38 MAP kinase pathway. These findings illustrate a novel regulatory mechanism by which Smad- dependent transcriptional activation cooperatively modulates Smad proteins through receptor- mediated phosphorylation and sumoylation.