Smads bind directly to the Jun family of AP-1 transcription factors

Smad3 and Smad4 are sequence-specific DNA-binding factors that bind to thei r consensus DNA-binding sites in response to transforming growth factor bet a (TGF beta) and activate transcription. Recent evidence implicates Smad3 a nd Smad4 in the transcriptional activation of consensus AP-1 DNA-binding si tes that do not interact with Smads directly. Here, we report that Smad3 an d Smad4 can physically interact with AP-1 family members. In vitro binding studies demonstrate that both Smad3 and Smad4 bind all three Jun family mem bers: JunB, cJun, and JunD, The Smad interacting region of JunB maps to a C -terminal 20-amino acid sequence that is partially conserved in cJun and Ju nD, We show that Smad3 and Smad4 also associate with an endogenous form of cJun that is rapidly phosphorylated in response to TGF beta, Providing evid ence for the importance of this interaction between Smad and Jun proteins, we demonstrate that Smad3 is required for the activation of concatamerized AP-1 sites in a reporter construct that has previously been characterized a s unable to bind Smad proteins directly. Together, these data suggest that TGF beta-mediated transcriptional activation through AP-1 sites may involve a regulated interaction between Smads and AP-1 transcription factors.