Pokemon, an important proto-oncoprotein, is a transcriptional repressor that belongs to the POK (POZ and Kruppel) family. Smad4, a key component of TGF-beta pathway, plays an essential role in TGF-beta-induced transcriptional responses. In this study, we show that Pokemon can interact directly with Smad4 both in vitro and in vivo. Overexpression of Pokemon decreases TGF-beta-induced transcriptional activities, whereas knockdown of Pokemon increases these activities. Interestingly, Pokemon does not affect activation of Smad2/3, formation of Smads complex, or DNA binding activity of Smad4. TGF-beta 1 treatment increases the interaction between Pokemon and Smad4, and also enhances the recruitment of Pokemon to Smad4-DNA complex. In addition, we also find that Pokemon recruits HDAC1 to Smad4 complex but decreases the interaction between Smad4 and p300/CBP. Taken together, all these data suggest that Pokemon is a new partner of Smad4 and plays a negative role in TGF-beta pathway. (C) 2014 Published by Elsevier B.V.