Can function as a specific repressor of TGF-beta, activin, and BMP signaling through its interaction with the R-SMAD proteins. Antagonizes TGF-beta-induced cell proliferation arrest (By similarity). Interacts with SMAD1, SMAD2, SMAD3 and SMAD5. Binds to both phosphorylated and unphosphorylated R-SMADS (By similarity).