Acts as a coactivator of transcriptional activity. Required to increase TGFB1/Smad-mediated transactivation. Acts through SMAD2, SMAD3 and SMAD4 to increase transcriptional activity. Increases phosphorylation of SMAD2 and SMAD3 on their C-terminal SSXS motif, possibly through recruitment of TGFBR1. Promotes the nuclear accumulation of SMAD2, SMAD3 and SMAD4 proteins. Binds to poly(A) RNA. Homodimer; each protein chain binds one RNA molecule via the external surface of the homodimer. Interacts with SMAD2, SMAD3 and SMAD4; the interactions are direct. Translocates into cytoplasmic stress granules that probably correspond to P-bodies in response to oxidative stress. mRNA expressed in developing heart, with significantly higher expression in the atria relative to the ventricles. mRNA already detected at 9.5 dpc. The RRM domain is necessary for interaction with SMAD4. Both the RRM domain and the C-terminus are required for TGFB1/Smad-mediated transactivation activity (By similarity).