TMEPAI, a transmembrane TGF-β-inducible protein, sequesters Smad proteins in TGF-β signaling

Transforming growth factor-[beta] (TGF-[beta]) is a multifunctional cytokine of key importance for controlling embryogenesis and tissue homeostasis. How TGF-[beta] signals are attenuated and terminated is not well understood. Here, we show that TMEPAI, a direct target gene of TGF-[beta] signaling, antagonizes TGF-[beta] signaling by interfering with TGF-[beta] type I receptor (T[beta]RI)-induced R-Smad phosphorylation. TMEPAI can directly interact with R-Smads via a Smad interaction motif (SIM). TMEPAI competes with Smad anchor for receptor activation (SARA) for R-Smad binding, thereby sequestering R-Smads from T[beta]RI kinase activation. In mammalian cells, ectopic expression of TMEPAI inhibited TGF-[beta]-induced PAI-1 production, whereas specific siRNA-mediated knockdown of TMEPAI expression potentiated TGF-[beta]-induced Smad2 phosphorylation and cellular responsiveness by TGF-[beta]. Consistently, TMEPAI inhibits activin-mediated mesoderm formation in Xenopus embryos. Taken together, TMEPAI participates in a negative feedback loop to control the duration and intensity of TGF-[beta] signaling.