Smurf1 regulates the inhibitory activity of Smad7 by targeting Smad7 to the plasma membrane

Smad ubiquitin regulatory factor 1 (Smurf1), a HECT-type E3 ubiquitin ligase, interacts with inhibitory Smad7 and induces cytoplasmic localization of Smad7. Smurf1 then associates with transforming growth factor-beta type I receptor (TbetaR-1) and enhances the turnover of this receptor. However, the mechanisms of the nuclear export and plasma membrane localization of the Smurf1-Smad7 complex have not been elucidated. We show here that Smurf1 targets Smad7 to the plasma membrane through its N-terminal conserved 2 (C2) domain. Both wild-type Smurf1 (Smurf1(WT)) and Smurf1 lacking the C2 domain (Smurf1(DeltaC2)) bound to Smad7 and translocated nuclear Smad7 to the cytoplasm. However, unlike Smurf1(WT), Smurf1(DeltaC2) did not move to the plasma membrane and failed to recruit Smad7 to the cell surface TbetaR-II-TbetaR-I complex. Moreover, although Smurf1(DeltaC2) induced ubiquitination of Smad7, it failed to induce the ubiquitination and degradation of TbetaR-1 and did not enhance the inhibitory activity of Smad7. Thus, these results suggest that the plasma membrane localization of Smad7 by Smurf1 requires the C2 domain of Smurf1 and is essential for the inhibitory effect of Smad7 in the transforming growth factor-beta signaling pathway.