Endofin, a FYVE Domain Protein, Interacts with Smad4 and Facilitates Transforming Growth Factor-β Signaling

Transforming growth factor-beta (TGF-beta) signaling is facilitated by scaffold proteins such as SARA (Smad anchor for receptor activation). Endofin, a member of the FYVE domain protein family, has been suggested to regulate membrane trafficking. In this study, we report that endofin functions as a scaffold protein to facilitate TGF-beta signaling. Overexpression of endofin FYVE domain-deletion mutants inhibited TGF-beta-induced expression of CAGA-luciferase. Knockdown of endogenous endofin expression by RNA interference specifically led to reduction of the transcriptional responses of TGF-beta, but had no effect on BMP- or Wnt1-induced reporter expression. Furthermore, in endofin small interfering RNA-expressing stable cells, TGF-beta-mediated expression of plasminogen activator inhibitor-1 and p21(Cip1) was significantly reduced, and TGF-beta-promoted apoptosis was also impaired. We further showed that endofin could interact with Smad4 and TGF-beta type I receptors. Reduction of endogenous endofin expression resulted in a decrease of TGF-beta-induced Smad2 phosphorylation and Smad2-Smad4 complex formation. Together, our findings suggest that endofin facilitates TGF-beta signaling as a scaffold protein to promote the R-Smad-Smad4 complex formation by bringing Smad4 to the proximity of the receptor complex.