Smurf2 is a ubiquitin E3 ligase mediating proteasome-dependent degradationof Smad2 in transforming growth factor-beta signaling

Smads are important intracellular signaling effecters for transforming grow th factor-beta (TGF-beta) and related factors. Proper TGF-beta signaling re quires precise control of Smad functions. In this study, we have identified a novel HECT class ubiquitin E3 ligase, designated Smurf2, that negatively regulates Smad2 signaling. In both yeast two-hybrid and in vitro binding a ssays, we found that Smurf2 could interact with receptor-activated Smads (R -Smads), including Smad1, Smad2, and Smad3 but not Smad4. Ectopic expressio n of Smurf2 was sufficient to reduce the steady-state levels of Smad1 and S mad2 but not Smad8 or Smad4. Significantly, Smurf2 displayed preference to Smad2 as its target for degradation. Furthermore, Smurf2 exhibited higher b inding affinity to activated Smad2 upon TGF-beta stimulation. The ability o f Smurf2 to promote Smad2 destruction required the HECT catalytic activity of Smurf2 and depended on the proteasome-dependent pathway. Consistent with these results, Smurf2 potently reduced the transcriptional activity of Sma d2. These data suggest that a ubiquitin/proaeasome-dependent mechanism is i mportant for proper regulation of TGF-beta signaling.