Smad4 Protein Stability Is Regulated by Ubiquitin Ligase SCFβ-TrCP1*
- Mei Wan‡,
- Yi Tang‡,
- Ewan M. Tytler§,
- Chongyuan Lu‡,
- Bingwen Jin‡,
- Selwyn M. Vickers§,
- Lei Yang‡,
- Xingming Shi‡ and
- Xu Cao‡¶
- ‡Department of Pathology, §Department of Surgery, University of Alabama at Birmingham, School of Medicine, Birmingham, Alabama 35294
- ↵¶ To whom correspondence should be addressed: Dept. of Pathology, School of Medicine, University of Alabama at Birmingham, Birmingham, AL 35294. Tel.: 205-934-0162; Fax: 205-934-1775; E-mail: cao{at}path.uab.edu.
Abstract
Smad4 is a key intracellular mediator for the transforming growth factor-β (TGF-β) superfamily of growth factors and is also an important tumor suppressor. The receptor-regulated Smad (R-Smad) proteins are regulated by ubiquitin-mediated degradation, yet the precise control of Smad4 protein stability is unclear. We have identified SCFβ-TrCP1, a ubiquitin (E3) ligase, as a critical determinant for the protein degradation of Smad4 protein. F-box protein β-TrCP1 in this E3 ligase interacts with Smad4 both in yeast and in mammalian cells, but has no interaction with Smad2 and has weak interaction with Smad3. The β-TrCP1/Smad3 interaction was abolished by Smad4 gene silencing, indicating the interaction is indirect and is through Smad4. Ectopic expression of SCF complex containing β-TrCP1 is sufficient to induce the ubiquitination and degradation of Smad4. Furthermore, small interfering RNA-triggered endogenous β-TrCP1 suppression increases the expression of Smad4 protein. Consistent with these results, cells that overexpress the SCF complex display an inhibited TGF-β-dependent transcriptional activity and an impaired cell cycle arrest function. Thus, SCFβ-TrCP1 abrogates TGF-β function in vivo by decreasing Smad4 stability.
- Received January 7, 2004.
- Revision received February 26, 2004.
- The American Society for Biochemistry and Molecular Biology, Inc.
