The TGF-beta superfamily of proteins regulates many different biological processes, including cell growth, differentiation and embryonic pattern formation. TGF-beta-like factors signal across cell membranes through complexes of transmembrane receptors known as type I and type II serine/threonine-kinase receptors, which in turn activate the SMAD signaling pathway. On the inside of the cell membrane, a receptor-regulated class of SMADs are phosphorylated by the type-I-receptor kinase. In this way, receptors for different factors are able to pass on specific signals along the pathway: for example, receptors for bone morphogenetic protein (BMP) target SMADs 1, 5 and 8, whereas receptors for activin and TGF-beta target SMADs 2 and 3. Phosphorylation of receptor-regulated SMADs induces their association with Smad4, the 'common-partner' SMAD, and stimulates accumulation of this complex in the nucleus, where it regulates transcriptional responses. Smurf1, a new member of the Hect family of E3 ubiquitin ligases, has been identified. Smurf1 selectively interacts with receptor-regulated SMADs specific for the BMP pathway in order to trigger their ubiquitination and degradation, and hence their inactivation. In the amphibian Xenopus laevis, Smurf1 messenger RNA is localized to the animal pole of the egg; in Xenopus embryos, ectopic Smurf1 inhibits the transmission of BMP signals and thereby affects pattern formation. Smurf1 also enhances cellular responsiveness to the Smad2 (activin/TGF-beta) pathway. Thus, targeted ubiquitination of SMADs may serve to control both embryonic development and a wide variety of cellular responses to TGF-beta signals (Zhu, 1999).

Smads are important intracellular signaling effectors for transforming growth factor-beta (TGF-beta) and related factors. Proper TGF-beta signaling requires precise control of Smad functions. A novel HECT class ubiquitin E3 ligase, designated Smurf2, has been identified that negatively regulates Smad2 signaling. In both yeast two-hybrid and in vitro binding assays, Smurf2 could interact with receptor-activated Smads (R-Smads), including Smad1, Smad2, and Smad3 but not Smad4. Ectopic expression of Smurf2 is sufficient to reduce the steady-state levels of Smad1 and Smad2 but not Smad3 or Smad4. Significantly, Smurf2 displays preference to Smad2 as its target for degradation. Furthermore, Smurf2 exhibits higher binding affinity to activated Smad2 upon TGF-beta stimulation. The ability of Smurf2 to promote Smad2 destruction requires the HECT catalytic activity of Smurf2 and depends on the proteasome-dependent pathway. Consistent with these results, Smurf2 potently reduces the transcriptional activity of Smad2. These data suggest that a ubiquitin/proteasome-dependent mechanism is important for proper regulation of TGF-beta signaling (Lin, 2000).

Smad proteins are key intracellular signaling effectors for the TGF-beta superfamily of peptide growth factors. Following receptor-induced activation, Smads move into the nucleus to activate transcription of a select set of target genes. The activity of Smad proteins must be tightly regulated to exert the biological effects of different ligands in a timely manner. Smurf2, a new member of the Hect family of E3 ubiquitin ligases, has been identified. Smurf2 selectively interacts with receptor-regulated Smads and preferentially targets Smad1 for ubiquitination and proteasome-mediated degradation. At higher expression levels, Smurf2 also decreases the protein levels of Smad2, but not Smad3. In Xenopus embryos, ectopic Smurf2 expression specifically inhibits Smad1 responses and thereby affects embryonic patterning by bone morphogenetic protein signals. These findings suggest that Smurf2 may regulate the competence of a cell to respond to transforming growth factor-beta/bone morphogenetic protein signaling through a distinct degradation pathway that is similar to, yet independent of, Smurf1 (Zhang, 2001).

Smad7 is an inhibitory Smad that acts as a negative regulator of signaling by the TGF-beta superfamily proteins. Smad7 is induced by TGF-beta, stably interacts with activated TGF-beta type I receptor (TbetaR-I), and interferes with the phosphorylation of receptor-regulated Smads. Smurf1, an E3 ubiquitin ligase for bone morphogenetic protein-specific Smads, also interacts with Smad7 and induces Smad7 ubiquitination and translocation into the cytoplasm. In addition, Smurf1 associates with TbetaR-I via Smad7, with subsequent enhancement of turnover of TbetaR-I and Smad7. These results thus reveal a novel function of Smad7, i.e. induction of degradation of TbetaR-I through recruitment of an E3 ligase to the receptor (Ebisawa, 2001).

The receptor-regulated Smad proteins are essential intracellular mediators of signal transduction by the TGF-beta superfamily of growth factors and are also important as regulators of gene transcription. A new role for TGF-beta-regulated Smad2 and Smad3 as components of a ubiquitin ligase complex is described. In the presence of TGF-beta signaling, Smad2 interacts through its proline-rich PPXY motif with the tryptophan-rich WW domains of Smurf2, a recently identified E3 ubiquitin ligase. TGF-beta also induces the association of Smurf2 with the transcriptional co-repressor SnoN and Smad2 can function to mediate this interaction. This allows Smurf2 HECT domain to target SnoN for ubiquitin-mediated degradation by the proteasome. Thus, stimulation by TGF-beta can induce the assembly of a Smad2-Smurf2 ubiquitin ligase complex that functions to target substrates for degradation (Bonni, 2001).

Smurf: Biological Overview | Regulation | Developmental Biology | Effects of Mutation | References

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