title: Degradation of the tumor suppressor Smad4 by WW and HECT domain ubiquitin ligases.
abstract: Smad4 mediates signaling by the transforming growth factor-beta (TGF-beta) superfamily of cytokines. Smad signaling is negatively regulated by inhibitory (I) Smads and ubiquitin-mediated processes. Known mechanisms of proteasomal degradation of Smads depend on the direct interaction of specific E3 ligases with Smads. Alternatively, I-Smads elicit degradation of the TGF-beta receptor by recruiting the WW and HECT domain E3 ligases, Smurfs, WWP1, or NEDD4-2. We describe an equivalent mechanism of degradation of Smad4 by the above E3 ligases, via formation of ternary complexes between Smad4 and Smurfs, mediated by R-Smads (Smad2) or I-Smads (Smad6/7), acting as adaptors. Smurfs, which otherwise cannot directly bind to Smad4, mediated poly-ubiquitination of Smad4 in the presence of Smad6 or Smad7. Smad4 co-localized with Smad7 and Smurf1 primarily in the cytoplasm and in peripheral cell protrusions. Smad2 or Smad7 mutants defective in Smad4 interaction failed to induce Smurf1-mediated down-regulation of Smad4. A Smad4 mutant defective in Smad2 or Smad7 interaction could not be effectively down-regulated by Smurf1. We propose that Smad4 is targeted for degradation by multiple ubiquitin ligases that can simultaneously act on R-Smads and signaling receptors. Such mechanisms of down-regulation of TGF-beta signaling may be critical for proper physiological response to this pathway.
authors.affiliation: Ludwig Institute for Cancer Research, Box 595, Biomedical Center, Uppsala University, SE-751 24 Uppsala, Sweden.
authors.forename: Anita
authors.initials: A
authors.lastname: Morén
authors.forename: Takeshi
authors.initials: T
authors.lastname: Imamura
authors.forename: Kohei
authors.initials: K
authors.lastname: Miyazono
authors.forename: Carl-Henrik
authors.initials: CH
authors.lastname: Heldin
authors.forename: Aristidis
authors.initials: A
authors.lastname: Moustakas
mesh_terms: D000256:Adenoviridae / Q000378:metabolism; D002460:Cell Line; D003593:Cytoplasm / Q000378:metabolism; D004268:DNA-Binding Proteins / Q000737:chemistry* / Q000378:metabolism; D015536:Down-Regulation; D056827:Endosomal Sorting Complexes Required for Transport; D005822:Genetic Vectors; D005982:Glutathione Transferase / Q000378:metabolism; D049452:Green Fluorescent Proteins / Q000378:metabolism; D006801:Humans; D008856:Microscopy, Fluorescence; D009154:Mutation; D000075702:Nedd4 Ubiquitin Protein Ligases; D010957:Plasmids / Q000378:metabolism; D046988:Proteasome Endopeptidase Complex / Q000378:metabolism; D011485:Protein Binding; D017434:Protein Structure, Tertiary; D018125:Receptors, Transforming Growth Factor beta / Q000378:metabolism; D015398:Signal Transduction; D051899:Smad2 Protein; D051901:Smad4 Protein; D051906:Smad7 Protein; D015534:Trans-Activators / Q000737:chemistry* / Q000378:metabolism; D014162:Transfection; D016212:Transforming Growth Factor beta / Q000378:metabolism; D025801:Ubiquitin / Q000737:chemistry / Q000378:metabolism; D044767:Ubiquitin-Protein Ligases / Q000737:chemistry* / Q000378:metabolism
chemical_list: D004268:DNA-Binding Proteins; D056827:Endosomal Sorting Complexes Required for Transport; D018125:Receptors, Transforming Growth Factor beta; C497782:SMAD2 protein, human; C497796:SMAD4 protein, human; C497806:SMAD7 protein, human; D051899:Smad2 Protein; D051901:Smad4 Protein; D051906:Smad7 Protein; D015534:Trans-Activators; D016212:Transforming Growth Factor beta; D025801:Ubiquitin; D049452:Green Fluorescent Proteins; D000075702:Nedd4 Ubiquitin Protein Ligases; C000618483:Nedd4 protein, human; C000618485:Nedd4L protein, human; C498342:SMURF1 protein, human; C471641:SMURF2 protein, human; C471642:WWP1 protein, human; D044767:Ubiquitin-Protein Ligases; D005982:Glutathione Transferase; D046988:Proteasome Endopeptidase Complex