title: Competition between Ski and CREB-binding protein for binding to Smad proteins in transforming growth factor-beta signaling.
abstract: The family of Smad proteins mediates transforming growth factor-beta (TGF-beta) signaling in cell growth and differentiation. Smads repress or activate TGF-beta signaling by interacting with corepressors (e.g. Ski) or coactivators (e.g. CREB-binding protein (CBP)), respectively. Specifically, Ski has been shown to interfere with the interaction between Smad3 and CBP. However, it is unclear whether Ski competes with CBP for binding to Smads and whether they can interact with Smad3 at the same binding surface on Smad3. We investigated the interactions among purified constructs of Smad, Ski, and CBP in vitro by size-exclusion chromatography, isothermal titration calorimetry, and mutational studies. Here, we show that Ski-(16-192) interacted directly with a homotrimer of receptor-regulated Smad protein (R-Smad), e.g. Smad2 or Smad3, to form a hexamer; Ski-(16-192) interacted with an R-Smad.Smad4 heterotrimer to form a pentamer. CBP-(1941-1992) was also found to interact directly with an R-Smad homotrimer to form a hexamer and with an R-Smad.Smad4 heterotrimer to form a pentamer. Moreover, these domains of Ski and CBP competed with each other for binding to Smad3. Our mutational studies revealed that domains of Ski and CBP interacted with Smad3 at a portion of the binding surface of the Smad anchor for receptor activation. Our results suggest that Ski negatively regulates TGF-beta signaling by replacing CBP in R-Smad complexes. Our working model suggests that Smad protein activity is delicately balanced by Ski and CBP in the TGF-beta pathway.
authors.affiliation: Department of Biochemistry and Molecular Pharmacology, University of Massachusetts Medical School, Worcester, Massachusetts 01605, USA. Weijun.Chen@umassmed.edu
authors.forename: Weijun
authors.initials: W
authors.lastname: Chen
authors.forename: Suvana S
authors.initials: SS
authors.lastname: Lam
authors.forename: Hema
authors.initials: H
authors.lastname: Srinath
authors.forename: Celia A
authors.initials: CA
authors.lastname: Schiffer
authors.forename: William E
authors.initials: WE
authors.lastname: Royer
authors.forename: Kai
authors.initials: K
authors.lastname: Lin
mesh_terms: D001667:Binding, Competitive; D050882:CREB-Binding Protein / Q000737:chemistry* / Q000378:metabolism*; D018360:Crystallography, X-Ray; D004268:DNA-Binding Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D006801:Humans; D008958:Models, Molecular; D011485:Protein Binding; D020836:Protein Structure, Quaternary; D011518:Proto-Oncogene Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D015398:Signal Transduction; D051785:Smad Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D016212:Transforming Growth Factor beta / Q000378:metabolism*
chemical_list: D004268:DNA-Binding Proteins; D011518:Proto-Oncogene Proteins; D051785:Smad Proteins; D016212:Transforming Growth Factor beta; C060614:SKI protein, human; D050882:CREB-Binding Protein