title: Nuclear targeting of transforming growth factor-beta-activated Smad complexes.
abstract: Upon stimulation by the transforming growth factor beta (TGF-beta), Smad2 and Smad3 are phosphorylated at their C termini and assemble into stable heteromeric complexes with Smad4. These complexes are the functional entities that translocate into the nucleus and regulate the expression of TGF-beta target genes. Here we report that the TGF-beta-activated phospho-Smad3/Smad4 complex utilizes an importin-independent mechanism for nuclear import and engages different nucleoporins for nuclear import compared with the monomeric Smad4. Within the heteromeric complex, phospho-Smad3 appears to dominate over Smad4 in the nuclear import process and guides the complex to its nuclear destination. We also demonstrate that the binding of phospho-Smad3 to Smad4 prevents Smad4 from interacting with the nuclear export receptor chromosome region maintenance 1. In this way, TGF-beta signaling suppresses nuclear export of Smad4 by chromosome region maintenance 1 and thereby targets Smad4 into the nucleus. Indeed tumorigenic mutations in Smad4 that affect its interaction with Smad2 or Smad3 impair nuclear accumulation of Smad4 in response to TGF-beta.
authors.affiliation: Program in Molecular Medicine, University of Massachusetts Medical School, Worcester, Massachusetts 01605, USA.
authors.forename: Hong Bing
authors.initials: HB
authors.lastname: Chen
authors.forename: Jonathan G
authors.initials: JG
authors.lastname: Rud
authors.forename: Kai
authors.initials: K
authors.lastname: Lin
authors.forename: Lan
authors.initials: L
authors.lastname: Xu
mesh_terms: D021581:Active Transport, Cell Nucleus; D000818:Animals; D019556:COS Cells; D002466:Cell Nucleolus / Q000378:metabolism; D002467:Cell Nucleus / Q000378:metabolism*; D003600:Cytosol / Q000378:metabolism; D004268:DNA-Binding Proteins / Q000378:metabolism*; D019281:Dimerization; D005982:Glutathione Transferase / Q000378:metabolism; D006367:HeLa Cells; D006801:Humans; D007150:Immunohistochemistry; D047468:Immunoprecipitation; D028884:Karyopherins / Q000737:chemistry; D008856:Microscopy, Fluorescence; D009154:Mutation; D010766:Phosphorylation; D011485:Protein Binding; D017434:Protein Structure, Tertiary; D021381:Protein Transport; D011994:Recombinant Proteins / Q000737:chemistry; D015398:Signal Transduction; D051785:Smad Proteins; D051900:Smad3 Protein; D051901:Smad4 Protein; D015534:Trans-Activators / Q000378:metabolism*; D014162:Transfection; D016212:Transforming Growth Factor beta / Q000378:metabolism*
chemical_list: D004268:DNA-Binding Proteins; D028884:Karyopherins; D011994:Recombinant Proteins; C497790:SMAD3 protein, human; C497796:SMAD4 protein, human; D051785:Smad Proteins; D051900:Smad3 Protein; D051901:Smad4 Protein; D015534:Trans-Activators; D016212:Transforming Growth Factor beta; D005982:Glutathione Transferase