Formation of a stable heterodimer between Smad2 and Smad4

J Biol Chem. 2001 Jun 8;276(23):20688-94. doi: 10.1074/jbc.M100174200. Epub 2001 Mar 27.


Smad proteins mediate transforming growth factor beta signaling from the cell membrane to the nucleus. Upon phosphorylation by the activated receptor kinases, the receptor-regulated Smad, such as Smad2, forms a heterocomplex with the co-mediator Smad, Smad4. This heterocomplex is then translocated into the nucleus, where it associates with other transcription factors and regulates expression of ligand-responsive genes. The stoichiometry between receptor-regulated Smad and co-mediator Smad is important for understanding the molecular mechanisms of the signaling process. Using purified recombinant proteins, we demonstrate that Smad2 and Smad4 form a stable heterodimer and that the Smad4 activation domain is important for the formation of this complex. Many tumor-derived missense mutations disrupt the formation of this heterocomplex in in vitro interaction assays. Mapping these mutations onto the structures of Smad4 and Smad2 identifies a symmetric interface between these two Smad proteins. Importantly, two previous models on the formation of a heterocomplex are incompatible with our observations and other reported evidence.

Publication types

  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Chromatography, Gel
  • DNA-Binding Proteins / chemistry
  • DNA-Binding Proteins / genetics
  • DNA-Binding Proteins / metabolism*
  • Dimerization
  • Models, Molecular
  • Mutagenesis, Site-Directed
  • Protein Binding
  • Smad2 Protein
  • Trans-Activators / chemistry
  • Trans-Activators / genetics
  • Trans-Activators / metabolism*
  • Ultracentrifugation


  • DNA-Binding Proteins
  • Smad2 Protein
  • Trans-Activators