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Differential ubiquitination defines the functional status of the tumor suppressor Smad4

Anita Morén1, Ulf Hellman, Yuri Inada

  • 1Ludwig Institute for Cancer Research, Box 595, Biomedical Center, SE-751 24 Uppsala, Sweden.

The Journal of Biological Chemistry
|June 10, 2003
PubMed

Insights

Oligoubiquitination of Smad4 protein enhances its function and oligomerization with R-Smads. In contrast, polyubiquitination of cancer-associated Smad4 mutants leads to their degradation, highlighting distinct roles of ubiquitination in Smad4 regulation.

Area of Science:

  • Molecular Biology
  • Cell Signaling
  • Cancer Biology

Background:

  • Smad4 is a critical signal transducer for transforming growth factor-beta (TGF-beta) superfamily pathways involved in cell growth and differentiation.
  • Inactivation of Smad4 is observed in human cancers, often linked to protein instability and degradation.
  • Ubiquitination of Receptor-activated (R-) Smads regulates their stability and signaling pathway activity.

Purpose of the Study:

  • To investigate the role of ubiquitination in the stability and function of Smad4, particularly in cancer-associated mutants.
  • To identify specific ubiquitination sites on Smad4 and their impact on protein interactions and transcriptional activity.

Main Methods:

  • Analysis of missense mutants of Smad4 derived from human cancers.
  • Assessment of protein ubiquitination (mono-, oligo-, and poly-) and proteasomal degradation.
  • Mass spectrometry to identify ubiquitination targets.
  • Mutagenesis studies to evaluate the functional significance of specific lysine residues.
  • Analysis of Smad4/R-Smad hetero-oligomerization and transcriptional activity.

Main Results:

  • Cancer-derived Smad4 mutants exhibit enhanced polyubiquitination and proteasomal degradation compared to wild-type Smad4.
  • Wild-type Smad4 undergoes mono- or oligo-ubiquitination, primarily on the C-terminal MH2 domain, specifically at lysine 507.
  • Mono- or oligo-ubiquitination at lysine 507 enhances Smad4 oligomerization with R-Smads and promotes transcriptional activity.
  • Mutagenesis of lysine 507 impairs Smad4/R-Smad hetero-oligomerization and transcriptional function.
  • Overexpression of a mono-ubiquitination-specific ubiquitin mutant enhances Smad4 transcriptional activity.

Conclusions:

  • Oligoubiquitination positively regulates Smad4 function by promoting R-Smad interaction and transcriptional activity.
  • Polyubiquitination is primarily associated with the degradation of unstable, cancer-associated Smad4 mutants.
  • Lysine 507 is a key site for regulatory ubiquitination of Smad4.

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