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Functional mapping of the MH1 DNA-binding domain of DPC4/SMAD4
1Predoctoral Program in Human Genetics, Johns Hopkins University, Baltimore, MD, USA.
Abstract:
The transcription factor Smad4 binds DNA in response to a TGF-beta ligand-initiated intracellular signaling cascade. SMAD4 is deleted or mutated during tumorigenesis in many human tumors. Some of these mutations occur in the N-terminal portion of the protein, the Mad homology 1 (MH1) region, which exhibits sequence-specific DNA-binding. We used alanine scanning mutagenesis and natural mutations to map the subregion of the MH1 domain necessary for that function. We created 20 individual mutations in the MH1 region of human Smad4 and assayed their effect on DNA-binding in vitro. Mutation of residues in the less conserved N- and C-terminal areas of the MH1 region had no effect on DNA-binding. However, mutations in the domain from L43 to R135 caused a dramatic reduction of the ability of Smad4 to bind DNA. Previous work demonstrated a beta-hairpin protein motif within this region to be responsible for DNA-binding, but suggested that the tumorigenic mutations occurring outside this motif may target a separate function of the MH1 domain. Our results demonstrate that the MH1 domain as a whole is very sensitive to changes in overall structure, and that tumorigenic mutations within the region of L43-R135 indeed would target DNA-binding.
Insights
Smad4 protein mutations in the Mad homology 1 (MH1) region disrupt DNA binding, a critical function for the Smad4 transcription factor. This finding clarifies how cancer-related Smad4 mutations impact its DNA-binding ability.
Area of Science:
- Molecular Biology
- Cancer Biology
- Biochemistry
Background:
- Smad4 is a crucial transcription factor in TGF-beta signaling.
- SMAD4 gene mutations and deletions are common in human tumors.
- The Mad homology 1 (MH1) region of Smad4 mediates sequence-specific DNA-binding.
Purpose of the Study:
- To identify the specific subregion within the Smad4 MH1 domain essential for DNA-binding.
- To investigate the impact of naturally occurring mutations on Smad4 DNA-binding function.
Main Methods:
- Alanine scanning mutagenesis was employed to create 20 individual mutations in the Smad4 MH1 region.
- In vitro assays were performed to assess the DNA-binding ability of mutated Smad4 proteins.
Main Results:
- Mutations outside the L43-R135 region of MH1 did not affect DNA-binding.
- Mutations within the L43-R135 region significantly reduced Smad4's DNA-binding capacity.
- Tumorigenic mutations within L43-R135 directly impair DNA-binding, rather than a separate function.
Conclusions:
- The entire Smad4 MH1 domain is sensitive to structural alterations affecting DNA-binding.
- Cancer-associated mutations in the L43-R135 region of Smad4 directly compromise its DNA-binding function.