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Identification and characterization of constitutively active Smad2 mutants: evaluation of formation of Smad complex
1Department of Biological Chemistry, University of Michigan, Ann Arbor 48109-0606, USA. funaba@azabu-u.ac.jp
Abstract:
Smads mediate activin, transforming growth factor beta (TGFbeta), and bone morphogenetic protein signaling from receptors to nuclei. According to the current model, activated activin/TGFbeta receptors phosphorylate the carboxyl-terminal serines of Smad2 and Smad3 (SSMS-COOH); phosphorylated Smad2/3 oligomerizes with Smad4, translocates to the nucleus, and modulates transcription of defined genes. To test key features of this model in detail, we explored the construction of constitutively active Smad2 mutants. To mimic phosphorylated Smad2, we made two Smad2 mutants with acidic amino acid substitutions of carboxyl-terminal serines: Smad2-2E (Ser465, 467Glu) and Smad2-3E (Ser464, 465, 467Glu). The mutants enhanced basal transcriptional activity in a mink lung epithelial cell line, L17. In a Smad4-deficient cell line, SW480.7, Smad2-2E did not affect basal signaling; however, cotransfection with full-length Smad4, but not transfection of Smad4 alone, resulted in enhanced basal transcriptional activity, suggesting that the constitutively active Smad2 mutant also requires Smad4 for function. In vitro protein interaction analysis revealed that Smad2-2E bound more tightly to Smad4 than did wild-type Smad2; dissociation constants were 270 +/- 66 nM for wild-type Smad2:Smad4 complexes and 79 +/- 18 nM for Smad2-2E:Smad4 complexes. Determination of the subcellular localization of Smad2 revealed that a greater percentage of Smad2-2E was localized in the nucleus than wild-type Smad2. These results suggest that Smad2 phosphorylation results in both tighter binding to Smad4 and increased nuclear concentration; those changes may be responsible for transcriptional activation by Smad2.
Insights
Constitutively active Smad2 mutants mimic phosphorylation, enhancing basal transcription. These Smad2 mutants require Smad4 for function, bind more tightly, and increase nuclear concentration, suggesting key roles in TGF-beta signaling pathways.
Area of Science:
- Molecular Biology
- Cell Signaling
- Gene Regulation
Background:
- Smad proteins are key mediators of activin, transforming growth factor beta (TGF-beta), and bone morphogenetic protein signaling.
- The canonical model describes receptor-mediated phosphorylation of Smad2/3, followed by Smad4 complex formation, nuclear translocation, and target gene modulation.
Purpose of the Study:
- To investigate the functional consequences of Smad2 phosphorylation by creating constitutively active Smad2 mutants.
- To elucidate the roles of Smad4 and subcellular localization in Smad2-mediated transcriptional activation.
Main Methods:
- Construction of Smad2 mutants (Smad2-2E, Smad2-3E) with acidic substitutions mimicking phosphorylation.
- Assays for basal transcriptional activity in epithelial cell lines (L17, SW480.7).
- In vitro protein interaction analysis and subcellular localization studies.
Main Results:
- Smad2 mutants enhanced basal transcriptional activity.
- Smad2-2E required Smad4 co-expression for enhanced signaling in Smad4-deficient cells.
- Smad2-2E exhibited tighter binding to Smad4 and increased nuclear localization compared to wild-type Smad2.
Conclusions:
- Smad2 phosphorylation likely increases its affinity for Smad4 and enhances its nuclear concentration.
- These molecular changes are proposed mechanisms for Smad2-mediated transcriptional activation in TGF-beta signaling pathways.