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Distinct oligomeric states of SMAD proteins in the transforming growth factor-beta pathway
1Cell Biology Program and Howard Hughes Medical Institute, Memorial Sloan-Kettering Cancer Center, New York, New York 10021, USA.
Abstract:
Protein interactions are critical for the function of SMADs as mediators of transforming growth factor-beta (TGF-beta) signals. TGF-beta receptor phosphorylation of SMAD2 or SMAD3 causes their association with SMAD4 and accumulation in the nucleus where the SMAD complex binds cofactors that determine the choice of target genes. We provide evidence that in the basal state, SMADs 2, 3, and 4 form separate, strikingly different complexes. SMAD2 is found mostly as monomer, whereas the closely related SMAD3 exists in multiple oligomeric states. This difference is due to a unique structural element in the MH1 domain of SMAD2 that inhibits protein-protein interactions in the basal state. In contrast to SMAD2 and SMAD3, SMAD4 in the basal state is found mostly as a homo-oligomer, most likely a trimer. Upon cell stimulation with TGF-beta, SMAD proteins become engaged in a multitude of complexes ranging in size from SMAD2-SMAD4 heterodimers to assemblies of >650 kDa. The latter display the highest DNA binding affinity for the TGF-beta-response elements of JUNB and collagen 7. These observations, all validated with endogenous SMAD proteins, modify previous models regarding the assembly and activity of SMAD complexes in the TGF-beta pathway.
Insights
Transforming growth factor-beta (TGF-beta) signaling relies on SMAD proteins forming distinct complexes. TGF-beta stimulation induces SMADs to assemble into larger, high-affinity DNA-binding complexes, altering cellular responses.
Area of Science:
- Molecular Biology
- Cell Signaling
- Protein Interactions
Background:
- SMAD proteins are crucial mediators of transforming growth factor-beta (TGF-beta) signaling pathways.
- TGF-beta receptor-mediated phosphorylation of SMAD2 or SMAD3 initiates their association with SMAD4.
- The SMAD complex translocates to the nucleus, where it interacts with cofactors to regulate gene expression.
Purpose of the Study:
- To investigate the distinct complex formations of SMAD proteins in the basal state.
- To characterize the dynamic changes in SMAD complex assembly upon TGF-beta stimulation.
- To determine the functional implications of these SMAD complex dynamics on DNA binding affinity.
Main Methods:
- Analysis of SMAD protein complex formation using biochemical assays.
- Investigation of SMAD protein structures, focusing on the MH1 domain of SMAD2.
- Validation of findings using endogenous SMAD proteins in response to TGF-beta stimulation.
Main Results:
- In basal conditions, SMAD2, SMAD3, and SMAD4 exist as distinct complexes: SMAD2 as a monomer, SMAD3 in various oligomeric states, and SMAD4 as a homo-oligomer (likely a trimer).
- A unique structural element in SMAD2's MH1 domain inhibits basal protein-protein interactions.
- TGF-beta stimulation leads to the formation of diverse SMAD complexes, from SMAD2-SMAD4 heterodimers to large assemblies (>650 kDa) with high DNA binding affinity for specific TGF-beta-responsive elements.
Conclusions:
- The basal state assembly of SMAD proteins is more complex than previously modeled, with SMAD2's MH1 domain playing a key inhibitory role.
- TGF-beta signaling dynamically remodels SMAD complexes, leading to the formation of high-affinity DNA-binding entities.
- These findings provide a revised understanding of SMAD complex assembly and activity in TGF-beta signaling.