Unveiling the dimer/monomer propensities of Smad MH1-DNA complexes

R-Smads are effectors of the transforming growth factor β (TGFβ) superfamily and along with Smad4 form trimers to interact with DNA. The 5GC-DNA complexes determined here by X-ray crystallography for Smad5 and Smad8 proteins corroborate that all MH1 domains bind SBE and 5GC sites similarly, although Smad2/3/4 MH1 domains bind DNA as monomers whereas Smad1/5/8 form helix-swapped dimers. To examine the relevance of the dimerization phenomenon and to exclude a possible crystallography-induced dimeric state, we studied these MH1 domains in solution. As in the crystals, Smad5/8 domains populate dimers and open monomers in equilibrium, whereas Smad/3/4 ones adopt monomeric closed conformations. We also found that swapping the loop1-sequence between Smad5 and Smad3 results in the chimera-DNA complex crystallizing as a monomer, revealing that the loop1-sequence determines the monomer/dimer propensity of Smad MH1-domains. We propose that distinct MH1-dimerization status of TGFβ and BMP activated Smads influences the interaction with specific loci genome-wide by distinct R-Smad and Smad4 complexes.