Conformational landscape of full-length Smad proteins

Smad transcription factors, the main effectors of the TGF{beta} (transforming growth factor {beta}) network, have been shaped along the evolution of multicellular animals to regulate essential processes. Smad proteins have a mixed architecture of globular domains and flexible linkers and adopt distinct quaternary structures depending on their activation state and cellular context. Here we studied the structures of full-length Smad4 and Smad2 proteins through an integrative approach combining small-angle X-ray scattering and detailed atomic information obtained from Nuclear Magnetic Resonance spectroscopy, X-ray and molecular dynamic simulations. Both Smad4 and Smad2 populate ensembles of expanded/compact conformations, with the MH1 and MH2 domains tethered by intrinsically disordered linkers that provide conformational freedom to the proteins. In solution, Smad4 is monomeric, whereas Smad2 coexists as monomer-dimer-trimer association states, even without activation. Smad2 dimers, which were previously overlooked, are proposed as key building blocks that define the functional quaternary structures of Smad proteins. HighlightsO_LISolution ensembles of full-length Smad2 and Smad4 proteins C_LIO_LISmad4 populates dynamic extended and compact monomeric conformations C_LIO_LISmad2 dimers may be the intermediate state to form heterotrimers with Smad4 C_LIO_LINew integrative methodological strategy for examining multi-domain and flexible proteins C_LI