Conformational landscape of full-length Smad proteins

Abstract

AbstractSmad transcription factors, the main effectors of the TGFβ (transforming growth factor β) 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.HighlightsSolution ensembles of full-length Smad2 and Smad4 proteinsSmad4 populates dynamic extended and compact monomeric conformationsSmad2 dimers may be the intermediate state to form heterotrimers with Smad4New integrative methodological strategy for examining multi-domain and flexible proteins