Distinct oligomeric states of SMAD proteins in the transforming growth factor-beta pathway

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.