Figure 1. TGF-β pathway and activin receptors ligand traps. A) Canonical signaling trough Smad2/3 activation.

Ligand binding induces dimerization of type II receptors and to oligomerization with type I receptors; the activated multimers activate Smad2/3s by phosphorylating them and triggering the formation of the complex with Smad4. pSmad2/3-Smad4 complex translocates to the nucleus regulating specific gene expression. Dimeric ligands and receptors appear as monomers only to simplify the picture.