Crystal structure of the human TβR2 ectodomain-TGF-β3 complex

Transforming growth factor-β (TGF-β) is the prototype of a large family of structurally related cytokines that play key roles in maintaining cellular homeostasis by signaling through two classes of functionally distinct Ser/Thr kinase receptors, designated as type I and type II. TGF-β initiates receptor assembly by binding with high affinity to the type II receptor. Here, we present the 2.15 Å crystal structure of the extracellular ligand-binding domain of the human TGF-β type II receptor (ecTβR2) in complex with human TGF-β3. ecTβR2 interacts with homodimeric TGF-β3 by binding identical finger segments at opposite ends of the growth factor. Relative to the canonical 'closed' conformation previously observed in ligand structures across the superfamily, ecTβR2-bound TGF-β3 shows an altered arrangement of its monomeric subunits, designated the 'open' conformation. The mode of TGF-β3 binding shown by ecTβR2 is compatible with both ligand conformations. This, in addition to the predicted mode for TGF-β binding to the type I receptor ectodomain (ecTβR1), suggests an assembly mechanism in which ecTβR1 and ecTβR2 bind at adjacent positions on the ligand surface and directly contact each other via proteinprotein interactions.