Solution structure of the chick TGFβ type II receptor ligand-binding domain

The transforming growth factor β (TGFβ) signaling pathway influences cell proliferation, immune responses, and extracellular matrix reorganization throughout the vertebrate life cycle. The signaling cascade is initiated by ligand-binding to its cognate type II receptor. Here, we present the structure of the chick type II TGFβ receptor determined by solution NMR methods. Distance and angular constraints were derived from ¹⁵N and ¹³C edited NMR experiments. Torsion angle dynamics was used throughout the structure calculations and refinement. The 20 final structures were energy minimized using the generalized Born solvent model. For these 20 structures, the average backbone root-mean-square distance from the average structure is below 0.6Å. The overall fold of this 109-residue domain is conserved within the superfamily of these receptors. Chick receptors fully recognize and respond to human TGFβ ligands despite only 60% identity at the sequence level. Comparison with the human TGFβ receptor determined by X-ray crystallography reveals different conformations in several regions. Sequence divergence and crystal packing interactions under low pH conditions are likely causes. This solution structure identifies regions were structural changes, however subtle, may occur upon ligand-binding. We also identified two very well conserved molecular surfaces. One was found to bind ligand in the crystallized human TGFβ3:TGFβ type II receptor complex. The other, newly identified area can be the interaction site with type I and/or type III receptors of the TGFβ signaling complex.