In the absence of type III receptor, the transforming growth factor (TGF)-β type II-B receptor requires the type I receptor to bind TGF-β2

Transforming growth factor beta( TGF-beta) ligands exert their biological effects through type II ( TbetaRII) and type I receptors ( TbetaRI). Unlike TGF-beta1 and - beta3, TGF- beta2 appears to require the co- receptor betaglycan ( type III receptor, TbetaRIII) for high affinity binding and signaling. Recently, the TbetaRIII null mouse was generated and revealed significant non- overlapping phenotypes with the TGF-beta2 null mouse, implying the existence of TbetaRIII independent mechanisms for TGF-beta2 signaling. Because a variant of the type II receptor, the type II- B receptor ( TbetaRII- B), has been suggested to mediate TGF-beta2 signaling in the absence of TbetaRIII, we directly tested the ability of TbetaRII- B to bind TGF-beta2. Here we show that the soluble extracellular domain of the type II- B receptor ( sTbetaRII- B. Fc) bound TGF- beta1 and TGF- beta3 with high affinity ( K-d values = 31.7 +/- 22.8 and 74.6 +/- 15.8 pM, respectively), but TGF-beta2 binding was undetectable at corresponding doses. Similar results were obtained for the soluble type II receptor ( sTbetaRII. Fc). However, sTbetaRII. Fc or sTbetaRII- B. Fc in combination with soluble type I receptor ( sTbetaRI. Fc) formed a high affinity complex that bound TGF-beta2, and this complex inhibited TGF-beta2 in a biological inhibition assay. These results show that TGF-beta2 has the potential to signal in the absence of TbetaRIII when sufficient TGF-beta2, TbetaRI, and TbetaRII or TbetaRII- B are present. Our data also support a cooperative model for receptor- ligand interactions, as has been suggested by crystallization studies of TGF-beta receptors and ligands. Our cell- free binding assay system will allow for testing of models of receptor- ligand complexes prior to actual solution of crystal structures.