BINDING-AFFINITY OF TRANSFORMING GROWTH-FACTOR-BETA FOR ITS TYPE-II RECEPTOR IS DETERMINED BY THE C-TERMINAL REGION OF THE MOLECULE

Transforming growth factor-beta (TGF-beta) isoforms have differential binding affinities for the TGF-beta type II receptor (T beta RII). In most cells, TGF-beta 1 and TGF-beta 3 bind to T beta RII with much hig her affinity than TGF-beta 2. Here, we report an analysis of the effec t of TGF-beta structure on its binding to T beta RII by using TGF-beta mutants with domain deletions, amino acid replacements, and isoform c himeras, Examination of the binding of TGF-beta mutants to the recombi nant extracellular domain of T beta RII by a solid-phase TGF-beta/T be ta RII assay demonstrated that only those TGF-beta mutants containing the C terminus of TGF-beta 1 (TGF-beta 1-(Delta 69-73), TGF-beta 1-(Tr p(71)), and TGF-beta 2/beta 1-(83-112)) bind with high affinity to T b eta RII, similar to native TGF-beta 1. Moreover, replacement of only 6 amino acids in the C terminus of TGF-beta 1 with the corresponding se quence of TGF-beta 2 (TGF-beta 1/beta 2-(91-96)) completely eliminated the high affinity binding of TGF-beta 1. Proliferation of fetal bovin e heart endothelial (FBHE) cells was inhibited to a similar degree by all of the TGF-beta mutants. However, recombinant soluble T beta RII b locked the inhibition of FBHE cell proliferation induced by TGF-beta m utants retaining the C terminus of TGF-beta 1, consistent with the hig h binding affinity between these TGF-beta molecules and T beta RII, It was further confirmed that the TGF-beta 2 mutant with its C terminus replaced by that of TGF-beta 1 (TGF-beta 2/beta 1-(83-112)) competed a s effectively as TGF-beta 1 with I-125-TGF-beta 1 for binding to membr ane T beta RI and T beta RII on FBHE cells. These observations clearly indicate that the domain in TGF-beta 1 responsible for its high affin ity binding to T beta RII, both the soluble and membrane-bound forms, is located at C terminus of the molecule.