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 higher affinity than TGF-beta 2. Here, we report an analysis of the effect of TGF-beta structure on its binding to T beta RII by using TGF-beta mutants with domain deletions, amino acid replacements, and isoform chimeras, Examination of the binding of TGF-beta mutants to the recombinant extracellular domain of T beta RII by a solid-phase TGF-beta/T beta 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-(Trp(71)), and TGF-beta 2/beta 1-(83-112)) bind with high affinity to T beta 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 sequence of TGF-beta 2 (TGF-beta 1/beta 2-(91-96)) completely eliminated the high affinity binding of TGF-beta 1. Proliferation of fetal bovine heart endothelial (FBHE) cells was inhibited to a similar degree by all of the TGF-beta mutants. However, recombinant soluble T beta RII blocked the inhibition of FBHE cell proliferation induced by TGF-beta mutants retaining the C terminus of TGF-beta 1, consistent with the high 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 as effectively as TGF-beta 1 with I-125-TGF-beta 1 for binding to membrane 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 affinity binding to T beta RII, both the soluble and membrane-bound forms, is located at C terminus of the molecule.