Three key residues underlie the differential affinity of the TGFβ isoforms for the TGFβ type II receptor

TGF beta 1, beta 2, and beta 3 are 25 kDa homodimeric polypeptides that play crucial non-overlapping roles in development, tumor suppression, and wound healing. They exhibit 70-82% sequence identity and transduce their signals by binding and bringing together the TGF beta type I and type II receptors, T beta RI and T beta RII. TGF beta 2 differs from the other isoforms in that it binds T beta RII weakly and is dependent upon the co-receptor betaglycan for function. To explore the physicochemical basis underlying these differences, we generated a series of single amino acid T beta RII variants based on the crystal structure of the T beta RII:TGF beta 3 complex and examined these in terms of their TGF beta isoform binding affinity and their equilibrium stability. The results showed that T beta RII IIe53 and Glu119, which contact TGF beta 3 Va192 and Arg25, respectively, together with T beta RII Asp32, Glu55, and Glu75, which contact TGF beta 3 Arg94, each contribute significantly, between 1 kcal mol(-1) to 1.5 kcal mol(-1), to ligand binding affinities. These contacts likely underlie the estimated 4.1 kcal mol(-1) lower affinity with which T beta RII binds TGF beta 2 as these three ligand residues are unchanged in TGF beta 1 but are conservatively substituted in TGF beta 2 (Lys25, lle92, and Lys94). To test this hypothesis, a TGF beta 2 variant was generated in which these three residues were changed to those in TGF beta s 1 and 3. This variant exhibited receptor binding affinities comparable to those of TGF beta s 1 and 3. Together, these results show that these three residues underlie the lowered affinity of TGF beta 2 for T beta RII and that all isoforms likely induce assembly of the TGF beta signaling receptors in the same overall manner. (c) 2005 Elsevier Ltd. All rights reserved.