The soluble exoplasmic domain of the type II transforming growth factor (TGF)-β receptor: A heterogeneously glycosylated protein with high affinity and selectivity for TGF-β ligands

The transforming growth factor (TGF)-β type II receptor is a transmembrane serine/threonine kinase which is essential for all TGF-β-induced signals. In several cell types TGF-β2 is as potent as TGF-β1 or TGF-β3 in inducing cellular responses, yet TGF-β2 does not bind to the majority of expressed type II receptors. Here we characterized the properties of the soluble extracellular domain of the human TGF-β type II receptor synthesized in COS-7 cells. Like the membrane-attached type II receptor, the soluble receptor contains complex N-linked oligosaccharides as well as additional sialic acid residues that cause it to migrate heterogenously upon SDS-polyacrylamide gel electrophoresis. ¹²⁵I-TGF-β1 binds to and is chemically cross-linked to this protein. Unlabeled TGF-β1 inhibits the binding of ¹²⁵I-TGF-β1 with an apparent dissociation constant (K_d) of ∼200 pM, similar to the apparent K_d (∼ 50 pM) of the cell-surface type II receptor. TGF-β3 inhibits the binding of ¹²⁵I-TGF-β1 to the soluble type II receptor with a similar dissociation constant, ∼500 pM. In contrast, ¹²⁵I-TGF-β2 cannot bind and be chemically cross-linked to the soluble type II receptor, nor does as much as a 125-fold excess of unlabeled TGF-β2 inhibit the binding of ¹²⁵I-TGF-β1 to the soluble receptor. This is the first demonstration of the binding affinities of the type II receptor in the absence of the other cell-surface molecules known to bind TGF-β. Expressed alone in COS-7 cells the type II receptor also cannot bind TGF-β2; co-expression of type III receptor enables the type II receptor to bind TGF-β2. Thus, the type III receptor or some other component is required for transmission of TGF-β2-induced signals by the type II receptor.