MAPPING OF THE LIGAND-BINDING DOMAIN OF THE TRANSFORMING GROWTH-FACTOR-BETA RECEPTOR TYPE-III BY DELETION MUTAGENESIS

Transforming growth factor beta (TGF-beta) receptor type III is a memb rane-anchored proteoglycan that binds TGF-beta via the core protein. W e have determined, by deletion mutagenesis of the receptor type III, t he minimal essential region of the extracellular domain that is capabl e of binding TGF-beta. Nine deletion mutants were produced, six of whi ch are expressed on the cell surface and bind TGF-beta. We find that t he shortest of these active mutants, which retains only 253 of the 785 amino acids of the extracellular domain, binds TGF-beta with the same affinity as the full-length receptor. These results indicate that the ligand binding domain lies proximal to the transmembrane domain and i s functionally independent from the rest of the extracellular domain. We have determined from the mutants that one of the potential glycosam inoglycan attachment sites in the receptor type III is not utilized. R esults from the nonglycosylated mutants confirm that the glycosaminogl ycan chains are not required for the folding, targeting, and TGF-beta binding activity of the receptor. Moreover, we present evidence for di merization and multimerization of the receptor.