LIGAND-DEPENDENT AND LIGAND-INDEPENDENT INTERACTIONS WITH THE TRANSFORMING-GROWTH-FACTOR TYPE-II AND TYPE-I RECEPTOR SUBUNITS RESIDE IN THEAMINOTERMINAL PORTION OF THE ECTODOMAIN OF THE TYPE-III SUBUNIT

The type III receptor for transforming growth factor beta (TGF beta), which exhibits no kinase activity, binds TGF beta 1 and TGF beta 2 and is involved in assembly and activity of the multi-subunit TGF beta si gnal transduction complex. Recently we showed that TGF beta receptor t ype III (T beta RIII) can participate in a complex composed of the dim eric TGF beta ligand and a type III, II, and I receptor subunit. The i nteraction of the T beta RIII subunit with T beta RII is TGF beta-depe ndent, whereas interaction with T beta RI is TGF beta-independent. Her e we use coexpression of the three types of TGF beta receptors in bacu loviral-infected insect cells to determine which parts of the unglycos ylated T beta RIII receptor participate in the binding of TGF beta, th e TGF beta-dependent interaction with T beta RII and the TGF beta-inde pendent interaction with T beta RI. The results suggest that the first 500 amino acid residues in the aminoterminal portion of T beta RIII e xhibit all three properties.