Determinants of ligand binding specificity in the glial cell line-derived neurotrophic factor family receptor αs

The glial cell line-derived neurotrophic factor (GDNF) family comprise a subclass of cystine-knot superfamily ligands that interact with a multisubunit receptor complex formed by the c-Ret tyrosine kinase and a cystine-rich glycosyl phosphatidylinositol-anchored binding subunit called GDNF family receptor a (GFRα). All four GDNF family ligands utilize c-Ret as a common signaling receptor, whereas specificity is conferred by differential binding to four distinct GFRα homologues. To understand how the different GFRαs discriminate ligands, we have constructed a large set of chimeric and truncated receptors and analyzed their ligand binding and signaling capabilities. The major determinant of ligand binding was found in the most conserved region of the molecule, a central domain predicted to contain four conserved a helices and two β strands. Distinct hydrophobic and positively charged residues in this central region were required for binding of GFRα1 to GDNF. Interaction of GFRα1 and GFRα2 with GDNF and neurturin required distinct subsegments within this central domain, which allowed the construction of chimeric receptors that responded equally well to both ligands. C-terminal segments adjacent to the central domain are necessary and have modulatory function in ligand binding. In contrast, the N-terminal domain was dispensable without compromising ligand binding specificity. Ligand-independent interaction with c-Ret also resides in the central domain of GFRα1, albeit within a distinct and smaller region than that required for ligand binding. Our results indicate that the central region of this class of receptors constitutes a novel binding domain for cystine-knot superfamily ligands.