RET Recognition of GDNF-GFRaα1 Ligand by a Composite Binding Site Promotes Membrane-Proximal Self-Association

The RET receptor tyrosine kinase is essential to vertebrate development and implicated in multiple human diseases. RET binds a cell surface bipartite ligand comprising a GDNF family ligand and a GFR alpha coreceptor, resulting in RET transmembrane signaling. We present a hybrid structural model, derived from electron microscopy (EM) and low-angle X-ray scattering (SAXS) data, of the RET extracellular domain (RETECD), GDNF, and GFR alpha 1 ternary complex, defining the basis for ligand recognition. RET ECD envelopes the dimeric ligand complex through a composite binding site comprising four discrete contact sites. The GFR alpha 1-mediated contacts are crucial, particularly close to the invariant RET calcium-binding site, whereas few direct contacts are made by GDNF, explaining how distinct ligand/coreceptor pairs are accommodated. The RET ECD cysteine-rich domain (CRD) contacts both ligand components and makes homotypic membrane-proximal interactions occluding three different antibody epitopes. Coupling of these CRD-mediated interactions suggests models for ligand-induced RET activation and ligand-independent oncogenic deregulation.