RET Recognition of GDNF-GFRα1 Ligand by a Composite Binding Site Promotes Membrane-Proximal Self-Association

• Extracellular architecture of RETECD is revealed by SAXS
• EM structure for a RETECD-GDNF-GFRα complex reveals a composite ligand-binding site
• A GFRα1-binding hotspot contacts the invariant RETCLD2-3 calcium sites
• RETCRD couples ligand recognition and receptor homodimerization, exploited by MEN2A

SummaryThe 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 GFRα 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 GFRα1 ternary complex, defining the basis for ligand recognition. RETECD envelopes the dimeric ligand complex through a composite binding site comprising four discrete contact sites. The GFRα1-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 RETECD 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.

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