AbstractRET receptor tyrosine kinase plays vital developmental and neuroprotective roles in metazoans.GDNF family ligands (GFLs) when bound to cognate GFRα co-receptors recognise and activate RET stimulating its cytoplasmic kinase function. The principles for RET ligand-co-receptor recognition are incompletely understood. Here we report a crystal structure of the cadherin-like module (CLD1-4) from zebrafish RET revealing interdomain flexibility between CLD2-CLD3. Comparison with a cryo-EM structure of a ligand-engaged zebrafish RETECD-GDNF-GFRα1 complex indicates conformational changes within a clade-specific CLD3 loop adjacent to co-receptor. Our observations indicate RET is a molecular clamp with a flexible calcium-dependent arm that adapts to different GFRα co-receptors, while its rigid arm recognises a GFL dimer to align both membrane-proximal cysteine-rich domains. We also visualise linear arrays of RETECD-GDNF-GFRα1 suggesting a conserved contact stabilises higher-order species. Our study reveals ligand-co-receptor recognition by RET involves both receptor plasticity and strict spacing of receptor dimers by GFL ligands.HighlightsCrystal structure of zebrafish RET cadherin-like module reveals conformational flexibility at the calcium-dependent CLD2-CLD3 interfaceComparison of X-ray and cryo-EM structures indicate conformational differences between unliganded and liganded RET involving a clade-specific CLD3 loopStrict spatial separation of RETECDC-termini is imposed by each cysteine-rich domain interaction with GFL dimerDifferences in co-receptor engagement and higher-order ligand-bound RET complexes indicate potentially divergent signalling mechanisms