Distinct structure elements in GDNF mediate binding to GFRα1 and activation of the GFRα1-c-Ret receptor complex

Ligand-induced receptor oligomerization is a widely accepted mechanism for activation of cell-surface receptors. We investigated ligand-receptor interactions in the glial cell-line derived neurotrophic factor (GDNF) receptor complex, formed by the c-Ret receptor tyrosine kinase and the glycosylphosphatidylinositol (GPI)-anchored subunit GDNF family receptor alpha-1 (GFR alpha 1), As only GFR alpha 1 can bind GDNF directly, receptor complex formation is thought to be initiated by GDNF binding to this receptor. Here we identify an interface in GDNF formed by exposed acidic and hydrophobic residues that is critical for binding to GFR alpha 1. Unexpectedly, several GDNF mutants deficient in GFR alpha 1 binding retained the ability to bind and activate c-Ret at normal levels. Although impaired in binding GFR alpha 1 efficiently, these mutants still required GFR alpha 1 for c-Ret activation. These findings support a role for c-Ret in ligand binding and indicate that GDNF does not initiate receptor complex formation, but rather interacts with a pre-assembled GFR alpha 1-c-Ret complex.