Distinct structure elements in GDNF mediate binding to GFR alpha 1 and activation of the GFR alpha 1-c-Ret receptor complex

Ligand-induced receptor oligomerization is a widely accepted mechanism for activation of cell-surface receptors. We investigated ligand-receptor inter actions in the glial cell-line derived neurotrophic factor (GDNF) receptor complex, formed by the c-Ret receptor tyrosine kinase and the glycosylphosp hatidylinositol (GPI)-anchored subunit GDNF family receptor alpha-1 (GFR al pha 1), As only GFR alpha 1 can bind GDNF directly, receptor complex format ion is thought to be initiated by GDNF binding to this receptor. Here we id entify an interface in GDNF formed by exposed acidic and hydrophobic residu es 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 a ctivate c-Ret at normal levels. Although impaired in binding GFR alpha 1 ef ficiently, these mutants still required GFR alpha 1 for c-Ret activation. T hese findings support a role for c-Ret in ligand binding and indicate that GDNF does not initiate receptor complex formation, but rather interacts wit h a pre-assembled GFR alpha 1-c-Ret complex.