Mapping ligand binding domains in chimeric fibroblast growth factor receptor molecules - Multiple regions determine ligand binding specificity

Fibroblast growth factors (FGFs) mediate essential cellular functions by ac tivating one of four alternatively spliced FGF receptors (FGFRs). To determ ine the mechanism regulating ligand binding affinity and specificity, solub le FGFR1 and FGFR3 binding domains were compared for activity. FGFR1 bound well to FGF2 but poorly to FGF8 and FGF9. In contrast, FGFR3 bound well to FGF8 and FGF9 but poorly to FGF2. The differential ligand binding specifici ty of these two receptors was exploited to map specific ligand binding regi ons in mutant and chimeric receptor molecules. Deletion of immunoglobulin-l ike (Ig) domain I did not effect ligand binding, thus localizing the bindin g region(s) to the distal two Ig domains. Mapping studies identified two re gions that contribute to FGF binding. Additionally, FGF2 binding showed pos itive cooperativity, suggesting the presence of two binding sites on a sing le FGFR or two interacting sites on an FGFR dimer. Analysis of FGF8 and FGF 9 binding to chimeric receptors showed that a broad region spanning Ig doma in II and sequences further N-terminal determines binding specificity for t hese ligands, These data demonstrate that multiple regions of the FGFR regu late ligand binding specificity and that these regions are distinct with re spect to different members of the FGF family.