Identification of residues important both for primary receptor binding andspecificity in fibroblast growth factor-7

Fibroblast growth factors (FGFs) mediate a multitude of physiological and p athological processes by activating a family of tyrosine kinase receptors ( FGFRs), Each FGFR binds to a unique subset of FGFs and ligand binding speci ficity is essential in regulating FGF activity, FGF-7 recognizes one FGFR i soform known as the FGFR2 IIIb isoform or keratinocyte growth factor recept or (KGFR), whereas FGF-2 binds well to FGFR1, FGFR2, and FGFR4 but interact s poorly with KGFR, Previously, mutations in FGF-2 identified a set of resi dues that are important for high affinity receptor binding, known as the pr imary receptor-binding site. FGF-7 contains this primary site as well as a region that restricts interaction with FGFR1. The sequences that confer on FGF-7 its specific binding to KGFR have not been identified. By utilizing d omain swapping and site-directed mutagenesis we have found that the loop co nnecting the beta4-beta5 strands of FGF-7 contributes to high affinity rece ptor binding and is critical for KGFR recognition. Replacement of this loop with the homologous loop from FGF-2 dramatically reduced both the affinity of FGF-7 for KGFR and its biological potency but did not result in the abi lity to bind FGFR1. Point mutations in residues comprising this loop of FGF -7 reduced both binding affinity and biological potency. The reciprocal loo p replacement mutant (FGF2-L4/7) retained FGF-2 like affinity for FGFR1 and for KGFR, Our results show that topologically similar regions in these two FGFs have different roles in regulating receptor binding specificity and s uggest that specificity may require the concerted action of distinct region s of an FGF.