Mutations uncouple human fibroblast growth factor (FGF)-7 biological activity and receptor binding and support broad specificity in the secondary receptor binding site of FGFs

The fibroblast growth factor (FGF) family plays a key role in a multitude o f physiological and pathological processes. The activities of FGFs are medi ated by a family of tyrosine kinase receptors, designated FGFRs, The mechan ism by which FGFs induce receptor activation is controversial. Despite thei r structural similarity, FGFs display distinct receptor binding characteris tics and cell type specificity. Previous studies with FGF-2 identified a lo w affinity receptor binding site that is located within a loop connecting i ts 9th and 10th beta-strands, The corresponding residues in the other famil y members are highly variable, and it was proposed that the variability mig ht confer on FGFs unique receptor binding characteristics. We studied the r ole of this loop in FGF-7 by both site-directed mutagenesis and loop replac ement. Unlike the other members of the FGF family, FGF-7 recognizes only on e FGFR isoform and is, therefore, ideal for studies of how the specificity in the FGF-FGFR interaction is conferred at the structural level, Point mut ations in the loop of FGF-7 did not change receptor binding affinity but re sulted in reduced mitogenic potency and reduced ability to induce receptor- mediated phosphorylation events. These results suggest that the loop of FGF -I? fulfills the role of low affinity binding site required for receptor ac tivation. The observation that it is possible to uncouple FGF-7 receptor bi nding and biological activity favors a bivalent model for FGFR dimerization , and it may be clinically relevant to the design of FGF-7 antagonists. Rec iprocal loop replacement between FGF-7 and FGF-S had no effect on their kno wn receptor binding affinities nor did it alter their known specificity in eliciting a mitogenic response. In conclusion, these results suggest that, despite the diversity in the loop structure of FGF-2 and FGF-7, the loop ha s a similar function in both growth factors.