Identification and concerted function of two receptor binding surfaces on basic fibroblast growth factor required for mitogenesis.

Members of the fibroblast growth factor (FGF) family promote angiogenesis and wound repair, modulate early developmental events and survival of neurons, and have been associated with the pathogenesis of various diseases. FGFs interact with specific FGF receptors (FGFRs) and heparan sulfate proteoglycans on cell surfaces to mediate mitogenesis. Using protein structure-based site-directed mutagenesis of basic FGF (bFGF), we have identified two FGFR binding sites on bFGF which act in concert to initiate signal transduction. Both FGFR binding surfaces are distinct from the heparan sulfate proteoglycan binding domain. The primary, higher affinity, binding interaction comprises a cluster of solvent exposed hydrophobic amino acids (Tyr-24, Tyr-103, Leu-140, and Met-142), and two polar residues (Arg-44 and Asn-101). The hydrophobic contacts dominate the primary binding interaction and provide approximately 75% of the binding affinity. The secondary FGFR binding site on bFGF has an approximately 250-fold lower affinity and is composed of amino acids Lys-110, Tyr-111, and Trp-114 in a surface-exposed type I beta-turn (formerly known as the putative receptor binding loop). Binding of FGFR to both bFGF surfaces in a stoichiometry of 2FGFR:1bFGF is required for growth factor mediated cell proliferation. This represents a mechanism for the fibroblast growth factor/receptor family in which FGF facilitates FGFR dimerization and subsequent signal transduction events as a monomeric ligand.