IDENTIFICATION AND CHARACTERIZATION OF A FIBROBLAST GROWTH-FACTOR (FGF) BINDING DOMAIN IN THE CYSTEINE-RICH FGF RECEPTOR

Three distinct transmembrane glycoproteins bind fibroblast growth fact or (FGF) family members. These include heparan sulfate proteoglycans, the tyrosine kinase-containing FGF receptors (FGFRs), and a cysteine-r ich FGF receptor (CFR), The four FGFRs are thought to mediate FGF-sign aling events but require the participation of the heparan sulfate prot eoglycans to bind FGFs and transduce intracellular signals. However, a number of groups have proposed that FGF action requires events indepe ndent of FGFR activation, CFR, a high affinity FGF binding protein, wa s first isolated from chicken embryos. To better understand the intera ctions between CFR and FGFs, we have constructed a series of CFR delet ion mutants and CFR fragments, Analysis of these has identified a simi lar to 200-amino acid domain that constitutes a CFR FGF binding site, A CFR fragment of 450 residues, CFR(290-740), binds FGF-2 with an affi nity indistinguishable from the full-length molecule, whereas smaller fragments display greatly reduced FGF binding. Although CFR binds hepa rin with high affinity, an analysis of the heparin CFR interaction fai led to identify a linear sequence containing a heparin binding site, T wo types of FGF binding sites were identified: an ionic strength and h eparin-independent site that represents FGF binding to CFR(290-740), a nd an additional FGF binding site that is heparan sulfate-dependent an d sensitive to high ionic strength, This latter site is likely to bind FGF indirectly via heparan sulfate binding to CFR. FGF-2 peptides tha t encompass a sequence implicated in FGF-2 binding to FGFRs also block FGF-S binding to CFR, Our data suggest that binding of FGFs to CFR an d FGFRs is mutually exclusive, since the CFR FGF binding site does not require heparan sulfate, and similar regions on FGF-2 interact with b oth FGFRs and CFR.