Structural interactions of fibroblast growth factor receptor with its ligands

Abstract

Fibroblast growth factors (FGFs) effect cellular responses by binding to FGF receptors (FGFRs). FGF bound to extracellular domains on the FGFR in the presence of heparin activates the cytoplasmic receptor tyrosine kinase through autophosphorylation. We have crystallized a complex between human FGF1 and a two-domain extracellular fragment of human FGFR2. The crystal structure, determined by multiwavelength anomalous diffraction analysis of the selenomethionyl protein, is a dimeric assemblage of 1:1 ligand: receptor complexes. FGF is bound at the junction between the two domains of one FGFR, and two such units are associated through receptor:receptor and secondary ligand:receptor interfaces. Sulfate ion positions appear to mark the course of heparin binding between FGF molecules through a basic region on receptor D2 domains. This dimeric assemblage provides a structural mechanism for FGF signal transduction. Fibroblast growth factors (FGFs) stimulate a variety of cellular functions by binding to cell surface FGF receptors (FGFRs) in the presence of heparin proteoglycans. FGFRs are single-chain receptor tyrosine kinases that become activated through autophosphorylation that is thought to be induced through a mechanism of ligand-mediated receptor oligomerization (1). Receptor activation gives rise to a signal transduction cascade that leads to gene activation and diverse biological responses (2, 3). Both FGFs and FGFRs are expressed in defined spatial and temporal patterns, and they are involved in differentiation of both epithelial and neuronal cells. FGFs are potent mitogens for many cell types. Aberrant signaling through FGFR can lead to tumorigenesis and skeletal disorders (4).The FGF system has appreciable diversity in both ligands and receptors. The FGF family contains at least 15 distinct factors, highly conserved across mammalian species but divergent (30-70% sequence identity) among paralogs (2). The FGFR family includes four identified genes and numerous subtypes of alternatively spliced isoforms, particularly within the well characterized FGFR1 and FGFR2 types. Differential responses follow from this diversity.FGFRs have an extracellular portion imbued with the ligandbinding potential, a transmembrane segment, and a tyrosine kinase domain in the cytoplasm (2). The extracellular portion comprises three Ig-like domains, D1, D2, and D3, with an acidic stretch of approximately 30 residues, the acid box, between D1 and D2. Isoforms generated by alternate splicing events include receptors that lack Ig-like domain D1 or both D1 and the acid box, as well as variants having two alternative sequences for the C-terminal half of the third Ig-like domain. The FGF binding site has been mapped to domains D2, D3, and the interdomain linker.FGFs are secreted factors originally identified based on their mitogenicity toward fibroblasts. They are small proteins for which several FGF crystal structures have been determined; all have 12 ␤ strands in a ␤-trefoil fold (2). Mutational analyses have mapped the sites of interaction...

Connecting signaling and cell cycle progression in growth factor-stimulated cells

“…Members of the ®broblast growth factor (FGF) family have two binding sites for the receptor within a single molecule of FGF. Resolution of the crystal structure and FGF ligand-receptor complex indicated that each receptor molecule also has two binding sites for the ligand (Plotnikov et al, 1999(Plotnikov et al, , 2000Stauber et al, 2000). Hence the ligand-receptor complex consists of two molecules of receptor and two molecules of FGF.…”

Structure of fibroblast growth factor 9 shows a symmetric dimer with unique receptor- and heparin-binding interfaces

“…Moreover, chemical crosslinking, ultracentrifugation experiments (Herr et al, 1997) and mass-spectrometric techniques (Davis et al, 1999) provided evidence of self-oligomerization for FGF2 in the presence and in the absence of heparin. Nevertheless, in the structures of the FGF2±receptor complex (Plotnikov et al, 1999) and the FGF1±receptor complex (Stauber et al, 2000) both FGF molecules are separate and are only linked via the receptor molecules. In these structures, heparin is postulated to bind into a positively charged groove created in the receptor dimer, with the two termini bound to the heparin-binding domains of the FGF2 molecules (Plotnikov et al, 1999;Stauber et al, 2000).…”

“…Using de®ned heparin fragments and soluble FGF receptors further demonstrated that ligand dimerization can signi®cantly enhance the binding of FGF2 to FGFR1, the dimerization of the receptor and the induction of downstream signal transduction pathways (Safran et al, 2000). Recently, however, several studies (Plotnikov et al, 1999(Plotnikov et al, , 2000Stauber et al, 2000) exploring the crystal structure of a complex between FGF2 and FGF1 with the extracellular domains of FGFR1 and FGFR2 have shown a 1:2 molecular ratio of ligands to receptors, with no evidence for ligand dimerization or with ligand association only through interaction with heparin (Pellegrini et al, 2000).…”

A novel mutation within the extracellular domain of TrkA causes constitutive receptor activation

“…Those proline residues are located in the linker region between domains Ig-2 and Ig-3, and the receptors bind slightly more radiolabeled FGF than wild type receptors (Neilson and Friesen, 1996). Recent analysis of the relative disposition of Ig2 and Ig3 domains in the FGF-FGFR structures predicts that these mutations result in aberrant interactions between FGFRs and FGFs leading to enhanced signaling when availability of ligand is limiting (Plotnikov et al, 1999(Plotnikov et al, , 2000Stauber et al, 2000).…”