Structural basis for activation of fibroblast growth factor signaling by sucrose octasulfate

Sucrose octasulfate (SOS) is believed to stimulate fibroblast growth factor (FGF) signaling by binding and stabilizing FGFs. In this report, we show that SOS induces FGF-dependent dimerization of FGF receptors (FGFRs). The crystal structure of the dimeric FGF2-FGFR1-SOS complex at 2.6-A resolution reveals a symmetric assemblage of two 1:1:1 FGF2-FGFR1-SOS ternary complexes. Within each ternary complex SOS binds to FGF and FGFR and thereby increases FGF-FGFR affinity. SOS also interacts with the adjoining FGFR and thereby promotes protein-protein interactions that stabilize dimerization. This structural finding is supported by the inability of selectively desulfated SOS molecules to promote receptor dimerization. Thus, we propose that SOS potentiates FGF signaling by imitating the dual role of heparin in increasing FGF-FGFR affinity and promoting receptor dimerization. Hence, the dimeric FGF-FGFR-SOS structure substantiates the recently proposed "two-end" model, by which heparin induces FGF-FGFR dimerization. Moreover, the FGF-FGFR-SOS structure provides an attractive template for the development of easily synthesized SOS-related heparin agonists and antagonists that may hold therapeutic potential.     

A protein canyon in the FGF-FGF receptor dimer selects from an à la carte menu of heparan sulfate motifs

Heparan sulfate (HS) is an essential and dynamic regulator of fibroblast growth factor (FGF) signaling. Two fundamentally different crystallographic models have been proposed to explain, at the molecular level, how HS/heparin enables FGF and FGF receptor (FGFR) to assemble into a functional dimer on the cell surface. In the symmetric 'two-end' model, the heparin-binding sites of FGF and FGFR merge to form a basic canyon that recruits two HS for binding. Within this canyon, the HS molecules primarily act to orchestrate and fortify multivalent and cooperative protein-protein contacts within the dimer that are the foundations of dimerization. In contrast, in the asymmetric model, which mechanistically resembles the previously proposed trans FGF dimer model, a single heparin molecule facilitates dimerization by cross-linking two FGFs into a trans dimer that brings together the two FGFRs. Interestingly, the crystal structure upon which the asymmetric model is based contains a symmetric dimer reminiscent of the symmetric two-end model, suggesting that a different interpretation of the crystal structure has led to the postulation of the asymmetric model. Importantly, the symmetric two-end model provides an intriguing solution to the problem of how HS selectivity is achieved in FGF signaling. The model reveals that, within the canyon, FGF and FGFR no longer adhere to their individual HS binding specificities, but instead act in unison to search for a unique HS motif from a plethora of HS epitopes that are expressed in a tissue-specific and developmentally regulated fashion. Primary sequence differences within the heparin-binding sites of FGFs and FGFRs, together with ligand-induced changes in FGFR conformation, lead to the formation of distinct canyons with unique HS specificity for individual FGF-FGFR complexes.     

Kinetic model for FGF, FGFR, and proteoglycan signal transduction complex assembly

The current working model for fibroblast growth factor receptor (FGFR) dimerization and activation requires the assembly of a ternary complex of fibroblast growth factor (FGF), FGFR, and heparin or heparan sulfate proteoglycan (HSPG) on the plasma membrane. The recent FGF2-FGFR1-heparin crystal structure provides a detailed but static view of the FGF-FGFR-heparin complex. However, the kinetics of ternary complex assembly has yet to be investigated. Here, we characterize FGF2, FGFR1, and heparin interactions using surface plasmon resonance (SPR). Binding constants for binary FGF2/FGFR1 (KD = 62 nM), FGF2/heparin (KD = 39 nM), and FGFR1/heparin (KD = 3.2 microM) interactions correlate to the magnitude of binding interface observed in the FGF2-FGFR1-heparin crystal structure. Interestingly, comparison of sensorgrams of sequential injections of FGF2 and FGFR1 and equimolar FGF2-FGFR1 injections onto a heparin neoproteoglycan surface demonstrates that FGF2 dramatically enhances the association of FGFR1 with heparin and leads us to propose a model for the stepwise assembly of a ternary FGF-FGFR-HSPG complex. The weak binding affinity of the FGFR1-heparin interaction suggests that in this model, FGFR and HSPG are unbound in the absence of FGF ligand. The availability of FGF results in formation of initial FGF-HSPG complexes, which promotes the rapid binding of FGFR and creates a ternary complex capable of undergoing dimerization and subsequent FGFR activation. In contrast, alternative models for the kinetic assembly of a ternary complex in which binary FGF-FGFR or FGFR-HSPG complexes are intermediates do not conform well with the experimental data.     

Crystal structure of a ternary FGF-FGFR-heparin complex reveals a dual role for heparin in FGFR binding and dimerization

The crystal structure of a dimeric 2:2:2 FGF:FGFR:heparin ternary complex at 3 A resolution has been determined. Within each 1:1 FGF:FGFR complex, heparin makes numerous contacts with both FGF and FGFR, thereby augmenting FGF-FGFR binding. Heparin also interacts with FGFR in the adjoining 1:1 FGF:FGFR complex to promote FGFR dimerization. The 6-O-sulfate group of heparin plays a pivotal role in mediating both interactions. The unexpected stoichiometry of heparin binding in the structure led us to propose a revised model for FGFR dimerization. Biochemical data in support of this model are also presented. This model provides a structural basis for FGFR activation by small molecule heparin analogs and may facilitate the design of heparin mimetics capable of modulating FGF signaling.     

Structural basis for fibroblast growth factor receptor activation

FGF signaling plays a ubiquitous role in human biology as a regulator of embryonic development, homeostasis and regenerative processes. In addition, aberrant FGF signaling leads to diverse human pathologies including skeletal, olfactory, and metabolic disorders as well as cancer. FGFs execute their pleiotropic biological actions by binding, dimerizing and activating cell surface FGF receptors (FGFRs). Proper regulation of FGF-FGFR binding specificity is essential for the regulation of FGF signaling and is achieved through primary sequence variations among the 18 FGFs and seven FGFRs. The severity of human skeletal syndromes arising from mutations that violate FGF-FGFR specificity is a testament to the importance of maintaining precision in FGF-FGFR specificity. The discovery that heparin/heparan sulfate (HS) proteoglycans are required for FGF signaling led to numerous models for FGFR dimerization and heralded one of the most controversial issues in FGF signaling. Recent crystallographic analyses have led to two fundamentally different models for FGFR dimerization. These models differ in both the stoichiometry and minimal length of heparin required for dimerization, the quaternary arrangement of FGF, FGFR and heparin in the dimer, and in the mechanism of 1:1 FGF-FGFR recognition and specificity. In this review, we provide an overview of recent structural and biochemical studies used to differentiate between the two crystallographic models. Interestingly, the structural and biophysical analyses of naturally occurring pathogenic FGFR mutations have provided the most compelling and unbiased evidences for the correct mechanisms for FGF-FGFR dimerization and binding specificity. The structural analyses of different FGF-FGFR complexes have also shed light on the intricate mechanisms determining FGF-FGFR binding specificity and promiscuity and also provide a plausible explanation for the molecular basis of a large number craniosynostosis mutations.     

Cooperative dimerization of fibroblast growth factor 1 (FGF1) upon a single heparin saccharide may drive the formation of 2:2:1 FGF1.FGFR2c.heparin ternary complexes

The related glycosaminoglycans heparin and heparan sulfate are essential for the activity of the fibroblast growth factor (FGF) family as they form an integral part of the signaling complex at the cell surface. Using size-exclusion chromatography we have studied the capacities of a variety of heparin oligosaccharides to bind FGF1 and FGFR2c both separately and together in ternary complexes. In the absence of heparin, FGF1 had no detectable affinity for FGFR2c. However, 2:2:1 complexes formed spontaneously in solution between FGF1, FGFR2c, and heparin octasaccharide (dp8). The dp8 sample was the shortest chain length that bound FGFR2c, that dimerized FGF1, and that promoted a strong mitogenic response to FGF1 through FGFR2c. Heparin hexasaccharide and various selectively desulfated heparin dp12s failed to bind FGFR2c and could only interact with FGF1 monomerically. These saccharides formed 1:1:1 complexes with FGF1 and FGFR2c, which had no tendency to self-associate, suggesting that binding of two FGF1 molecules to the same saccharide chain is a prerequisite for subsequent FGFR2c dimerization. We found that FGF1 dimerization upon heparin was favored over monomeric interactions even when a large excess of saccharide was present. A cooperative mechanism of FGF1 dimerization could explain how 2:2:1 signaling complexes form at the cell surface, an environment rich in heparan sulfate.     

Molecular Modeling Studies on Binding of bFGF to Heparin and its Receptor FGFR1

Abstract

Sugar induced protein-protein interactions play an important role in several biological processes. The carbohydrate moieties of proteoglycans, the glycosaminoglycans, bind to growth factors with a high degree of specificity and induce interactions with growth factor receptors, thereby regulate the growth factor activity. We have used molecular modeling method to study the modes of binding of heparin or heparan sulfate proteoglycans (HSPGs) to bFGF that leads to the dimerization of FGF receptor 1 (FGFR1) and activation of receptor tyrosine kinase. Homology model of FGFR1 Ig D(II)-D(III) domains was built to investigate the interactions between heparin, bFGF and FGFR1. The structural requirements to bridge the two monomeric bFGF molecules by heparin or HSPGs and to simulate the dimerization and activation of FGFR1 have been examined. A structural model of the biologically functional dimeric bFGF-heparin complex is proposed based on: (a) the stability of dimeric complex, (b) the favorable binding energies between heparin and bFGF molecules, and (c) its accessibility to FGFR1. The modeled complex between heparin, bFGF and FGFR1 has a stoichiometry of 1 heparin: 2 bFGF: 2 FGFR1. The structural properties of the proposed model of bFGF/heparin/FGFR1 complex are consistent with the binding mechanism of FGF to its receptor, the receptor dimerization, and the reported site-specific mutagenesis and biochemical cross-linking data. In the proposed model heparin bridges the two bFGF monomers in a specific orientation and the resulting complex induces FGF receptor dimerization, suggesting that in the oligosaccharide induced recognition process sugars orient the molecules in a way that brings about specific protein-protein or protein-carbohydrate interactions.     

A dimeric ternary complex of FGFR [correction of FGFR1], heparin and FGF-1 leads to an 'electrostatic sandwich' model for heparin binding.

BACKGROUND: Fibroblastic growth factors (FGFs) are a family of cytokines involved in regulation of cell growth, differentiation and chemotaxis in a variety of tissue types. High-affinity FGF receptors (FGFRs) are transmembrane proteins that consist of three extracellular immunoglobulin-like domains, a transmembrane helix and an intracellular protein tyrosine kinase signalling domain. FGFRs are activated through ligand-dependent dimerization that allows trans-autophosphorylation of the tyrosine kinase domains. Heparin or heparin-like molecules, such as heparan sulphate proteoglycans, bind to both FGFs and FGFRs and are required for FGF signal transduction. At present no structure of the ternary complex for FGFR, FGF and heparin exists. RESULTS: We have used the type-1 interleukin-1 receptor-interleukin-1 beta complex crystal structure, in which both the ligand and the receptor are homologous to those of the FGF-FGFR pair, to identify potential interactions in the FGFR-heparin-FGF ternary complex. A key feature of the modelled complex is the 'electrostatic sandwich' that is formed between the positively charged surfaces of FGF and the receptor, with the negatively charged heparin captured in between. The ternary complex places limits on the range of likely modes of receptor dimerization: one of five different dimeric receptor complexes built from the ternary complex correlates best with the experimental data. CONCLUSIONS: The ternary complex of FGFR, FGF and heparin, derived on the basis of the homologous interleukin-1 receptor complex, is in agreement with much of the published experimental data, as is the dimeric receptor complex (FGFR-heparin-FGF)2. This work suggests that the FGF interactions seen in crystal structures, which have previously been used to predict the mode of FGF dimerization, might not be relevant to the biologically active dimeric FGFR-heparin-FGF complex.     

Differential effects of heparin saccharides on the formation of specific fibroblast growth factor (FGF) and FGF receptor complexes.

Heparan sulfates (HS) play an important role in the control of cell growth and differentiation by virtue of their ability to modulate the activities of heparin-binding growth factors, an issue that is particularly well studied for fibroblast growth factors (FGFs). HS/heparin co-ordinate the interaction of FGFs with their receptors (FGFRs) and are thought to play a critical role in receptor dimerization. Biochemical and crystallographic studies, conducted mainly with FGF-2 or FGF-1 and FGF receptors 1 and 2, suggests that an octasaccharide is the minimal length required for FGF- and FGFR-induced dimerization and subsequent activation. In addition, 6-O-sulfate groups are thought to be essential for binding of HS to FGFR and for receptor dimerization. We show here that oligosaccharides shorter than 8 sugar units support activation of FGFR2 IIIb by FGF-1 and interaction of FGFR4 with FGF-1. In contrast, only relatively long oligosaccharides supported receptor binding and activation in the FGF-1.FGFR1 or FGF-7.FGFR2 IIIb setting. In addition, both 6-O- and 2-O-desulfated heparin activated FGF-1 signaling via FGFR2 IIIb, whereas neither one stimulated FGF-1 signaling via FGFR1 or FGF-7 via FGFR2 IIIb. These findings indicate that the structure of HS required for activating FGFs is dictated by the specific FGF and FGFR combination. These different requirements may reflect the differences in the mode by which a given FGFR interacts with the various FGFs.     

Structural basis for FGF receptor dimerization and activation.

The crystal structure of FGF2 bound to a naturally occurring variant of FGF receptor 1 (FGFR1) consisting of immunoglobulin-like domains 2 (D2) and 3 (D3) has been determined at 2.8 A resolution. Two FGF2:FGFR1 complexes form a 2-fold symmetric dimer. Within each complex, FGF2 interacts extensively with D2 and D3 as well as with the linker between the two domains. The dimer is stabilized by interactions between FGF2 and D2 of the adjoining complex and by a direct interaction between D2 of each receptor. A positively charged canyon formed by a cluster of exposed basic residues likely represents the heparin-binding site. A general model for FGF- and heparin-induced FGFR dimerization is inferred from the crystal structure, unifying a wealth of biochemical data.     

A light scattering study of the interaction of fibroblast growth factor (FGF) with its receptor

Light scattering technique has been used to study the interaction between fibroblast growth factor (FGF) and its receptor. In this study, a general mathematical model has been developed where the concentration of product formed by the interaction of two proteins and its dependence on the initial concentration of interacting proteins have been determined using laser light scattering. Calculated hydrodynamic diameters reveal that both human fibroblast growth factor (hFGF-1) and its receptor domain (D2 domain) exist as monomers in solution. Titration of hFGF-1 and the D2 domain of FGFR show that they interact in a 1:1 stoichiometry in solution. The binding stoichiometry does not depend on the concentrations of the interacting proteins. The results of this study, for the first time to our knowledge, provide an unambiguous evidence that the 2:2 binary complex of FGF and FGFR observed in the crystal structures of the FGF-FGFR complex (in the absence of heparin) is possibly a crystallization artifact.     

Heparin differentially regulates the interaction of fibroblast growth factor-4 with FGF receptors 1 and 2.

Fibroblast growth factor-4 (FGF4), like other FGFs, shares a high affinity for the anionic glycosaminoglycans heparin and heparan sulfate (HS), which in turn enhance FGF-receptor (FGFR) binding and activation. Here we demonstrate using a cell free system that, at low concentrations of heparin, FGF4 binds only to FGFR-2, while much higher heparin levels are required for binding to FGFR-1. Chemical crosslinking of radiolabeled FGF4 to the soluble FGF receptors confirms the preferential formation of FGF4-FGFR-2 complexes under restricted heparin availability, with maximal ligand-receptor interactions at almost 20-fold lower heparin concentrations then those required for the affinity labeling of FGFR-1. In accordance, HS-deficient cells expressing FGFR-2 proliferate in response to FGF4 at extremely low exogenous heparin concentrations, while FGFR-1 expressing cells are completely unresponsive under the same conditions. We suggest that FGFR-2 is the preferred receptor for FGF4 under restricted HS conditions and that the bioavailability of structurally distinct HS motifs may differentially control receptor specificity of FGF4 in vivo.     

betaKlotho is required for fibroblast growth factor (FGF) 21 signaling through FGF receptor (FGFR) 1c and FGFR3c

Fibroblast growth factor (FGF) 21, a structural relative of FGF23 that regulates phosphate homeostasis, is a regulator of insulin-independent glucose transport in adipocytes and plays a role in the regulation of body weight. It also regulates ketogenesis and adaptive responses to starvation. We report that in a reconstituted receptor activation assay system using BaF3 cells, which do not endogenously express any type of FGF receptor (FGFR) or heparan sulfate proteoglycan, FGF21 alone does not activate FGFRs and that betaKlotho is required for FGF21 to activate two specific FGFR subtypes: FGFR1c and FGFR3c. Coexpression of betaKlotho and FGFR1c on BaF3 cells enabled FGF21, but not FGF23, to activate receptor signaling. Conversely, coexpression of FGFR1c and Klotho, a protein related to betaKlotho, enabled FGF23 but not FGF21 to activate receptor signaling, indicating that expression of betaKlotho/Klotho confers target cell specificity on FGF21/FGF23. In all of these cases, heparin enhanced the activation but was not essential. In 3T3-L1 adipocytes, up-regulation of glucose transporter (GLUT) expression by FGF21 was associated with expression of betaKlotho, which was absent in undifferentiated 3T3-L1 fibroblasts. It is thus suggested that betaKlotho expression is a crucial determinant of the FGF21 specificity of the target cells upon which it acts in an endocrine fashion.     

Crystal structures of two FGF-FGFR complexes reveal the determinants of ligand-receptor specificity

To elucidate the structural determinants governing specificity in fibroblast growth factor (FGF) signaling, we have determined the crystal structures of FGF1 and FGF2 complexed with the ligand binding domains (immunoglobulin-like domains 2 [D2] and 3 [D3]) of FGF receptor 1 (FGFR1) and FGFR2, respectively. Highly conserved FGF-D2 and FGF-linker (between D2-D3) interfaces define a general binding site for all FGF-FGFR complexes. Specificity is achieved through interactions between the N-terminal and central regions of FGFs and two loop regions in D3 that are subject to alternative splicing. These structures provide a molecular basis for FGF1 as a universal FGFR ligand and for modulation of FGF-FGFR specificity through primary sequence variations and alternative splicing.     

Evidence that heparin saccharides promote FGF2 mitogenesis through two distinct mechanisms

Heparin-like saccharides play an essential role in binding to both fibroblast growth factors (FGF) and their receptors at the cell surface. In this study we prepared a series of heparin oligosaccharides according to their size and sulfation level. We then investigated their affinity for FGF2 and their ability to support FGF2 mitogenesis of heparan sulfate-deficient cells expressing FGFR1c. Tetra- and hexasaccharides bound FGF2, but failed to dimerize the growth factor. Nevertheless, these saccharides promoted FGF2-mediated cell growth. Furthermore, whereas enzymatic removal of the non-reducing end 2-O-sulfate group had little effect on the 1:1 interaction with FGF2, it eliminated the mitogenic activity of these saccharides. This evidence supports the symmetric two-end model of ternary complex formation. In contrast, even at very low concentrations, octasaccharide and larger heparin fragments conferred a potent mitogenic activity that was independent of terminal 2-O-sulfation. This correlated with the ability to dimerize FGF2 in an apparently cooperative manner. This data suggests that potent mitogenic signaling results from heparin-mediated trans-dimerization of FGF2, consistent with the asymmetric model of ternary complex formation. We propose that, depending on saccharide structure, there are different architectures and modes of ternary complex assembly that differ in stability and/or efficiency of transmembrane signaling.     

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 of physiological and pathological processes. The activities of FGFs are mediated by a family of tyrosine kinase receptors, designated FGFRs. The mechanism by which FGFs induce receptor activation is controversial. Despite their structural similarity, FGFs display distinct receptor binding characteristics and cell type specificity. Previous studies with FGF-2 identified a low affinity receptor binding site that is located within a loop connecting its 9th and 10th beta-strands. The corresponding residues in the other family members are highly variable, and it was proposed that the variability might confer on FGFs unique receptor binding characteristics. We studied the role of this loop in FGF-7 by both site-directed mutagenesis and loop replacement. Unlike the other members of the FGF family, FGF-7 recognizes only one FGFR isoform and is, therefore, ideal for studies of how the specificity in the FGF-FGFR interaction is conferred at the structural level. Point mutations in the loop of FGF-7 did not change receptor binding affinity but resulted in reduced mitogenic potency and reduced ability to induce receptor-mediated phosphorylation events. These results suggest that the loop of FGF-7 fulfills the role of low affinity binding site required for receptor activation. The observation that it is possible to uncouple FGF-7 receptor binding and biological activity favors a bivalent model for FGFR dimerization, and it may be clinically relevant to the design of FGF-7 antagonists. Reciprocal loop replacement between FGF-7 and FGF-2 had no effect on their known 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 has a similar function in both growth factors.     

Evidence that the intracellular domain of FGF receptor 2IIIb affects contact of the ectodomain with two FGF7 ligands

Models of the oligomeric FGF signaling complex, including those derived from crystal structures, vary in stoichiometry and arrangement of the three subunits comprised of heparin/heparan sulfate chains, FGFR tyrosine kinase and activating FGF. Here, using covalent affinity crosslinking of radiolabeled FGF7 to binary complexes of FGFR2IIIb and heparin, we show that two molecules of FGF7 contact each FGFR2IIIb. This supports models that propose a dimeric complex of two units with stoichiometry 1 FGF:1 FGFR in which each FGF contacts both FGFR. The bivalent FGF7 contact was dependent on the full-length amino terminus of FGF7alpha and the intracellular domain of FGFR2IIIb extending through the juxtamembrane domain and the beta1 and beta2 strands of the kinase which is required for ATP binding. We propose that the differences in crosslinking report differences in relationships among subunits in the ectodomain of the complex that are affected by the amino terminus of FGF and the FGFR intracellular domain. From this, we suggest the corollary that conformational relationships among subunits in the ectodomain are transmitted to the intracellular and ATP binding domains during activation of the complex.     

The strong dimerization of the transmembrane domain of the fibroblast growth factor receptor (FGFR) is modulated by C‐terminal juxtamembrane residues

The fibroblast growth factor receptor 3 (FGFR3) is a member of the FGFR subfamily of the receptor tyrosine kinases (RTKs) involved in signaling across the plasma membrane. Generally, ligand binding leads to receptor dimerization and activation. Dimerization involves the transmembrane (TM) domain, where mutations can lead to constitutive activation in certain cancer types and also in skeletal malformations. Thus, it has been postulated that FGFR homodimerization must be inherently weak to allow regulation, a feature reminiscent of α and β integrin TM interactions. However, we show herein that in FGFR3‐TM, four C‐terminal residues, CRLR, have a profound destabilizing effect in an otherwise strongly dimerizing TM peptide. In the absence of these four residues, the dimerizing propensity of FGFR3‐TM is comparable to glycophorin, as shown using various detergents. In addition, the expected enhanced dimerization induced by the mutation associated to the Crouzon syndrome A391E, was observed only when these four C‐terminal residues were present. In the absence of these four residues, A391E was dimer‐destabilizing. Finally, using site specific infrared dichroism and convergence with evolutionary conservation data, we have determined the backbone model of the FGFR3‐TM homodimer in model lipid bilayers. This model is consistent with, and correlates with the effects of, most known pathological mutations found in FGFR‐TM.     

Quantitative assessment of FGF regulation by cell surface heparan sulfates

Heparin/heparan sulfate-like glycosaminoglycans (HSGAGs) modulate the activity of the fibroblast growth factor (FGF) family of proteins. Through interactions with both FGFs and FGF receptors (FGFRs), HSGAGs mediate FGF-FGFR binding and oligomerization leading to FGFR phosphorylation and initiation of intracellular signaling cascades. We describe a methodology to examine the impact of heparan sulfate fine structure and source on FGF-mediated signaling. Mitogenic assays using BaF3 cells transfected with specific FGFR isoforms allow for the quantification of FGF1 and FGF2 induced responses independent of conflicting influences. As such, this system enables a systematic investigation into the role of cell surface HSGAGs on FGF signaling. We demonstrate this approach using cell surface-derived HSGAGs and find that distinct HSGAGs elicit differential FGF response patterns through FGFR1c and FGFR3c. We conclude that this assay system can be used to probe the ability of distinct HSGAG species to regulate the activity of specific FGF-FGFR pairs.     

Study of the interaction of the Ig2 module of the fibroblast growth factor receptor, FGFR Ig2, with the fibroblast growth factor 1, FGF1, by means of NMR spectroscopy

Fibroblast growth factor (FGF) receptor (FGFR) consists extracellularly of three immunoglobulin (Ig) modules (Ig1-3). Currently, there are two competing models (symmetric and asymmetric) of the FGF-FGFR-heparin complex based on crystal structures. Indirect evidence exists in support of both models. However, it is not clear which model is physiologically relevant. Our aim was to obtain direct, non-crystallographic evidence in support of them. We found by nuclear magnetic resonance that Ig2 could bind to FGF1 not only via the primary site (present in both models), but also via the secondary site (present only in the symmetric model). Thus, our data support the symmetric model.