Cooperative Heparin-Mediated Oligomerization of Fibroblast Growth Factor-1 (FGF1) Precedes Recruitment of FGFR2 to Ternary Complexes

Alan Brown,Christopher J Robinson,John T Gallagher,Tom L Blundell

doi:10.1016/j.bpj.2013.02.051

Alan Brown, Christopher J Robinson + Show 2 more

Open Access

https://doi.org/10.1016/j.bpj.2013.02.051

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Abstract

Fibroblast growth factors (FGFs) utilize cell surface heparan sulfate as a coreceptor in the assembly of signaling complexes with FGF-receptors on the plasma membrane. Here we undertake a complete thermodynamic characterization of the assembly of the FGF signaling complex using isothermal titration calorimetry. Heparin fragments of defined length are used as chemical analogs of the sulfated domains of heparan sulfate and examined for their ability to oligomerize FGF1. Binding is modeled using the McGhee-von Hippel formalism for the cooperative binding of ligands to a monodimensional lattice. Oligomerization of FGFs on heparin is shown to be mediated by positive cooperativity (α = 6). Heparin octasaccharide is the shortest length capable of dimerizing FGF1 and on longer heparin chains FGF1 binds with a minimal footprint of 4.2 saccharide units. The thermodynamics and stoichiometry of the ternary complex suggest that in solution FGF1 binds to heparin in a trans-dimeric manner before FGFR recruitment.

Highlights

Mammalian fibroblast growth factors (FGFs) are a family of 22 highly conserved polypeptides
In previous articles using size exclusion chromatography (SEC) to study FGF-FGFR-heparin interactions we suggested that formation of the ternary complex may be driven by cooperative binding of FGFs (FGF1 and FGF2) to heparin saccharides of sufficient length and sulfation to dimerize the growth factors [25,26]
By using defined-length heparin oligosaccharides similar to those used in structural studies, it is possible to link the thermodynamics of the FGF-heparin interaction to the known x-ray and NMR structures

Summary

Introduction

Mammalian fibroblast growth factors (FGFs) are a family of 22 highly conserved polypeptides. They are involved in a plethora of biological processes, including cell proliferation, differentiation, cell migration, and angiogenesis [1,2]. FGFs signal through four FGF receptors (FGFR1–4), which are high affinity cell surface receptor tyrosine kinases [3]. FGFRs have three extracellular immunoglobulin-like domains (D1, D2, and D3), of which only D2 and D3 are required for FGF binding [4]. Receptors 1–3 undergo alternative mRNA splicing to generate b- and c-isoforms with altered ligand specificities and affinities [5]. FGF1 is capable of binding all receptor isoforms, and is sometimes regarded as the universal ligand [6,7]

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