Basic fibroblast growth factor (FGF-2) is important in development, wound healing and angiogenesis. The human plasma proteinase inhibitor α2-macroglobulin (α2M) binds to and regulates the biological activity of various growth factors, including FGF-2. FGF-2 binds specifically and saturably to native α2M and conformationally modified α2M (α2M∗); however, the KD for FGF-2 binding to α2M∗ is 10-fold lower. This study investigates the biochemical nature of the interaction between FGF-2 and α2M∗ and localizes a possible FGF-2 binding site in the α2M subunit. FGF-2 binding to α2M∗ was not affected by shifts in pH between 6.5 and 10; however, increasing temperature decreased the KD for this interaction. The binding affinity of FGF-2 for α2M∗ also increased with increasing ionic strength. These results are consistent with the hypothesis that hydrophobic interactions predominate in promoting FGF-2 association with α2M∗. Consistent with this hypothesis, FGF-2 bound to a glutathione S-transferase fusion protein containing amino acids 591–774 of the α2M subunit (FP3) and to a hydrophobic 16-amino-acid peptide (amino acids 718–733) within FP3. Specific binding of FGF-2 to the 16-amino-acid peptide was inhibited by excess transforming growth factor-β1. When the 16-amino-acid peptide was chemically modified to neutralize the only two charged amino acids, FGF-2-binding activity was unaffected, supporting the predominant role of hydrophobic interactions. FGF-2 presentation to signalling receptors is influenced by growth factor binding to heparan sulphate proteoglycans (HSPGs), which is electrostatic in nature. Our results demonstrate that the interactions of FGF-2 with α2M∗ and HSPGs are biochemically distinct, suggesting that different FGF-2 sequences are involved.
Abbreviations used: FGF, fibroblast growth factor; HSPG, heparan sulphate proteoglycan; α2M, α2-macroglobulin; α2M∗, conformationally modified α2M; TGF-β, transforming growth factor-β; PDGF, platelet-derived growth factor; GST, glutathione S-transferase.