Basic fibroblast growth factor (FGF-2)

Basic fibroblast growth factor (FGF-2), a member of a protein family that includes three oncogenes (FGF-3, FGF-4 and FGF-5), exhibits angiogenic and a variety of growth and differentiation activities. Its diverse role in regulating cell growth and differentiation has suggested an involvement in wound healing, tumor growth and cancer. A common feature of the FGF family members is their high affinity toward heparin sulfate proteoglycans (HSPG). The interaction of FGF-2 with HSPG is required for high-affinity binding to its cell surface tyrosine kinase receptor (FGFR) and essential for mediating internalization and intracellular targeting through a proposed mechanism of receptor dimerization. It has been suggested that HSPG might interact directly with FGFR to facilitate the formation of a trimolecular complex and that the HSPG induced dimerization of FGF-2 may be important for receptor dimerization. Near complete ¹H, ¹⁵N, ¹³CO, and ¹³C assignments, solution secondary structure, dynamics, high-resolution structure of FGF-2 and its interaction with heparin have been analyzed by NMR. A helix-like structure was observed for residues 131-136 which is part of the heparin binding site (residues 128-138). The discovery of the helix-like region in the primary heparin binding site instead of the b-strand conformation described in the x-ray structures may have important implications in understanding the nature of heparin-FGF-2 interactions. A total of seven tightly bound water molecules were found in the FGF-2 structure, two of which are located in the heparin binding site. Presented the first direct experimental evidence obtained by NMR, independently confirmed by dynamic light scattering and biological relevance established in cell-based and cell-free assays to propose a specifically oriented heparin-FGF-2 complex. Since the FGF-2-tetrasaccharide trans-dimer is inactive in receptor binding and initiation of the biological response, we conclude that the minimum active structural unit of the FGF-2-heparin complex is the properly oriented cis-dimer component of the tetramer "sandwich" motif induced by the decasaccharide.