X-RAY CRYSTAL-STRUCTURE OF HUMAN ACIDIC FIBROBLAST GROWTH-FACTOR

Fibroblast growth factors (FGFs) are mitogenic and chemotactic agents for a wide variety of cell types and play a primary role in the regula tion of angiogenesis. Angiogenesis is involved in a variety of critica l physiological events including organogenesis, wound healing, ischemi c collateral circulation, and solid tumor growth. High-resolution stru ctural information is key to understanding the mechanism of action of these growth factors. We report here the X-ray crystal structure of hu man acidic FGF (aFGF), with data extending to 2.0 Angstrom resolution. The crystal contains four independent molecules in the asymmetric uni t. Each molecule contains a single bound sulfate ion, in similar juxta positions. The bound sulfate is stabilized through hydrogen-bond inter actions with residues Asn 18, Lys 113, and Lys 118 and defines a poten tial heparin binding site. The hydrogen bond with the N delta(2) moiet y of Asn 18 appears to be the most conserved interaction, being simila r to those observed for sulfate ion bound to human basic FGF (bFGF) an d similar but not identical to interactions observed for bovine aFGF w ith heparin analogs. Of the added solvent groups, five ordered water m olecules are conserved in each of the four independent structures of h uman aFGF. These water molecules, located at buried positions, provide hydrogen bonding partnerships with several buried polar groups in the core of the protein. A central interior cavity exists in each of the four structures, with sizes ranging from approximately 20 to 50 Angstr om(3). The cavity sizes appear to be significantly smaller than that o bserved in the related protein interleukin-1 beta. The region comprisi ng the high affinity FGF receptor binding site is structurally very si milar to the corresponding region from human bFGF, whereas the low aff inity site is structurally quite different. The results provide a stru ctural basis for the role of the low affinity binding site in FGF rece ptor discrimination.