PROPERLY ORIENTED HEPARIN-DECASACCHARIDE-INDUCED DIMERS ARE THE BIOLOGICALLY-ACTIVE FORM OF BASIC FIBROBLAST GROWTH-FACTOR

Interaction of basic fibroblast growth factor (FGF-2) with heparin or heparan sulfate proteoglycans (HSPGs) is required for receptor activat ion and initiation of biological responses. To gain insight into the m echanism of activation of the FGF receptor by FGF-2 and heparin, we ha ve used NMR, dynamic light scattering, and HSPG-deficient cells and ce ll-free systems. The first 28 N-terminal residues in FGF-2 were found to be highly mobile and flexible, consistent with the disorder found i n both the NMR and X-ray structures. The structure of an FGF-2-heparin -decasaccharide complex that binds to and activates the FGF receptor w as compared to a heparin-tetrasaccharide-induced complex that does not promote an interaction with the receptor. The major change observed u pon the addition of the tetrasaccharide to FGF-2 was an increase in th e correlation time consistent with the formation of an FGF-2 dimer. Th e NMR line widths of FGF-2 in the presence of the decasaccharide are s everely broadened relative to the tetrasaccharide, consistent with dyn amic light scattering results which indicate FGF-2 is a tetramer. The interaction of these heparin species with FGF-2 does not induce a sign ificant conformational change in the overall structure of FGF-2, but s mall chemical shift changes are observed in both heparin and receptor binding sites. A trans-oriented symmetric dimer of FGF-2 is formed in the presence of the tetrasaccharide whereas two cis-oriented dimers in a symmetric tetramer are formed in the presence of the decasaccharide . This suggests that the cis-oriented FGF-2 dimer is the minimal biolo gically active structural unit of FGF-2. These data allow us to propos e a novel mechanism to explain the functional interaction of FGF-2 wit h heparin and its transmembrane receptor.