Engineering of Fibroblast Growth Factor: Alteration of Receptor Binding Specificity

A five-residue loop structure in basic fibroblast growth factor (FGF-2) which extends from amino acid residue 118 to residue 122 was replaced, by cassette mutagenesis, with the corresponding seven-residue loop structure from the structural homologue acidic fibroblast growth factor (FGF-1) or the corresponding five-residue loop from interleukin-1 beta to give FGF-2LA and FGF-2LI, respectively. The mutants were expressed in Escherichia coli and purified to homogeneity, and their heparin and receptor binding and biological properties were examined. The ability of FGF-2LA to induce endothelial cell proliferation was the same as that of FGF-2. Affinities of the mutants to heparin and to cells that express FGF receptor-1 (FGFR-1) were identical to those of the wild-type protein. The role of the loop structure in FGF-1 and FGF-2 was elucidated by using soluble FGF receptor systems, which display distinct ligand binding specificities. Thus, FGF-2LA bound, with the same affinity as FGF-1 and FGF-2, to FGFR-1 and FGFR-2, whereas only FGF-1 and the FGF-1 loop-containing mutant, FGF-2LA, bound to the keratinocyte growth factor receptor. A change in receptor binding specificity was not observed with the FGF-2LI engineered mutant. That the binding specificity of FGF-2 was dramatically altered by transfer of a loop structure from FGF-1 to resemble the binding profile of the donor protein provides strong evidence that this motif is a receptor binding specificity determinant of fibroblast growth factors.