Site-directed mutagenesis of cysteine to serine residues affects heparin binding and mitogenicity in fibroblast growth factor 4 produced in Escherichia coli

Abstract

The binding activity to heparan sulphate is crucial for the mitogenic activity of fibroblast growth factor 4 (FGF4) in developing mammalian embryos. There are two conserved cysteine residues in FGF family proteins, Cys-84 and Cys-151 in mouse FGF4, and these constitute all of the cysteine residues in FGF4. However, the relationships among the heparin binding activity, growth promoting activity, and the two conserved cysteine residues in FGF4 are still unclear. Consequently, we generated in Escherichia coli three kinds of point-mutated mouse FGF4, namely C84S, C151S, and C84S;C151S, by converting the cysteine residues to serine residues. In heparin column chromatography, the heparin binding activities of these mutants were attenuated. In particular, the activity of the double-mutated C84S;C151S was weakened considerably. The growth promoting activities of these mutants correlated well with their heparin binding activities. We also demonstrated that an octapeptide, the Leu-76 to Tyr-83 region, which contained four basic amino acid residues and flanked Cys-84 in mouse FGF4, enhanced the heparin binding activity when fused to glutathione-S-transferase as a recombinant protein. Overall, our findings imply that the two conserved cysteine residues in FGF4 are both involved in the heparin binding activity and mitogenicity likely by affecting the configuration of heparin binding sites in FGF4.