Reductive methylation of lysine residues in acidic fibroblast growth factor: effect on mitogenic activity and heparin affinity.

Reductive methylation of bovine brain derived acidic fibroblast growth factor (aFGF) with formaldehyde and sodium cyanoborohydride reduces its capacity to stimulate mitogenesis in Balb/C 3T3 cells, and this correlates with the modification of less than 3 of its 12 lysine residues. Fractionation of methylated aFGF on immobilized heparin shows that the affinity of the modified mitogen for heparin is also decreased substantially. The capacity of methylated mitogen of low heparin affinity (LA-aFGF) to stimulate mitogenesis is also reduced, and this correlates with a reduced affinity for its cell surface receptor. Structural characterization of LA-aFGF using peptide mapping and sequencing procedures demonstrates that Lys-118 is the primary site of modification. The results indicate that in aFGF, Lys-118 plays an important role in heparin binding and suggest that this residue and its local environment are involved in the interaction of aFGF with both heparin and its cell surface receptor.