Conversion of cysteine to serine residues alters the activity, stability, and heparin dependence of acidic fibroblast growth factor

Acidic fibroblast growth factor (aFGF) is a broad spectrum mitogen that is stabilized by complexation with heparin and heparan proteoglycans. The monomeric human protein contains 3 reduced cysteine residues of unknown function, the first 2 of which are conserved among all seven known fibroblast growth factors. The influence of these free sulfhydryl groups on the level, stability, and heparin dependence of the mitogenic activity at physiological temperature and pH is characterized using a complete set of site-directed mutants in which either any 1, 2, or all 3 of the cysteine residues are converted to serines. Mutants of aFGF in which either any 2 or all 3 cysteine residues are substituted by serines are more active, have longer activity half-lives, and are less heparin dependent than wild-type aFGF. In contrast, wild-type aFGF and the three mutants that each retain 2 cysteine residues inactivate more rapidly in the absence of heparin by a nonproteolytic mechanism but are markedly stabilized by heparin. This cysteine-mediated destabilization of aFGF not only diminishes its activity in the absence of heparin in tissue culture but also could functionally restrict its activity in vivo to the vicinity of mast cell-derived heparins and heparan proteoglycans associated with cell surfaces and basement membranes.