Structure‐function studies of FGF‐1: Dissociation and partial reconstitution of certain of its biological activities

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Abstract
We reported previously that the mitogenic activities of FGF‐1 (acidic FGF) could be dissociated from its receptor‐binding activities by site‐directed mutagenesis of lysine 132 to a glutamic acid. Although the mutant FGF‐1 protein binds to the high‐affinity tyrosinekinase receptors, stimulates tyrosine‐kinase activity, and promotes expression of immediate‐early genes, it is not mitogenic for a variety of tested cell lines. Interestingly, the mutant FGF‐1 is capable of other functions associated with the wild‐type protein such as promotion of mesoderm formation in Xenopus animal caps. The mutant exhibits a reduced apparent affinity for heparin‐Sepharose compared to the wild‐type protein. The relationship between the reduced heparin affinity and lack of mitogenic activity of this mutant is not clear. Recent data indicates the relationship is not as simple as reduced stability of the protein. When NIH 3T3 cells are transfected with expression vectors encoding either wild‐type or mutant FGF‐1, a transformed phenotype can be seen in cells overexpressing the wild‐type FGF‐1, whereas cells overexpressing mutant FGF‐1 appear normal. Analysis of lysates of these cells indicates that a tyrosine‐kinase cascade, distinct from that associated with the high‐affinity cell surface receptors, has been activated in the wild‐type transfected cells but not in the mutant transfected cells. Although both transfected cell lines contain FGF‐1 cell surface receptors as judged by crosslinking studies, the wild‐type transfects are refractory to exogenous FGF‐1, whereas the mutant transfectants respond normally. Together these results support an intracellular role of wild‐type FGF‐1 in mediating certain of its functions. In addition, they demonstrate that certain functions of the growth factor can be dissociated at the structural level. Additional mutagenesis studies have resulted in the identification of mutants with heparin‐binding or mitogenic deficiencies that do not correlate as well as those of the 132 mutant. It appears that the inactivity of the lysine 132 mutant is related, in part, to cysteine 131.

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