Structure-function studies of FGF-1: dissociation and partial reconstitution of certain of its biological activities.

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 tyrosine-kinase 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 transfectants are refractory to exogenous FGF-1, whereas the mutant transfectants respond normally.(ABSTRACT TRUNCATED AT 250 WORDS)