Multiple forms of bFGF: differential nuclear and cell surface localization

The single copy gene for human basic fibroblast growth factor (bFGF) has been shown to encode not one but multiple proteins of 24, 23, 22 and 18 kD. Although bioactivities of the 18 kD protein are currently used to define bFGF gene function, it is not yet known if the three larger proteins have these same bioactivities or whether they will serve to define new bFGF gene functions. In this report we present a comparative study describing the de novo synthesis, transport, processing and intracellular location of individual bFGF isoforms. Data from cDNA mutagenesis and COS cell expression experiments show that individual isoforms are differentially localized to either the cell surface or to the nucleus. The 24, 23 and 22 kD proteins (CUG-mediated initiation) exclusively localize in the nucleus while the 18 kD protein (AUG-mediated initiation) is preferentially exported onto the cell surface, but is not released into the surrounding culture medium. Specific CUG or AUG translation initiation codons are necessary and sufficient for the synthesis of each isoform examined and thereby, indirectly, mediate differential localization. Since bFGF does not contain the characteristic signals predicted for cell surface or nuclear targeting, our continuing studies will either unmask its functionally equivalent domain(s) or will identify the requisite participation of yet unknown cellular components.