Heparin-binding fibroblast growth factor (FGF)-1 is a potent modifier of angiogenic and neurotrophic phenomena in vivo which signals these responses by including tyrosine phosphorylation of a family of high affinity FGF receptors (R) on the surface of target cells. Because FGF-1 does not contain a classical signal peptide sequence to direct its secretion through the endoplasmic reticulum-Golgi apparatus, yet requires FGFR signaling for function, it is important to understand the mechanism by which FGF-1 is secreted. They have previously shown that FGF-1 is released as a biologically inactive and poor heparin-binding protein in response to temperature stress in vitro. Additional studies from this laboratory have shown that FGF-1 (i) can be activated by reduced glutathione, (ii) is released as a cys30-cys30 FGF-1 homodimer, (iii) is able to associate with phosphatidylserine with high affinity, and (iv) is complexed to a cytosolic p45 fragment of p65 synaptotagmin (stg)-1 in both temperature-conditioned medium and in ovine brain. Interestingly, p65 stg-1 is a transmembrane component of the conventional exocytotic pathway which serves as a docking mechanism for classical exocytotic secretory vesicles. These observations have generated a working hypothesis which assumes that FGF-1 utilizes the cytosolic face of the conventional exocytotic pathway to gain access to a novel secretion pathway which employees a protease to cleave stg-1 free of its transmembrane support. Thus, the goal of this application is to confirm and expand their preliminary data that FGF-1 is secreted as a FGF-1 homodimer:stg-1 complex and to determine the structural basis for the interaction between FGF-1 and stg-1 including the identification of the stg-1-cleaving enzyme. They anticipate that this effort will not only elucidate a novel secretion pathway, but will also yield new insight into the mechanism by which FGF-1 modifies human physiologic and pathophysiologic states.
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