THE EXTRAVESICULAR DOMAIN OF SYNAPTOTAGMIN-1 IS RELEASED WITH THE LATENT FIBROBLAST GROWTH-FACTOR-I HOMODIMER IN RESPONSE TO NEAT SHOCK

The heparin-binding fibroblast growth factor (FGF) prototypes lack a c lassical signal sequence, yet their presence is required in the extrac ellular compartment for the activation of cell-surface receptor-depend ent signaling. Early studies with FGF-1 demonstrated its presence in b ovine brain as a novel high molecular weight complex, and subsequent s tudies identified a second heparin-binding protein that co-purified wi th FGF-1. Polypeptide sequence analysis revealed that this heparin-bin ding protein corresponded to the extravesicular domain of bovine synap totagmin (Syn)-1, a transmembrane component of synaptic vesicles invol ved in the regulation of organelle traffic. Since FGF-1 is released in response to heat shock as a mitogenically inactive Cys-SO homodimer, we sought to determine whether this heparin-binding protein was involv ed in the release of FGF-1. We report that a proteolytic fragment of t he extravesicular domain of Syn-1 is associated with FGF-1 in the extr acellular compartment of FGF-l-transfected NIH 3T3 cells following tem perature stress. By using heparin-Sepharose affinity to discriminate b etween the monomer and homodimer forms of FGF-1 and resolution by conv entional and limited denaturant gel shift immunoblot analysis, it was possible to identify FGF-1 and Syn-1 as potential components of a dena turant- and reducing agent-sensitive extracellular complex. It was als o possible to demonstrate that the expression of an antisense-Syn-l ge ne represses the release of FGF-1 in response to heat shock. These dat a indicate that FGF-1 may be able to utilize the cytosolic face of con ventional exocytotic vesicles to traffic to the inner surface of the p lasma membrane where it may gain access to the extracellular compartme nt as a complex with Syn-1.