Synexin (annexin VII) and membrane fusion during the process of exocytotic secretion

Synexin is a cytosolic protein, which in the presence of increased calcium can undergo a conformational change that renders it highly hydrophobic. This chapter discusses some of the assumptions to model the fusion process in vitro, the ways in which synexin promotes different aspects of the model, and some of the physical and chemical evidence underlying the present understanding of synexin action during membrane fusion. The ability of synexin to fuse target membranes lies in the fact that once activated by calcium, the synexin monomers form oligomers, which are capable of interacting with two membranes simultaneously. While the hydrophobic bridge hypothesis has been developed to explain the process of fusion driven by synexin, it need not be limited in principle to just this fusion event. It may be anticipated that any fusion event in which a hydrophobic environment is introduced to the potential fusion site, may proceed by the hydrophobic bridge mechanism and is exemplified by studies on calcium-dependent, synexin driven membrane contact and fusion of chromaffin granules, chromaffin granule ghosts, and phospholipid bilayer liposomes.