Synexin facilitates fusion of specific phospholipid membranes at divalent cation concentrations found intracellularly.

The effect of synexin (an adrenal medullary protein) on the kinetics of Ca2+- and Mg2+-mediated membrane fusion was examined. Membrane fusion was studied by monitoring intermixing of the aqueous contents of phospholipid vesicles. Synexin facilitated Ca2+-mediated, but not Mg2+-mediated, fusion of phosphatidate/phosphatidylethanolamine (1:3) and phosphatidate/phosphatidylserine/phosphatidylethanolamine/cholesterol (1:2:3:2) vesicles. The threshold concentration of Ca2+ for fusion was decreased to approximately equal to 10 microM in the presence of synexin at 6 micrograms/ml and 1.5 mM Mg2+ in vesicle suspensions containing 50 microM lipid. This effect of synexin was drastically inhibited by including 25% phosphatidylcholine (mol/mol) in the vesicle membrane. It is proposed that the Ca2+-dependent lipid-specific enhancement of membrane fusion by synexin contributes to an increase in the sensitivity of specific intracellular membranes to Ca2+ with respect to fusion.