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 ≈ 10 μ M in the presence of synexin at 6 μ g/ml and 1.5 mM Mg2+in vesicle suspensions containing 50 μ M 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.