title: Synexin facilitates fusion of specific phospholipid membranes at divalent cation concentrations found intracellularly.
abstract: 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.
mesh_terms: D000818:Animals; D017310:Annexin A7; D002118:Calcium / Q000494:pharmacology; D002417:Cattle; D004305:Dose-Response Relationship, Drug; D005089:Exocytosis; D008274:Magnesium / Q000494:pharmacology; D008561:Membrane Fusion / Q000187:drug effects*; D008563:Membrane Lipids; D010712:Phosphatidic Acids; D010713:Phosphatidylcholines; D010714:Phosphatidylethanolamines; D010743:Phospholipids; D011506:Proteins / Q000494:pharmacology*
chemical_list: D017310:Annexin A7; D008563:Membrane Lipids; D010712:Phosphatidic Acids; D010713:Phosphatidylcholines; D010714:Phosphatidylethanolamines; D010743:Phospholipids; D011506:Proteins; D008274:Magnesium; D002118:Calcium