Direct vesicle-vesicle exchange of phospholipids mediated by polymyxin B.

Direct and rapid intermembrane exchange of phospholipids without fusion is shown to occur across stable vesicle-vesicle contacts formed by stoichiometric amounts of polymyxin B. The exchange is selective for monoanionic, but not dianionic, glycerophospholipids irrespective of their chain length or phase properties. Selective transfer mediated by protein contacts between membranes could serve as a model for stable fusion intermediates and provide a structural and organizational basis for direct phospholipid transfer as implicated in entry of enveloped viruses, secretion, endocytosis, and intracellular transport and targeting of lipids. It is also proposed that the loss of the specificity of phospholipid composition in the inner and outer membranes of gram negative bacteria mediated by polymyxin B could be the basis for its bactericidal action.