Plasmenylethanolamine Facilitates Rapid Membrane Fusion: A Stopped-Flow Kinetic Investigation Correlating the Propensity of a Major Plasma Membrane Constituent To Adopt an H_II Phase with Its Ability To Promote Membrane Fusion

A critical step in membrane fusion involves the formation of a lipid intermediate which shares a conformational similarity with an inverted hexagonal phase (H_II). Since plasmenylethanolamines possess a marked propensity for hexagonal phase formation and represent a major lipid constituent of several membrane systems which undergo rapid membrane fusion (e.g., plasma membranes and synaptic vesicle membranes), we compared the relative fusogenicity of lipid vesicles containing plasmenylethanolamine to that of vesicles containing their diacyl phospholipid counterpart (i.e., phosphatidylethanolamine). Vesicles comprised of equimolar mixtures of phosphatidylcholine and phosphatidylethanolamine fused slowly with phosphatidylserine vesicles in the presence of 10 mM CaCl₂, as assessed either by lipid mixing (dequenching of octadecyl rhodamine fluorescence, 7.4F_max% s⁻¹) or internal contents mixing (fluorescence enhancement from the resultant Tb/dipicolinic acid charge transfer complex, 8.7F_max% s⁻¹). In stark contrast, vesicles comprised of equimolar mixtures of phosphatidylcholine and plasmenylethanolamine fused three times more rapidly, as assessed by both lipid mixing (22.1F_max% s⁻¹) and internal contents mixing (21.4F_max% s⁻¹) assays. The importance of an H_II-like intermediate in membrane fusion was further substantiated by demonstration that plasmenylethanolamines containing arachidonic acid at the sn-2 position (which demonstrate a greater propensity for H_II phase formation) exhibited the most rapid rate of membrane fusion (five times greater than phosphatidylethanolamine containing oleic acid at the sn-2 position). Furthermore, vesicles containing plasmenylethanolamines in physiologic ratios with other phospholipids (i.e., PC/PE/PS, 45:45:10, mol/mol) underwent fusion six times more rapidly (4.4F_max% min⁻¹) than corresponding vesicles in which plasmenylethanolamine was replaced with phosphatidylethanolamine (0.7.F_max% min⁻¹). Collectively, these studies demonstrate the importance of plasmalogens containing arachidonic acid in facilitating membrane fusion and further substantiate the importance of an H_II-like intermediate in membrane fusion events.