Membrane Fusion and Inverted Phases

Abstract

We have found a correlation between liposome fusion kinetics and lipid phase behavior for several inverted phase forming lipids. N-Methylated dioleoylphosphatidylethanolamine (DOPE-Me), or mixtures of dioleoylphosphatidylethanolamine (DOPE) and dioleoylphosphatidylcholine (DOPC), will form an inverted hexagonal phase (HII) at high temperatures (above TH), a lamellar phase (Lα) at low temperatures, and an isotropic/inverted cubic phase at intermediate temperatures, which is defined by the appearance of narrow isotropic 31P NMR resonances. The phase behavior has been verified by using high-sensitivity DSC, 31P NMR, freeze-fracture electron microscopy, and X-ray diffraction. The temperature range over which the narrow isotropic resonances occur is defined as ΔTI and the range ends at TH. Extruded liposomes (~0.2 μm in diameter) composed of these lipids show fusion and leakage kinetics which are strongly correlated with the temperatures of these phase transitions. At temperatures below ΔTI where the lipid phase is Lα, there is little or no fusion, i.e., mixing of aqueous contents, or leakage. However, as the temperature reaches ΔTI, there is a rapid increase in both fusion and leakage rates. At temperatures above TH, the liposomes show aggregation-dependent lysis, as the rapid formation of HII phase precursors disrupts the membranes. We show that the correspondence between the fusion and leakage kinetics and the observed phase behavior is easily rationalized in terms of a recent kinetic theory of Lα/inverted phase transitions. In particular, it is likely that membrane fusion and the Lα/inverted cubic phase transition proceed via a common set of intermembrane intermediates. © 1989, American Chemical Society. All rights reserved.