Bilayer curvature and certain amphipaths promote poly(ethylene glycol)-induced fusion of dipalmitoylphosphatidylcholine unilamellar vesicles

Abstract

Unilamellar vesicles of varying and reasonably uniform size were prepared from 1,2-dipalmitoyl-3-sn-phosphatidylcholine (DPPC) by the extrusion procedure and sonication. Quasi-elastic light scattering was used to show that different vesicle preparations had mean (Z-averaged) diameters of 1340, 900, 770, 630, and 358 A (sonicated). Bilayer-phase behavior as detected by differential scanning calorimetry was consistent with the existence of essentially uniform vesicle populations of different sizes. The response of these different vesicles to treatment with poly(ethylene glycol) (PEG) was monitored using fluorescence assays for lipid transfer, contents leakage, and contents mixing, as well as quasi-elastic light scattering. No fusion, as judged by vesicle contents mixing and change in vesicle size, was detected for vesicles of diameter greater than 770 A. The diameters of smaller vesicles increased dramatically when treated with high concentrations of PEG, although mixing of their contents could not be detected both because of their small trapped volumes and because of the extensive leakage induced in small vesicles by high concentrations of PEG. Lipid transfer was detected between vesicles of all sizes. We conclude the high bilayer curvature does encourage fusion of closely juxtaposed membrane bilayers but that highly curved vesicles appear also to rupture and form larger structures when diluted from high PEG concentration, a process that can be confused with fusion. Despite the failure of PEG to induce fusion of large, uncurved vesicles composed of a single phosphatidylcholine, these vesicles can be induced to fuse when they contain small amounts of certain amphiphathic compounds thought to play a role in cellular fusion processes. Thus, vesicles which contained 0.5 mol % L-alpha-lysopalmitoylphosphatidylcholine, 5 mol % platelet activating factor, or 0.5 mol % palmitic acid fused in the presence of 30%, 25%, and 20% (w/w) PEG, respectively. However, vesicles containing 1,2-dipalmitoyl-sn-glycerol, 1,2-dioleoyl-sn-glycerol, 1-oleoyl-2-acetyl-sn-glycerol, or monooleoyl-rac-glycerol at surface concentrations up to 5 mol % did not fuse in the presence or absence of PEG. There was no correlation between the abilities of these amphipaths to induce phase separation or nonlamellar phases and their abilities to support fusion of pure DPPC unilamellar vesicles in the presence of high concentrations of PEG. The results are discussed in terms of the type of disrupted lipid packing that could be expected to favor PEG-mediated fusion.

SNAREpin/Munc18 promotes adhesion and fusion of large vesicles to giant membranes

“…In particular, membrane tension and/or curvature, and the formation of distinct localized architectures (including protein clusters, membrane dimples, and lipid rafts) all appear to influence the rate and extent of membrane fusion. For example, high bilayer curvature significantly increases fusion efficiency in various protein-free liposome fusion reactions, driven by divalent cations or poly(ethylene glycol) (PEG) (34,35). There is no single reconstitution method that puts each of these variables in play, and as such, there continues to be a need for new and multiple assay systems.…”

Microscopic observations reveal that fusogenic peptides induce liposome shrinkage prior to membrane fusion

“…LUVs are 0.1-1 m in diameter, whereas giant liposomes are 1-20 m, which results in different membrane curvatures. Fusogenic peptides may penetrate more efficiently into membranes with larger curvatures and may destabilize the lipid bilayer structures, making fusion easier (26). Consequently, fusogenic peptides possess few charges, and HA causes fusion only between LUVs, but not giant liposomes, because only LUVs possess a large curvature.…”

Tetrameric Bacterial Sodium Channels: Characterization of Structure, Stability, and Drug Binding

“…Lipids were then rehydrated to 20 mg/mL lipid in 1 mL of 20 mM Tris (pH 7.5) containing 10 µM sodium green dye (20). Liposomes with a diameter of ∼100 nm were formed by extrusion using a mini extruder (Avanti Polar Lipids) (21).…”