1 Hemifusion of Giant Lipid Vesicles Induced by a Small Transient Osmotic Depletion Force

We observed hemifusion of giant unilamellar vesicles (GUVs) by injecting a small volume of 5 % polyethylene glycol solution in the vicinity for a short time. The induced transient osmotic depletion attraction between two bilayers was smaller than the corresponding van der Waals force, yet hemifusion readily occurred for some lipid compositions. This might imply that the energy barrier for hemifusion is very low if the lipid compositions were primed for the reaction. The barrier for hemifusion is then the task of bringing two membranes into contact. We found that hemifusion is a stochastic event very much like pore formation by tension or by pore-forming peptides. This method provides an operational definition for hemifusogenicity which so far has been defined only qualitatively. Surprisingly the configuration of hemifusion depends on the lipid composition, ranging from a stalk-like hemifusion to a large hemifusion diaphragm. The formation of a large hemifusion diaphragm requires trans-bilayer lipid mixing in the participating GUVs. We also found that hemifusogenicity correlates with the water activity for the stalk phase in the lipid water mixture. The effect of cholesterol on hemifusogenicity was clearly demonstrated