On the correlation between HII phase and the contact-induced destabilization of phosphatidylethanolamine-containing membranes.

The abundance of phosphatidylethanolamine (PtdEtn) in biological membranes and the capacity of this lipid to sustain nonbilayer structures have been promoted as evidence for a role of PtdEtn in biological fusion processes. To date there has been no direct evidence of a connection between the kinetics of bilayer destabilization and the polymorphism accessible to PtdEtn. We have developed a model system to examine this point directly using the proton-induced destabilization of PtdEtn/cholesterylhemisuccinate unilamellar liposomes. We find that the initial rate of bilayer mixing rapidly increases with temperature and reaches a maximal level just below the HII-phase transition temperature. The leakage from these liposomes rapidly increases, both in rate and extent, within the HII-phase transition temperature range. Of an even greater significance is that at no temperature is there any mixing of aqueous contents within the liposomes. Thus, these lipids can begin to undergo the lamellar- to HII-phase transition at the stage of two apposed liposomes. However, the nonbilayer structures formed do not cause fusion--i.e., the concomitant mixing of aqueous contents.