AREA EXPANSION AND PERMEATION OF PHOSPHOLIPID MEMBRANE BILAYERS BY INFLUENZA FUSION PEPTIDES AND MELITTIN

The fusion of membrane bilayers is an ubiquitous cellular process. Exo cytosis, organelle formation, cellular trafficking, cell division, fer tilization, and numerous other cellular activities all involve membran e fusion. Fusion can be rapid, occurring on the order of seconds or mi nutes. In viral infection, specific glycoproteins mediate the fusion o f the viral lipid envelope with a cellular membrane. Viral fusion glyc oproteins typically contain a segment which embeds into the target cel lular membrane, referred to as the fusion peptide. The mechanism of ac tion of viral fusion proteins is still not certain; in particular, the extent and rate of insertion of the fusion peptide are not well quant ified. In this report, we use micropipet aspiration and video microsco py of large unilamellar phosphatidylcholine vesicles to determine the membrane area expansion resulting from the insertion of fusion peptide s into the lipid bilayer. The fusion peptide of the viral fusion prote in, influenza hemagglutinin, inserts into phosphatidylcholine bilayers , resulting in an increase in the membrane area on a time scale (i.e., seconds) similar to that of viral fusion. Following peptide insertion , porous defects form in minutes. We show that chemical changes in the Nand C-termini of this peptide can either eliminate, decrease, or enh ance surface activity of the peptide; particularly the propensity to f orm pores can be diminished. In control studies, the well-studied lyri c peptide, melittin, similarly increases the membrane area and forms p ores in the membrane. The observation that the wild-type influenza fus ion peptide and melittin each form pores below the areal expansion lim it of 5% suggest that membrane disrupting proteins act through specifi c and localized perturbation.