Membrane fusion promoters and inhibitors have contrasting effects on lipidbilayer structure and undulations

It has been established that the fusion of both biological membranes and ph ospholipid bilayers can be modulated by altering their lipid composition (C hernomordik et al., 1995. J. Membr. Biol. 146:3). In particular, when added exogenously between apposing membranes, monomyristoylphosphatidylcholine ( MMPC) inhibits membrane fusion, whereas glycerol monoleate (GMO), oleic aci d (OA), and arachidonic acid (AA) promote fusion. This present study uses x -ray diffraction to investigate the effects of MMPC, GMO, OA, and AA on the bending and stability of lipid bilayers when bilayers are forced together with applied osmotic pressure. The addition of 10 and 30 mol% MMPC to egg p hosphatidylcholine (EPC) bilayers maintains the bilayer structure, even whe n the interbilayer;fluid spacing is reduced to similar to 3 Angstrom, and i ncreases the repulsive pressure between bilayers so that the fluid spacing in excess water increases by 5 and 15 Angstrom, respectively. Thus MMPC inc reases the undulation pressure, implying that the addition of MMPC promotes out-of-plane bending and decreases the adhesion energy between bilayers. I n contrast, the addition of GMO has minor effects on the undulation pressur e; 10 and 50 mol% GMO increase the fluid spacing of EPC in excess water by 0 and 2 Angstrom, respectively. However, x-ray diffraction indicates that, at small interbilayer separations, GMO, OA, or AA converts the bilayer to a structure containing hexagonally packed scattering units similar to 50 Ang strom in diameter. Thus GMO, OA, or AA destabilizes bilayer structure as ap posing bilayers are brought into contact, which could contribute to their r ole in promoting membrane fusion.