Membrane fusion is a vital process in key cellular events. The fusion capability of a membrane depends on its elastic properties and varies with its lipid composition. It is believed that as the composition varies, the consequent change in C 0 (monolayer spontaneous curvature) is the major factor dictating fusion, owing to the associated variation in G E s (elastic energies) of the fusion intermediates (e.g. stalk). By exploring the correlations among fusion, C 0 and K cp (monolayer bending modulus), we revisit this long-held belief and re-examine the fusogenic contributions of some relevant factors. We observe that not only C 0 but also K cp variations affect fusion, with depression in K cp leading to suppression in fusion. Variations in G E s and inter-membrane interactions cannot account for the K cp -fusion correlation; fusion is suppressed even as the G E s decrease with K cp , indicating the presence of factor(s) with fusogenic importance overtaking that of G E . Furthermore, analyses find that the C 0 influence on fusion is effected via modulating G E of the pre-fusion planar membrane, rather than stalk. The results support a recent proposition calling for a paradigm shift from the conventional view of fusion and may reshape our understanding to the roles of fusogenic proteins in regulating cellular fusion machineries.Membrane fusion is vital for living organisms. Many cellular events, such as the release of neurotransmitters, the invasion of enveloped viruses, the intracellular trafficking of proteins and the conception for sexual reproduction, involve membrane fusion 1,2 . Complete of the fusion process sees two membrane-bound entities merge into a single one, with the initially discrete membranes and the enclosed contents mixed together. Cellular implementation of fusion requires the concerted action of an intricate machinery consisting of lipids, fusogenic proteins and fusion-triggering stimulants (e.g., Ca 2+ ) 3,4 . While the wide diversity of the lipids, proteins and other biomolecules involved in cellular fusion often complicates the attempts to explore the inner working shared by various fusion machineries, protein-free model membranes with defined lipid compositions [e.g., liposome, also known as unilamellar vesicle (ULV), a hollow spherical structure bound with a single lipid bilayer] have been proven an indispensable tool in uncovering the universal mechanism for all sorts of fusion 3,5 . It is known from model membrane studies that initiating and advancing the fusion process demand the overcoming of several energy barriers; recognizing these barriers has provided insight on how proteins regulate cellular fusion machineries 3,5 . The first energy barrier arises from the need to bring two fusion-destined membranes into close proximity to initiate fusion 6,7 . The barrier, an inter-membrane interaction known as hydration repulsion, results from the resistance to removing inter-membrane water needed for shortening the inter-membrane distance 8 . Once fusion is initiated, the next energy barriers ...

cyclopropane and/or cyclohexane rings. Such structure of the hydrophobic part of the molecule not only helps the archaea to live at high temperatures, but also protects their membranes from harmful influence of various phospholipases, secreted by other organisms, and from oxidative stress. However, the role of the branched chain structure in the permeability of archaeal membranes to various ions, gases and water is still an open question. Pores, being conducting defects in a membrane, are the one of the most general causes of loss of membrane barrier function leading to cell death. So structural peculiarities of archaeal lipid should have some influence on membrane resistance to pore formation. We have studied process of pore formation in bilayer lipid membranes formed by ether and ester branched and unbranched lipids using electrical breakdown technique and molecular dynamics simulations. We have shown that branched lipid tails have a great influence on probability of pore formation and dynamics of its growth. A theoretical model connecting pore edge line tension and dynamics of pore widening was proposed. 1204-Pos Board B155Effect of Electrostatic Repulsion on DMPG Bilayers It is well known that ionic strength plays a fundamental role in the structure of DMPG (dimyristoyl phosphatidylglycerol) anionic vesicles in water medium. In buffer, at pH values above 4 and at high ionic strength (above~100 mM), the morphology of DMPG vesicles are rather similar to that of DMPC (dimyristoyl phosphatidylcholine) vesicles. However, at low ionic strength (~4 mM), DMPG dispersions display several anomalous characteristics, which were interpreted as the opening of bilayer pores along the wide bilayer gel-fluid transition region (from~18 o C to 30 o C) 1 . Here, we revisit DMPG in pure water 2 , to emphasize electrostatic interactions between the polar head-groups, which will not be shielded by ions in solution. For comparison, we used several techniques that have been recently applied to DMPG in buffer: light scattering, both static (SLS) and dynamic (DLS); differential scanning calorimetry (DSC); electron spin resonance (ESR) of spin labels incorporated into the aggregates; and viscosity, turbidity and electrical conductivity measurements. DSC and spin labels indicate that, in water, the bilayer gel-fluid transition is even wider, starting around 10 o C but still ending~30 o C. However, high electric conductivity, high viscosity and low turbidity found only in the gel-fluid transition region for DMPG in buffer, are found at higher temperatures in water, when lipid bilayers are already in the fluid state. Moreover, different from DMPG in buffer, in water, vesicles were found to fuse along the transition region. Data suggest that the strong PG --PGelectrostatic repulsion in water leads not only to pore formation in DMPG bilayers, but also to the opening of the vesicles and vesicle fusion.[1]Enoki,T. A.; Henriques, V. B.; M. T. Lamy, Chem. Electrochemistry is a technique that can be used to detect the contents of neurotransmitter...