Diacylglycerol and hexadecane increase divalent cation-induced lipid mixing rates between phosphatidylserine large unilamellar vesicles

Triacylglycerol-droplet-induced bilayer spontaneous curvature in giant unilamellar vesicles

This study investigated the incorporation of triacylglycerol droplets in the bilayers of giant unilamellar vesicles (GUVs) using four triacylglycerols and four phosphatidylcholines by confocal laser scanning microscopy. The triacylglycerol droplets were incorporated between the monolayer leaflets of the GUVs. Among the spherical droplets protruding on only one side of the bilayers, the droplets bound to the outer leaflets outnumbered those bound to the inner leaflets. The more frequent droplet binding to the outer leaflet caused transbilayer asymmetry in the droplet surface density. A vesicle consisting of a single-bilayer spherical segment and a double-bilayer spherical segment was also observed. The yield of these vesicles was comparable with or higher than that of the droplet-incorporating GUVs for many of the phosphatidylcholine-triacylglycerol combinations. In a vesicle consisting of single-bilayer and double-bilayer segments, most of the triacylglycerol droplets were localized on the outermost membrane surface along the segment boundary and in the double-bilayer segment. To rationalize the formation of these vesicle structures, we propose that the transbilayer asymmetry in the droplet surface density induces spontaneous curvature of the bilayer, with the bilayer spontaneously bending away from the droplets. Energy calculations performed assuming the existence of spontaneous curvature of the bilayer corroborated the experimentally determined membrane shapes for the vesicles consisting of unilamellar and bilamellar regions.

Free energies of membrane stalk formation from a lipidomics perspective

Many biological membranes are asymmetric and exhibit complex lipid composition, comprising hundreds of distinct chemical species. Identifying the biological function and advantage of this complexity is a central goal of membrane biology. Here, we study how membrane complexity controls the energetics of the first steps of membrane fusions, that is, the formation of a stalk. We first present a computationally efficient method for simulating thermodynamically reversible pathways of stalk formation at coarse-grained resolution. The method reveals that the inner leaflet of a typical plasma membrane is far more fusogenic than the outer leaflet, which is likely an adaptation to evolutionary pressure. To rationalize these findings by the distinct lipid compositions, we computed ~200 free energies of stalk formation in membranes with different lipid head groups, tail lengths, tail unsaturations, and sterol content. In summary, the simulations reveal a drastic influence of the lipid composition on stalk formation and a comprehensive fusogenicity map of many biologically relevant lipid classes.

Fusion of cellular membranes begins with the formation of a stalk. Here, the authors develop a computationally efficient method for coarse-grained simulations of stalk formation and apply this approach to comprehensively analyse how stalk formation is influenced by the membrane lipid composition.

Membrane Composition Modulates Fusion by Altering Membrane Properties and Fusion Peptide Structure

Membrane fusion, one of the most essential processes in the life of eukaryotes, occurs when two separate lipid bilayers merge into a continuous bilayer and internal contents of two separated membranes mingle. There is a certain class of proteins that assist the binding of the viral envelope to the target host cell and catalyzing fusion. All class I viral fusion proteins contain a highly conserved 20–25 amino-acid amphipathic peptide at the N-terminus, which is essential for fusion activity and is termed as the ‘fusion peptide’. It has been shown that insertion of fusion peptides into the host membrane and the perturbation in the membrane generated thereby is crucial for membrane fusion. Significant efforts have been given in the last couple of decades to understand the lipid-dependence of structure and function of the fusion peptide in membranes to understand the role of lipid compositions in membrane fusion. In addition, the lipid compositions further change the membrane physical properties and alter the mechanism and extent of membrane fusion. Therefore, lipid compositions modulate membrane fusion by changing membrane physical properties and altering structure of the fusion peptide.

Exploring the Mechanism of Viral Peptide-Induced Membrane Fusion

Membrane fusion is essential in several cellular processes in the existence of eukaryotic cells such as cellular trafficking, compartmentalization, intercellular communication, sexual reproduction, cell division, and endo- and exocytosis. Membrane fusion proceeds in model membranes as well as biological membranes through the rearrangement of lipids. The stalk hypothesis provides a picture of the general nature of lipid rearrangement based on mechanical properties and phase behavior of water-lipid mesomorphic systems. In spite of extensive research on exploring the mechanism of membrane fusion, a clear molecular understanding of intermediate and pore formation is lacking. In addition, the mechanism by which proteins and peptides reduce the activation energy for stalk and pore formation is not yet clear though there are several propositions on how they catalyze membrane fusion. In this review, we have discussed about various putative functions of fusion peptides by which they reduce activation barrier and thus promote membrane fusion. A careful analysis of the discussed effects of fusion peptides on membranes might open up new possibilities for better understanding of the membrane fusion mechanism.

Membrane permeabilization induced by sphingosine: effect of negatively charged lipids

Sphingosine [(2S, 3R, 4E)-2-amino-4-octadecen-1, 3-diol] is the most common sphingoid long chain base in sphingolipids. It is the precursor of important cell signaling molecules, such as ceramides. In the last decade it has been shown to act itself as a potent metabolic signaling molecule, by activating a number of protein kinases. Moreover, sphingosine has been found to permeabilize phospholipid bilayers, giving rise to vesicle leakage. The present contribution intends to analyze the mechanism by which this bioactive lipid induces vesicle contents release, and the effect of negatively charged bilayers in the release process. Fluorescence lifetime measurements and confocal fluorescence microscopy have been applied to observe the mechanism of sphingosine efflux from large and giant unilamellar vesicles; a graded-release efflux has been detected. Additionally, stopped-flow measurements have shown that the rate of vesicle permeabilization increases with sphingosine concentration. Because at the physiological pH sphingosine has a net positive charge, its interaction with negatively charged phospholipids (e.g., bilayers containing phosphatidic acid together with sphingomyelins, phosphatidylethanolamine, and cholesterol) gives rise to a release of vesicular contents, faster than with electrically neutral bilayers. Furthermore, phosphorous 31-NMR and x-ray data show the capacity of sphingosine to facilitate the formation of nonbilayer (cubic phase) intermediates in negatively charged membranes. The data might explain the pathogenesis of Niemann-Pick type C1 disease.

End-products diacylglycerol and ceramide modulate membrane fusion induced by a phospholipase C/sphingomyelinase from Pseudomonas aeruginosa

A phospholipase C/sphingomyelinase from Pseudomonas aeruginosa has been assayed on vesicles containing phosphatidylcholine, sphingomyelin, phosphatidylethanolamine and cholesterol at equimolar ratios. The enzyme activity modifies the bilayer chemical composition giving rise to diacylglycerol (DAG) and ceramide (Cer). Assays of enzyme activity, enzyme-induced aggregation and fusion have been performed. Ultrastructural evidence of vesicle fusion at various stages of the process is presented, based on cryo-EM observations. The two enzyme lipidic end-products, DAG and Cer, have opposite effects on the bilayer physical properties; the former abolishes lateral phase separation, while the latter generates a new gel phase [Sot et al., FEBS Lett. 582, 3230-3236 (2008)]. Addition of either DAG, or Cer, or both to the liposome mixture causes an increase in enzyme binding to the bilayers and a decrease in lag time of hydrolysis. These two lipids also have different effects on the enzyme activity, DAG enhancing enzyme-induced vesicle aggregation and fusion, Cer inhibiting the hydrolytic activity. These effects are explained in terms of the different physical properties of the two lipids. DAG increases bilayers fluidity and decreases lateral separation of lipids, thus increasing enzyme activity and substrate accessibility to the enzyme. Cer has the opposite effect mainly because of its tendency to sequester sphingomyelin, an enzyme substrate, into rigid domains, presumably less accessible to the enzyme.

Diacylglycerols, multivalent membrane modulators

Diacylglycerols are second messengers confined to biomembranes and, although relatively simple molecules from the structural point of view, they are able of triggering a surprisingly wide range of biological responses. Diacylglycerols are recognized by a well conserved protein motif, such as the C1 domain. This domain was observed for the first time in protein kinases C but is now known to be present in many other proteins. The effect of diacylglycerols is not limited to binding to C1 domains and they are able to alter the biophysical properties of biomembranes and hence modulate the activity of membrane associated proteins and also facilitate some processes like membrane fusion.

The formation of TGN-to-plasma-membrane transport carriers

In the trans-Golgi network (TGN), proteins are sorted for transport to the endosomes, plasma membrane, preceding Golgi cisternae, and endoplasmic reticulum. The formation of clathrin-coated vesicles for transport to the endosomes and of COP-I-coated vesicles for retrograde trafficking is fairly well characterized at the molecular level. We describe our current understanding of the TGN-to-cell-surface carriers, with a specific focus on the components involved in membrane fission. Inhibiting the fission machinery promotes growth of transport carriers into large tubules that remain attached to the TGN. Overactivating this machinery, on the other hand, vesiculates the TGN. To understand how membrane fission is regulated by cargo to form transport carriers yet prevents complete vesiculation of the TGN remains a daunting challenge. We discuss these issues with regard to TGN-to-cell-surface transport carriers.

Energetics of vesicle fusion intermediates: comparison of calculations with observed effects of osmotic and curvature stresses

We reported previously the effects of both osmotic and curvature stress on fusion between poly(ethylene glycol)-aggregated vesicles. In this article, we analyze the energetics of fusion of vesicles of different curvature, paying particular attention to the effects of osmotic stress on small, highly curved vesicles of 26 nm diameter, composed of lipids with negative intrinsic curvature. Our calculations show that high positive curvature of the outer monolayer "charges" these vesicles with excess bending energy, which then releases during stalk expansion (increase of the stalk radius, r(s)) and thus "drives" fusion. Calculations based on the known mechanical properties of lipid assemblies suggest that the free energy of "void" formation as well as membrane-bending free energy dominate the evolution of a stalk to an extended transmembrane contact. The free-energy profile of stalk expansion (free energy versus r(s)) clearly shows the presence of two metastable intermediates (intermediate 1 at r(s) approximately 0 - 1.0 nm and intermediate 2 at r(s) approximately 2.5 - 3.0 nm). Applying osmotic gradients of +/-5 atm, when assuming a fixed trans-bilayer lipid mass distribution, did not significantly change the free-energy profile. However, inclusion in the model of an additional degree of freedom, the ability of lipids to move into and out of the "void", made the free-energy profile strongly dependent on the osmotic gradient. Vesicle expansion increased the energy barrier between intermediates by approximately 4 kT and the absolute value of the barrier by approximately 7 kT, whereas compression decreased it by nearly the same extent. Since these calculations, which are based on the stalk hypothesis, correctly predict the effects of both membrane curvature and osmotic stress, they support the stalk hypothesis for the mechanism of membrane fusion and suggest that both forms of stress alter the final stages, rather than the initial step, of the fusion process, as previously suggested.

Roles of Curvature and Hydrophobic Interstice Energy in Fusion: Studies of Lipid Perturbant Effects†

We have examined the effects of small amounts (1-4 mol %) of lipids of different molecular shapes, long chain lipids, and hydrocarbon on the kinetics of PEG-mediated fusion of 1,2-dioleoyl-3-sn-phosphatidylcholine/1,2-dioleoyl-3-sn-phosphatidylethanolamine/sphingomyelin/cholesterol (DOPC/DOPE/SM/CH, 35:30:15:30) sonicated vesicles. The effects of these lipid perturbants were different for different steps in the fusion process and varied with the ratio of the cross-sectional areas of headgroup to acyl chain moieties. For lipids with a ratio <1 (negative intrinsic curvature), a decrease in this ratio led to a dramatic increase in the initial rate of vesicle contents mixing but left the initial rate of lipid mixing roughly unchanged. For lipids with ratios >1 (positive intrinsic curvature), the initial rates of both lipid and contents mixing decreased mildly with increasing ratio. In the context of the "stalk model" for fusion, lipid mixing reflects mainly formation of the initial fusion intermediate (stalk), while contents mixing reflects conversion of this intermediate either to a second intermediate or to a fusion pore. Results with positively curved lipids (ganglioside, GM1; lysophosphatidylcholine, LPCs) and negatively curved lipids (dioleoylglycerol, DOG, and 1,2-diphytanoyl-sn-glyvero-3-phosphatidylcholine, DPhPC) can be taken as supportive of the usual interpretation of the stalk model in terms of bending energy, but enhancement of fusion in the presence of long-chain phospholipids, hexadecane, as well as a mixture of GM1 plus hexadecane could not be explained by their curvature alone. We propose that the ability of a lipid perturbant to compensate for lipid packing mismatch, that is, to lower "void" energy, must be taken into account, along with intrinsic curvature, to explain the ability of lipid perturbants to promote pore formation.

VSV transmembrane domain (TMD) peptide promotes PEG-mediated fusion of liposomes in a conformationally sensitive fashion

Helical instability induced by gly residues in the transmembrane domain (TMD) of G protein, the fusion protein of vesicular stomatitis virus (VSV), was speculated to aid in the later steps of the fusion process, because G protein with ala's substituted for the two TMD gly's was inactive (Cleverley, D. Z., and Lenard, J. (1998) Proc. Natl. Acad. Sci. U. S. A. 95, 3425-30). Here we examine the conformations of synthetic peptides corresponding to fusion-active (GGpep) and inactive (AApep; G's replaced by A's) TMDs by CD spectroscopy, and then their effects on the kinetics of poly (ethyleneglycol) (PEG)-mediated fusion of small unilamellar vesicles. GGpep and AApep both assumed history-dependent, non-interconvertible ordered structures. Both peptides were largely helical under all conditions if derived from trifluoroethanol solutions, and aggregated in a beta-sheet form if derived from acetonitrile solutions. In solvent, detergents or lipid bilayers, GGpep showed a greater range of secondary structural features than did AApep. The two peptides had large but different effects on PEG-mediated fusion. Both enhanced the rate but not the extent of lipid mixing. AApep significantly inhibited the extent of fusion pore formation while GGpep had no effect. The initial rate of fusion was enhanced 6-fold by GGpep and less than 2-fold by AApep. Addition of 5 mol % hexadecane overrode all peptide-induced effects. We suggest that both GGpep and hexadecane promote pore formation by stabilizing the nonlamellar structures in fusion intermediates or initial small pores. AApep, which had fewer nonhelical features in its CD spectrum than GGpep, actually inhibited fusion pore formation.

Lipid intermediates in membrane fusion: formation, structure, and decay of hemifusion diaphragm

Lipid bilayer fusion is thought to involve formation of a local hemifusion connection, referred to as a fusion stalk. The subsequent fusion stages leading to the opening of a fusion pore remain unknown. The earliest fusion pore could represent a bilayer connection between the membranes and could be formed directly from the stalk. Alternatively, fusion pore can form in a single bilayer, referred to as hemifusion diaphragm (HD), generated by stalk expansion. To analyze the plausibility of stalk expansion, we studied the pathway of hemifusion theoretically, using a recently developed elastic model. We show that the stalk has a tendency to expand into an HD for lipids with sufficiently negative spontaneous splay, (~)J(s)< 0. For different experimentally relevant membrane configurations we find two characteristic values of the spontaneous splay. (~)J*(s) and (~)J**(s), determining HD dimension. The HD is predicted to have a finite equilibrium radius provided that the spontaneous splay is in the range (~)J**(s)< (~)J(s)<(~)J*(s), and to expand infinitely for (~)J(s)<(~)J**(s). In the case of common lipids, which do not fuse spontaneously, an HD forms only under action of an external force pulling the diaphragm rim apart. We calculate the dependence of the HD radius on this force. To address the mechanism of fusion pore formation, we analyze the distribution of the lateral tension emerging in the HD due to the establishment of lateral equilibrium between the deformed and relaxed portions of lipid monolayers. We show that this tension concentrates along the HD rim and reaches high values sufficient to rupture the bilayer and form the fusion pore. Our analysis supports the hypothesis that transition from a hemifusion to a fusion pore involves radial expansion of the stalk.

Influence of gp41 fusion peptide on the kinetics of poly(ethylene glycol)-mediated model membrane fusion

The fusion peptide of the HIV fusion protein gp41 is required for viral fusion and entry into a host cell, but it is unclear whether this 23-residue peptide can fuse model membranes. We address this question for model membrane vesicles in the presence and absence of aggregating concentrations of poly(ethylene glycol) (PEG). PEG had no effect on the physical properties of peptide bound to membranes or free in solution. We tested for fusion of both highly curved and uncurved PC/PE/SM/CH (35:30:15:20 mol %) vesicles and highly curved PC/PE/CH (1:1:1) vesicles treated with peptide in the presence and absence of PEG. Fusion was never observed in the absence of PEG, although high peptide concentrations led to aggregation and rupture, especially in unstable PC/PE/CH (1:1:1) vesicles. When 5 wt % PEG was present to aggregate vesicles, peptide enhanced the rate of lipid mixing between curved PC/PE/SM/CH vesicles in proportion to the peptide concentration, with this effect leveling off at peptide/lipid (P/L) ratios approximately 1:200. Peptide produced an even larger effect on the rate of contents mixing but inhibited contents mixing at P/L ratios >1:200. No fusion enhancement was seen with uncurved vesicles. The rate of fusion was also enhanced by the presence of hexadecane, and peptide-induced rate enhancement was not observed in the presence of hexadecane. We conclude that gp41 fusion peptide does not induce vesicle fusion at subrupturing concentrations but can enhance fusion between highly curved vesicles induced to fuse by PEG. The different effects of peptide on the rates of lipid mixing and fusion pore formation suggest that, while gp41 fusion peptide does affect hemifusion, it mainly affects pore formation.

Osmotic and curvature stress affect PEG-induced fusion of lipid vesicles but not mixing of their lipids

Poly (ethylene glycol) (PEG) in the external environment of membrane vesicles creates osmotic imbalance that leads to mechanical stress in membranes and may induce local membrane curvature. To determine the relative importance of membrane stress and curvature in promoting fusion, we monitored contents mixing (CM) and lipid mixing (LM) between different sized vesicles under a variety of osmotic conditions. CM between highly curved vesicles (SUV, 26 nm diameter) was up to 10 times greater than between less curved vesicles (LUV, 120 nm diameter) after 5 min incubation at a low PEG concentration (<10 wt%), whereas LM was only approximately 30% higher. Cryo-electron microscopy showed that PEG at 10 wt% did not create high curvature contacts between membranes in LUV aggregates. A negative osmotic gradient (-300 mOs/kg, hypotonic inside) increased CM two- to threefold for both types of vesicles, but did not affect LM. A positive gradient (+220 mOs/kg, hypertonic inside) nearly eliminated CM and had no effect on LM. Hexadecane added to vesicles had no effect on LM but enhanced CM and reduced the inhibitory effect on CM of a positive osmotic gradient, but had little influence on results obtained under a negative osmotic gradient. We conclude that the ability of closely juxtaposed bilayers to form an initial intermediate ("stalk") as soon as they come into close contact was not influenced by osmotic stress or membrane curvature, although pore formation was critically dependent on these stresses. The results also suggest that hexadecane affects the same part of the fusion process as osmotic stress. We interpret this result to suggest that both a negative osmotic gradient and hexadecane reduce the unfavorable free energy of hydrophobic interstices associated with the intermediates of the fusion process.

Stalk model of membrane fusion: solution of energy crisis

Membrane fusion proceeds via formation of intermediate nonbilayer structures. The stalk model of fusion intermediate is commonly recognized to account for the major phenomenology of the fusion process. However, in its current form, the stalk model poses a challenge. On one hand, it is able to describe qualitatively the modulation of the fusion reaction by the lipid composition of the membranes. On the other, it predicts very large values of the stalk energy, so that the related energy barrier for fusion cannot be overcome by membranes within a biologically reasonable span of time. We suggest a new structure for the fusion stalk, which resolves the energy crisis of the model. Our approach is based on a combined deformation of the stalk membrane including bending of the membrane surface and tilt of the hydrocarbon chains of lipid molecules. We demonstrate that the energy of the fusion stalk is a few times smaller than those predicted previously and the stalks are feasible in real systems. We account quantitatively for the experimental results on dependence of the fusion reaction on the lipid composition of different membrane monolayers. We analyze the dependence of the stalk energy on the distance between the fusing membranes and provide the experimentally testable predictions for the structural features of the stalk intermediates.

Lipids favoring inverted phase enhance the ability of aerolysin to permeabilize liposome bilayers

Channel formation by the bacterial toxin aerolysin follows oligomerization of the protein to produce heptamers that are capable of inserting into lipid bilayers. How insertion occurs is not understood, not only for aerolysin but also for other proteins that can penetrate membranes. We have studied aerolysin channel formation by measuring dye leakage from large unilamellar egg phosphatidylcholine vesicles containing varying amounts of other lipids. The rate of leakage was enhanced in a dose-dependent manner by the presence of phosphatidylethanolamine, diacylglycerol, cholesterol, or hexadecane, all of which are known to favor a lamellar-to-inverted hexagonal (L-H) phase transition. Phosphatidylethanolamine molecular species with low L-H transition temperatures had the largest effects on aerolysin activity. In contrast, the presence in the egg phosphatidylcholine liposomes of lipids that are known to stabilize the lamellar phase, such as sphingomyelin and saturated phosphatidylcholines, reduced the rate of channel formation, as did the presence of lysophosphatidylcholine, which favors positive membrane curvature. When two different lipids that favor hexagonal phase were present with egg PC in the liposomes, their stimulatory effects were additive. Phosphatidylethanolamine and lysophosphatidylcholine canceled each other's effect on channel formation.

Short-chain alcohols promote an early stage of membrane hemifusion

Hemifusion, the linkage of contacting lipid monolayers of two membranes before the opening of a fusion pore, is hypothesized to proceed through the formation of a stalk intermediate, a local and strongly bent connection between membranes. When the monolayers' propensity to bend does not support the stalk (e.g., as it is when lysophosphatidylcholine is added), hemifusion is inhibited. In contrast, short-chain alcohols, reported to affect monolayer bending in a manner similar to that of lysophosphatidylcholine, were here found to promote hemifusion between fluorescently labeled liposomes and planar lipid bilayers. Single hemifusion events were detected by fluorescence microscopy. Methanol or ethanol (1.2-1.6 w/w %) added to the same compartment of the planar bilayer chamber as liposomes caused a 5-50 times increase in the number of hemifusion events. Alcohol-induced hemifusion was inhibited by lysophosphatidylcholine. Promotion of membrane hemifusion by short-chain alcohol was also observed for cell-cell fusion mediated by influenza virus hemagglutinin (HA). Alcohol promoted a fusion stage subsequent to the low pH-dependent activation of HA. We propose that binding of short-chain alcohol to the surface of membranes promotes hemifusion by facilitating the transient breakage of the continuity of each of the contacting monolayers, which is required for their subsequent merger in the stalk intermediate.

Structure and functional properties of diacylglycerols in membranes

1. 1,2-Diacyl-sn-glycerols (DAG) are minor components of cell membranes (about 1 mole% of the lipids) and yet they are potent regulators of both the physical properties of the lipid bilayer and the catalytic behaviour of several membrane-related enzymes. 2. In the pure state DAG's present a considerable polymorphism, with several crystalline phases in addition to the neat fluid phase. The most stable crystalline phase is the so-called beta' phase, a monoclinic crystalline form with orthorhombic perpendicular subcell chain packing, in which both acyl chains lie parallel to each other in a hairpinlike configuration about the sn-1 and sn-2 glycerol carbon atoms. The molecules are organized in a bilayer, with the glycerol backbone roughly parallel to the plane of the bilayer, and the acyl chains tilted at approximately 60 degrees with respect to that plane. Acyl chain unsaturation, and particularly a single cis unsaturation, impairs chain packing in mixed-chain DAG's, and this results in an increased number of metastable crystalline phases. 3. DAG's mix with phospholipids in fluid bilayers when their melting temperature is below or close enough to the melting temperature of the bilayer system. When incorporated in phospholipid bilayers, the conformation of DAG is such that the glycerol backbone is nearly perpendicular to the bilayer, with the sn-1 chain extending from the glycerol Cl carbon into the hydrophobic matrix of the bilayer and the sn-2 chain first extending parallel to the bilayer surface, then making a 90 degrees bend at the position of the sn-1 carbonyl to become parallel to the sn-1 chain. DAG's are located in phospholipid bilayers about two CH2 units deeper than the adjacent phospholipids. DAG's mix nonideally with phospholipids, giving rise to in-plane separations of DAG-rich and -poor domains, even in the fluid state. DAG molecules also increase the separation between phospholipid headgroups, and decrease the hydration of the bilayer surface. Also, because the transversal section of the DAG headgroup is small when compared to that of the acyl chains, DAG favours the (negative) curvature of the lipid monolayers, and DAG-phospholipid mixtures tend to convert into inverted nonlamellar hexagonal or cubic phases. 4. A number of membrane enzyme activities are modulated (activated) by DAG, most notably protein kinase C, phospholipases and other enzymes of lipid metabolism. Protein kinase C activation (and perhaps that of other enzymes as well) occurs as the combined result of a number of DAG-induced modifications of lipid bilayers that include: changes in lipid headgroup conformation, interspacing and hydration, changes in the bilayer propensity to form inverted nonlamellar phases, and lateral phase separations of DAG-rich and -poor domains. Among the DAG-activated enzymes, phospholipases C show the peculiarity of yielding the activator DAG as their reaction product, and this allows the self-induced transition from a low- to a high-activity status. 5. DAG's induce or enhance membrane fusion in a number of ways, mainly through partial dehydration of the bilayer surface, increase in lipid monolayer curvature and perhaps lateral phase separation. DAG-increased fusion rates have been demonstrated in several instances of cation-induced fusion of model membranes, as well as in Ca(2+)-induced fusion of chromaffin granules with plasma membrane vesicles. Also phospholipase C has been shown to induce vesicle aggregation and fusion through the catalytic generation of DAG in the bilayers. A rather general property of DAG is that it promotes vesicular or interparticle aggregation. 6. In the living cell, DAG is often generated through phospholipid degradation in response to an extracellular agonist binding a specific receptor in the cell surface. DAG is said to act as an intracellular second messenger. (ABSTRACT TRUNCATED)

Effect of single chain lipids on phospholipase C-promoted vesicle fusion. A test for the stalk hypothesis of membrane fusion

The effect of low proportions (up to 5 mol %) of single-chain lipids on phospholipase C-promoted fusion of large unilamellar vesicles has been investigated with the aim of testing the so-called stalk model of membrane fusion. This model is known in two main versions, the one originally published by Kozlov and Markin [Kozlov, M. M. and Markin, V. S. (1983) Biofizika 28, 255-261] and what is known as the "modified stalk model" [Siegel, D. P. (1993) Biophys. J. 65, 2124-2140], that differ in a number of predictions. In the view of the latter author, hydrocarbons or other nonpolar lipids should help fusion by decreasing the interstitial energy of the stalk connecting the two apposed bilayers. We show that small amounts of hexadecane or squalene increase significantly the fusion rates in our system. Changes in monolayer curvature are the object of different predictions by the original and modified stalk theories. According to the original form, fusion would be promoted by lipids inducing a negative curvature in the closest (cis) monolayers of the fusing membranes and inhibited by the same lipids in the trans monolayers; the opposite would happen with lipids inducing a positive curvature. The modified stalk model predicts that fusion is helped by increasing the negative curvature of both monolayers. In our system, symmetrically distributed arachidonic acid, which increases the negative curvature, enhances lipid and content mixing, and the opposite is found with symmetrically distributed lysophosphatidylcholine or palmitoylcarnitine, which facilitate a positive monolayer curvature. In addition, fluorescence polarization and 31P NMR studies of the lamellar-to-isotropic (Q224 cubic) thermotropic transition of a lipid mixture corresponding to our liposomal composition reveal that all lipids that facilitate fusion decrease the transition temperature, while fusion inhibitors increase the transition temperature. Moreover, fusion (content mixing) rates show a maximum at the lamellar-to-isotropic transition temperature. These observations support the involvement of inverted lipid structures, as occurring in the inverted cubic phases, in membrane fusion. All these data are in full agreement with the stalk model of membrane fusion, particularly in its modified version.

Lipid mixing in dimyristoyl phosphatidylcholine-dimyristoyl glycerol dispersions: spin label ESR studies

The lipid transfer and mixing properties in hydrated dispersions of dimyristoyl phosphatidylcholine (DMPC)/dimyristoyl glycerol (DMG) binary mixtures have been investigated by using electron spin resonance spectroscopy of the spin-labelled lipid components. The assay system is based on the reduction in spectral broadening from spin-spin interactions that takes place by dilution of the spin-labelled lipid on transfer to a 9-fold excess of dispersions that contain no spin label. Lipid dispersions with DMPC:DMG compositions of 70:30, 40:60 and 20:80 mol/mol, for which the fluid phases have lamellar, inverted hexagonal and isotropic structures, respectively, have been studied. Essentially no transfer of spin-labelled lipid takes place for any of the lipid mixtures in the lamellar gel phase, or in dispersions of DMPC alone at all temperatures studied. The greatest degrees of transfer are found in the fluid phase of the DMPC/DMG mixtures. In general, the extent of lipid transfer is greater for the diacylglycerol component than for the phosphatidylcholine component. The extent of transfer of phosphatidylcholine is very low in the fluid lamellar phase of the 70:30 mol/mol DMPC/DMG, as compared with that of diacylglycerol. Only in the case of the 40:60 mol/mol DMPC/DMG mixture, in the inverted hexagonal phase, are the extents of transfer comparable for both phosphatidylcholine and diacylglycerol components, indicating a bulk transfer of lipid within the dispersions. The largest extent of transfer is found for diacylglycerol in the 20:80 mol/mol DMPC/DMG mixture in the isotropic phase.

Effect of physical constraints on the mechanisms of membrane fusion: bolaform lipid vesicles as model systems

Bolaform lipid vesicles were used to study the effect of physical constraints on membrane fusion. In these vesicles the membrane is organized in a single monolayer, because of the presence of covalent bonds in its middle plane. Therefore, the formation of fusion intermediates is subject to higher energy barriers and greater geometrical constraints than is usual in bilayer membranes. Bolaform lipids were extracted from the thermophilic archaeon Sulfolobus solfataricus. These lipids can be divided into two classes, the monosubstituted molecules, in which one of the polar heads is glycerol, and the bisubstituted molecules, endowed with two complex polar heads. The fusion process in vesicles composed of different mixtures of monosubstituted/bisubstituted molecules was studied by means of fluorescence techniques. Ca2+ or poly(ethylene glycol) was employed as a fusogenic agent. We found that fusion of such constrained membranes is still possible, provided that molecules able to mediate a structural rearrangement of the membrane are present. This condition is fulfilled by monosubstituted molecules, which are able to partition the glycerol headgroup in the apolar moiety. In addition, the presence of traces (approximately 5%) of the monopolar compound diphytanylglycerol is an important factor for fusion to occur. On the contrary, vesicles formed by bisubstituted molecules are unable to fuse, irrespective of the fusogen employed.

Diacylglycerol and the promotion of lamellar-hexagonal and lamellar-isotropic phase transitions in lipids: implications for membrane fusion

Changes in steady-state fluorescence anisotropy of 1 -(4-trimethylaminophenyl)-6-phenyl-1,3,5-hexatriene TMA-DPH) are applied to the detection of lamellar-hexagonal transitions in egg phosphatidylethanolamine. Even low (2 mole%) proportions of diacylglycerol decrease the hexagonal transition temperature considerably, as confirmed by differential scanning calorimetry. Diacylglycerol is also found to promote a lamellar to "isotropic" (Q(224) cubic) transition in mixtures of phosphatidylcholine: phosphatidylethanolamine:cholesterol. This nonreversible transition is also observed by (31)P nuclear magnetic resonance and detected as a large increase in TMA-DPH steady-state anisotropy. The same technique reveals as well that lysophosphatidylcholine counteracts the effect of diacylglycerol and stabilizes the lamellar phase in both transitions. Diacylglycerol and lysophosphatidylcholine are known to respectively promote and inhibit membrane fusion in a variety of systems. These data are interpreted in support of the hypothesis of a highly bent structural fusion intermediate ("stalk"). They also show the interest of lipid-phase studies in predicting and rationalizing membrane fusion mechanisms.

Influenza-virus-liposome lipid mixing is leaky and largely insensitive to the material properties of the target membrane

Monolayer intrinsic curvature, void stabilization, and membrane rupture tension have been suggested as important factors determining the rate of membrane fusion. Here, we have studied the kinetics of fusion between influenza virus and target liposomes as a function of various target membrane material properties. In order to examine the fusion process directly, a simple prebinding step is used and proven to be adequate to achieve fusion-rate-limiting kinetics. To test the hypothesis about membrane curvature and void stabilization, we studied the lipid mixing kinetics with dioleoylphosphatidylcholine (DOPC)/ganglioside GD1a (GD1a) liposomes containing lysooleoylphosphatidylcholine (LPC, positive curvature), dioleoyglycerol (DOG, negative curvature), arachidonic acid (AA, negative curvature), and hexadecane (HD, void stabilization). DOG, AA, and HD (at 4 mol%) showed no significant effect on the fusion kinetics, while LPC reversibly inhibited influenza HA mediated fusion only at very high concentrations. Using target liposomes with different membrane rupture tension values, no obvious correlation between membrane rupture tension and the rate of lipid mixing was observed. Moreover, a reported potential antiviral compound, tert-butylhydroquinone (t-b-HQ) (Bodian et al., 1993), showed no significant effect on the kinetics of influenza fusion. Finally leakage of liposome contents was detected during lipid mixing. For encapsulated molecules smaller than 450 MW, the kinetics of leakage is very similar to the kinetics of lipid mixing. In fact, leakage was also detected for encapsulated molecules up to 10 000 MW, suggesting that HA mediated lipid mixing is a very leaky process. Since "nonleaky fusion" has been the foundation of influenza fusion models, our work suggests the need for a major revision in the modeling of this process.

The hemifusion intermediate and its conversion to complete fusion: regulation by membrane composition

To fuse, membranes must bend. The energy of each lipid monolayer with respect to bending is minimized at the spontaneous curvature of the monolayer. Two lipids known to promote opposite spontaneous curvatures, lysophosphatidylcholine and arachidonic acid, were added to different sides of planar phospholipid membranes. Lysophosphatidylcholine added to the contacting monolayers of fusing membranes inhibited the hemifusion we observed between lipid vesicles and planar membranes. In contrast, fusion pore formation depended upon the distal monolayer of the planar membrane; lysophosphatidylcholine promoted and arachidonic acid inhibited. Thus, the intermediates of hemifusion and fusion pores in phospholipid membranes involve different membrane monolayers and may have opposite net curvatures, Biological fusion may proceed through similar intermediates.

Bending membranes to the task: structural intermediates in bilayer fusion

Merger of lipid bilayers plays a central role in diverse biological fusion reactions. Recent studies suggest that different membrane fusion systems, including fusion of purely lipid bilayers, involve formation of similar stalk-type intermediates--highly bent (net negative curvature) and transient lipidic connections between fusing membranes.

The dissimilar effect of diacylglycerols on Ca(2+)-induced phosphatidylserine vesicle fusion

We have studied the effect of physiological concentrations of different diacylglycerols on Ca(2+)-induced fusion between phosphatidylserine vesicles. We monitored vesicle fusion as mixing of membrane lipids under conditions where the limiting factor was the aggregation and also in conditions where this aggregation was not the limiting factor. We found that diacylglycerols have a different modulating effect on the Ca(2+)-induced fusion: i) depending on their interfacial conformation, so that 1,2-isomers of diacylglycerols containing unsaturated or short saturated acyl chains stimulated fusion and their 1,3-isomers did not, and ii) depending on their specific type of bilayer interior perturbation, so that diacylglycerols containing unsaturated or short chain saturated acyl chains stimulated fusion but those containing long-chain saturated acyl chains did not. These requirements resembled those required for the diacylglycerol activation of protein kinase C, suggesting that diacylglycerol acts in both the specific activation of this enzyme and the induction of membrane fusion through the same perturbation of lipid structure. We found that polylysine affected the stimulatory role of 1,2-dioleoylglycerol differently, depending on whether aggregation was the limiting factor of fusion. When we studied the effect of very low concentrations of diacylglycerols on the bulk structural properties of phosphatidylserine, we found that they neither significantly perturbed the thermotropic transitions of phosphatidylserine nor affected the interaction of Ca2+ with the phosphate group of phosphatidylserine. The underlying mechanism of fusion between phosphatidylserine vesicles is discussed.