Structural effects of neutral lipids on divalent cation-induced interactions of phosphatidylserine-containing bilayers

Bridges of Calcium Bicarbonate Tightly Couple Dipolar Lipid Membranes

The interaction between lipid membranes and ions is associated with a range of key physiological processes. Most earlier studies have focused on the interaction of lipids with cations, while the specific effects of the anions have been largely overlooked. Owing to dissolved atmospheric carbon dioxide, bicarbonate is an important ubiquitous anion in aqueous media. In this paper, we report on the effect of bicarbonate anions on the interactions between dipolar lipid membranes in the presence of previously adsorbed calcium cations. Using a combination of solution X-ray scattering, osmotic stress, and molecular dynamics simulations, we followed the interactions between 1,2-didodecanoyl-sn-glycero-3-phosphocholine (DLPC) lipid membranes that were dialyzed against CaCl2 solutions in the presence and absence of bicarbonate anions. Calcium cations adsorbed onto DLPC membranes, charge them, and lead to their swelling. In the presence of bicarbonate anions, however, the calcium cations can tightly couple one dipolar DLPC membrane to the other and form a highly condensed and dehydrated lamellar phase with a repeat distance of 3.45 ± 0.02 nm. Similar tight condensation and dehydration has only been observed between charged membranes in the presence of multivalent counterions. Bridging between bilayers by calcium bicarbonate complexes induced this arrangement. Furthermore, in this condensed phase, lipid molecules and adsorbed ions were arranged in a two-dimensional oblique lattice.

Multivalent ions and biomolecules: Attempting a comprehensive perspective

Ions are ubiquitous in nature. They play a key role for many biological processes on the molecular scale, from molecular interactions, to mechanical properties, to folding, to self-organisation and assembly, to reaction equilibria, to signalling, to energy and material transport, to recognition etc. Going beyond monovalent ions to multivalent ions, the effects of the ions are frequently not only stronger (due to the obviously higher charge), but qualitatively different. A typical example is the process of binding of multivalent ions, such as Ca2+ , to a macromolecule and the consequences of this ion binding such as compaction, collapse, potential charge inversion and precipitation of the macromolecule. Here we review these effects and phenomena induced by multivalent ions for biological (macro)molecules, from the "atomistic/molecular" local picture of (potentially specific) interactions to the more global picture of phase behaviour including, e. g., crystallisation, phase separation, oligomerisation etc. Rather than attempting an encyclopedic list of systems, we rather aim for an embracing discussion using typical case studies. We try to cover predominantly three main classes: proteins, nucleic acids, and amphiphilic molecules including interface effects. We do not cover in detail, but make some comparisons to, ion channels, colloidal systems, and synthetic polymers. While there are obvious differences in the behaviour of, and the relevance of multivalent ions for, the three main classes of systems, we also point out analogies. Our attempt of a comprehensive discussion is guided by the idea that there are not only important differences and specific phenomena with regard to the effects of multivalent ions on the main systems, but also important similarities. We hope to bridge physico-chemical mechanisms, concepts of soft matter, and biological observations and connect the different communities further.

Structure and Interactions between Charged Lipid Membranes in the Presence of Multivalent Ions

When aqueous salt solutions contain multivalent ions (like Ca²⁺ or Mg²⁺), strong correlation effects may lead to ion-bridging, net attraction, and tight-coupling between like-charged interfaces. To examine the effects of surface charge density, temperature, salt type, and salt concentration on the structures of tightly coupled charged interfaces, we have used mixed lipid membranes, containing either saturated or unsaturated tails in the presence of multivalent ions. We discovered that tightly coupled membrane lamellar phases, dominated by attractive interactions, coexisted with weakly coupled lamellar phases, dominated by repulsive interactions. To control the membrane charge density, we mixed lipids with negatively charged headgroups, DLPS and DOPS, with their zwitterionic analogue having the same tails, DLPC and DOPC, respectively. Using solution X-ray scattering we measured the lamellar repeat distance, D, at different ion concentrations, temperatures, and membrane charge densities. The multivalent ions tightly coupled the mixed lipid bilayers whose charged lipid molar fraction was between 0.1 and 1. The repeat distance of the tightly coupled phase was about 4 nm for the DLPS/DLPC mixtures and about 5 nm for the DOPS/DOPC mixtures. In this phase, the repeat distance slightly increased with increasing temperature and decreased with increasing charge density. When the molar fraction of charged lipid was 0.1 or 0.25, a less tightly coupled phase coexisted with the tightly coupled phase. The weakly coupled lamellar phase had significantly larger D values, although they were consistently shorter than the D values in monovalent salt solutions with similar screening lengths.

Membrane Charging and Swelling upon Calcium Adsorption as Revealed by Phospholipid Nanodiscs

Direct binding of calcium ions (Ca²⁺) to phospholipid membranes is an unclarified yet critical signaling pathway in diverse Ca²⁺-regulated cellular phenomena. Here, high-pressure-liquid-chromatography, small-angle X-ray scattering (SAXS), UV-vis absorption, and differential refractive index detections are integrated to probe Ca²⁺-binding to the zwitterionic lipid membranes in nanodiscs. The responses of the membranes upon Ca²⁺-binding, in composition and conformation, are quantified through integrated data analysis. The results indicate that Ca²⁺ binds specifically into the phospholipid headgroup zone, resulting in membrane charging and membrane swelling, with a saturated Ca²⁺-lipid binding ratio of 1:8. A Ca²⁺-binding isotherm to the nanodisc is further established and yields an unexpectedly high binding constant K = 4260 M^-1 and a leaflet potential of ca. 100 mV based on a modified Gouy-Chapman model. The calcium-lipid binding ratio, however, drops to 40% when the nanodisc undergoes a gel-to-fluid phase transition, leading to an effective charge capacity of a few μF/cm².

Local electrostatic interactions determine the diameter of fusion pores

In regulated exocytosis vesicular and plasma membranes merge to form a fusion pore in response to stimulation. The nonselective cation HCN channels are involved in the regulation of unitary exocytotic events by at least 2 mechanisms. They can affect SNARE-dependent exocytotic activity indirectly, via the modulation of free intracellular calcium; and/or directly, by altering local cation concentration, which affects fusion pore geometry likely via electrostatic interactions. By monitoring membrane capacitance, we investigated how extracellular cation concentration affects fusion pore diameter in pituitary cells and astrocytes. At low extracellular divalent cation levels predominantly transient fusion events with widely open fusion pores were detected. However, fusion events with predominately narrow fusion pores were present at elevated levels of extracellular trivalent cations. These results show that electrostatic interactions likely help determine the stability of discrete fusion pore states by affecting fusion pore membrane composition.

Behavior of human cells in contact with a poly(d,l-lactic acid) porous matrix after calcification using phosphatidylserine

As part of a strategy aimed at improving bioresorbable scaffolds for the engineering of bony tissues, a route to deposit calcium phosphate onto surfaces of poly(dl-lactic acid)–based porous matrices was investigated. Porosity was generated using the NaCl-leaching technique. Calcification was achieved after deposition of phosphatidylserine, a nucleating agent of natural origin, onto pore surfaces, followed by incubation of the phospholipid-coated matrix in a pH 6.5 aqueous medium consisted of 3.5 mmol CaCl2 and 2.6 mmol KH2PO4 for 3 days. Calcified matrices were noncytotoxic according to the ISO10993-5 standard test and exhibited low inflammatory potential. To compare responses of human cells of different types, human osteogenic bone marrow cells from the femoral head, human chondrocytes from femoral cartilage collected after hip surgery, and human vascular endothelial cells isolated from an umbilical cord were allowed to grow in the presence of the calcified matrices in vitro. Articular chondrocytes adhered to and grew on the calcified matrices up to colony formation. In contrast, the other two types of cells attached and proliferated for approximately 3 days and then detached. These different cell behaviors are discussed with respect to the nature of the cells and to the release of calcium ions from the coating.

Anionic lipids in Ca(2+)-triggered fusion

Anionic lipids are native membrane components that have a profound impact on many cellular processes, including regulated exocytosis. Nonetheless, the full nature of their contribution to the fast, Ca(2+)-triggered fusion pathway remains poorly defined. Here we utilize the tightly coupled quantitative molecular and functional analyses enabled by the cortical vesicle model system to elucidate the roles of specific anionic lipids in the docking, priming and fusion steps of regulated release. Studies with cholesterol sulfate established that effectively localized anionic lipids could contribute to Ca(2+)-sensing and even bind Ca(2+) directly as effectors of necessary membrane rearrangements. The data thus support a role for phosphatidylserine in Ca(2+) sensing. In contrast, phosphatidylinositol would appear to serve regulatory functions in the physiological fusion machine, contributing to priming and thus the modulation and tuning of the fusion process. We note the complexities associated with establishing the specific roles of (anionic) lipids in the native fusion mechanism, including their localization and interactions with other critical components that also remain to be more clearly and quantitatively defined.

The structure of ions and zwitterionic lipids regulates the charge of dipolar membranes

In pure water, zwitterionic lipids form lamellar phases with an equilibrium water gap on the order of 2 to 3 nm as a result of the dominating van der Waals attraction between dipolar bilayers. Monovalent ions can swell those neutral lamellae by a small amount. Divalent ions can adsorb onto dipolar membranes and charge them. Using solution X-ray scattering, we studied how the structure of ions and zwitterionic lipids regulates the charge of dipolar membranes. We found that unlike monovalent ions that weakly interact with all of the examined dipolar membranes, divalent and trivalent ions adsorb onto membranes containing lipids with saturated tails, with an association constant on the order of ∼10 M(-1). One double bond in the lipid tail is sufficient to prevent divalent ion adsorption. We suggest that this behavior is due to the relatively loose packing of lipids with unsaturated tails that increases the area per lipid headgroup, enabling their free rotation. Divalent ion adsorption links two lipids and limits their free rotation. The ion-dipole interaction gained by the adsorption of the ions onto unsaturated membranes is insufficient to compensate for the loss of headgroup free-rotational entropy. The ion-dipole interaction is stronger for cations with a higher valence. Nevertheless, polyamines behave as monovalent ions near dipolar interfaces in the sense that they interact weakly with the membrane surface, whereas in the bulk their behavior is similar to that of multivalent cations. Advanced data analysis and comparison with theory provide insight into the structure and interactions between ion-induced regulated charged interfaces. This study models biologically relevant interactions between cell membranes and various ions and the manner in which the lipid structure governs those interactions. The ability to monitor these interactions creates a tool for probing systems that are more complex and forms the basis for controlling the interactions between dipolar membranes and charged proteins or biopolymers for encapsulation and delivery applications.

Pb2+ as modulator of protein-membrane interactions

Lead is a potent environmental toxin that mimics the effects of divalent metal ions, such as zinc and calcium, in the context of specific molecular targets and signaling processes. The molecular mechanism of lead toxicity remains poorly understood. The objective of this work was to characterize the effect of Pb(2+) on the structure and membrane-binding properties of C2α. C2α is a peripheral membrane-binding domain of Protein Kinase Cα (PKCα), which is a well-documented molecular target of lead. Using NMR and isothermal titration calorimetry (ITC) techniques, we established that C2α binds Pb(2+) with higher affinity than its natural cofactor, Ca(2+). To gain insight into the coordination geometry of protein-bound Pb(2+), we determined the crystal structures of apo and Pb(2+)-bound C2α at 1.9 and 1.5 Å resolution, respectively. A comparison of these structures revealed that the metal-binding site is not preorganized and that rotation of the oxygen-donating side chains is required for the metal coordination to occur. Remarkably, we found that holodirected and hemidirected coordination geometries for the two Pb(2+) ions coexist within a single protein molecule. Using protein-to-membrane Förster resonance energy transfer (FRET) spectroscopy, we demonstrated that Pb(2+) displaces Ca(2+) from C2α in the presence of lipid membranes through the high-affinity interaction with the membrane-unbound C2α. In addition, Pb(2+) associates with phosphatidylserine-containing membranes and thereby competes with C2α for the membrane-binding sites. This process can contribute to the inhibitory effect of Pb(2+) on the PKCα activity.

Ca(2+) bridging of apposed phospholipid bilayers

In an effort to provide insight into the mechanism of Ca(2+)-induced fusion of lipid vesicles, molecular dynamics simulations in the isobaric-isothermal ensemble are used to investigate interactions of Ca(2+) with apposed lipid bilayers in close proximity. Simulations reveal the formation of a Ca(2+)-phospholipid "anhydrous complex" between apposed bilayers, whereas similar calculations performed with Na(+) display only complexation between neighboring lipids within the same bilayer. The binding of Ca(2+) to apposed phospholipids brings large regions of the bilayers into close contact (<4 Å), displacing water from phospholipid head groups in the process and creating regions of local dehydration. Dehydration of the apposed bilayers leads to ordering of the phospholipid tails, which is partially disrupted by the presence of Ca(2+)-phospholipid bridges.

Asymmetric distribution of phosphatidyl serine in supported phospholipid bilayers on titanium dioxide

Supported phospholipid bilayers (SPBs) are useful for studying cell adhesion, cell-cell interactions, protein-lipid interactions, protein crystallization, and applications in biosensor and biomaterial areas. We have recently reported that SPBs could be formed on titanium dioxide, an important biomaterial, from vesicles containing anionic phospholipid phosphatidyl serine (PS) in the presence of calcium. Here, we show that the mobility of the fluorescently labeled PS present in these bilayers is severely restricted, whereas that of the zwitterionic phosphatidyl choline is not affected. Removal of calcium alleviated the restriction on the mobility of PS. Both components were found to be mobile in SPBs of identical compositions prepared in the presence of calcium on silica. To explain these results, we propose that, on TiO2, PS is trapped in the proximal leaflet of the bilayers. This proposal is supported by the results of protein adsorption experiments carried out on bilayers containing various amounts of PS prepared on silica and titania.

Formation of cubic phases from large unilamellar vesicles of dioleoylphosphatidylglycerol/monoolein membranes induced by low concentrations of Ca2+

We developed a new method for the transformation of large unilamellar vesicles (LUVs) into the cubic phase. We found that the addition of low concentrations of Ca(2+) to suspensions of multilamellar vesicles (MLVs) of membranes of monoolein (MO) and dioleoylphosphatidylglycerol (DOPG) mixtures (DOPG/MO) changed their L(alpha) phase to the cubic phases. For instance, the addition of 15-25 mM Ca(2+) to 30%-DOPG/70%-MO-MLVs induced the Q(229) phase, whereas the addition of > or =28 mM Ca(2+) induced the Q(224) phase. LUVs of DOPG/MO membranes containing > or =25 mol % DOPG were prepared easily. Low concentrations of Ca(2+) transformed these LUVs in excess buffer into the Q(224) or the Q(229) phase, depending on the Ca(2+) concentration. For example, 15 and 50 mM Ca(2+) induced the Q(224) and Q(229) phase in the 30%-DOPG/70%-MO-LUVs at 25 degrees C, respectively. This finding is the first demonstration of transformation of LUVs of lipid membranes into the cubic phase under excess water condition.

Cholesterol facilitates the native mechanism of Ca2+-triggered membrane fusion

The process of regulated exocytosis is defined by the Ca2+-triggered fusion of two apposed membranes, enabling the release of vesicular contents. This fusion step involves a number of energetically complex steps and requires both protein and lipid membrane components. The role of cholesterol has been investigated using isolated release-ready native cortical secretory vesicles to analyze the Ca2+-triggered fusion step of exocytosis. Cholesterol is a major component of vesicle membranes and we show here that selective removal from membranes, selective sequestering within membranes, or enzymatic modification causes a significant inhibition of the extent, Ca2+ sensitivity and kinetics of fusion. Depending upon the amount incorporated, addition of exogenous cholesterol to cholesterol-depleted membranes consistently recovers the extent, but not the Ca2+ sensitivity or kinetics of fusion. Membrane components of comparable negative curvature selectively recover the ability to fuse, but are unable to recover the kinetics and Ca2+ sensitivity of vesicle fusion. This indicates at least two specific positive roles for cholesterol in the process of membrane fusion: as a local membrane organizer contributing to the efficiency of fusion, and, by virtue of its intrinsic negative curvature, as a specific molecule working in concert with protein factors to facilitate the minimal molecular machinery for fast Ca2+-triggered fusion.

Regulated secretion: SNARE density, vesicle fusion and calcium dependence

SNAREs such as VAMP, SNAP-25 and syntaxin are essential for intracellular trafficking, but what are their exact molecular roles and how are their interactions with other proteins manifest? Capitalizing on the differential sensitivity of SNAREs to exogenous proteases, we quantified the selective removal of identified SNAREs from native secretory vesicles without loss of fusion competence. Using previously established fusion assays and a high sensitivity immunoblotting protocol, we analyzed the relationship between these SNARE proteins and Ca2+-triggered membrane fusion. Neither the extent of fusion nor the number of intermembrane fusion complexes per vesicle were correlated with the measured density of identified egg cortical vesicle (CV) SNAREs. Without syntaxin, CVs remained fusion competent. Surprisingly, for one (but not another) protease the Ca2+ dependence of fusion was correlated with CV SNARE density, suggesting a native protein complex that associates with SNAREs, the architecture of which ensures high Ca2+ sensitivity. As SNAREs may function during CV docking in vivo, and as further proteolysis after SNARE removal eventually ablates fusion, we hypothesize that the triggered steps of regulated fusion (Ca2+ sensitivity and the catalysis and execution of fusion) require additional proteins that function downstream of SNAREs.

The lipid-water interface: revelations by osmotic stress

Lipids at the bilayer-water interface are highly disordered and mobile, and large areas of the bilayer undergo thermal undulations. Water penetrates significantly down to the hydrocarbon chain level. This water, and water out to about 10 A from the surface, is perturbed by the lipid surface in a way that produces a strong hydration repulsion and precludes molecular contact between bilayers. Its removal costs work, but most of this water is neither a permeable barrier nor unavailable to solvate other solutes. All hydrophilic surfaces show this "hydration force." Most lipids have an excess higher free energy when packed within a bilayer membrane since in isolation they pack into high curvature assemblies with polar groups on the concave side. Osmotic stress measurements of those curved assemblies yield a measure of monolayer elastic parameters and the excess higher free energy, which likely controls embedded proteins. Osmotic stress experiments can determine whether water is energetically significant, or not, in almost any system. The osmotic effect of solutes, independent of specific binding, is to compete with lipids and proteins for water. Solute affinity for water can modify lipid packing and protein conformation, coupling lipid and protein structure and function to osmolality at the molecular level.

Interactions of Ca(2+) with sphingomyelin and dihydrosphingomyelin

The changes induced by Ca(2+) on human lens sphingolipids, sphingomyelin (SM), and dihydrosphingomyelin were investigated by infrared spectroscopy. Ca(2+)-concentration-dependent studies of the head group region revealed that, for both sphingolipids, Ca(2+) partially dehydrates some of the phosphate groups and binds to others. Ca(2+) affects the interface of each sphingolipid differently. In SM, Ca(2+) shifts the amide I' band to frequencies lower than those in dehydrated samples of SM alone. This could be attributed to the direct binding of Ca(2+) to carbonyl groups and/or strong tightening of interlipid H-bonds to levels beyond those in dehydrated samples of SM only. In contrast, Ca(2+) induces relatively minor dehydration around the amide groups of dihydrosphingomyelin and a slight enhancement of direct lipid-lipid interactions. Temperature-dependent studies reveal that 0.2 M Ca(2+) increases the transition temperature T(m) from 31.6 +/- 1.0 degrees C to 35.7 +/- 1.1 degrees C for SM and from 45.5 +/- 1.1 degrees C to 48.2 +/- 1.0 degrees C for dihydrosphingomyelin. Binding of Ca(2+) to some phosphate groups remains above T(m). The strength of the interaction is, however, weaker. This allows for the partial rehydration of these moieties. Similarly, above T(m), Ca(2+)-lipid and/or direct inter-lipid interactions are weakened and lead to the rehydration of amide groups.

Adsorption of polyvalent cations to bilayer membranes from negatively charged lipid: estimating the lipid accessibility in the case of complete binding

The effect of trivalent (Gd(3+) and Yb(3+)) and divalent (Be(2+) and Ca(2+)) cations on suspensions of multilamellar liposomes formed from brain PS and DMPS has been studied using microelectrophoresis and DSC techniques, respectively. The zeta potential values have been shown to strongly depend on the total lipid concentration in the suspension. At moderate concentrations of the polyvalent cations, the total cation concentration exceeds the bulk one several times due to adsorption of cations to the liposomes. A modification of the Gouy-Chapman-Stern theory in the case of unknown bulk concentration of the polyvalent cation is presented. An intrinsic association constant for Be(2+) ions was evaluated to be about K(2) approximately 50 M(-1). The algorithm for estimating the concentrations of the accessible (to exogenously added polyvalent cations) lipid-binding sites is described. These values are consistent with the subsurface concentrations of the polyvalent cations, which monotonously increase with the total concentration of the polyvalent cations. The calculated lipid accessibilities are shown to be in accordance with the DSC data.

Strength of Ca(2+) binding to retinal lipid membranes: consequences for lipid organization

There is evidence that membranes of rod outer segment (ROS) disks are a high-affinity Ca(2+) binding site. We were interested to see if the high occurrence of sixfold unsaturated docosahexaenoic acid in ROS lipids influences Ca(2+)-membrane interaction. Ca(2+) binding to polyunsaturated model membranes that mimic the lipid composition of ROS was studied by microelectrophoresis and (2)H NMR. Ca(2+) association constants of polyunsaturated membranes were found to be a factor of approximately 2 smaller than constants of monounsaturated membranes. Furthermore, strength of Ca(2+) binding to monounsaturated membranes increased with the addition of cholesterol, while binding to polyunsaturated lipids was unaffected. The data suggest that the lipid phosphate groups of phosphatidylcholine (PC), phosphatidylethanolamine (PE), and phosphatidylserine (PS) in PC/PE/PS (4:4:1, mol/mol) are primary targets for Ca(2+). Negatively charged serine in PS controls Ca (2+) binding by lowering the electric surface potential and elevating cation concentration at the membrane/water interface. The influence of hydrocarbon chain unsaturation on Ca(2+) binding is secondary compared to membrane PS content. Order parameter analysis of individual lipids in the mixture revealed that Ca(2+) ions did not trigger lateral phase separation of lipid species as long as all lipids remained liquid-crystalline. However, depending on temperature and hydrocarbon chain unsaturation, the lipid with the highest chain melting temperature converted to the gel state, as observed for the monounsaturated phosphatidylethanolamine (PE) in PC/PE/PS (4:4:1, mol/mol) at 25 degrees C.

Membrane fusion

Although catalyzed by different proteins, the energy barriers for lipid bilayer fusion in exocytosis, viral fusion, and trafficking seem to be the same as those for the fusion of protein-free phospholipid membranes. To minimize this energy, fusion will proceed through a minimal number of lipid molecules, probably localized in bent non-bilayer intermediates. Experiments on phospholipid bilayer membrane fusion show the pathway of contact, hemifusion, flickering fusion pore formation, and fusion pore enlargement caused by swelling of the vesicle. Lipid curvature determines the barriers to hemifusion and fusion pore formation, while swelling-induced membrane tension drives fusion pore enlargement. Experiments on viral protein-induced cell-cell fusion and exocytosis show the same pathway with the same fundamental effects of lipid curvature and membrane tension. Thus while proteins control these reactions, lipid energetics determine the basic reaction scheme for membrane fusion.

Biochemical and functional studies of cortical vesicle fusion: the SNARE complex and Ca2+ sensitivity

Cortical vesicles (CV) possess components critical to the mechanism of exocytosis. The homotypic fusion of CV centrifuged or settled into contact has a sigmoidal Ca2+ activity curve comparable to exocytosis (CV-PM fusion). Here we show that Sr2+ and Ba2+ also trigger CV-CV fusion, and agents affecting different steps of exocytotic fusion block Ca2+, Sr2+, and Ba2+-triggered CV-CV fusion. The maximal number of active fusion complexes per vesicle, <n\>Max, was quantified by NEM inhibition of fusion, showing that CV-CV fusion satisfies many criteria of a mathematical analysis developed for exocytosis. Both <n\>Max and the Ca2+ sensitivity of fusion complex activation were comparable to that determined for CV-PM fusion. Using Ca2+-induced SNARE complex disruption, we have analyzed the relationship between membrane fusion (CV-CV and CV-PM) and the SNARE complex. Fusion and complex disruption have different sensitivities to Ca2+, Sr2+, and Ba2+, the complex remains Ca2+- sensitive on fusion-incompetent CV, and disruption does not correlate with the quantified activation of fusion complexes. Under conditions which disrupt the SNARE complex, CV on the PM remain docked and fusion competent, and isolated CV still dock and fuse, but with a markedly reduced Ca2+ sensitivity. Thus, in this system, neither the formation, presence, nor disruption of the SNARE complex is essential to the Ca2+-triggered fusion of exocytotic membranes. Therefore the SNARE complex alone cannot be the universal minimal fusion machine for intracellular fusion. We suggest that this complex modulates the Ca2+ sensitivity of fusion.

Lipid bilayer electrostatic energy, curvature stress, and assembly of gramicidin channels

Hydrophobic interactions between lipid bilayers and imbedded membrane proteins couple protein conformation to the mechanical properties of the bilayer. This coupling is widely assumed to account for the regulation of membrane protein function by the membrane lipids' propensity to form nonbilayer phases, which will produce a curvature stress in the bilayer. Nevertheless, there is only limited experimental evidence for an effect of bilayer curvature stress on membrane protein structure. We show that alterations in curvature stress, due to alterations in the electrostatic energy of dioleoylphosphatidylserine bilayers, modulate the structurally well-defined gramicidin A monomer <--> dimer reaction. Maneuvers that decrease the electrostatic energy of the unperturbed bilayer promote channel dissociation; we measure the change in interaction energy. The bilayer electrostatic energy thus can affect membrane protein structure by a mechanism that does not involve the electrostatic field across the bilayer, but rather electrostatic interactions among the phospholipid head groups in each monolayer which affect the bilayer curvature stress. These results provide further evidence for the importance of mechanical interactions between a bilayer and its imbedded proteins for protein structure and function.

Encapsulation of bilayer vesicles by self-assembly

Vesicles of lipid bilayers have been investigated as drug-delivery vehicles for almost 20 years. The vesicles' interior space is separated from the surrounding solution because small molecules have only limited permeability through the bilayer. Single-walled (unilamellar) vesicles are made by a variety of non-equilibrium techniques, including mechanical disruption of lamellar phases by sonication or extrusion through filters, or chemical disruption by detergent dialysis or solvent removal. These techniques do not, however, allow the encapsulation of a specific volume, nor can they be used to encapsulate other vesicles. Here we show that molecular-recognition processes mediated by lipophilic receptors and substrates (here the biotin-streptavidin complex) can be used to produce a multicompartmental aggregate of tethered vesicles encapsulated within a large bilayer vesicle. We call these encapsulated aggregates vesosomes. Encapsulation is achieved by unrolling bilayers from cochleate cylinderss which are tethered to the aggregate by biotin-streptavidin coupling. These compartmentalized vesosomes could provide vehicles for multicomponent or multifunctional drug delivery; in particular, the encapsulating membrane could significantly modify permeation properties, or could be used to enhance the biocompatibility of the system.

The interaction of alpha-tocopherol with phosphatidylserine vesicles and calcium

The interaction of alpha-tocopherol with dimyristoylphosphatidylserine (DMPS) has been studied in the presence and in the absence of Ca2+ by using differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FT-IR) and 45Ca2+-binding. In the absence of Ca2+, DSC showed that alpha-tocopherol decreases the temperature of the lamellar gel to lamellar liquid crystalline phase transition as well as it decreases delta H of this transition. Two different peaks were detected at 10 mol% of alpha-tocopherol and probably one of the peaks correspond to pure or nearly pure DMPS and the other to DMPS incorporating most of the alpha-tocopherol. The phase transition was totally abolished at 30 mol% of alpha-tocopherol. In the presence of Ca2+ this L(beta) to L(alpha) phase transition of DMPS was even more perturbed by alpha-tocopherol, so that it was totally abolished by only 7 mol% of alpha-tocopherol, at Ca2+ concentrations which were clearly non-saturating, like those giving DMPS/Ca2+ molar ratio of 4:1 and 10:1. Furthermore, the transition of the DMPS/Ca2+ complex observed at 91.6 degrees C was perturbed by the presence of alpha-tocopherol, indicating a change in the structure of the crystalline complex. The FT-IR analysis of the effect of alpha-tocopherol on DPMS phase transition confirmed the decrease in the phase transition temperature of the phospholipid, and also that alpha-tocopherol increases the number of gauche isomers in the gel state but has no effect in the liquid crystalline state. The binding of 45Ca2+ was also affected by the presence of alpha-tocopherol, so that the number of binding sites was decreased, and this may be interesting for situations in which phosphatidylserine and Ca2+ are simultaneously implicated in biological functions, such as membrane fusion and enzyme activation.

Resonance energy transfer imaging of phospholipid vesicle interaction with a planar phospholipid membrane: undulations and attachment sites in the region of calcium-mediated membrane--membrane adhesion

Membrane fusion of a phospholipid vesicle with a planar lipid bilayer is preceded by an initial prefusion stage in which a region of the vesicle membrane adheres to the planar membrane. A resonance energy transfer (RET) imaging microscope, with measured spectral transfer functions and a pair of radiometrically calibrated video cameras, was used to determine both the area of the contact region and the distances between the membranes within this zone. Large vesicles (5-20 microns diam) were labeled with the donor fluorophore coumarin-phosphatidylethanolamine (PE), while the planar membrane was labeled with the acceptor rhodamine-PE. The donor was excited with 390 nm light, and separate images of donor and acceptor emission were formed by the microscope. Distances between the membranes at each location in the image were determined from the RET rate constant (kt) computed from the acceptor:donor emission intensity ratio. In the absence of an osmotic gradient, the vesicles stably adhered to the planar membrane, and the dyes did not migrate between membranes. The region of contact was detected as an area of planar membrane, coincident with the vesicle image, over which rhodamine fluorescence was sensitized by RET. The total area of the contact region depended biphasically on the Ca2+ concentration, but the distance between the bilayers in this zone decreased with increasing [Ca2+]. The changes in area and separation were probably related to divalent cation effects on electrostatic screening and binding to charged membranes. At each [Ca2+], the intermembrane separation varied between 1 and 6 nm within each contact region, indicating membrane undulation prior to adhesion. Intermembrane separation distances < or = 2 nm were localized to discrete sites that formed in an ordered arrangement throughout the contact region. The area of the contact region occupied by these punctate attachment sites was increased at high [Ca2+]. Membrane fusion may be initiated at these sites of closest membrane apposition.