Polymer-induced membrane contraction, phase separation, and fusion via Marangoni flow.

Experiments have shown that the depletion of polymer in the region between two apposed (contacting or nearly contacting) bilayer membranes leads to fusion. In this paper we show theoretically that the addition of nonadsorbing polymer in solution can promote lateral contraction and phase separation of the lipids in the outer monolayers of the membranes exposed to the polymer solution, i.e., outside the contact zone. This initial phase coexistence of higher- and lower-density lipid domains in the outer monolayer results in surface tension gradients in the outer monolayer. Initially, the inner layer lipids are not exposed to the polymer solution and remain in their original "unstressed" state. The differential stresses on the bilayers give rise to a Marangoni flow of lipid from the outer monolayers in the "contact zone" (where there is little polymer and hence a uniform phase) to the outer monolayers in the "reservoir" (where initially the surface tension gradients are large due to the polymer-induced phase separation). As a result, the low-density domains of the outer monolayers in the contact zone expose their hydrophobic chains, and those of the inner monolayers, to the solvent and to each other across the narrow water gap, allowing fusion to occur via a hydrophobic interaction. More generally, this type of mechanism suggests that fusion and other intermembrane interactions may be triggered by Marangoni flows induced by surface tension gradients that provide "action at a distance" far from the fusion or interaction zone.     

Transbilayer effects of raft-like lipid domains in asymmetric planar bilayers measured by single molecule tracking

Cell membranes have complex lipid compositions, including an asymmetric distribution of phospholipids between the opposing leaflets of the bilayer. Although it has been demonstrated that the lipid composition of the outer leaflet of the plasma membrane is sufficient for the formation of raft-like liquid-ordered (l(o)) phase domains, the influence that such domains may have on the lipids and proteins of the inner leaflet remains unknown. We used tethered polymer supports and a combined Langmuir-Blodgett/vesicle fusion (LB/VF) technique to build asymmetric planar bilayers that mimic plasma membrane asymmetry in many ways. We show that directly supported LB monolayers containing cholesterol-rich l(o) phases are inherently unstable when exposed to water or vesicle suspensions. However, tethering the LB monolayer to the solid support with the lipid-anchored polymer 1,2-dimyristoyl phophatidylethanolamine-N-[poly(ethylene glycol)-triethoxysilane] significantly improves stability and allows for the formation of complex planar-supported bilayers that retain >90% asymmetry for 1-2 h. We developed a single molecule tracking (SPT) system for the study of lipid diffusion in asymmetric bilayers with coexisting liquid phases. SPT allowed us to study in detail the diffusion of individual lipids inside, outside, or directly opposed to l(o) phase domains. We show here that l(o) phase domains in one monolayer of an asymmetric bilayer do not induce the formation of domains in the opposite leaflet when this leaflet is composed of palmitoyl-oleoyl phosphatidylcholine and cholesterol but do induce domains when this leaflet is composed of porcine brain phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, and cholesterol. The diffusion of lipids is similar in l(o) and liquid-disordered phase domains and is not affected by transbilayer coupling, indicating that lateral and transverse lipid interactions that give rise to the domain structure are weak in the biological lipid mixtures that were employed in this work.     

Transbilayer Colocalization of Lipid Domains Explained via Measurement of Strong Coupling Parameters

When micron-scale compositional heterogeneity develops in membranes, the distribution of lipids on one face of the membrane strongly affects the distribution on the other. Specifically, when lipid membranes phase separate into coexisting liquid phases, domains in each monolayer leaflet of the membrane are colocalized with domains in the opposite leaflet. Colocalized domains have never been observed to spontaneously move out of registry. This result indicates that the lipid compositions in one leaflet are strongly coupled to compositions in the opposing leaflet. Predictions of the interleaflet coupling parameter, Λ, vary by a factor of 50. We measure the value of Λ by applying high shear forces to supported lipid bilayers. This causes the upper leaflet to slide over the lower leaflet, moving domains out of registry. We find that the threshold shear stress required to deregister domains in the upper and lower leaflets increases with the inverse length of domains. We derive a simple, closed-form expression relating the threshold shear to Λ, and find Λ = 0.016 ± 0.004 k_BT/nm².     

Fluid-gel coexistence in lipid membranes under differential stress

A widely conserved property of many biological lipid bilayers is their asymmetry. In addition to having distinct compositions on its two sides, a membrane can also exhibit different tensions in its two leaflets, a state known as differential stress. Here, we examine how this stress can influence the phase behavior of the constituent lipid monolayers of a single-component membrane. For temperatures sufficiently close to, but still above, the main transition, molecular dynamics simulations show the emergence of finite gel domains within the compressed leaflet. We describe the thermodynamics of this phenomenon by adding two empirical single-leaflet free energies for the fluid-gel transition, each evaluated at its respective asymmetry-dependent lipid density. Finite size effects arising in simulation are included in the theory through a geometry-dependent interfacial term. Our model reproduces the phase coexistence observed in simulation. It could therefore be used to connect the “hidden variable” of differential stress to experimentally observable properties of the main phase transition. These ideas could be generalized to any first-order bilayer phase transition in the presence of asymmetry, including liquid-ordered/liquid-disordered phase separation.     

Improving our picture of the plasma membrane: Rafts induce ordered domains in a simplified model cytoplasmic leaflet

By study of asymmetric membranes, models of the cell plasma membrane (PM) have improved, with more realistic properties of the asymmetric lipid composition of the membrane being explored. We used hemifusion of symmetric giant unilamellar vesicles (GUVs) with a supported lipid bilayer (SLB) to engineer bilayer leaflets of different composition. During hemifusion, only the outer leaflets of GUV and SLB are connected, exchanging lipids by simple diffusion. aGUVs were detached from the SLB for study. In general these aGUVs are formed with one leaflet that phase-separates into Ld (liquid disordered) + Lo (liquid ordered) phases, and another leaflet with lipid composition that would form a single fluid phase in a symmetric bilayer. We observed that ordered phases of either Lo or Lβ (gel phase) induce an ordered domain in the apposed fluid leaflet that lacks high melting lipids. Results suggest both an inter-leaflet and an intra-leaflet redistribution of cholesterol. We used C-Laurdan spectral images to investigate the lipid packing/order of aGUVs, finding that cholesterol partitions into the induced ordered domains. We suggest this behavior to be commonplace, that when Ld + Lo phase separation occurs in a cell PM exoplasmic leaflet, an induced order domain forms in the cytoplasmic leaflet.     

Effect of externally applied electrostatic fields on the surface topography of ceramide-enriched domains in mixed monolayers with sphingomyelin

Lipid and protein molecules anisotropically oriented at a hydrocarbon-aqueous interface configure a dynamic array of self-organized molecular dipoles. Electrostatic fields applied to lipid monolayers have been shown to induce in-plane migration of domains or phase separation in a homogeneous system. In this work, we have investigated the effect of externally applied electrostatic fields on the distribution of the condensed ceramide-enriched domains in mixed monolayers with sphingomyelin. In these monolayers, the lipids segregate in different phases at all pressures. This allows analyzing by epifluorescence microscopy the effect of the electrostatic field at all lateral pressure because coexistence of lipid domains in condensed state are always present. Our observations indicate that a positive potential applied to an electrode placed over the monolayer promotes a repulsion of the ceramide-enriched domains which is rather insensitive to the film composition, depends inversely on the lateral pressure and exhibits threshold dependence on the in-plane elasticity.     

Many length scales surface fractality in monomolecular films of whole myelin lipids and proteins

Monomolecular films prepared with all the lipid and protein components of myelin were spread at the air/aqueous buffer interface from isolated bovine spinal cord myelin fully dissolved in chloroform:methanol (2:1) or by surface free energy shock of myelin membrane microvesicles. These monolayers show indistinguishable surface behavior, with similar compositional phase coexistence through all the compression isotherm on several subphase conditions. The domains were observed through epifluorescence and Brewster angle microscopy on the air/water interface and on Langmuir-Blodgett films. Their thickness was measured ellipsometrically. Under molecular packing conditions resembling those found in the natural membrane, the morphology and size of the domains are highly self-similar, displaying no characteristic length scale. These properties are the hallmark of fractal objects. The fractality extends at least three orders of magnitudes, from the micrometer to the millimeter range, the fractal dimension being about 1.7. A possible implication of fractality in membrane structure and/or function is demonstrated through the high fluctuation of the propagation of signals through constrained diffusion in corrals formed by domains in the plane of the monolayer, which restricts the diffusion of a fluorescent probe over many length scale domains.     

The Mechanism of Collapse of Heterogeneous Lipid Monolayers

Collapse of homogeneous lipid monolayers is known to proceed via wrinkling/buckling, followed by folding into bilayers in water. For heterogeneous monolayers with phase coexistence, the mechanism of collapse remains unclear. Here, we investigated collapse of lipid monolayers with coexisting liquid-liquid and liquid-solid domains using molecular dynamics simulations. The MARTINI coarse-grained model was employed to simulate monolayers of ∼80 nm in lateral dimension for 10–25 μs. The monolayer minimum surface tension decreased in the presence of solid domains, especially if they percolated. Liquid-ordered domains facilitated monolayer collapse due to the spontaneous curvature induced at a high cholesterol concentration. Upon collapse, bilayer folds formed in the liquid (disordered) phase; curved domains shifted the nucleation sites toward the phase boundary. The liquid (disordered) phase was preferentially transferred into bilayers, in agreement with the squeeze-out hypothesis. As a result, the composition and phase distribution were altered in the monolayer in equilibrium with bilayers compared to a flat monolayer at the same surface tension. The composition and phase behavior of the bilayers depended on the degree of monolayer compression. The monolayer-bilayer connection region was enriched in unsaturated lipids. Percolation of solid domains slowed down monolayer collapse by several orders of magnitude. These results are important for understanding the mechanism of two-to-three-dimensional transformations in heterogeneous thin films and the role of lateral organization in biological membranes. The study is directly relevant for the function of lung surfactant, and can explain the role of nanodomains in its surface activity and inhibition by an increased cholesterol concentration.     

Lipid asymmetry in DLPC/DSPC-supported lipid bilayers: a combined AFM and fluorescence microscopy study

A fundamental attribute of cell membranes is transmembrane asymmetry, specifically the formation of ordered phase domains in one leaflet that are compositionally different from the opposing leaflet of the bilayer. Using model membrane systems, many previous studies have demonstrated the formation of ordered phase domains that display complete transmembrane symmetry; but there have been few reports on the more biologically relevant asymmetric membrane structures. Here we report on a combined atomic force microscopy and fluorescence microscopy study whereby we observe three different states of transmembrane symmetry in phase-separated supported lipid bilayers formed by vesicle fusion. We find that if the leaflets differ in gel-phase area fraction, then the smaller domains in one leaflet are in registry with the larger domains in the other leaflet and the system is dynamic. In a presumed lipid flip-flop process similar to Ostwald ripening, the smaller domains in one leaflet erode away whereas the large domains in the other leaflet grow until complete compositional asymmetry is reached and remains stable. We have quantified this evolution and determined that the lipid flip-flop event happens most frequently at the interface between symmetric and asymmetric DSPC domains. If both leaflets have identical area fraction of gel-phase, gel-phase domains are in registry and are static in comparison to the first state. The stability of these three DSPC domain distributions, the degree of registry observed, and the domain immobility have biological significance with regards to maintenance of lipid asymmetry in living cell membranes, communication between inner leaflet and outer leaflet, membrane adhesion, and raft mobility.     

Lipid domains in the exoplasmic and cytoplasmic leaflet of the human erythrocyte membrane: a spin label approach

The existence of different lipid domains in the monolayers of the human erythrocyte membrane was investigated at 4 °C by employing spin-labelled phospholipid analogues. Spectra of analogues located exclusively either in the exoplasmic or in the cytoplasmic leaflet of erythrocyte membranes were recorded. Spectra were simulated by variation of order parameter describing the average amplitude of motion of the long molecular axis of the nitrogen 2pπ orbital of the spin label and of the respective correlation times. For both leaflets at least three components were required to fit the experimental spectra, differing mainly in the order parameter. While the parameters of each component are not very different between both membrane halves, the relative contribution of each component to the spectrum is different between the exoplasmic and cytoplasmic leaflet. The order parameter of the most fluid component, presumably resembling the lipid bulk phase, is smaller in the cytoplasmic leaflet in comparison to the exoplasmic one. The lateral coexistence of different lipid domains in the human red blood cell membrane is concluded. The molecular nature of those domains is discussed. Received: 6 November 1998 / Revised version: 25 January 1999 / Accepted: 29 January 1999     

Nonequilibrium behavior in supported lipid membranes containing cholesterol

We investigate lateral organization of lipid domains in vesicles versus supported membranes and monolayers. The lipid mixtures used are predominantly DOPC/DPPC/Chol and DOPC/BSM/Chol, which have been previously shown to produce coexisting liquid phases in vesicles and monolayers. In a monolayer at an air-water interface, these lipids have miscibility transition pressures of approximately 12-15 mN/m, which can rise to 32 mN/m if the monolayer is exposed to air. Lipid monolayers can be transferred by Langmuir-Sch?fer deposition onto either silanized glass or existing Langmuir-Blodgett supported monolayers. Micron-scale domains are present in the transferred lipids only if they were present in the original monolayer before deposition. This result is valid for transfers at 32 mN/m and also at lower pressures. Domains transferred to glass supports differ from liquid domains in vesicles because they are static, do not align in registration across leaflets, and do not reappear after temperature is cycled. Similar static domains are found for vesicles ruptured onto glass surfaces. Although supported membranes on glass capture some aspects of vesicles in equilibrium (e.g., gel-liquid transition temperatures and diffusion rates of individual lipids), the collective behavior of lipids in large liquid domains is poorly reproduced.     

Liquid crystalline/gel state phase separation in docosahexaenoic acid-containing bilayers and monolayers

The phase behavior of lipid mixtures containing 1-stearoyl-2-docosahexaenoyl-sn-glycero-3-phosphocholine (18:0, 22:6 PC) with 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) was studied with bilayers using differential scanning calorimetry (DSC), and with monolayers monitoring pressure/area isotherms and surface elasticity, and lipid domain formation followed by epifluorescence microscopy. From DSC studies it is concluded that DPPC/18:0, 22:6 PC phase separates into DPPC-rich and 18:0, 22:6 PC-rich phases. In monolayers, phase separation is indicated by changes in pressure-area isotherms implying phase separation where 18:0, 22:6 PC is 'squeezed out' of the remaining DPPC monolayer. Phase separation into lipid domains in the mixed PC monolayer is quantified by epifluorescence microscopy using the fluorescently labeled phospholipid membrane probe, 1, 2-dioleoyl-sn-glycero-3-phosphoethanolamine-N-(lissamine rhodamine B sulfonyl). These results further describe the ability of docosahexaenoic acid to participate in lipid phase separations in membranes.     

The Mechanism of Collapse of Heterogeneous Lipid Monolayers

Collapse of homogeneous lipid monolayers is known to proceed via wrinkling/buckling, followed by folding into bilayers in water. For heterogeneous monolayers with phase coexistence, the mechanism of collapse remains unclear. Here, we investigated collapse of lipid monolayers with coexisting liquid-liquid and liquid-solid domains using molecular dynamics simulations. The MARTINI coarse-grained model was employed to simulate monolayers of ∼80 nm in lateral dimension for 10–25 μs. The monolayer minimum surface tension decreased in the presence of solid domains, especially if they percolated. Liquid-ordered domains facilitated monolayer collapse due to the spontaneous curvature induced at a high cholesterol concentration. Upon collapse, bilayer folds formed in the liquid (disordered) phase; curved domains shifted the nucleation sites toward the phase boundary. The liquid (disordered) phase was preferentially transferred into bilayers, in agreement with the squeeze-out hypothesis. As a result, the composition and phase distribution were altered in the monolayer in equilibrium with bilayers compared to a flat monolayer at the same surface tension. The composition and phase behavior of the bilayers depended on the degree of monolayer compression. The monolayer-bilayer connection region was enriched in unsaturated lipids. Percolation of solid domains slowed down monolayer collapse by several orders of magnitude. These results are important for understanding the mechanism of two-to-three-dimensional transformations in heterogeneous thin films and the role of lateral organization in biological membranes. The study is directly relevant for the function of lung surfactant, and can explain the role of nanodomains in its surface activity and inhibition by an increased cholesterol concentration.     

Spontaneous domain formation of phospholipase A2 at interfaces: fluorescence microscopy of the interaction of phospholipase A2 with mixed monolayers of lecithin, lysolecithin and fatty acid.

Fluorescence microscopy has recently been proven to be an ideal tool to investigate the specific interaction of phospholipase A2 with oriented substrate monolayers. Using a dual labeling technique, it could be shown that phospholipase A2 can specifically attack and hydrolyze solid analogous L-alpha-DPPC domains. After a critical extent of monolayer hydrolysis the enzyme itself starts to aggregate forming regular shaped protein domains (Grainger et al. (1990) Biochim. Biophys. Acta 1023, 365-379). In order to confirm that the existence of hydrolysis products in the monolayer is necessary for the observed aggregation of phospholipase A2, mixed monolayers of D- and L-alpha-DPPC, L-alpha-lysoPPC and palmitic acid in different ratios were examined. The phase behavior and the interaction of these films with phospholipase A2 were directly visualized with an epifluorescence microscope. Above a certain critical concentration of lysolecithin and palmitic acid in the monolayer, compression of these mixed films leads to phase separation and formation of mixed domains of unknown composition. Their high negative charge density is evidenced by preferential binding of a cationic dye to these phase-separated areas. Introduction of fluorescence-labeled phospholipase A2 underneath these mixed domains results in rapid binding of the protein to the domains without visible hydrolytic activity, regardless of whether the L-form or the D-form of the DPPC were used. In binary mixtures, only those with DPPC/palmitic acid show formation of phase-separated areas which can be specifically targeted by phospholipase A2 leading to a rapid formation (within 2 min) of protein domains. Experiments with pyrenedecanoic acid containing monolayers give the first direct evidence that acid is located above the enzyme domains. These results show that a locally high negative charge density of the phase-separated domains is one of the prerequisites for the binding of phospholipase A2. In addition, however, small amounts of D- or L-alpha-DPPC headgroups within the domains of the monolayer seem to be necessary for recognition followed by fast binding of the protein to the domains. This is confirmed by experiments with mixed monolayers of diacetylene carboxylic acid and D-alpha-DPPC. The acid--immiscible with lecithin--forms well defined pure acid domains in the monolayer. While the cationic dye can be docked rapidly to these phase separated areas, no preferential enzyme binding and thus no protein domain formation below these acid domains can be induced.     

Membrane fusion: overcoming of the hydration barrier and local restructuring

The early stages of membrane fusion have been investigated theoretically. It has been shown that the hydration repulsion, operating between apposed membranes, is overcome locally under the action of out-of-plane thermal fluctuations of the bilayers. The fluctuations lead to the formation of close (less than 0.5 nm) contact between the membranes within a small area (approximately 10 nm2). Increasing hydration repulsion between apposed polar heads of lipid molecules in this area causes the rupture of interacting monolayers. The rupture results in monolayer fusion of the membranes, i.e. in the formation of a bridge connecting the monolayers, which is usually named the monolayer stalk. The influence of degree of hydration of the monolayers and their spontaneous curvature on conditions of monolayer fusion have been analysed. The proposed mechanism of early stages of fusion process can proceed without preliminary formation of tight dehydrated contact between the membranes and even without any dehydration.     

Domain formation in models of the renal brush border membrane outer leaflet.

The plasma membrane outer leaflet plays a key role in determining the existence of rafts and detergent-resistant membrane domains. Monolayers with lipid composition mimicking that of the outer leaflet of renal brush border membranes (BBM) have been deposited on mica and studied by atomic force microscopy. Sphingomyelin (SM) and palmitoyloleoyl phosphatidylcholine (POPC) mixtures, at molar ratios varying from 2:1 to 4:1, were phase-separated into liquid condensed (LC) SM-enriched phase and liquid expanded (LE) POPC-enriched phase. The LC phase accounted for 33 and 58% of the monolayers surface for 2:1 and 4:1 mixtures, respectively. Addition of 20-50 mol % cholesterol (Chl) to the SM/POPC (3:1) mixtures induced marked changes in the topology of monolayers. Whereas Chl promoted the connection between SM domains at 20 mol %, increasing Chl concentration progressively reduced the size of domains and the height differences between the phases. Lateral heterogeneity was, however, still present at 33 mol % Chl. The results indicate that the lipid composition of the outer leaflet is most likely responsible for the BBM thermotropic transition properties. They also strongly suggest that the common maneuver that consists of depleting membrane cholesterol to suppress rafts does not abolish the lateral heterogeneity of BBM membranes.     

Hydrolytic action of phospholipase A2 in monolayers in the phase transition region: direct observation of enzyme domain formation using fluorescence microscopy

Phospholipase A2, a ubiquitous lipolytic enzyme highly active in the hydrolysis of organized phospholipid substrates, has been characterized optically in its action against a variety of phospholipid monolayers using fluorescence microscopy. By labeling the enzyme with a fluorescent marker and introducing it into the subphase of a Langmuir film balance, the hydrolysis of lipid monolayers in their liquid-solid phase transition region could be directly observed with the assistance of an epifluorescence microscope. Visual observation of hydrolysis of different phospholipid monolayers in the phase transition region in real-time could differentiate various mechanisms of hydrolytic action against lipid solid phase domains. DPPC solid phase domains were specifically targeted by phospholipase A2 and were observed to be hydrolyzed in a manner consistent with localized packing density differences. DPPE lipid domain hydrolysis showed no such preferential phospholipase A2 response but did demonstrate a preference for solid/lipid interfaces. DMPC solid lipid domains were also hydrolyzed to create large circular areas in the monolayer cleared of solid phase lipid domains. In all cases, after critical extents of monolayer hydrolysis in the phase transition region, highly stabile, organized domains of enzyme of regular sizes and morphologies were consistently seen to form in the monolayers. Enzyme domain formation was entirely dependent upon hydrolytic activity in the monolayer phase transition region and was not witnessed otherwise.     

Distribution of ganglioside GM1 in L-alpha-dipalmitoylphosphatidylcholine/cholesterol monolayers: a model for lipid rafts

The distribution of low concentrations of ganglioside GM1 in L-alpha-dipalmitoylphosphatidylcholine (DPPC) and DPPC/cholesterol monolayers supported on mica has been studied using atomic force microscopy (AFM). The monolayers studied correspond to a pure gel phase and a mixture of liquid-expanded (LE) and liquid-condensed (LC) phases for DPPC and to a single homogeneous liquid-ordered phase for 2:1 DPPC/cholesterol. The addition of 2.5-5% GM1 to phase-separated DPPC monolayers resulted in small round ganglioside-rich microdomains in the center and at the edges of the LC domains. Higher amounts of GM1 (10%) give numerous filaments in the center of the LC domains and larger patches at the edges. A gel phase DPPC monolayer containing GM1 showed large domains containing a network of GM1-rich filaments. The addition of GM1 to a liquid-ordered 2:1 DPPC/cholesterol monolayer gives small, round domains that vary in size from 50 to 150 nm for a range of surface pressures. Larger amounts of GM1 lead to coalescence of the small, round domains to give longer filaments that cover 30-40% of the monolayer surface for 10 mol % GM1. The results indicate that biologically relevant GM1 concentrations lead to submicron-sized domains in a cholesterol-rich liquid-ordered phase that is analogous to that found in detergent-insoluble membrane fractions, and are thought to be important in membrane microdomains or rafts. This demonstrates that AFM studies of model monolayers and bilayers provide a powerful method for the direct detection of microdomains that are too small for study with most other techniques.     

α-Synuclein interacts differently with membranes mimicking the inner and outer leaflets of neuronal membranes

The toxicity of α-synuclein (α-syn), the amyloidogenic protein responsible for Parkinson's disease, is likely related to its interaction with the asymmetric neuronal membrane. α-Syn exists as cytoplasmatic and as extracellular protein as well. To shed light on the different interactions occurring at the different α-syn localizations, we have here modelled the external and internal membrane leaflets of the neuronal membrane with two complex lipid mixtures, characterized by phase coexistence and with negative charge confined to either the ordered or the disordered phase, respectively. To this purpose, we selected a five-component (DOPC/SM/DOPE/DOPS/chol) and a four-component (DOPC/SM/GM1/chol) lipid mixtures, which contained the main membrane lipid constituents and exhibited a phase separation with formation of ordered domains. We have compared the action of α-syn in monomeric form and at different concentrations (1 nM, 40 nM, and 200 nM) with respect to lipid systems with different composition and shape by AFM, QCM-D, and vesicle leakage experiments. The experiments coherently showed a higher stability of the membranes composed by the internal leaflet mixture to the interaction with α-syn. Damage to membranes made of the external leaflet mixture was detected in a concentration-dependent manner. Interestingly, the membrane damage was related to the fluidity of the lipid domains and not to the presence of negatively charged lipids.     

Fluorescence, polarized fluorescence, and Brewster angle microscopy of palmitic acid and lung surfactant protein B monolayers

Fluorescence, polarized fluorescence, and Brewster angle microscopy reveal that human lung surfactant protein SP-B and its amino terminus (SP-B[1-25]) alter the phase behavior of palmitic acid monolayers by inhibiting the formation of condensed phases and creating a new fluid protein-rich phase. This fluid phase forms a network that separates condensed phase domains at coexistence and persists to high surface pressures. The network changes the monolayer collapse mechanism from heterogeneous nucleation/growth and fracturing processes to a more homogeneous process through isolating individual condensed phase domains. This results in higher surface pressures at collapse, and monolayers easier to respread on expansion, factors essential to the in vivo function of lung surfactant. The network is stabilized by a low-line tension between the coexisting phases, as confirmed by the observation of extended linear domains, or "stripe" phases, and a Gouy-Chapman analysis of protein-containing monolayers. Comparison of isotherm data and observed morphologies of monolayers containing SP-B(1-25) with those containing the full SP-B sequence show that the shortened peptide retains most of the native activity of the full-length protein, which may lead to cheaper and more effective synthetic replacement formulations.