Surface topography of membrane domains

Studying biological membranes with extended range high-speed atomic force microscopy

High—speed atomic force microscopy has proven to be a valuable tool for the study of biomolecular systems at the nanoscale. Expanding its application to larger biological specimens such as membranes or cells has, however, proven difficult, often requiring fundamental changes in the AFM instrument. Here we show a way to utilize conventional AFM instrumentation with minor alterations to perform high-speed AFM imaging with a large scan range. Using a two—actuator design with adapted control systems, a 130 × 130 × 5 μm scanner with nearly 100 kHz open—loop small-signal Z—bandwidth is implemented. This allows for high-speed imaging of biologically relevant samples as well as high-speed measurements of nanomechanical surface properties. We demonstrate the system performance by real-time imaging of the effect of charged polymer nanoparticles on the integrity of lipid membranes at high imaging speeds and peak force tapping measurements at 32 kHz peak force rate.

Autonomous buckling of micrometer-sized lipid-protein membrane patches constructed by Dictyostelium discoideum

Background

The cytosol of amoeba cells controls the membrane deformation during their motion in vivo. To investigate such ability of the cytosol of amoeba cell, Dictyostelium discoideum (Dictyostelium), in vitro, we used lipids extracted from Dictyostelium and commercially available phospholipids, and prepared substrate-supported lipid membrane patches on the micrometer scale by spin coating.

Results

We found that the spin coater holder, which has pores (pore size = 3.1 mm) of negative pressure to hold the cover glass induced the concave surface of the cover glass. The membrane lipid patches were formed at each position in the vicinity of the holder pores and their sizes were in the range of 2.7 to 3.2 × 10⁴ μm². After addition of the cytosol extracted from Dictyostelium to the lipid membrane patches, through time-lapse observation with a confocal laser scanning fluorescence microscope, we observed an autonomous buckling of the Dictyostelium lipid patches and localized behaviours of proteins found within.

Conclusion

The current method serves as the novel technique for the preparation of film patches in which the positions of patches are controlled by the holder pores without fabricating, modifying, and arranging the chemical properties of the solution components of lipids. The findings imply that lipid-binding proteins in the cytosol were adsorbed and accumulated within the Dictyostelium lipid patches, inducing the transformation of the cell-sized patch.

Electronic supplementary material

The online version of this article (doi:10.1186/1754-1611-9-3) contains supplementary material, which is available to authorized users.

N-nervonoylsphingomyelin (C24:1) prevents lateral heterogeneity in cholesterol-containing membranes

This study was conducted to explore how the nature of the acyl chains of sphingomyelin (SM) influence its lateral distribution in the ternary lipid mixture SM/cholesterol/1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), focusing on the importance of the hydrophobic part of the SM molecule for domain formation. Atomic force microscopy (AFM) measurements showed that the presence of a double bond in the 24:1 SM molecule in mixtures with cholesterol (CHO) or in pure bilayers led to a decrease in the molecular packing. Confocal microscopy and AFM showed, at the meso- and nanoscales respectively, that unlike 16:0 and 24:0 SM, 24:1 SM does not induce phase segregation in ternary lipid mixtures with DOPC and CHO. This ternary lipid mixture had a nanomechanical stability intermediate between those displayed by liquid-ordered (Lo) and liquid-disordered (Ld) phases, as reported by AFM force spectroscopy measurements, demonstrating that 24:1 SM is able to accommodate both DOPC and CHO, forming a single phase. Confocal experiments on giant unilamellar vesicles made of human, sheep, and rabbit erythrocyte ghosts rich in 24:1 SM and CHO, showed no lateral domain segregation. This study provides insights into how the specific molecular structure of SM affects the lateral behavior and the physical properties of both model and natural membranes. Specifically, the data suggest that unsaturated SM may help to keep membrane lipids in a homogeneous mixture rather than in separate domains.

Interaction of cholesterol in ternary lipid mixtures investigated using single-molecule fluorescence

Fluorescence measurements of the sterol analog 23-(dipyrrometheneboron difluoride)-24-norcholesterol (BODIPY-cholesterol) are used to compare the effects of cholesterol (Chol) in monolayers of 1,2-dipalmitoyl-sn-glycero-3-phosphatidylcholine (DPPC)/1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC)/Chol and chicken egg sphingomyelin (SM)/DOPC/Chol. Monolayers are formed using the Langmuir-Blodgett technique and compared at surface pressures of 8 and 30 mN/m. In particular, these ternary lipid mixtures are compared using both ensemble and single-molecule fluorescence measurements of BODIPY-cholesterol. In mixed monolayers incorporating 0.10 mol % BODIPY-cholesterol, fluorescence microscopy measurements as a function of cholesterol added reveal similar trends in monolayer phase structure for both DPPC/DOPC/Chol and SM/DOPC/Chol films. With a probe concentration reduced to ∼10(-8) mol % BODIPY-cholesterol, single-molecule fluorescence measurements using defocused polarized total internal reflection microscopy are used to characterize the orientations of BODIPY-cholesterol in the monolayers. Population histograms of the BODIPY emission dipole tilt angle away from the membrane normal reveal distinct insertion geometries with a preferred angle observed near 78°. The measured angles and populations are relatively insensitive to added cholesterol and changes in surface pressure for monolayers of SM/DOPC/Chol. For monolayers of DPPC/DOPC/Chol, however, the single-molecule measurements reveal significant changes in the BODIPY-cholesterol insertion geometry when the surface pressure is increased to 30 mN/m. These changes are discussed in terms of a squeeze-out mechanism for BODIPY-cholesterol in these monolayers and provide insight into the partitioning and arrangement of BODIPY-cholesterol in ternary lipid mixtures.

Phase transitions in supported lipid bilayers studied by AFM

We review the capabilities of Atomic Force Microscopy (AFM) in the study of phase transitions in Supported Lipid Bilayers (SLBs). AFM represents a powerful technique to cover the resolution range not available to fluorescence imaging techniques and where spectroscopic data suggest what the relevant lateral scale for domain formation might be. Phase transitions of lipid bilayers involve the formation of domains characterized by different heights with respect to the surrounding phase and are therefore easily identified by AFM in liquid solution once the bilayer is confined to a flat surface. Even if not endowed with high time resolution, AFM allows light to be shed on some aspects related to lipid phase transitions in the case of both a single lipid component and lipid mixtures containing sterols also. We discuss here the obtained results in light of the peculiarities of supported lipid bilayer model systems.

Label-free characterization of biomembranes: from structure to dynamics

We review recent progress in the study of the structure and dynamics of phospholipid membranes and associated proteins, using novel label-free analytical tools. We describe these techniques and illustrate them with examples highlighting current capabilities and limitations. Recent advances in applying such techniques to biological and model membranes for biophysical studies and biosensing applications are presented, and future prospects are discussed.

Modeling the interplay between protein and lipid aggregation in supported membranes

We present a theoretical model that deals with the complex interplay between lipid segregation and the self-aggregation of lipid-attached proteins. The model, in contrast to previous ones that consider proteins only as passive elements affecting the lipid distribution, describes the system including three terms: the dynamic interactions between protein monomers, the interactions between lipid components, and a mixed term considering protein-lipid interactions. It is used to explain experimental results performed on a well-defined system in which a self-aggregating soluble bacterial cytoskeletal protein polymerizes on a lipid bilayer containing two lipid components. All the elements considered in a previously described protein model, including torsion of the monomers within the filament, are needed to account for the observed filament shapes. The model also points out that lipid segregation can affect the length and curvature of the filaments and that the dynamic behavior of the lipids and proteins can have different time scales, giving rise to memory effects. This simple model that considers a dynamic protein assembly on a fluid and active lipid surface can be easily extended to other biologically relevant situations in which the interplay between protein and lipid aggregation is needed to fully describe the system.

Milk sphingomyelin domains in biomimetic membranes and the role of cholesterol: morphology and nanomechanical properties investigated using AFM and force spectroscopy

Milk sphingomyelin (MSM) and cholesterol segregate into domains in the outer bilayer membrane surrounding milk fat globules. To elucidate the morphology and mechanical properties of theses domains, supported lipid bilayers with controlled molar proportions of MSM, dioleoylphosphatidylcholine (DOPC) and cholesterol were produced in buffer mimicking conditions of the milk aqueous phase. Atomic force microscopy imaging showed that (i) for T < 35 °C MSM segregated in gel phase domains protruding above the fluid phase, (ii) the addition of 20 mol % cholesterol resulted in smaller and more elongated l(o) phase domains than in equimolar MSM/DOPC membranes, (iii) the MSM/cholesterol-enriched l(o) phase domains were less salient than the MSM gel phase domains. Force spectroscopy measurements furthermore showed that cholesterol reduced the resistance of MSM/DOPC membrane to perforation. The results are discussed with respect to the effect of cholesterol on the biophysical properties of lipid membranes. The combination of AFM imaging and force mapping provides unprecedented insight into the structural and mechanical properties of milk lipid membranes, and opens perspectives for investigation of the functional properties of MSM domains during milk fat processing or digestion.

Preparation of mica supported lipid bilayers for high resolution optical microscopy imaging

Supported lipid bilayers (SLBs) are widely used as a model for studying membrane properties (phase separation, clustering, dynamics) and its interaction with other compounds, such as drugs or peptides. However SLB characteristics differ depending on the support used. Commonly used techniques for SLB imaging and measurements are single molecule fluorescence microscopy, FCS and atomic force microscopy (AFM). Because most optical imaging studies are carried out on a glass support, while AFM requires an extremely flat surface (generally mica), results from these techniques cannot be compared directly, since the charge and smoothness properties of these materials strongly influence diffusion. Unfortunately, the high level of manual dexterity required for the cutting and gluing thin slices of mica to the glass slide presents a hurdle to routine use of mica for SLB preparation. Although this would be the method of choice, such prepared mica surfaces often end up being uneven (wavy) and difficult to image, especially with small working distance, high numerical aperture lenses. Here we present a simple and reproducible method for preparing thin, flat mica surfaces for lipid vesicle deposition and SLB preparation. Additionally, our custom made chamber requires only very small volumes of vesicles for SLB formation. The overall procedure results in the efficient, simple and inexpensive production of high quality lipid bilayer surfaces that are directly comparable to those used in AFM studies.

Morphology change and detachment of lipid bilayers from the mica substrate driven by graphene oxide sheets

Understanding the interaction between graphene oxide (GO) and a lipid membrane is significant for exploring the biocompatibility and cytotoxicity of GO, which is the basis for utilizing GO in the fields of biosensors, bioimaging, drug delivery, antibacterials, and so on. In this article, we monitored the dynamic process of the morphology change and detachment of lipid bilayers on mica substrates prompted by GO sheets by in situ atomic force microscope (AFM) imaging. It was found that the bare lipid bilayer dramatically expanded in height and would be unstable and detachable from the mica substrates as induced by GO. The detached lipid molecules were found to bind to the GO surface. The results also imply that GO is likely to influence the height and stability of the supported lipid bilayers (SLBs) by adsorbing metal ions such as calcium ions that were used to stabilize the bilayer structures on the mica substrate. These findings illustrate a complicated effect of GO on the SLBs and should be helpful in future applications of GO in biotechnology.

Coexistence of probe conformations in lipid phases-a polarized fluorescence microspectroscopy study

Several well-established fluorescence methods depend on environment-sensitive probes that report about molecular properties of their local environment. For reliable interpretation of experiments, careful characterization of probes' behavior is required. In this study, bleaching-corrected polarized fluorescence microspectroscopy with nanometer spectral peak position resolution was applied to characterize conformations of two alkyl chain-labeled 7-nitro-2-1,3-benzoxadiazol-4-yl phospholipids in three model membranes, representing the three main lipid phases. The combination of polarized and spectral detection revealed two main probe conformations with their preferential fluorophore dipole orientations roughly parallel and perpendicular to membrane normal. Their peak positions were separated by 2-6 nm because of different local polarities and depended on lipid environment. The relative populations of conformations, estimated by a numerical model, indicated a specific sensitivity of the two probes to molecular packing with cholesterol. The coexistence of probe conformations could be further exploited to investigate membrane organization below microscopy spatial resolution, such as lipid rafts. With the addition of polarized excitation or detection to any environment-sensitive fluorescence imaging technique, the conformational analysis can be directly applied to explore local membrane complexity.

Modification of plasma membrane organization in tobacco cells elicited by cryptogein

Lipid mixtures within artificial membranes undergo a separation into liquid-disordered and liquid-ordered phases. However, the existence of this segregation into microscopic liquid-ordered phases has been difficult to prove in living cells, and the precise organization of the plasma membrane into such phases has not been elucidated in plant cells. We developed a multispectral confocal microscopy approach to generate ratiometric images of the plasma membrane surface of Bright Yellow 2 tobacco (Nicotiana tabacum) suspension cells labeled with an environment sensitive fluorescent probe. This allowed the in vivo characterization of the global level of order of this membrane, by which we could demonstrate that an increase in its proportion of ordered phases transiently occurred in the early steps of the signaling triggered by cryptogein and flagellin, two elicitors of plant defense reactions. The use of fluorescence recovery after photobleaching revealed an increase in plasma membrane fluidity induced by cryptogein, but not by flagellin. Moreover, we characterized the spatial distribution of liquid-ordered phases on the membrane of living plant cells and monitored their variations induced by cryptogein elicitation. We analyze these results in the context of plant defense signaling, discuss their meaning within the framework of the "membrane raft" hypothesis, and propose a new mechanism of signaling platform formation in response to elicitor treatment.

Electrochemical impedance spectroscopy as a method for electrical characterization of the bilayers formed from lipid-amino acid systems

Bilayer lipid membranes composed of phosphatidylcholine and isoleucine or phosphatidylcholine and tyrosine were investigated using electrochemical impedance spectroscopy. Interaction between membrane components causes significant deviations from the additivity rule which can be explained by formation of the domain structures. The surface area of domains was calculated based on derived equations. We suggest that the stoichiometry of phosphatidylcholine-isoleucine domain is equal 3:1. In the case of tyrosine-modified phosphatidylcholine membranes, domain with stoichiometry 7:1 should be taken into consideration.

Interaction of antibiotics with lipid vesicles on thin film porous silicon using reflectance interferometric Fourier transform spectroscopy

The ability to observe interactions of drugs with cell membranes is an important area in pharmaceutical research. However, these processes are often difficult to understand due to the dynamic nature of cell membranes. Therefore, artificial systems composed of lipids have been used to study membrane properties and their interaction with drugs. Here, lipid vesicle adsorption, rupture, and formation of planar lipid bilayers induced by various antibiotics (surfactin, azithromycin, gramicidin, melittin and ciprofloxacin) and the detergent dodecyl-b-D-thiomaltoside (DOTM) was studied using reflective interferometric Fourier transform spectroscopy (RIFTS) on an oxidized porous silicon (pSi) surface as a transducer. The pSi transducer surfaces are prepared as thin films of 3 μm thickness with pore dimensions of a few nanometers in diameter by electrochemical etching of crystalline silicon followed by passivation with a thermal oxide layer. Furthermore, the sensitivity of RIFTS was investigated using three different concentrations of surfactin. Complementary techniques including atomic force microscopy, fluorescence recovery after photobleaching, and fluorescence microscopy were used to validate the RIFTS-based method and confirm adsorption and consequent rupture of vesicles to form a phospholipid bilayer upon the addition of antibiotics. The method provides a sensitive and real-time approach to monitor the antibiotic-induced transition of lipid vesicles to phospholipid bilayers.

High-speed AFM and applications to biomolecular systems

Directly observing individual protein molecules in action at high spatiotemporal resolution has long been a holy grail for biological science. This is because we long have had to infer how proteins function from the static snapshots of their structures and dynamic behavior of optical makers attached to the molecules. This limitation has recently been removed to a large extent by the materialization of high-speed atomic force microscopy (HS-AFM). HS-AFM allows us to directly visualize the structure dynamics and dynamic processes of biological molecules in physiological solutions, at subsecond to sub-100-ms temporal resolution, without disturbing their function. In fact, dynamically acting molecules such as myosin V walking on an actin filament and bacteriorhodopsin in response to light are successfully visualized. In this review, we first describe theoretical considerations for the highest possible imaging rate of this new microscope, and then highlight recent imaging studies. Finally, the current limitation and future challenges to explore are described.

Structure and lateral interaction in mixed monolayers of dioctadecyldimethylammonium chloride (DOAC) and stearyl alcohol

π-A isotherms, atomic force microscopy (AFM), and sum frequency generation (SFG) vibrational spectroscopy are employed to investigate the molecular structure and lateral interactions in mixed monolayers of dioctadecyldimethylammonium chloride (DOAC) and stearyl alcohol (SA) at air/water and air/solid interfaces. To avoid possible interference between the two molecules in the SFG spectroscopic measurements, perprotonated DOAC and perdeuterated SA (dSA) were used. The thermodynamic analyses for the π-A isotherms show that DOAC is miscible with dSA. SFG observations reveal that DOAC molecules become conformationally ordered as dSA molecules are introduced into the monolayer. AFM observations demonstrate coexistence of DOAC-rich and dSA-rich domains in the mixed monolayer with ratios different from their initial composition in the subphase. The present study suggests that DOAC molecules in the mixed monolayer are condensed by mixing with dSA in which the repulsive interactions between positively charged head groups of the DOAC molecules are largely reduced along with an increase of van der Waals interactions with dSA.

The lipid composition and physical properties of the yeast vacuole affect the hemifusion-fusion transition

Yeast vacuole fusion requires the formation of SNARE bundles between membranes. Although the function of vacuolar SNAREs is controlled in part by regulatory lipids, the exact role of the membrane in regulating fusion remains unclear. Because SNAREs are membrane-anchored and transmit the force required for fusion to the bilayer, we hypothesized that the lipid composition and curvature of the membrane aid in controlling fusion. Here, we examined the effect of altering membrane fluidity and curvature on the functionality of fusion-incompetent SNARE mutants that are thought to generate insufficient force to trigger the hemifusion-fusion transition. The hemifusion-fusion transition was inhibited by disrupting the 3Q:1R stoichiometry of SNARE bundles with the mutant SNARE Vam7p(Q283R) . Similarly, replacing the transmembrane domain of the syntaxin homolog Vam3p with a lipid anchor allowed hemifusion, but not content mixing. Hemifusion-stalled reactions containing either of the SNARE mutants were stimulated to fuse with chlorpromazine, an amphipathic molecule that alters membrane fluidity and curvature. The activity of mutant SNAREs was also rescued by the overexpression of SNAREs, thus multiplying the force transferred to the membrane. Thus, we conclude that either increasing membrane fluidity, or multiplying SNARE-generated energy restored the fusogenicity of mutant SNAREs that are stalled at hemifusion. We also found that regulatory lipids differentially modulated the complex formation of wild-type SNAREs. Together, these data indicate that the physical properties and the lipid composition of the membrane affect the function of SNAREs in promoting the hemifusion-fusion transition.

Molecular machines directly observed by high-speed atomic force microscopy

Molecular machines made of proteins are highly dynamic and carry out sophisticated biological functions. The direct and dynamic high-resolution visualization of molecular machines in action is considered to be the most straightforward approach to understanding how they function but this has long been infeasible until recently. High-speed atomic force microscopy has recently been realized, making such visualization possible. The captured images of myosin V, F1-ATPase, and bacteriorhodopsin have enabled their dynamic processes and structure dynamics to be revealed in great detail, giving unique and deep insights into their functional mechanisms.

High-speed atomic force microscopy

High-speed atomic force microscopy (HS-AFM) has been developed as a nano-dynamics visualization technique. This microscopy permits direct observation of structure dynamics and dynamic processes of biological molecules in physiological solutions, at a subsecond to sub-100 ms temporal resolution and an ∼2 nm lateral and a 0.1 nm vertical resolution. Importantly, tip-sample interactions do not disturb the biomolecules' functions. Various functioning proteins including myosin V walking on an actin filament and bacteriorhodopsin responding to light have been successfully visualized with HS-AFM. In the quest for understanding the functional mechanisms of proteins, inferences no longer have to be made from static snapshots of molecular structures and dynamic behavior of optical markers attached to proteins. High-resolution molecular movies obtained from HS-AFM observations reveal the details of molecules' dynamic behavior in action, without the need for intricate analyses and interpretations. In this review, I first describe the fundamentals behind the achieved high imaging rate and low invasiveness to samples, and then highlight recent imaging studies. Finally, future studies are briefly described.

Imaging of transmembrane proteins directly incorporated within supported lipid bilayers using atomic force microscopy

Structural analysis of transmembrane proteins remains a challenge in biology, mainly due to their difficulty in being overexpressed and the required use of detergents that impair different steps of biochemistry classically used to obtain 3D crystals. In this context, we have developed a new technique for protein incorporation within supported lipid bilayers that only requires a few picomoles of protein per assay. Proteins are directly inserted into a detergent-destabilized bilayer that can be imaged in buffer with atomic force microscopy (AFM) allowing structural analysis down to sub-nanometer lateral resolution. In this chapter, we describe the main guidelines for this technique, from the choice of detergent to the requirements for AFM high-resolution imaging.

Substrate Effects on the Formation Process, Structure and Physicochemical Properties of Supported Lipid Bilayers

Supported lipid bilayers are artificial lipid bilayer membranes existing at the interface between solid substrates and aqueous solution. Surface structures and properties of the solid substrates affect the formation process, fluidity, two-dimensional structure and chemical activity of supported lipid bilayers, through the 1–2 nm thick water layer between the substrate and bilayer membrane. Even on SiO₂/Si and mica surfaces, which are flat and biologically inert, and most widely used as the substrates for the supported lipid bilayers, cause differences in the structure and properties of the supported membranes. In this review, I summarize several examples of the effects of substrate structures and properties on an atomic and nanometer scales on the solid-supported lipid bilayers, including our recent reports.

Atomic force microscopy: a versatile tool to probe the physical and chemical properties of supported membranes at the nanoscale

Atomic force microscopy (AFM) was developed in the 1980s following the invention of its precursor, scanning tunneling microscopy (STM), earlier in the decade. Several modes of operation have evolved, demonstrating the extreme versatility of this method for measuring the physicochemical properties of samples at the nanoscopic scale. AFM has proved an invaluable technique for visualizing the topographic characteristics of phospholipid monolayers and bilayers, such as roughness, height or laterally segregated domains. Implemented modes such as phase imaging have also provided criteria for discriminating the viscoelastic properties of different supported lipid bilayer (SLB) regions. In this review, we focus on the AFM force spectroscopy (FS) mode, which enables determination of the nanomechanical properties of membrane models. The interpretation of force curves is presented, together with newly emerging techniques that provide complementary information on physicochemical properties that may contribute to our understanding of the structure and function of biomembranes. Since AFM is an imaging technique, some basic indications on how real-time AFM imaging is evolving are also presented at the end of this paper.

Miniaturised technologies for the development of artificial lipid bilayer systems

Artificially reproducing cellular environments is a key aim of synthetic biology, which has the potential to greatly enhance our understanding of cellular mechanisms. Microfluidic and lab-on-a-chip (LOC) techniques, which enable the controlled handling of sub-microlitre volumes of fluids in an automated and high-throughput manner, can play a major role in achieving this by offering alternative and powerful methodologies in an on-chip format. Such techniques have been successfully employed over the last twenty years to provide innovative solutions for chemical analysis and cell-, molecular- and synthetic- biology. In the context of the latter, the formation of artificial cell membranes (or artificial lipid bilayers) that incorporate membrane proteins within miniaturised LOC architectures offers huge potential for the development of highly sensitive molecular sensors and drug screening applications. The aim of this review is to give a comprehensive and critical overview of the field of microsystems for creating and exploiting artificial lipid bilayers. Advantages and limitations of three of the most popular approaches, namely suspended, supported and droplet-based lipid bilayers, are discussed. Examples are reported that show how artificial cell membrane microsystems, by combining together biological procedures and engineering techniques, can provide novel methodologies for basic biological and biophysical research and for the development of biotechnology tools.

High-speed atomic force microscopy coming of age

High-speed atomic force microscopy (HS-AFM) is now materialized. It allows direct visualization of dynamic structural changes and dynamic processes of functioning biological molecules in physiological solutions, at high spatiotemporal resolution. Dynamic molecular events unselectively appear in detail in an AFM movie, facilitating our understanding of how biological molecules operate to function. This review describes a historical overview of technical development towards HS-AFM, summarizes elementary devices and techniques used in the current HS-AFM, and then highlights recent imaging studies. Finally, future challenges of HS-AFM studies are briefly discussed.

Antimicrobial mechanism of monocaprylate

Monoglyceride esters of fatty acids occur naturally and encompass a broad spectrum of antimicrobial activity. Monocaprylate is generally regarded as safe (GRAS) and can function both as an emulsifier and as a preservative in food. However, knowledge about its mode of action is lacking. The aim of this study was therefore to elucidate the mechanism behind monocaprylate's antimicrobial effect. The cause of cell death in Escherichia coli, Staphylococcus xylosus, and Zygosaccharomyces bailii was investigated by examining monocaprylate's effect on cell structure, membrane integrity, and its interaction with model membranes. Changes in cell structure were visible by atomic force microscopy (AFM), and propidium iodide staining showed membrane disruption, indicating the membrane as a site of action. This indication was confirmed by measuring calcein leakage from membrane vesicles exposed to monocaprylate. AFM imaging of supported lipid bilayers visualized the integration of monocaprylate into the liquid disordered, and not the solid ordered, phase of the membrane. The integration of monocaprylate was confirmed by quartz crystal microbalance measurements, showing an abrupt increase in mass and hydration of the membrane after exposure to monocaprylate above a threshold concentration. We hypothesize that monocaprylate destabilizes membranes by increasing membrane fluidity and the number of phase boundary defects. The sensitivity of cells to monocaprylate will therefore depend on the lipid composition, fluidity, and curvature of the membrane.

Methylation of glycosylated sphingolipid modulates membrane lipid topography and pathogenicity of Cryptococcus neoformans

In previous studies we showed that the replication of Cryptococcus neoformans in the lung environment is controlled by the glucosylceramide (GlcCer) synthase gene (GCS1), which synthesizes the membrane sphingolipid GlcCer from the C9-methyl ceramide. Here, we studied the effect of the mutation of the sphingolipid C9 methyltransferase gene (SMT1), which adds a methyl group to position 9 of the sphingosine backbone of ceramide. The C. neoformans Δsmt1 mutant does not make C9-methyl ceramide and, thus, any methylated GlcCer. However, it accumulates demethylated ceramide and demethylated GlcCer. The Δsmt1 mutant loses more than 80% of its virulence compared with the wild type and the reconstituted strain. Interestingly, growth of C. neoformans Δsmt1 in the lung was decreased and C. neoformans cells were contained in lung granulomas, which significantly reduced the rate of their dissemination to the brain reducing the onset of meningoencephalitis. Thus, using fluorescent spectroscopy and atomic force microscopy we compared the wild type and Δsmt1 mutant and found that the altered membrane composition and GlcCer structure affects fungal membrane rigidity, suggesting that specific sphingolipid structures are required for proper fungal membrane organization and integrity. Therefore, we propose that the physical structure of the plasma membrane imparted by specific classes of sphingolipids represents a critical factor for the ability of the fungus to establish virulence.

Real-time imaging of lipid domains and distinct coexisting membrane protein clusters

A detailed understanding of biomembrane architecture is still a challenging task. Many in vitro studies have shown lipid domains but much less information is known about the lateral organization of membrane proteins because their hydrophobic nature limits the use of many experimental methods. We examined lipid domain formation in biomimetic Escherichia coli membranes composed of phosphatidylethanolamine and phosphatidylglycerol in the absence and presence of 1% and 5% (mol/mol) membrane multidrug resistance protein, EmrE. Monolayer isotherms demonstrated protein insertion into the lipid monolayer. Subsequently, Brewster angle microscopy was applied to image domains in lipid matrices and lipid-protein mixtures. The images showed a concentration dependent impact of the protein on lipid domain size and shape and more interestingly distinct coexisting protein clusters. Whereas lipid domains varied in size (14-47μm), protein clusters exhibited a narrow size distribution (2.6-4.8μm) suggesting a non-random process of cluster formation. A 3-D display clearly indicates that these proteins clusters protrude from the membrane plane. These data demonstrate distinct co-existing lipid domains and membrane protein clusters as the monofilm is being compressed and illustrate the significant mutual impact of lipid-protein interactions on lateral membrane architecture.

Atomic force microscopy studies of a floating-bilayer lipid membrane on a Au(111) surface modified with a hydrophilic monolayer

The surface of a gold electrode was functionalized with a hydrophilic monolayer of 1-thio-β-D-glucose formed by spontaneous self-assembly. The Langmuir-Blodgett/Langmuir-Schaefer (LB/LS) method was then used to assemble a bilayer onto the modified Au(111) surface. The bilayer lipid membrane (BLM) was separated from the Au(111) electrode surface by incorporating the monosialoganglioside GM1 into the inner leaflet of a bilayer composed of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and cholesterol. To make the inner leaflet, monolayers of GM1/DMPC/cholesterol with mole ratios of 1:6:3, 2:5:3, and 3:4:3 were used. The outer leaflet was composed of a 7:3 mole ratio of DMPC/cholesterol. Because of the amphiphilic properties of GM1, the hydrophobic acyl chains were incorporated into the BLM, whereas the large hydrophilic carbohydrate headgroups were physically adsorbed to the Au(111) electrode surface, creating a "floating" BLM (fBLM). This model contained a water-rich reservoir between the BLM and the gold surface. In addition, because of the bilayer being physically adsorbed onto the support, the fluidity of the BLM was maintained. The compression isotherms were measured at the air/water interface to determine the phase behavior and optimal transfer conditions. The images acquired using atomic force microscopy (AFM) and the force-distance measurements showed that the structure of the fBLM evolved with increasing GM1 content from 10 to 30 mol %, undergoing a transition from a corrugated to a homogeneous phase. This change was associated with a significant increase in bilayer thickness (from ∼5.3 to 7.3 nm). The highest-quality fBLM was produced with 30 mol % GM1.

Anomalous diffusion in supported lipid bilayers induced by oxide surface nanostructures

Hierarchic structure and anomalous diffusion on submicrometer scale were introduced into an artificial cell membrane, and the spatiotemporal dependence of lipid diffusion was visualized on nanostructured oxide surfaces. We observed the lipid diffusion in supported lipid bilayers (SLBs) on step-and-terrace TiO(2)(100) and amorphous SiO(2)/Si surfaces by single molecule tracking (SMT) method. The SMT at the time resolution of 500 μs to 30 ms achieved observation of the lipid diffusion over the spatial and temporal ranges of 100 nm/millisecond to 1 μm/second. The temporal dependence of the diffusion coefficient in the SLB on TiO(2)(100) showed that the crossover from anomalous diffusion to random diffusion occurred around 10 ms. The surface fine architecture on substrates will be applicable to induce hierarchic structures on the order of 100 nm or less, which correspond to the microcompartment size in vivo.

Hydration effects on membrane structure probed by single molecule orientations

Single molecule fluorescence measurements are used to probe the structural changes in glass-supported DPPC bilayers as a function of relative humidity (RH). Defocused polarized total internal reflection fluorescence microscopy is employed to determine the three-dimensional orientation of the fluorescent lipid analogue BODIPY-PC, doped into DPPC membranes in trace amounts. Supported DPPC bilayers formed using vesicle fusion and Langmuir-Blodgett/Langmuir-Schäfer (LB/LS) transfer are compared and show similar trends as a function of relative humidity. Population histograms of the emission dipole tilt angle reveal bimodal distributions as observed previously for BODIPY-PC in DPPC. These distributions are dominated by large populations of BODIPY-PC molecules with emission dipoles oriented parallel (≥81°) and normal (≤10°) to the membrane plane, with less than 25% oriented at intermediate tilts. As the relative humidity is increased from 13% to 95%, the population of molecules oriented normal to the surface decreases with a concomitant increase in those oriented parallel to the surface. The close agreement in trends observed for bilayers formed from vesicle fusion and LB/LS transfer supports the assignment of an equivalent surface pressure of 23 mN/m for bilayers formed from vesicle fusion. At each RH condition, a small population of BODIPY-PC dye molecules are laterally mobile in both bilayer preparations. This population exponentially increases with RH but never exceeds 6% of the total population. Interestingly, even under conditions where there is little lateral diffusion, fluctuations in the single molecule orientations can be observed which suggests there is appreciable freedom in the acyl chain region. Dynamic measurements of single molecule orientation changes, therefore, provide a new view into membrane properties at the single molecule level.

Temperature-dependent phase transitions in zeptoliter volumes of a complex biological membrane

Phase transitions in purple membrane have been a topic of debate for the past two decades. In this work we present studies of a reversible transition of purple membrane in the 50-60 °C range in zeptoliter volumes under different heating regimes (global heating and local heating). The temperature of the reversible phase transition is 52 ± 5 °C for both local and global heating, supporting the hypothesis that this transition is mainly due to a structural rearrangement of bR molecules and trimers. To achieve high resolution measurements of temperature-dependent phase transitions, a new scanning probe microscopy-based method was developed. We believe that our new technique can be extended to other biological systems and can contribute to the understanding of inhomogeneous phase transitions in complex systems.

A mitochondrial late embryogenesis abundant protein stabilizes model membranes in the dry state

Late embryogenesis abundant (LEA) proteins are a highly diverse group of polypeptides expected to play important roles in desiccation tolerance of plant seeds. They are also found in other plant tissues and in some anhydrobotic invertebrates, fungi, protists and prokaryotes. The LEA protein LEAM accumulates in the matrix space of pea (Pisum sativum) mitochondria during late seed maturation. LEAM is an intrinsically disordered protein folding into amphipathic alpha-helix upon desiccation. This suggests that it could interact with the inner mitochondrial membrane, providing structural protection in dry seeds. Here, we have used Fourier-transform infrared and fluorescence spectroscopy to gain insight into the molecular details of interactions of LEAM with phospholipid bilayers in the dry state and their effects on liposome stability. LEAM interacted specifically with negatively charged phosphate groups in dry phospholipids, increasing fatty acyl chain mobility. This led to an enhanced stability of liposomes during drying and rehydration, but also upon freezing. Protection depended on phospholipid composition and was strongly enhanced in membranes containing the mitochondrial phospholipid cardiolipin. Collectively, the results provide strong evidence for a function of LEAM as a mitochondrial membrane protectant during desiccation and highlight the role of lipid composition in the interactions between LEA proteins and membranes.