Atomic force microscopy of cholera toxin B-oligomers bound to bilayers of biologically relevant lipids

Atomic force microscopy applied to study macromolecular content of embedded biological material

We demonstrate that atomic force microscopy represents a powerful tool for the estimation of structural preservation of biological samples embedded in epoxy resin, in terms of their macromolecular distribution and architecture. The comparison of atomic force microscopy (AFM) and transmission electron microscopy (TEM) images of a biosample (Caenorhabditis elegans) prepared following to different types of freeze-substitution protocols (conventional OsO4 fixation, epoxy fixation) led to the conclusion that high TEM stainability of the sample results from a low macromolecular density of the cellular matrix. We propose a novel procedure aimed to obtain AFM and TEM images of the same particular organelle, which strongly facilitates AFM image interpretation and reveals new ultrastructural aspects (mainly protein arrangement) of a biosample in addition to TEM data.

In situ scanning probe microscopy studies of tetanus toxin-membrane interactions

Despite the considerable information available with regards to the structure of the clostridial neurotoxins, and their inherent threat as biological warfare agents, the mechanisms underpinning their interactions with and translocation through the cell membrane remain poorly understood. We report herein the results of an in situ scanning probe microscopy study of the interaction of tetanus toxin C-fragment (Tet C) with supported planar lipid bilayers containing the ganglioside receptor G(T1b). Our results show that Tet C preferentially binds to the surface of fluid phase domains within biphasic membranes containing G(T1b) and that with an extended incubation period these interactions lead to dramatic changes in the morphology of the lipid bilayer, including the formation of 40-80 nm diameter circular cavities. Combined atomic force microscopy/total internal reflection fluorescence microscopy experiments confirmed the presence of Tet C in the membrane after extended incubation. These morphological changes were found to be dependent upon the presence of G(T1b) and the solution pH.

Enzyme association with lipidic Langmuir-Blodgett films: interests and applications in nanobioscience

This review presents the recent advances in the achievement of organized proteo-lipidic nanostructures based on Langmuir-Blodgett technology and their potential applications in the nanobioscience area. By using the self-assembled properties of amphiphilic biomolecules at the air-water interface, the Langmuir-Blodgett (LB) technique offers the possibility to prepare ultrathin layers suitable for biomolecule immobilization at the molecular level. This review will provide a general overview of the enzyme association with preformed Langmuir-Blodgett films in connection with their potential applications in biosensing device developments, and then introduce the design of a new functionalised biomimetic nanostructure with oriented recognition site. The potential applications of such an organized proteo-lipidic nanostructure for biocatalysis investigations of an immobilised enzyme in a biomimetic situation and for the development of bioelectronic devices are finally discussed.

Imaging and detecting molecular interactions of single transmembrane proteins

Single-molecule atomic force microscopy (AFM) provides novel ways to characterize structure-function relationships of native membrane proteins. High-resolution AFM-topographs allow observing substructures of single membrane proteins at sub-nanometer resolution as well as their conformational changes, oligomeric state, molecular dynamics and assembly. Complementary to AFM imaging, single-molecule force spectroscopy experiments allow detecting molecular interactions established within and between membrane proteins. The sensitivity of this method makes it possible to detect the interactions that stabilize secondary structures such as transmembrane alpha-helices, polypeptide loops and segments within. Changes in temperature or protein-protein assembly do not change the position of stable structural segments, but influence their stability established by collective molecular interactions. Such changes alter the probability of proteins to choose a certain unfolding pathway. Recent examples have elucidated unfolding and refolding pathways of membrane proteins as well as their energy landscapes. We review current and future potential of these approaches to reveal insights into membrane protein structure, function, and unfolding as we recognize that they could help answering key questions in the molecular basis of certain neuro-pathological dysfunctions.

Watching the components of photosynthetic bacterial membranes and their in situ organisation by atomic force microscopy

The atomic force microscope has developed into a powerful tool in structural biology allowing information to be acquired at submolecular resolution on the protruding structures of membrane proteins. It is now a complementary technique to X-ray crystallography and electron microscopy for structure determination of individual membrane proteins after extraction, purification and reconstitution into lipid bilayers. Moving on from the structures of individual components of biological membranes, atomic force microscopy has recently been demonstrated to be a unique tool to identify in situ the individual components of multi-protein assemblies and to study the supramolecular architecture of these components allowing the efficient performance of a complex biological function. Here, recent atomic force microscopy studies of native membranes of different photosynthetic bacteria with different polypeptide contents are reviewed. Technology, advantages, feasibilities, restrictions and limits of atomic force microscopy for the acquisition of highly resolved images of up to 10 A lateral resolution under native conditions are discussed. From a biological point of view, the new insights contributed by the images are analysed and discussed in the context of the strongly debated organisation of the interconnected network of membrane-associated chlorophyll-protein complexes composing the photosynthetic apparatus in different species of purple bacteria.

Sample preparation procedures for biological atomic force microscopy

Since the late 1980s, atomic force microscopy (AFM) has been increasingly used in biological sciences and it is now established as a versatile tool to address the structure, properties and functions of biological specimens. AFM is unique in that it provides three-dimensional images of biological structures, including biomolecules, lipid films, 2D protein crystals and cells, under physiological conditions and with unprecedented resolution. A crucial prerequisite for successful, reliable biological AFM is that the samples need to be well attached to a solid substrate using appropriate, nondestructive methods. In this review, we discuss common techniques for immobilizing biological specimens for AFM studies.

Measurement of the anchorage force between GPI-anchored alkaline phosphatase and supported membranes by AFM force spectroscopy

Mammalian alkaline phosphatases (AP) are glycosylphosphatidylinositol (GPI) anchored proteins that are localized on the outer layer of the plasma membrane. The GPI anchors are covalently attached to the C-termini of proteins and consist of a glycan chain bonded to phosphatidylinositol with two acyl chains anchored into the membrane bilayer. Force spectroscopy, based on atomic force microscope (AFM) technology, was used to determine the adhesion of alkaline phosphatase in the absence and presence of anchors. The GPI anchors increase markedly the adhesion frequency (i.e., the protein affinity for the membrane). An adhesion force of 350 +/- 200 pN is measured between GPI-anchored AP (AP(GPI)) and supported phospholipid bilayers of dipalmitoylphosphatidylcholine (DPPC) presenting structural defects (holes). In the absence of defects, the adhesion force (103 +/- 17 pN) and the adhesion frequency are reduced. These results indicate that AP(GPI) poorly spontaneously insert into membranes in vivo and open new perspectives for the characterization of the interactions between GPI proteins and membranes.

Patterned supported lipid bilayers and monolayers on poly(dimethylsiloxane)

A simple and practical method for patterning supported lipid bilayers on poly(dimethylsiloxane) is presented. By using electron microscopy grids to laterally control the extent of plasma oxidation, the substrate is partitioned into regions of different hydrophilicities. Addition of vesicles then results in the spontaneous formation of lipid bilayers and monolayers side-by-side on the surface, separated by regions that contain no lipid and/or a region with adhering vesicles. By using millimeter-sized plastic masks we are able to control the formation of these lipid structures on macroscopic patches by simply varying the plasma-cleaning time. For the first time, we are able to influence, in a controlled fashion, the chemical composition of a substrate in such a way that it supports fluid lipid monolayers, rejects lipid adhesion, adsorbs intact lipid vesicles, or supports fluid bilayers.

Manipulating forces between surfaces: applications in colloid science and biophysics

It is the forces between the microscopic constituents of materials which to a large extent determine the macroscopic properties. For example, it is the differences in bonding between the carbon atoms which determines the different physical properties of carbon and graphite. The same is true in colloidal systems. In colloidal systems, there are three common types of long-range interactions between particles: van der Waals forces, electrical double layer forces and steric forces. In this paper, examples as to how these forces can be modified and even manipulated will be given. To convincingly demonstrate these effects, it is necessary to measure these interaction forces. We have achieved this by using the principles of atomic force microscopy (AFM). The principle is simple, a small particle, 5-30 microm, is attached onto a small weak cantilever spring. The interaction between this particle and another particle or a surface is measured by monitoring the deflection of the spring as the two particles are moved together. In this paper, I shall give examples of direct measurements of van der Waals, electrical double layer and steric forces and show how they can be modified and how these modifications affect the properties of bulk suspensions. Similar principles are involved in the interactions of biological materials. However, nature is much cleverer than man such that many of the macromolecules on cell surfaces are able to specifically recognise only one other molecule. An example of this recognition-type interaction, namely, cholera toxin interacting with the glycolipid Gm1, will also be presented. Finally, the adhesion of cells to surfaces of different surface chemistries has been determined; this is of significance in many fields ranging from fouling of filtration membranes on the one hand to the biocompatibility of surgical implants on the other.

Correlation of AFM and SFA measurements concerning the stability of supported lipid bilayers

Phospholipid bilayers were studied by means of atomic force microscopy (AFM) and a surface force apparatus (SFA). The stability of the supported bilayers was described by the amount of irregularities in the topography of the membrane by means of AFM and by the occurrence of hemifusion in the SFA, which is an indicator of defective bilayers. The bilayers, composed of lipids having the same headgroup but different chain lengths in the two leaflets, were prepared by Langmuir-Blodgett deposition and transferred at different surface pressures. The topography of the supported bilayers in aqueous solution, as imaged by AFM, revealed an increasing number of defects in the supported lipid membranes with decreased deposition pressure of the outer lipid layer. These defects, which appeared in the form of monolayer and bilayer (self-assembled) thick holes within the membrane, were energetically favorable over an evenly depleted bilayer. We found that the quantity of these defects (holes of </=0.5 micro m diameter and covering up to 30% of the surface area) correlated well with the stability of the bilayers as measured by SFA, a truly complementary instrument.

An overview of the biophysical applications of atomic force microscopy

The potentialities of the atomic force microscopy (AFM) make it a tool of undeniable value for the study of biologically relevant samples. AFM is progressively becoming a usual benchtop technique. In average, more than one paper is published every day on AFM biological applications. This figure overcomes materials science applications, showing that 17 years after its invention, AFM has completely crossed the limits of its traditional areas of application. Its potential to image the structure of biomolecules or bio-surfaces with molecular or even sub-molecular resolution, study samples under physiological conditions (which allows to follow in situ the real time dynamics of some biological events), measure local chemical, physical and mechanical properties of a sample and manipulate single molecules should be emphasized.

Atomic force microscopy studies of interaction of the 20S proteasome with supported lipid bilayers

The 20S proteasome plays important roles in degradation of intracellular proteins. Mechanisms of its activation, its localization in cells, and its binding to biomembranes are not well understood. In this study, we used atomic force microscopy (AFM) to investigate interactions between the 20S proteasome and supported bilayers of various lipids in a buffer. We found that the 20S proteasome specifically bound to supported bilayers containing phosphatidylinositol (PI), but did not bind to supported bilayers containing phosphatidylcholine, phosphatidic acid or dioleoyltrimethylammonium propane. Binding of the 20S proteasomes had a high orientation; almost all were in a top view position. The specific and orientational binding of the 20S proteasome with PI may play important roles inside cells such as endoplasmic reticulum (ER) membrane. Use of AFM to study supported bilayers provides new information on ligand-receptor interactions.

Growth of cationic lipid toward bilayer lipid membrane by solution spreading: scanning probe microscopy study

The growth of cationic lipid dioctadecyldimethylammonium bromide (DODAB) toward bilayer lipid membrane (BLM) by solution spreading on cleaved mica surface was studied by atomic force microscopy (AFM). Bilayer of DODAB was formed by exposing mica to a solution of DODAB in chloroform and subsequently immersing into potassium chloride solution for film developing. AFM studies showed that at the initial stage of the growth, the adsorbed molecules exhibited the small fractal-like aggregates. These aggregates grew up and expanded laterally into larger patches with time and experienced from monolayer to bilayer, finally a close-packed bilayer film (5.4+/-0.2 nm) was approached. AFM results of the film growth process indicated a growth mechanism of nucleation, growth and coalescence of dense submonolayer, it revealed the direct information about the film morphology and confirmed that solution spreading was an effective technique to prepare a cationic bilayer in a short time.

Material studies of lipid vesicles in the L(alpha) and L(alpha)-gel coexistence regimes

In this work, we utilize micropipette aspiration and fluorescence imaging to examine the material properties of lipid vesicles made from mixtures of palmitoyloleoylphosphocholine (POPC) and dipalmitoylphosphatidylcholine (DPPC). At elevated temperatures/low DPPC fractions, these lipids are in a miscible liquid crystalline (L(alpha)) state, whereas at lower temperatures/higher DPPC fractions they phase-separate into L(alpha) and gel phases. We show that the elastic modulus, K, and critical tension, tau(c), of L(alpha) vesicles are independent of DPPC fraction. However, as the sample temperature is increased from 15 degrees C to 45 degrees C, we measure decreases in both K and tau(c) of 20% and 50%, respectively. The elasticity change is likely driven by a change in interfacial tension. We describe the reduction in critical tension using a simple model of thermally activated membrane pores. Vesicles with two-phase coexistence exhibit material properties that differ from L(alpha) vesicles including critical tensions that are 20-40% lower. Fluorescence imaging of phase coexistent POPC/DPPC vesicles shows that the DPPC-rich domains exist in an extended network structure that exhibits characteristics of a solid. This gel network explains many of the unusual material properties of two-phase membranes.

Supported lipid bilayers as effective substrates for atomic force microscopy

Supported lipid bilayers offer a diverse set of substrates for AFM investigations of both water-soluble samples and integral membrane proteins. Although their amphipathic nature is necessary for the latter specimens, it is their robustness and easily changed surface characteristics that make these surfaces particularly attractive for the former ones. When starting any AFM investigation of water-soluble biological complexes, it is probably best to try mica first, owing to its remarkable effectiveness with a wide range of samples. Yet, if mica should prove inadequate, supported lipid bilayers are, as demonstrated here, a sensible second choice.

A relocated technique of atomic force microscopy (AFM) samples and its application in molecular biology

A kind of simple atomic force microscopy (AFM) relocated technique, which takes advantage of homemade sample locator system, is used for investigating repeatedly imaging of some specific species on the whole substrate (over 1 x 1 cm2) with resolution about 400 nm. As applications of this sample locator system, single extended DNA molecules and plasmid DNA network are shown in different AFM operational modes: tapping mode and contact mode with different tips after the substrates have been moved.

Placental alkaline phosphatase is efficiently targeted to rafts in supported lipid bilayers

Evidence is growing that biological membranes contain lipid microdomains or "rafts" that may be involved in processes such as cellular signaling and protein trafficking. In this study, we have used atomic force microscopy to examine the behavior of rafts in supported lipid bilayers. We show that bilayers composed of equimolar dioleoylphosphatidylcholine and sphingomyelin spontaneously form rafts, which are detectable as raised features. A comparison of the extents of protrusion of the rafts in monolayers and bilayers indicates that the rafts in the two leaflets of the bilayer coincide. The rafts were observed both in the absence and presence of cholesterol (33 mol %). Cholesterol reduced raft protrusion presumably by increasing the thickness of the non-raft bilayer. PLAP (glycosylphosphatidylinositol-anchored protein placental alkaline phosphatase) was purified and shown to exist as a dimer. Following its incorporation into supported lipid bilayers, PLAP was found to be targeted efficiently to rafts, both in the absence and presence of cholesterol. We suggest that atomic force microscopy provides a powerful tool for the study of raft structure and properties.

The size of lipid rafts: an atomic force microscopy study of ganglioside GM1 domains in sphingomyelin/DOPC/cholesterol membranes

Atomic force microscopy has been used to study the distribution of ganglioside GM1 in model membranes composed of ternary lipid mixtures that mimic the composition of lipid rafts. The results demonstrate that addition of 1% GM1 to 1:1:1 sphingomyelin/dioleoylphosphatidylcholine/cholesterol monolayers leads to the formation of small ganglioside-rich microdomains (40-100 nm in size) that are localized preferentially in the more ordered sphingomyelin/cholesterol-rich phase. With 5% GM1 some GM1 microdomains are also detected in the dioleoylphosphatidylcholine-rich phase. A similar preferential localization of GM1 in the ordered phase is observed for bilayers with the same ternary lipid mixture in the upper leaflet. The small GM1-rich domains observed in these experiments are similar to the sizes for lipid rafts in natural membranes but considerably smaller than the ordered bilayer domains that have been shown to be enriched in GM1 in recent fluorescence microscopy studies of lipid bilayers. The combined data from a number of studies of model membranes indicate that lateral organization occurs on a variety of length scales and mimics many of the properties of natural membranes.

Observing structure, function and assembly of single proteins by AFM

Single molecule experiments provide insight into the individuality of biological macromolecules, their unique function, reaction pathways, trajectories and molecular interactions. The exceptional signal-to-noise ratio of the atomic force microscope allows individual proteins to be imaged under physiologically relevant conditions at a lateral resolution of 0.5-1nm and a vertical resolution of 0.1-0.2nm. Recently, it has become possible to observe single molecule events using this technique. This capability is reviewed on various water-soluble and membrane proteins. Examples of the observation of function, variability, and assembly of single proteins are discussed. Statistical analysis is important to extend conclusions derived from single molecule experiments to protein species. Such approaches allow the classification of protein conformations and movements. Recent developments of probe microscopy techniques allow simultaneous measurement of multiple signals on individual macromolecules, and greatly extend the range of experiments possible for probing biological systems at the molecular level. Biologists exploring molecular mechanisms will benefit from a burgeoning of scanning probe microscopes and of their future combination with molecular biological experiments.

Scanning force microscopy in the applied biological sciences

Fifteen years after its invention, the scanning force microscope (SFM) is rooted deep in the biological sciences. Here we discuss the use of SFM in biotechnology and biomedical research. The spectrum of applications reviewed includes imaging, force spectroscopy and mapping, as well as sensor applications. It is our hope that this review will be useful for researchers considering the use of SFM in their studies but are uncertain about its scope of capabilities. For the benefit of readers unfamiliar with SFM technology, the fundamentals of SFM imaging and force measurement are also briefly introduced.

Imaging domains in model membranes with atomic force microscopy

Lateral segregation in biomembranes can lead to the formation of biologically functional domains. This paper reviews atomic force microscopy studies on domain formation in model membranes, with special emphasis on transbilayer asymmetry, and on lateral domains induced by lipid-lipid interactions or by peptide-lipid interactions.

Atomic force microscopy studies of ganglioside GM1 domains in phosphatidylcholine and phosphatidylcholine/cholesterol bilayers

The distribution of ganglioside in supported lipid bilayers has been studied by atomic force microscopy. Hybrid dipalmitoylphosphatidylcholine (DPPC)/dipalmitoylphosphatidylethanolamine (DPPE) and (2:1 DPPC/cholesterol)/DPPE bilayers were prepared using the Langmuir Blodgett technique. Egg PC and DPPC bilayers were prepared by vesicle fusion. Addition of ganglioside GM1 to each of the lipid bilayers resulted in the formation of heterogeneous surfaces that had numerous small raised domains (30--200 nm in diameter). Incubation of these bilayers with cholera toxin B subunit resulted in the detection of small protein aggregates, indicating specific binding of the protein to the GM1-rich microdomains. Similar results were obtained for DPPC, DPPC/cholesterol, and egg PC, demonstrating that the overall bilayer morphology was not dependent on the method of bilayer preparation or the fluidity of the lipid mixture. However, bilayers produced by vesicle fusion provided evidence for asymmetrically distributed GM1 domains that probably reflect the presence of ganglioside in both inner and outer monolayers of the initial vesicle. The results are discussed in relation to recent inconsistencies in the estimation of sizes of lipid rafts in model and natural membranes. It is hypothesized that small ganglioside-rich microdomains may exist within larger ordered domains in both natural and model membranes.

Visualizing detergent resistant domains in model membranes with atomic force microscopy

Evidence is accumulating that in cell membranes microdomains exist, also referred to as rafts or detergent resistant membranes. In this study, atomic force microscopy is used to study supported lipid bilayers, consisting of a fluid phosphatidylcholine, sphingomyelin and cholesterol. Domains were visualized of which the morphology and size depended on the cholesterol concentration. The presence of cholesterol was found to induce bilayer coupling. At 30 mol% cholesterol, a change in percolation phase was observed, and at 50 mol%, when both fluid lipids and solid lipids are saturated with cholesterol, phase separation was still observed. In addition, we were able to directly visualize the resistance of domains against non-ionic detergent.

Cell adhesion to protein-micropatterned-supported lipid bilayer membranes

A new method for constructing controlled interfaces between cells and synthetic supported lipid bilayer membranes is reported. Microcontact printing is used to define squares and grid lines of fibronectin onto glass, which subsequently direct the self-assembly of fluid lipid bilayers onto the complementary, uncoated regions of the surface. Features of fibronectin as small as 5 microm effectively control the lateral organization of the lipid bilayers. These fibronectin barriers also facilitate the adhesion of endothelial cells, which exhibit minimal adhesion to fluid supported lipid bilayers alone. Cells selectively adhere to the features of fibronectin, spanning over and exposing the cells to the intervening regions of supported lipid bilayer. Cell spreading is correlated with both the geometry and dimensions of the fibronectin barriers. Importantly, lipids underlying adherent cells are laterally mobile, suggesting that, in contrast to the regions of fibronectin, cells were not in direct contact with the supported membrane. Protein micropatterning thus provides a valuable tool for controlling supported membranes and for juxtaposing anchorage-dependent cells with lipid bilayers. These systems should be generally useful for studying specific interactions between cells and biomolecules incorporated into supported membranes, and as an approach for integrating living cells with synthetic, laterally complex surfaces.

Imaging molecular structure and physiological function of gap junctions and hemijunctions by multimodal atomic force microscopy

Gap junctions are specialized plasma membrane structures that join neighboring cells via specialized intercellular ion channels (hemichannels) and provide a direct pathway for cell-cell communication. They presumably mediate regulation of growth, transmission of developmental signals, coordination of muscle contraction, and maintenance of metabolic homeostasis. Hemichannels are also present in the non-junctional regions of the cell plasma membrane and they provide a direct pathway for communication between the cytoplasm and the extracellular region. Recent studies suggest that gap junctional communication is much more complex than previously anticipated, in terms of both its structure as well as its activity. While the mechanism of gap junction activity is being studied extensively, their quaternary structure, assembly, and conformational changes underlying gating of their activity as well as their physiological role are poorly understood because, due to their complex structure, these junctions are less amenable to existing techniques for high-resolution three-dimensional structure-function analyses. Atomic Force Microscopy (AFM) images molecular structure of biological specimens in an aqueous environment, allows on-line perturbations, and can be coupled with electrophysiological, biochemical, and other microscopic techniques. The present review examines the potential of AFM application for the study of the molecular structure of hydrated, native gap junctions and hemijunctions as well as their physiological functions. Special attention is paid to new, complementary, or provocative findings from AFM studies of both vertebrate and invertebrate gap junctions and hemijunctions.

Advances in the characterization of supported lipid films with the atomic force microscope

During the past decade, the atomic force microscope (AFM) has become a key technique in biochemistry and biophysics to characterize supported lipid films, as testified by the continuous growth in the number of papers published in the field. The unique capabilities of AFM are: (i) capacity to probe, in real time and in aqueous environment, the surface structure of lipid films; (ii) ability to directly measure physical properties at high spatial resolution; (iii) possibility to modify the film structure and biophysical processes in a controlled way. Such experiments, published up to June 2000, are the focus of the present review. First, we provide a general introduction on the preparation and characterization of supported lipid films as well as on the principles of AFM. The section 'Structural properties' focuses on the various applications of AFM for characterizing the structure of supported lipid films: visualization of molecular structure, formation of structural defects, effect of external agents, formation of supported films, organization of phase-separated films (coexistence region, mixed films) and, finally, the use of supported lipid bilayers for anchoring biomolecules such as DNA, enzymes and crystalline protein arrays. The section 'Physical properties' introduces the principles of force measurements by AFM, interpretation of these measurements and their recent application to supported lipid films and related structures. Finally, we highlight the major achievements brought by the technique and some of the current limitations.

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.

Surface topography of the p3 and p6 annexin V crystal forms determined by atomic force microscopy

Annexin V is a member of a family of structurally homologous proteins sharing the ability to bind to negatively charged phospholipid membranes in a Ca(2+)-dependent manner. The structure of the soluble form of annexin V has been solved by X-ray crystallography, while electron crystallography of two-dimensional (2D) crystals has been used to reveal the structure of its membrane-bound form. Two 2D crystal forms of annexin V have been reported to date, with either p6 or p3 symmetry. Atomic force microscopy has previously been used to investigate the growth and the topography of the p6 crystal form on supported phospholipid bilayers (Reviakine et al., 1998). The surface structure of the second crystal form, p3, is presented in this study, along with an improved topographic map of the p6 crystal form. The observed topography is correlated with the structure determined by X-ray crystallography.

Reversible loss of crystallinity on photobleaching purple membrane in the presence of hydroxylamine

Structural changes of purple membrane during photobleaching in the presence of hydroxylamine were monitored using atomic force microscopy (AFM). The process of bleaching was associated with the disassembly of the purple membrane crystal into smaller crystals. Imaging steps of the photobleaching progress showed that disassembly proceeds until the sample is fully bleached and its crystallinity is almost lost. As revealed from high resolution AFM topographs, the loss of crystallinity was initiated by loss of lattice forming contact between the individual bacteriorhodopsin trimers. The bacteriorhodopsin molecules, however, remained assembled into trimers during the entire photobleaching process. Regeneration of the photobleached sample into intact purple membrane resulted in the reassembly of the bacteriorhodopsin trimers into the trigonal lattice of purple membrane. The data provide novel insights into factors triggering purple membrane formation and structure.

Atomic force microscopy and electron microscopy analysis of retrovirus Gag proteins assembled in vitro on lipid bilayers

We have used an in vitro system that mimics the assembly of immature Moloney murine leukemia virus (M-MuLV) particles to examine how viral structural (Gag) proteins oligomerize at membrane interfaces. Ordered arrays of histidine-tagged Moloney capsid protein (his-MoCA) were obtained on membrane bilayers composed of phosphatidylcholine (PC) and the nickel-chelating lipid 1, 2-di-O-hexadecyl-sn-glycero-3-(1'-2"-R-hydroxy-3'N-(5-amino-1-carboxy pentyl)iminodiacetic acid)propyl ether (DHGN). The membrane-bound arrays were analyzed by electron microscopy (EM) and atomic force microscopy (AFM). Two-dimensional projection images obtained by EM showed that bilayer-bound his-MoCA proteins formed cages surrounding different types of protein-free cage holes with similar cage holes spaced at 81.5-A distances and distances between dissimilar cage holes of 45.5 A. AFM images, showing topological features viewed near the membrane-proximal domain of the his-MoCA protein, revealed a cage network of only symmetrical hexamers spaced at 79-A distances. These results are consistent with a model in which dimers constitute structural building blocks and where membrane-proximal and distal his-MoCA regions interact with different partners in membrane-bound arrays.

Nanometer scale organization of mixed surfactin/phosphatidylcholine monolayers

Mixed monolayers of the surface-active lipopeptide surfactin-C(15) and of dipalmitoyl phosphatidylcholine (DPPC) were deposited on mica and their nanometer scale organization was investigated using atomic force microscopy (AFM) and x-ray photoelectron spectroscopy (XPS). AFM topographic images revealed phase separation for mixed monolayers prepared at 0.1, 0.25, and 0.5 surfactin molar ratios. This was in agreement with the monolayer properties at the air-water interface indicating a tendency of the two compounds to form bidimensional domains in the mixed systems. The step height measured between the surfactin and the DPPC domains was 1.2 +/- 0.1 nm, pointing to a difference in molecular orientation: while DPPC had a vertical orientation, the large peptide ring of surfactin was lying on the mica surface. The N/C atom concentration ratios obtained by XPS for pure monolayers were compatible with two distinct geometric models: a random layer for surfactin and for DPPC, a layer of vertically-oriented molecules in which the polar headgroups are in contact with mica. XPS data for mixed systems were accounted for by a combination of the two pure monolayers, considering respective surface coverages that were in excellent agreement with those measured by AFM. These results illustrate the complementarity of AFM and XPS to directly probe the molecular organization of multicomponent monolayers.

Blistering of langmuir-blodgett bilayers containing anionic phospholipids as observed by atomic force microscopy

Asymmetric bilayers of different phospholipid compositions have been prepared by the Langmuir-Blodgett (L-B) method, and imaged by atomic force microscopy (AFM). Such bilayers can function as a model for biological membranes. The first leaflet consisted of zwitterionic phospholipids phosphatidylcholine (PC) or phosphatidylethanolamine (PE). The second leaflet consisted of the anionic phospholipid phosphatidylglycerol (PG), in either the condensed or liquid phase or, for comparison, of PC. Different bilayers showed different morphology. In all bilayers defects in the form of holes were present. In some bilayers with a first leaflet consisting of PC, polygonal line-shaped defects were observed, whereas when the first leaflet consisted of PE, mainly round defects were seen. Not only the shape, but also the amount of defects varied, depending on the condition and the composition of the second leaflet. In most of the PG-containing systems the defects were surrounded by elevations, which reversibly disappeared in the presence of divalent cations. This is the first time that such elevations have been observed on phospholipid bilayers. We propose that they are induced by phospholipid exchange between the two leaflets around the defects, leading to the presence of negatively charged phospholipids in the first leaflet. Because the substrate is also negatively charged, the bilayer around the edges is repelled and lifted up. Since it was found that the elevations are indeed detached from the substrate, we refer to this effect as bilayer blistering.

Images of oligomeric Kv beta 2, a modulatory subunit of potassium channels

The Shaker type voltage-gated potassium (K+) channel consists of four pore-forming Kv alpha subunits. The channel expression and kinetic properties can be modulated by auxiliary hydrophilic Kv beta subunits via formation of heteromultimeric Kv alpha-Kv beta complexes. Because each (Kv alpha)4 could recruit more than one Kv beta subunit and different Kv beta subunits could potentially interact, the stoichiometry of alpha-beta and beta-beta complexes is therefore critical for understanding the functional regulation of Shaker type potassium channels. We expressed and purified Kv beta 2 subunit in Sf9 insect cells. The purified Kv beta 2, examined by atomic force and electron microscopy techniques, is found predominately as a square-shaped tetrameric complex with side dimensions of 100 x 100 A2 and height of 51 A. Thus, Kv beta 2 is capable of forming a tetramer in the absence of pore-forming alpha subunits. The center of the Kv beta 2 complex was observed to be the most heavily stained region, suggesting that this region could be part of an extended tubular structure connecting the inner mouth of the ion permeation pathway to the cytoplasmic environment.

Electrostatically balanced subnanometer imaging of biological specimens by atomic force microscope

To achieve high-resolution topographs of native biological macromolecules in aqueous solution with the atomic force microscope (AFM) interactions between AFM tip and sample need to be considered. Short-range forces produce the submolecular information of high-resolution topographs. In contrast, no significant high-resolution information is provided by the long-range electrostatic double-layer force. However, this force can be adjusted by pH and electrolytes to distribute the force applied to the AFM tip over a large sample area. As demonstrated on fragile biological samples, adjustment of the electrolyte solution results in a local reduction of both vertical and lateral forces between the AFM tip and proteinous substructures. Under such electrostatically balanced conditions, the deformation of the native protein is minimized and the sample surface can be reproducibly contoured at a lateral resolution of 0.6 nm.

Watching amyloid fibrils grow by time-lapse atomic force microscopy

Late-onset diabetes is typically associated with amyloid deposits of fibrillar amylin in the pancreatic islets. Aqueous synthetic human amylin spontaneously forms polymorphic fibrils in vitro, and this system was used to examine the dynamics of fibril assembly. By time-lapse atomic force microscopy (AFM), the growth of individual amylin fibrils on a mica surface was observed over several hours. Prominent was the assembly of a protofibril with an elongation rate in these experiments of 1.1(+/-0.5) nm/minute. The assembly of higher order polymorphic fibrils was also observed. Growth of the protofibrils was bidirectional, i.e. it occurred by elongation at both ends. This ability of AFM to continuously monitor growth, directionality, and changes in morphology for individual fibrils, provides a significant advantage over spectroscopy-based bulk methods which average the growth of many fibrils and typically require 100 to 1000-fold more protein. The time-lapse AFM procedure used for human amylin here is thus likely to be applicable to fibril formation from other amyloid proteins and peptides.

Filipin-induced lesions in planar phospholipid bilayers imaged by atomic force microscopy

Filipin is a macrolide polyene with antifungal activity belonging to the same family of antibiotics as amphotericin B and nystatin. Despite the spectroscopy and electron microscopy studies of its interaction with natural membranes and membrane model systems, several aspects of its biochemical action, such as the role of membrane sterols, remain to be completely understood. We have used atomic force microscopy (AFM) to study the effect of filipin on dipalmitoylphosphatidylethanolamine bilayers in the presence and absence of cholesterol. The bilayers were prepared by Langmuir-Blodgett deposition over mica and imaged under water. It was shown that filipin-induced lesions could only be found in membranes with cholesterol. In close agreement with electron microscopy results, we have reported the presence of densely packed circular protrusions in the membrane with a mean diameter of 19 nm (corrected for convolution with AFM tip) and 0.4 nm height. Larger circular protrusions (90 nm diameter and 2.5 nm height) and doughnut-shaped lesions were also detected. These results demonstrate that filipin-induced lesions in membranes previously observed by electron microscopy are not biased by artifacts resulting from sample preparation. Filipin aggregates in aqueous solution could also be imaged for the first time. These polydisperse spherical structures were observed in samples with and without cholesterol.

Growth of Protein 2-D Crystals on Supported Planar Lipid Bilayers Imaged in Situ by AFM

Theories of crystallization, both in 3-D and 2-D, are still very limited, mainly due to the scarcity of experimental approaches providing pertinent data on elementary phenomena. We present here a novel experimental approach for following, in real time and in situ, the process of 2-D crystallization of proteins on solid supports. Using annexin V as a model of a protein binding by affinity to a lipid matrix, we show that 2-D crystals of proteins can be formed on supported planar lipid bilayers (SPBs). Atomic Force Microscopy (AFM) enables the process of 2-D crystal growth to be visualized. The submolecular organization of the crystals was characterized at a resolution of approximately 2 nanometers, and defects, hitherto not observed in protein crystals, were resolved. These results have potential applications in basic and applied sciences. Copyright 1998 Academic Press.