The effect of S-layer protein adsorption and crystallization on the collective motion of a planar lipid bilayer studied by dynamic light scattering

Dynamic Light Scattering Measurements for Soft Materials on Solid Substrates: Employing Evanescent-wave Illumination and Dark-field Collection with a High Numerical Aperture Microscope Objective

We developed an instrument that allows us to measure dynamic light scattering from soft materials on solid substrates by avoiding strong background due to the reflection light from substrates. In the instrument, samples on substrates are illuminated by evanescent-light field and the resultant scattered light from the samples is collected with a dark-field optical configuration by employing a high numerical aperture microscope objective. We applied the instrument to measure the dynamic properties of supported lipid bilayers (SLBs), which have been widely utilized in industries as functional materials such as biosensors. From the time course of the scattered light from the SLBs, the power spectrum with the broad peak ranging from 10 to 20 kHz is observed. The use of the microscope objectives enables us to apply the instrument to future light scattering imaging for dynamic properties of soft materials supported on various substrates by combining with conventional microscope systems.

Nanotechnology with S-layer Proteins

Nanosciences are distinguished by the cross-fertilization of biology, chemistry, material sciences, and solid-state physics and, hence, open up a great variety of new opportunities for innovation. The technological utilization of self-assembly systems, wherein molecules spontaneously associate under equilibrium conditions into reproducible supramolecular structures, is one key challenge in nanosciences for life and non-life science applications. The attractiveness of such processes is due to their ability to build uniform, ultra-small functional units with predictable properties down to the nanometer scale. Moreover, newly developed techniques and methods open up the possibility to exploit these structures at meso- and macroscopic scale. An immense significance at innovative approaches for the self-assembly of supramolecular structures and devices with dimensions of a few to tens of nanometers constitutes the utilization of crystalline bacterial cell surface proteins. The latter have proven to be particularly suited as building blocks in a molecular construction kit comprising of all major classes of biological molecules. The controlled immobilization of biomolecules in an ordered fashion on solid substrates and their directed confinement in definite areas of nanometer dimensions are key requirements for many applications including the development of bioanalytical sensors, biochips, molecular electronics, biocompatible surfaces, and signal processing between functional membranes, cells, and integrated circuits.

Variable Lysozyme Transport Dynamics on Oxidatively Functionalized Polystyrene Films

Tuning protein adsorption dynamics at polymeric interfaces is of great interest to many biomedical and material applications. Functionalization of polymer surfaces is a common method to introduce application-specific surface chemistries to a polymer interface. In this work, single-molecule fluorescence microscopy is utilized to determine the adsorption dynamics of lysozyme, a well-studied antibacterial protein, at the interface of polystyrene oxidized via UV exposure and oxygen plasma and functionalized by ligand grafting to produce varying degrees of surface hydrophilicity, surface roughness, and induced oxygen content. Single-molecule tracking indicates lysozyme loading capacities, and surface mobility at the polymer interface is hindered as a result of all functionalization techniques. Adsorption dynamics of lysozyme depend on the extent and the specificity of the oxygen functionalities introduced to the polystyrene surface. Hindered adsorption and mobility are dominated by hydrophobic effects attributed to water hydration layer formation at the functionalized polystyrene surfaces.

Probing peptide and protein insertion in a biomimetic S-layer supported lipid membrane platform

The most important aspect of synthetic lipid membrane architectures is their ability to study functional membrane-active peptides and membrane proteins in an environment close to nature. Here, we report on the generation and performance of a biomimetic platform, the S-layer supported lipid membrane (SsLM), to investigate the structural and electrical characteristics of the membrane-active peptide gramicidin and the transmembrane protein α-hemolysin in real-time using a quartz crystal microbalance with dissipation monitoring in combination with electrochemical impedance spectroscopy. A shift in membrane resistance is caused by the interaction of α-hemolysin and gramicidin with SsLMs, even if only an attachment onto, or functional channels through the lipid membrane, respectively, are formed. Moreover, the obtained results did not indicate the formation of functional α-hemolysin pores, but evidence for functional incorporation of gramicidin into this biomimetic architecture is provided.

S-layers: principles and applications

Monomolecular arrays of protein or glycoprotein subunits forming surface layers (S-layers) are one of the most commonly observed prokaryotic cell envelope components. S-layers are generally the most abundantly expressed proteins, have been observed in species of nearly every taxonomical group of walled bacteria, and represent an almost universal feature of archaeal envelopes. The isoporous lattices completely covering the cell surface provide organisms with various selection advantages including functioning as protective coats, molecular sieves and ion traps, as structures involved in surface recognition and cell adhesion, and as antifouling layers. S-layers are also identified to contribute to virulence when present as a structural component of pathogens. In Archaea, most of which possess S-layers as exclusive wall component, they are involved in determining cell shape and cell division. Studies on structure, chemistry, genetics, assembly, function, and evolutionary relationship of S-layers revealed considerable application potential in (nano)biotechnology, biomimetics, biomedicine, and synthetic biology.

Reassembly of S-layer proteins

Crystalline bacterial cell surface layers (S-layers) represent the outermost cell envelope component in a broad range of bacteria and archaea. They are monomolecular arrays composed of a single protein or glycoprotein species and represent the simplest biological membranes developed during evolution. They are highly porous protein mesh works with unit cell sizes in the range of 3 to 30 nm, and pore sizes of 2 to 8 nm. S-layers are usually 5 to 20 nm thick (in archaea, up to 70 nm). S-layer proteins are one of the most abundant biopolymers on earth. One of their key features, and the focus of this review, is the intrinsic capability of isolated native and recombinant S-layer proteins to form self-assembled mono- or double layers in suspension, at solid supports, the air-water interface, planar lipid films, liposomes, nanocapsules, and nanoparticles. The reassembly is entropy-driven and a fascinating example of matrix assembly following a multistage, non-classical pathway in which the process of S-layer protein folding is directly linked with assembly into extended clusters. Moreover, basic research on the structure, synthesis, genetics, assembly, and function of S-layer proteins laid the foundation for their application in novel approaches in biotechnology, biomimetics, synthetic biology, and nanotechnology.

Biomimetic interfaces based on S-layer proteins, lipid membranes and functional biomolecules

Designing and utilization of biomimetic membrane systems generated by bottom-up processes is a rapidly growing scientific and engineering field. Elucidation of the supramolecular construction principle of archaeal cell envelopes composed of S-layer stabilized lipid membranes led to new strategies for generating highly stable functional lipid membranes at meso- and macroscopic scale. In this review, we provide a state-of-the-art survey of how S-layer proteins, lipids and polymers may be used as basic building blocks for the assembly of S-layer-supported lipid membranes. These biomimetic membrane systems are distinguished by a nanopatterned fluidity, enhanced stability and longevity and, thus, provide a dedicated reconstitution matrix for membrane-active peptides and transmembrane proteins. Exciting areas in the (lab-on-a-) biochip technology are combining composite S-layer membrane systems involving specific membrane functions with the silicon world. Thus, it might become possible to create artificial noses or tongues, where many receptor proteins have to be exposed and read out simultaneously. Moreover, S-layer-coated liposomes and emulsomes copying virus envelopes constitute promising nanoformulations for the production of novel targeting, delivery, encapsulation and imaging systems.

Characterizing the S-layer structure and anti-S-layer antibody recognition on intact Tannerella forsythia cells by scanning probe microscopy and small angle X-ray scattering

Tannerella forsythia is among the most potent triggers of periodontal diseases, and approaches to understand underlying mechanisms are currently intensively pursued. A ~22-nm-thick, 2D crystalline surface (S-) layer that completely covers Tannerella forsythia cells is crucially involved in the bacterium-host cross-talk. The S-layer is composed of two intercalating glycoproteins (TfsA-GP, TfsB-GP) that are aligned into a periodic lattice. To characterize this unique S-layer structure at the nanometer scale directly on intact T. forsythia cells, three complementary methods, i.e., small-angle X-ray scattering (SAXS), atomic force microscopy (AFM), and single-molecular force spectroscopy (SMFS), were applied. SAXS served as a difference method using signals from wild-type and S-layer-deficient cells for data evaluation, revealing two possible models for the assembly of the glycoproteins. Direct high-resolution imaging of the outer surface of T. forsythia wild-type cells by AFM revealed a p4 structure with a lattice constant of ~9.0 nm. In contrast, on mutant cells, no periodic lattice could be visualized. Additionally, SMFS was used to probe specific interaction forces between an anti-TfsA antibody coupled to the AFM tip and the S-layer as present on T. forsythia wild-type and mutant cells, displaying TfsA-GP alone. Unbinding forces between the antibody and wild-type cells were greater than with mutant cells. This indicated that the TfsA-GP is not so strongly attached to the mutant cell surface when the co-assembling TfsB-GP is missing. Altogether, the data gained from SAXS, AFM, and SMFS confirm the current model of the S-layer architecture with two intercalating S-layer glycoproteins and TfsA-GP being mainly outwardly oriented.

Self-assembly of "S-bilayers", a step toward expanding the dimensionality of S-layer assemblies

Protein-based assemblies with ordered nanometer-scale features in three dimensions are of interest as functional nanomaterials but are difficult to generate. Here we report that a truncated S-layer protein assembles into stable bilayers, which we characterized using cryogenic-electron microscopy, tomography, and X-ray spectroscopy. We find that emergence of this supermolecular architecture is the outcome of hierarchical processes; the proteins condense in solution to form 2-D crystals, which then stack parallel to one another to create isotropic bilayered assemblies. Within this bilayered structure, registry between lattices in two layers was disclosed, whereas the intrinsic symmetry in each layer was altered. Comparison of these data to images of wild-type SbpA layers on intact cells gave insight into the interactions responsible for bilayer formation. These results establish a platform for engineering S-layer assemblies with 3-D architecture.

Nanotechnology with s-layer proteins

Nanosciences are distinguished by the cross-fertilization of biology, chemistry, material sciences, and solid-state physics and hence open up a great variety of new opportunities for innovation. The technological utilization of self-assembly systems, wherein molecules spontaneously associate under equilibrium conditions into reproducible supramolecular aggregates, is one key challenge in nanosciences for life and nonlife science applications. The attractiveness of such processes is due to their ability to build uniform, ultrasmall functional units and the possibility to exploit such structures at meso- and macroscopic scale very frequently by newly developed techniques and methods. By the utilization of crystalline bacterial cell-surface proteins (S-layer proteins) innovative approaches for the assembly of supramolecular structures and devices with dimensions of a few to tens of nanometers have been developed. S-layers have proven to be particularly suited as building blocks in a molecular construction kit involving all major classes of biological molecules. The controlled immobilization of biomolecules in an ordered fashion on solid substrates and their controlled confinement in definite areas of nanometer dimensions are key requirements for many applications including the development of bioanalytical sensors, biochips, molecular electronics, biocompatible surfaces, and signal processing between functional membranes, cells, and integrated circuits.

A review of protein adsorption on bioceramics

Bioceramics, because of its excellent biocompatible and mechanical properties, has always been considered as the most promising materials for hard tissue repair. It is well know that an appropriate cellular response to bioceramics surfaces is essential for tissue regeneration and integration. As the in vivo implants, the implanted bioceramics are immediately coated with proteins from blood and body fluids, and it is through this coated layer that cells sense and respond to foreign implants. Hence, the adsorption of proteins is critical within the sequence of biological activities. However, the biological mechanisms of the interactions of bioceramics and proteins are still not well understood. In this review, we will recapitulate the recent studies on the bioceramic-protein interactions.

Nanobiotechnology with S-layer proteins as building blocks

One of the key challenges in nanobiotechnology is the utilization of self- assembly systems, wherein molecules spontaneously associate into reproducible aggregates and supramolecular structures. In this contribution, we describe the basic principles of crystalline bacterial surface layers (S-layers) and their use as patterning elements. The broad application potential of S-layers in nanobiotechnology is based on the specific intrinsic features of the monomolecular arrays composed of identical protein or glycoprotein subunits. Most important, physicochemical properties and functional groups on the protein lattice are arranged in well-defined positions and orientations. Many applications of S-layers depend on the capability of isolated subunits to recrystallize into monomolecular arrays in suspension or on suitable surfaces (e.g., polymers, metals, silicon wafers) or interfaces (e.g., lipid films, liposomes, emulsomes). S-layers also represent a unique structural basis and patterning element for generating more complex supramolecular structures involving all major classes of biological molecules (e.g., proteins, lipids, glycans, nucleic acids, or combinations of these). Thus, S-layers fulfill key requirements as building blocks for the production of new supramolecular materials and nanoscale devices as required in molecular nanotechnology, nanobiotechnology, biomimetics, and synthetic biology.

Composite S-layer lipid structures

Designing and utilization of biomimetic membrane systems generated by bottom-up processes is a rapidly growing scientific and engineering field. Elucidation of the supramolecular construction principle of archaeal cell envelopes composed of S-layer stabilized lipid membranes led to new strategies for generating highly stable functional lipid membranes at meso- and macroscopic scale. In this review, we provide a state of the art survey how S-layer proteins, lipids, and polysaccharides may be used as basic building blocks for the assembly of S-layer supported lipid membranes. These biomimetic membrane systems are distinguished by a nanopatterned fluidity, enhanced stability and longevity and thus, provide a dedicated reconstitution matrix for membrane-active peptides and transmembrane proteins. Exciting areas for application of composite S-layer membrane systems concern sensor systems involving specific membrane functions.

S-layer stabilized lipid membranes (Review)

The present review focuses on a unique bio-molecular construction kit based on surface-layer (S-layer) proteins as building blocks and patterning elements, but also major classes of biological molecules such as lipids, membrane-active peptides and membrane proteins, and glycans for the design of functional supported lipid membranes. The biomimetic approach copying the supramolecular building principle of most archaeal cell envelopes merely composed of a plasma membrane and a closely associated S-layer lattice has resulted in robust and fluid lipid membranes. Most importantly, S-layer supported lipid membranes spanning an aperture or generated on solid and porous substrates constitute highly interesting model membranes for the reconstitution of responsive transmembrane proteins and membrane-active peptides. This is of particular challenge as one-third of all proteins are membrane proteins such as pore-forming proteins, ion channels, and receptors. S-layer supported lipid membranes are seen as one of the most innovative strategies in membrane protein-based nanobiotechnology with potential applications that range from pharmaceutical (high-throughput) drug screening over lipid chips to the detection of biological warfare agents.

Are S-layers exoskeletons? The basic function of protein surface layers revisited

Surface protein or glycoprotein layers (S-layers) are common structures of the prokaryotic cell envelope. They are either associated with the peptidoglycan or outer membrane of bacteria, and constitute the only cell wall component of many archaea. Despite their occurrence in most of the phylogenetic branches of microorganisms, the functional significance of S-layers is assumed to be specific for genera or groups of organisms in the same environment rather than common to all prokaryotes. Functional aspects have usually been investigated with isolated S-layer sheets or proteins, which disregards the interactions between S-layers and the underlying cell envelope components. This study discusses the synergistic effects in cell envelope assemblies, the hypothetical role of S-layers for cell shape formation, and the existence of a common function in view of new insights.

S-layers as a tool kit for nanobiotechnological applications

Crystalline bacterial cell surface layers (S-layers) have been identified in a great number of different species of bacteria and represent an almost universal feature of archaea. Isolated native S-layer proteins and S-layer fusion proteins incorporating functional sequences self-assemble into monomolecular crystalline arrays in suspension, on a great variety of solid substrates and on various lipid structures including planar membranes and liposomes. S-layers have proven to be particularly suited as building blocks and patterning elements in a biomolecular construction kit involving all major classes of biological molecules (proteins, lipids, glycans, nucleic acids and combinations of them) enabling innovative approaches for the controlled 'bottom-up' assembly of functional supramolecular structures and devices. Here, we review the basic principles of S-layer proteins and the application potential of S-layers in nanobiotechnology and biomimetics including life and nonlife sciences.

Probing bacterial interactions: integrated approaches combining atomic force microscopy, electron microscopy and biophysical techniques

Recent developments in the application of Atomic Force Microscopy (AFM) and other biophysical techniques for the study of bacterial interactions and adhesion are discussed in the light of established biological and microscopic approaches. Whereas molecular-biological techniques combined with electron microscopy allow the identification and localization of surface constituents mediating bacterial interactions, with AFM it has become possible to actually measure the forces involved in bacterial interactions. Combined with the flexibility of AFM in probing various types of physical interactions, such as electrostatic interactions, specific ligand-receptor interactions and the elastic forces of deformation and extension of bacterial surface polymers and cell wall, this provides prospects for the elucidation of the biophysical mechanism of bacterial interaction. However, because of the biochemical and a biophysical complexity of the bacterial cell wall, integrated approaches combining AFM with electron microscopy and biophysical techniques are needed to elucidate the mechanism by which a bacterium interacts with a host or material surface. The literature on electron microscopy of the bacterial cell wall is reviewed, with particular emphasis on the staining of specific classes of cell-wall constituents. The application of AFM in the analysis of bacterial surfaces is discussed, including AFM operating modes, sample preparation methods and results obtained on various strains. For various bacterial strains, the integration of EM and AFM data is discussed. Various biophysical aspects of the analysis of bacterial surface structure and interactions are discussed, including the theory of colloidal interactions and Bell's theory of cell-to-cell adhesion. An overview is given of biophysical techniques used in the analysis of the properties of bacterial surfaces and bacterial surface constituents and their integration with AFM. Finally, we discuss recent progress in the understanding of the role of bacterial interactions in medicine within the framework of the techniques and concepts discussed in the paper.

S-layer proteins as supporting scaffoldings for functional lipid membranes

A great challenge is the coupling of very sensitive microelectronic devices to wet biological systems in the generation of biomimetic sensors. Lipid membranes on solid supports (electrodes or semiconductors) may become the matrix of future bioelectronic devices probing and controlling biomolecular processes for scientific and technical applications. A sufficient electrical isolation between the coupling region and the surrounding electrolyte is mandatory. An attached lipid membrane with integral natural or designed proteins may be the material of choice if the biological components are kept in a proper environment. To retain the fluidity and stability of the lipid membrane and to provide an ion reservoir and space for bulky integral proteins, a separating layer composed of crystalline arrays of monomolecular proteinaceous subunits, termed S-layer, can be self-assembled on metal or semiconductor surfaces. In this way S-layer-supported lipid membranes are biomimetic structures mimicking the supramolecular principle of archeal cell envelopes. These composite architectures may ascend toward exciting new key devices, particularly in fields of membrane protein-based biosensors or lab-on-a-chip technology.

Differences in the modulation of collective membrane motions by ergosterol, lanosterol, and cholesterol: a dynamic light scattering study

A dynamic light scattering setup was used to study the undulations of freely suspended planar lipid bilayers, the so-called black lipid membranes, over a previously inaccessible range of frequency and wave number. A pure synthetic lecithin bilayer, 1,2-dielaidoyl-sn-3-glycero-phoshatidylcholine (DEPC), and binary mixtures of DEPC with 40 mol % of cholesterol, ergosterol, or lanosterol were studied. By analyzing the dynamic light scattering data (oscillation and damping curves) in terms of transverse shear motion, we extracted the lateral tension and surface viscosity of the composite bilayers for each sterol. Cholesterol gave the strongest increase in lateral tension (approximately sixfold) with respect to the DEPC control, followed by lanosterol (approximately twofold), and ergosterol (1.7-fold). Most interestingly, only cholesterol simultaneously altered the surface viscosity of the bilayer by almost two orders of magnitude, whereas the other two sterols did not affect this parameter. We interpret this unique behavior of cholesterol as a result of its previously established out-of-plane motion which allows the molecule to cross the bilayer midplane, thereby effectively coupling the bilayer leaflets to form a highly flexible but more stable composite membrane.

Exploring the interaction of the surfactant N-terminal domain of gamma-Zein with soybean phosphatidylcholine liposomes

Zeins are maize storage proteins that accumulate inside large vesicles called protein bodies. gamma-Zein lines the inner surface of the protein body membrane, and its N-terminal, proline-rich, repetitive domain with the sequence (VHLPPP)(8) appears to be necessary for the accumulation of the protein within the organelle. Synthetic (VHLPPP)(8) adopts an amphipathic polyproline II conformation and forms cylindrical micelles in aqueous solution. Here we explore the interaction of (VHLPPP)(8) with soybean phosphatidylcholine unilamellar lipid vesicles and examine its effect on the stability and permeability of the liposome membrane. The amphipathic N-terminal domain of gamma-zein interacts with the membrane and assembles to form extended domains over the phospholipid membrane. The interaction between the peptide and the membrane increases the stability and permeability of the liposome membrane. The spontaneous amphipathic aggregation of (VHLPPP)(8) on the membrane suggests a mechanism of gamma-zein deposition inside maize protein bodies.

The effect of hydrostatic pressure on S-layer-supported lipid membranes

We report on the behavior of unsupported and surface layer (S-layer)-supported lipid membranes at the application of a uniform hydrostatic pressure. At a hydrostatic pressure gradient higher than 6 N/m(2), unsupported lipid membranes, independent from which side pressurized and S-layer-supported lipid membranes pressurized from the lipid-faced side revealed a pronounced increase in capacitance. A maximal hydrostatic pressure gradient of 11.0 N/m(2) resulted in an almost doubling of the capacitance of the (composite) membranes. S-layer-supported lipid membranes showed a hysteresis in the capacitance versus pressure plot, indicating that this composite structure required a certain time to reorient when the pressure gradient acting from the lipid-faced side was balanced. By contrast, the S-layer-supported lipid membrane pressurized from the protein-faced side revealed only a minute increase in capacitance (C/C(0,max)=1.17+/-0.05), reflecting only minor pressure-induced area expansion. In addition, no hysteresis could be observed, indicating that no rearrangement of the composite membrane occurred. The maximal induced tension was with 4.3+/-0.2 mN/m, significantly higher than that of unsupported (2.5+/-0.3 mN/m) and S-layer-supported lipid membranes pressurized from the lipid-faced side (2.6+/-0.1 mN/m).

The 40th anniversary of bilayer lipid membrane research

This paper presents a historic perspective on the origin of the lipid bilayer concept and its experimental realization. Additionally, current studies in close collaboration with our colleagues on the use of supported BLMs as biosensors and molecular devices are delineated. Further, recent research of others on BLMs (planar lipid bilayers) is referenced.

S-layer-supported lipid membranes

Many prokaryotic organisms (archaea and bacteria) are covered by a regularly ordered surface layer (S-layer) as the outermost cell wall component. S-layers are built up of a single protein or glycoprotein species and represent the simplest biological membrane developed during evolution. Pores in S-layers are of regular size and morphology, and functional groups on the protein lattice are aligned in well-defined positions and orientations. Due to the high degree of structural regularity S-layers represent unique systems for studying the structure, morphogenesis, and function of layered supramolecular assemblies. Isolated S-layer subunits of numerous organisms are able to assemble into monomolecular arrays either in suspension, at air/water interfaces, on planar mono- and bilayer lipid films, on liposomes and on solid supports (e.g. silicon wafers). Detailed studies on composite S-layer/lipid structures have been performed with Langmuir films, freestanding bilayer lipid membranes, solid supported lipid membranes, and liposomes. Lipid molecules in planar films and liposomes interact via their head groups with defined domains on the S-layer lattice. Electrostatic interactions are the most prevalent forces. The hydrophobic chains of the lipid monolayers are almost unaffected by the attachment of the S-layer and no impact on the hydrophobic thickness of the membranes has been observed. Upon crystallization of a coherent S-layer lattice on planar and vesicular lipid membranes, an increase in molecular order is observed, which is reflected in a decrease of the membrane tension and an enhanced mobility of probe molecules within an S-layer-supported bilayer. Thus, the terminology 'semifluid membrane' has been introduced for describing S-layer-supported lipid membranes. The most important feature of composite S-layer/lipid membranes is an enhanced stability in comparison to unsupported membranes.