Composite S-layer lipid structures

Phase separation in polymer-based biomimetic structures containing planar membranes

Phase separation in biological membranes is crucial for proper cellular functions, such as signaling and trafficking, as it mediates the interactions of condensates on membrane-bound organelles and transmembrane transport to targeted destination compartments. The separation of a lipid bilayer into phases and the formation of lipid rafts involve the restructuring of molecular localization, their immobilization, and local accumulation. By understanding the processes underlying the formation of lipid rafts in a cellular membrane, it is possible to reconstitute this phenomenon in synthetic biomimetic membranes, such as hybrids of lipids and polymers or membranes composed solely of polymers, which offer an increased physicochemical stability and unlimited possibilities of chemical modification and functionalization. In this article, we relate the main lipid bilayer phase transition phenomenon with respect to hybrid biomimetic membranes, composed of lipids mixed with polymers, and fully synthetic membranes. Following, we review the occurrence of phase separation in biomimetic hybrid membranes based on lipids and/or direct lipid analogs, amphiphilic block copolymers. We further exemplify the phase separation and the resulting properties and applications in planar membranes, free-standing and solid-supported. We briefly list methods leading to the formation of such biomimetic membranes and reflect on their improved overall stability and influence on the separation into different phases within the membranes. Due to the importance of phase separation and compartmentalization in cellular membranes, we are convinced that this compiled overview of this phenomenon will be helpful for any researcher in the biomimicry area.

Electrochemical Biosensors Based on S-Layer Proteins

Designing and development of electrochemical biosensors enable molecule sensing and quantification of biochemical compositions with multitudinous benefits such as monitoring, detection, and feedback for medical and biotechnological applications. Integrating bioinspired materials and electrochemical techniques promote specific, rapid, sensitive, and inexpensive biosensing platforms for (e.g., point-of-care testing). The selection of biomaterials to decorate a biosensor surface is a critical issue as it strongly affects selectivity and sensitivity. In this context, smart biomaterials with the intrinsic self-assemble capability like bacterial surface (S-) layer proteins are of paramount importance. Indeed, by forming a crystalline two-dimensional protein lattice on many sensors surfaces and interfaces, the S-layer lattice constitutes an immobilization matrix for small biomolecules and lipid membranes and a patterning structure with unsurpassed spatial distribution for sensing elements and bioreceptors. This review aims to highlight on exploiting S-layer proteins in biosensor technology for various applications ranging from detection of metal ions over small organic compounds to cells. Furthermore, enzymes immobilized on the S-layer proteins allow specific detection of several vital biomolecules. The special features of the S-layer protein lattice as part of the sensor architecture enhances surface functionalization and thus may feature an innovative class of electrochemical biosensors.

Formation and characteristics of mixed lipid/polymer membranes on a crystalline surface-layer protein lattice

The implementation of self-assembled biomolecules on solid materials, in particular, sensor and electrode surfaces, gains increasing importance for the design of stable functional platforms, bioinspired materials, and biosensors. The present study reports on the formation of a planar hybrid lipid/polymer membrane on a crystalline surface layer protein (SLP) lattice. The latter acts as a connecting layer linking the biomolecules to the inorganic base plate. In this approach, chemically bound lipids provided hydrophobic anchoring moieties for the hybrid lipid/polymer membrane on the recrystallized SLP lattice. The rapid solvent exchange technique was the method of choice to generate the planar hybrid lipid/polymer membrane on the SLP lattice. The formation process and completeness of the latter were investigated by quartz crystal microbalance with dissipation monitoring and by an enzymatic assay using the protease subtilisin A, respectively. The present data provide evidence for the formation of a hybrid lipid/polymer membrane on an S-layer lattice with a diblock copolymer content of 30%. The hybrid lipid/polymer showed a higher stiffness compared to the pure lipid bilayer. Most interestingly, both the pure and hybrid membrane prevented the proteolytic degradation of the underlying S-layer protein by the action of subtilisin A. Hence, these results provide evidence for the formation of defect-free membranes anchored to the S-layer lattice.

Formation of planar hybrid lipid/polymer membranes anchored to an S-layer protein lattice by vesicle binding and rupture

Exploitation of biomolecular and biomimetic components on solid surfaces gain increasing importance for the design of stable functional platforms. The present study performed by quartz crystal microbalance with dissipation monitoring (QCM-D) reports on the formation of planar hybrid lipid/polymer membranes anchored to a crystalline surface (S-) layer protein lattice. In this approach, hybrid lipid/polymer vesicles were chemically bound to the S-layer protein lattice. Subsequently, to form a hybrid planar layer rupture and fusion was triggered either by (1) β- diketone - europium ion complex formation or (2) successive application of calcium ions, lowering the pH from 9 to 4, and the detergent CHAPS. As determined by QCM-D, method 1 resulted for a polymer content of 5% in a planar membrane with some imbedded intact vesicles, whereas method 2 succeeded in planar hybrid membranes with a polymer content of even up to 70%. These results provide evidence for the effective formation of planar lipid/polymer membranes varying in their composition on an S-layer protein lattice.

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.

S-Layer Protein-Based Biosensors

The present paper highlights the application of bacterial surface (S-) layer proteins as versatile components for the fabrication of biosensors. One technologically relevant feature of S-layer proteins is their ability to self-assemble on many surfaces and interfaces to form a crystalline two-dimensional (2D) protein lattice. The S-layer lattice on the surface of a biosensor becomes part of the interface architecture linking the bioreceptor to the transducer interface, which may cause signal amplification. The S-layer lattice as ultrathin, highly porous structure with functional groups in a well-defined special distribution and orientation and an overall anti-fouling characteristics can significantly raise the limit in terms of variety and the ease of bioreceptor immobilization, compactness of bioreceptor molecule arrangement, sensitivity, specificity, and detection limit for many types of biosensors. The present paper discusses and summarizes examples for the successful implementation of S-layer lattices on biosensor surfaces in order to give a comprehensive overview on the application potential of these bioinspired S-layer protein-based biosensors.

Analysis of self-assembly of S-layer protein slp-B53 from Lysinibacillus sphaericus

The formation of stable and functional surface layers (S-layers) via self-assembly of surface-layer proteins on the cell surface is a dynamic and complex process. S-layers facilitate a number of important biological functions, e.g., providing protection and mediating selective exchange of molecules and thereby functioning as molecular sieves. Furthermore, S-layers selectively bind several metal ions including uranium, palladium, gold, and europium, some of them with high affinity. Most current research on surface layers focuses on investigating crystalline arrays of protein subunits in Archaea and bacteria. In this work, several complementary analytical techniques and methods have been applied to examine structure-function relationships and dynamics for assembly of S-layer protein slp-B53 from Lysinibacillus sphaericus: (1) The secondary structure of the S-layer protein was analyzed by circular dichroism spectroscopy; (2) Small-angle X-ray scattering was applied to gain insights into the three-dimensional structure in solution; (3) The interaction with bivalent cations was followed by differential scanning calorimetry; (4) The dynamics and time-dependent assembly of S-layers were followed by applying dynamic light scattering; (5) The two-dimensional structure of the paracrystalline S-layer lattice was examined by atomic force microscopy. The data obtained provide essential structural insights into the mechanism of S-layer self-assembly, particularly with respect to binding of bivalent cations, i.e., Mg²⁺ and Ca²⁺. Furthermore, the results obtained highlight potential applications of S-layers in the fields of micromaterials and nanobiotechnology by providing engineered or individual symmetric thin protein layers, e.g., for protective, antimicrobial, or otherwise functionalized surfaces.

Functional Analysis of an S-Layer-Associated Fibronectin-Binding Protein in Lactobacillus acidophilus NCFM

Bacterial surface layers (S-layers) are crystalline arrays of self-assembling proteinaceous subunits called S-layer proteins (Slps) that comprise the outermost layer of the cell envelope. Many additional proteins that are associated with or embedded within the S-layer have been identified in Lactobacillus acidophilus NCFM, an S-layer-forming bacterium that is widely used in fermented dairy products and probiotic supplements. One putative S-layer-associated protein (SLAP), LBA0191, was predicted to mediate adhesion to fibronectin based on the in silico detection of a fibronectin-binding domain. Fibronectin is a major component of the extracellular matrix (ECM) of intestinal epithelial cells. Adhesion to intestinal epithelial cells is considered an important trait for probiotic microorganisms during transit and potential association with the intestinal mucosa. To investigate the functional role of LBA0191 (designated FbpB) in L. acidophilus NCFM, an fbpB-deficient strain was constructed. The L. acidophilus mutant with a deletion off bpB lost the ability to adhere to mucin and fibronectin in vitro Homologues off bpB were identified in five additional putative S-layer-forming species, but no homologues were detected in species outside theL. acidophilus homology group.

S-Layer-Based Nanocomposites for Industrial Applications

This chapter covers the fundamental aspects of bacterial S-layers: what are S-layers, what is known about them, and what are their main features that makes them so interesting for the production of nanostructures. After a detailed introduction of the paracrystalline protein lattices formed by S-layer systems in nature the chapter explores the engineering of S-layer-based materials. How can S-layers be used to produce "industry-ready" nanoscale bio-composite materials, and which kinds of nanomaterials are possible (e.g., nanoparticle synthesis, nanoparticle immobilization, and multifunctional coatings)? What are the advantages and disadvantages of S-layer-based composite materials? Finally, the chapter highlights the potential of these innovative bacterial biomolecules for future technologies in the fields of metal filtration, catalysis, and bio-functionalization.

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.

Inspired and stabilized by nature: ribosomal synthesis of the human voltage gated ion channel (VDAC) into 2D-protein-tethered lipid interfaces

The scheme of the cell-free, ribosomal synthesis of a VDAC protein in the presence of an S-layer supported lipid membrane. The VDAC protein is adapted from S. Hiller et al., Science, 2008, 321, 1206–1210.

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.

The grab-and-drop protocol: a novel strategy for membrane protein isolation and reconstitution from single cells

Samples of cell membrane were non-destructively removed from individual, live cells using optically trapped beads, and deposited into a supported lipid bilayer mounted on an S-layer protein-coated substrate.

Adhesion properties of potentially probiotic Lactobacillus kefiri to gastrointestinal mucus

We investigated the mucus-binding properties of aggregating and non-aggregating potentially probiotic strains of kefir-isolated Lactobacillus kefiri, using different substrates. All the strains were able to adhere to commercial gastric mucin (MUCIN) and extracted mucus from small intestine (SIM) and colon (CM). The extraction of surface proteins from bacteria using LiCl or NaOH significantly reduced the adhesion of three selected strains (CIDCA 8348, CIDCA 83115 and JCM 5818); although a significant proportion (up to 50%) of S-layer proteins were not completely eliminated after treatments. The surface (S-layer) protein extracts from all the strains of Lb. kefiri were capable of binding to MUCIN, SIM or CM, and no differences were observed among them. The addition of their own surface protein extract increased adhesion of CIDCA 8348 and 83115 to MUCIN and SIM, meanwhile no changes in adhesion were observed for JCM 5818. None of the seven sugars tested had the ability to inhibit the adhesion of whole bacteria to the three mucus extracts. Noteworthy, the degree of bacterial adhesion reached in the presence of their own surface protein (S-layer) extract decreased to basal levels in the presence of some sugars, suggesting an interaction between the added sugar and the surface proteins. In conclusion, the ability of these food-isolated bacteria to adhere to gastrointestinal mucus becomes an essential issue regarding the biotechnological potentiality of Lb. kefiri for the food industry.

S-layer coated emulsomes as potential nanocarriers

The present study introduces a novel nanocarrier system comprising lipidic emulsomes and S-layer (fusion) proteins as functionalizing tools coating the surface. Emulsomes composed of a solid tripalmitin core and a phospholipid shell are created reproducibly with an average diameter of approximately 300 nm using temperature-controlled extrusion steps. Both wildtype (wt) and recombinant (r) S-layer protein SbsB of Geobacillus stearothermophilus PV72/p2 are capable of forming coherent crystalline envelope structures with oblique (p1) lattice symmetry, as evidenced by transmission electron microscopy. Upon coating with wtSbsB, positive charge of emulsomes shifts to a highly negative zeta potential, whereas those coated with rSbsB become charge neutral. This observation is attributed to the presence of a negatively charged glycan, the secondary cell wall polymer (SCWP), which is associated only with wtSbsB. The present study shows for the first time the ability of recombinant and wildtype S-layer proteins to cover the entire surface of emulsomes with its characteristic crystalline lattice. Furthermore, in vitro cell culture studies reveal that S-layer coated emulsomes can be uptaken by human liver carcinoma cells (HepG2) without showing any significant cytotoxicity over a wide range of concentrations. The utilization of S-layer fusion proteins equipped in a nanopatterned fashion by identical or diverse functions may lead to further development of emulsomes in nanomedicine, especially for drug delivery and targeting.

Lactobacillus surface layer proteins: structure, function and applications

Bacterial surface (S) layers are the outermost proteinaceous cell envelope structures found on members of nearly all taxonomic groups of bacteria and Archaea. They are composed of numerous identical subunits forming a symmetric, porous, lattice-like layer that completely covers the cell surface. The subunits are held together and attached to cell wall carbohydrates by non-covalent interactions, and they spontaneously reassemble in vitro by an entropy-driven process. Due to the low amino acid sequence similarity among S-layer proteins in general, verification of the presence of an S-layer on the bacterial cell surface usually requires electron microscopy. In lactobacilli, S-layer proteins have been detected on many but not all species. Lactobacillus S-layer proteins differ from those of other bacteria in their smaller size and high predicted pI. The positive charge in Lactobacillus S-layer proteins is concentrated in the more conserved cell wall binding domain, which can be either N- or C-terminal depending on the species. The more variable domain is responsible for the self-assembly of the monomers to a periodic structure. The biological functions of Lactobacillus S-layer proteins are poorly understood, but in some species S-layer proteins mediate bacterial adherence to host cells or extracellular matrix proteins or have protective or enzymatic functions. Lactobacillus S-layer proteins show potential for use as antigen carriers in live oral vaccine design because of their adhesive and immunomodulatory properties and the general non-pathogenicity of the species.

Construction of silica-enhanced S-layer protein cages

The work presented here shows for the first time that it is possible to silicify S-layer coated liposomes and to obtain stable functionalized hollow nano-containers. For this purpose, the S-layer protein of Geobacillus stearothermophilus PV72/p2 was recombinantly expressed and used for coating positively charged liposomes composed of dipalmitoylphosphatidylcholine, cholesterol and hexadecylamine in a molar ratio of 10:5:4. Subsequently, plain (uncoated) liposomes and S-layer coated liposomes were silicified. Determination of the charge of the constructs during silicification allowed the deposition process to be followed. After the particles had been silicified, lipids were dissolved by treatment with Triton X-100 with the release of previously entrapped fluorescent dyes being determined by fluorimetry. Both, ζ-potential and release experiments showed differences between silicified plain liposomes and silicified S-layer coated liposomes. The results of the individual preparation steps were examined by embedding the respective assemblies in resin, ultrathin sectioning and inspection by bright-field transmission electron microscopy (TEM). Energy filtered TEM confirmed the successful construction of S-layer based silica cages. It is anticipated that this approach will provide a key to enabling technology for the fabrication of nanoporous protein cages for applications ranging from nano medicine to materials science.

S-layer protein self-assembly

Crystalline S(urface)-layers are the most commonly observed cell surface structures in prokaryotic organisms (bacteria and archaea). S-layers are highly porous protein meshworks with unit cell sizes in the range of 3 to 30 nm, and thicknesses of ~10 nm. One of the key features of S-layer proteins is their intrinsic capability to form self-assembled mono- or double layers in solution, and at interfaces. Basic research on S-layer proteins laid foundation to make use of the unique self-assembly properties of native and, in particular, genetically functionalized S-layer protein lattices, in a broad range of applications in the life and non-life sciences. This contribution briefly summarizes the knowledge about structure, genetics, chemistry, morphogenesis, and function of S-layer proteins and pays particular attention to the self-assembly in solution, and at differently functionalized solid supports.

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.

Deciphering the nanometer-scale organization and assembly of Lactobacillus rhamnosus GG pili using atomic force microscopy

In living cells, sophisticated functional interfaces are generated through the self-assembly of bioactive building blocks. Prominent examples of such biofunctional surfaces are bacterial nanostructures referred to as pili. Although these proteinaceous filaments exhibit remarkable structure and functions, their potential to design bioinspired self-assembled systems has been overlooked. Here, we used atomic force microscopy (AFM) to explore the supramolecular organization and self-assembly of pili from the Gram-positive probiotic bacterium Lactobacillus rhamnosus GG (LGG). High-resolution AFM imaging of cell preparations adsorbed on mica revealed pili not only all around the cells, but also in the form of remarkable star-like structures assembled on the mica surface. Next, we showed that two-step centrifugation is a simple procedure to separate large amounts of pili, even though through their synthesis they are covalently anchored to the cell wall. We also found that the centrifuged pili assemble as long bundles. We suggest that these bundles originate from a complex interplay of mechanical effects (centrifugal force) and biomolecular interactions involving the SpaC cell adhesion pilin subunit (lectin-glycan bonds, hydrophobic bonds). Supporting this view, we found that pili isolated from an LGG mutant lacking hydrophilic exopolysaccharides show an increased tendency to form tight bundles. These experiments demonstrate that AFM is a powerful platform for visualizing individual pili on bacterial surfaces and for unravelling their two-dimensional assembly on solid surfaces. Our data suggest that bacterial pili may provide a generic approach in nanobiotechnology for elaborating functional supramolecular interfaces assembled from bioactive building blocks.

S-layer fusion proteins--construction principles and applications

Crystalline bacterial cell surface layers (S-layers) are the outermost cell envelope component of many bacteria and archaea. S-layers are monomolecular arrays composed of a single protein or glycoprotein species and represent the simplest biological membrane developed during evolution. The wealth of information available on the structure, chemistry, genetics and assembly of S-layers revealed a broad spectrum of applications in nanobiotechnology and biomimetics. By genetic engineering techniques, specific functional domains can be incorporated in S-layer proteins while maintaining the self-assembly capability. These techniques have led to new types of affinity structures, microcarriers, enzyme membranes, diagnostic devices, biosensors, vaccines, as well as targeting, delivery and encapsulation systems.

Artificial membrane-like environments for in vitro studies of purified G-protein coupled receptors

Functional reconstitution of transmembrane proteins remains a significant barrier to their biochemical, biophysical, and structural characterization. Studies of seven-transmembrane G-protein coupled receptors (GPCRs) in vitro are particularly challenging because, ideally, they require access to the receptor on both sides of the membrane as well as within the plane of the membrane. However, understanding the structure and function of these receptors at the molecular level within a native-like environment will have a large impact both on basic knowledge of cell signaling and on pharmacological research. The goal of this article is to review the main classes of membrane mimics that have been, or could be, used for functional reconstitution of GPCRs. These include the use of micelles, bicelles, lipid vesicles, nanodiscs, lipidic cubic phases, and planar lipid membranes. Each of these approaches is evaluated with respect to its fundamental advantages and limitations and its applications in the field of GPCR research. This article is part of a Special Issue entitled: Membrane protein structure and function.

Bilayer lipid membrane formation on a chemically modified S-layer lattice

The present paper describes the generation of a biomimetic model lipid membrane on bacterial surface (S-)layer which covered the entire surface of various sensors. The S-layer lattice allows one to be independent from the underlying solid material and provides a biological surface and anchoring structure for lipid membranes. S-layer proteins were chemically modified via binding of two amine-terminated phospholipids. Subsequently, a bimolecular lipid membrane anchored to the previously generated viscoelastic lipid monolayer was generated by the rapid solvent exchange technique. Characterization of the intermediate (monolayer) and final membrane structures (bilayer) was performed by imaging, surface-sensitive, and electrochemical techniques. This bilayer lipid membrane generated on an S-layer lattice revealed a thickness of ∼6 nm and constitutes a stable supported model membrane system with highly isolating properties showing a membrane resistance of 8.5 MΩ × cm(2).

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.

Fluorescence energy transfer in the bi-fluorescent S-layer tandem fusion protein ECFP-SgsE-YFP

This work reports for the first time on the fabrication of a bi-functional S-layer tandem fusion protein which is able to self-assemble on solid supports without losing its functionality. Two variants of the green fluorescent protein (GFP) were genetically combined with a self-assembly system having the remarkable opportunity to interact with each other and act as functional nanopatterning biocoating. The S-layer protein SgsE of Geobacillus stearothermophilus NRS 2004/3a was fused with the cyan ECFP donor protein at the SgsE N-terminus and with the yellow YFP acceptor protein at the C-terminus. The fluorescence energy transfer was studied with spectrofluorimetry, confocal microscopy and flow cytometry, whilst protein self-assembly (on silicon dioxide particles) and structural investigations were carried out with atomic force microscopy (AFM). The fluorescence resonance energy transfer efficiency of reassembled SgsE tandem protein was 20.0 ± 6.1% which is almost the same transfer efficiency shown in solution (19.6 ± 0.1%). This work shows that bi-fluorescent S-layer fusion proteins self-assemble on silica particles retaining their fluorescent properties.