Relevance of charged groups for the integrity of the S-layer from Bacillus coagulans E38-66 and for molecular interactions

Patterns in Nature—S-Layer Lattices of Bacterial and Archaeal Cells

Bacterial surface layers (S-layers) have been observed as the outermost cell envelope component in a wide range of bacteria and most archaea. S-layers are monomolecular lattices composed of a single protein or glycoprotein species and have either oblique, square or hexagonal lattice symmetry with unit cell dimensions ranging from 3 to 30 nm. They are generally 5 to 10 nm thick (up to 70 nm in archaea) and represent highly porous protein lattices (30–70% porosity) with pores of uniform size and morphology in the range of 2 to 8 nm. Since S-layers can be considered as one of the simplest protein lattices found in nature and the constituent units are probably the most abundantly expressed proteins on earth, it seems justified to briefly review the different S-layer lattice types, the need for lattice imperfections and the discussion of S-layers from the perspective of an isoporous protein network in the ultrafiltration region. Finally, basic research on S-layers laid the foundation for applications in biotechnology, synthetic biology, and biomimetics.

S-Layer Ultrafiltration Membranes

Monomolecular arrays of protein subunits forming surface layers (S-layers) are the most common outermost cell envelope components of prokaryotic organisms (bacteria and archaea). Since S-layers are periodic structures, they exhibit identical physicochemical properties for each constituent molecular unit down to the sub-nanometer level. Pores passing through S-layers show identical size and morphology and are in the range of ultrafiltration membranes. The functional groups on the surface and in the pores of the S-layer protein lattice are accessible for chemical modifications and for binding functional molecules in very precise fashion. S-layer ultrafiltration membranes (SUMs) can be produced by depositing S-layer fragments as a coherent (multi)layer on microfiltration membranes. After inter- and intramolecular crosslinking of the composite structure, the chemical and thermal resistance of these membranes was shown to be comparable to polyamide membranes. Chemical modification and/or specific binding of differently sized molecules allow the tuning of the surface properties and molecular sieving characteristics of SUMs. SUMs can be utilized as matrices for the controlled immobilization of functional biomolecules (e.g., ligands, enzymes, antibodies, and antigens) as required for many applications (e.g., biosensors, diagnostics, enzyme- and affinity-membranes). Finally, SUM represent unique supporting structures for stabilizing functional lipid membranes at meso- and macroscopic scale.

Slp-coated liposomes for drug delivery and biomedical applications: potential and challenges

Slp forms a crystalline array of proteins on the outermost envelope of bacteria and archaea with a molecular weight of 40-200 kDa. Slp can self-assemble on the surface of liposomes in a proper environment via electrostatic interactions, which could be employed to functionalize liposomes by forming Slp-coated liposomes for various applications. Among the molecular characteristics, the stability, adhesion, and immobilization of biomacromolecules are regarded as the most meaningful. Compared to plain liposomes, Slp-coated liposomes show excellent physicochemical and biological stabilities. Recently, Slp-coated liposomes were shown to specifically adhere to the gastrointestinal tract, which was attributed to the "ligand-receptor interaction" effect. Furthermore, Slp as a "bridge" can immobilize functional biomacromol-ecules on the surface of liposomes via protein fusion technology or intermolecular forces, endowing liposomes with beneficial functions. In view of these favorable features, Slp-coated liposomes are highly likely to be an ideal platform for drug delivery and biomedical uses. This review aims to provide a general framework for the structure and characteristics of Slp and the interactions between Slp and liposomes, to highlight the unique properties and drug delivery as well as the biomedical applications of the Slp-coated liposomes, and to discuss the ongoing challenges and perspectives.

Synthesis of S-layer conjugates and evaluation of their modifiability as a tool for the functionalization and patterning of technical surfaces

Chemical functional groups of surface layer (S-layer) proteins were chemically modified in order to evaluate the potential of S-layer proteins for the introduction of functional molecules. S-layer proteins are structure proteins that self-assemble into regular arrays on surfaces. One general feature of S-layer proteins is their high amount of carboxylic and amino groups. These groups are potential targets for linking functional molecules, thus producing reactive surfaces. In this work, these groups were conjugated with the amino acid tryptophan. In another approach, SH-groups were chemically inserted in order to extend the spectrum of modifiable groups. The amount of modifiable carboxylic groups was further evaluated by potentiometric titration in order to evaluate the potential efficiency of S-layer proteins to work as matrix for bioconjugations. The results proved that S-layer proteins can work as effective matrices for the conjugation of different molecules. The advantage of using chemical modification methods over genetic methods lies in its versatile usage enabling the attachment of biomolecules, as well as fluorescent dyes and inorganic molecules. Together with their self-assembling properties, S-layer proteins are suitable as targets for bioconjugates, thus enabling a nanostructuring and bio-functionalization of surfaces, which can be used for different applications like biosensors, filter materials, or (bio)catalytic surfaces.

S-layer based biomolecular imprinting

AFM image of an S-layer protein array used for making molecular imprints.

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.

Protective effect caused by the exopolymer excreted by Pseudoalteromonas antarctica NF(3) on liposomes against the action of octyl glucoside

The capacity of the glycoprotein (GP) excreted by Pseudoalteromonas antarctica NF(3), to protect phosphatidylcholine (PC) liposomes against the action of octyl glucoside (OG) was studied in detail. Increasing amounts of GP assembled with liposomes resulted for the same interaction step in a linear increase in the effective surfactant to PC molar ratios (Re) and in a linear fall in the surfactant partitioning between bilayer and the aqueous phase (partition coefficients K). Thus, the higher the proportion of GP assembled with liposomes the lower the surfactant ability to alter the permeability of vesicles and the lower its affinity with these bilayer structures. In addition, increasing GP proportions resulted in a progressive increase of the free surfactant concentration (S(W)) needed to produce the same alterations in liposomes. The fact that S(W) was always lower than the surfactant critical micelle concentration indicates that the interaction was mainly ruled by the action of surfactant monomers, regardless of the amount of assembled GP.

S-layer-coated liposomes as a versatile system for entrapping and binding target molecules

In the present study, unilamellar liposomes coated with the crystalline bacterial cell surface layer (S-layer) protein of Bacillus stearothermophilus PV72/p2 were used as matrix for defined binding of functional molecules via the avidin- or streptavidin-biotin bridge. The liposomes were composed of dipalmitoyl phosphatidylcholine, cholesterol and hexadecylamine in a molar ratio of 10:5:4 and they had an average size of 180 nm. For introducing specific functions into the S-layer lattice without affecting substances encapsulated within the liposomes, crosslinking and activation reagents had to be identified which did not penetrate the liposomal membrane. Among different reagents, a hydrophilic dialdehyde generated by periodate cleavage of raffinose and a sulfo-succinimide activated dicarboxylic acid were found to be impermeable for the liposomal membrane. Both reagents completely crosslinked the S-layer lattice without interfering with its regular structure. Biotinylation of S-layer-coated liposomes was achieved by coupling p-diazobenzoyl biocytin which preferably reacts with the phenolic residue of tyrosine or with the imidazole ring of histidine. By applying this method, two biotin residues accessible for subsequent avidin binding were introduced per S-layer subunit. As visualized by labeling with biotinylated ferritin, an ordered monomolecular layer of streptavidin was formed on the surface of the S-layer-coated liposomes. As a second model system, biotinylated anti-human IgG was attached via the streptavidin bridge to the biotinylated S-layer-coated liposomes. The biological activity of the bound anti-human IgG was confirmed by ELISA.

Biopolymer excreted by pseudoalteromonas antarctica NF(3), as a coating and protective agent of liposomes against dodecyl maltoside

The ability of an exopolymer of glycoproteic character (GP) excreted by a new gram-negative species Pseudoalteromonas antarctica NF(3), to coat phosphatidylcholine (PC) liposomes and to protect these bilayers against the action of the nonionic surfactant dodecyl maltoside was investigated. Transmission electron microscopy (TEM) micrographs of freeze fractured liposome/GP aggregates reveal that the addition of the glycoprotein to liposomes led to the formation of a film (polymer adsorbed onto the bilayers) that tightly coated PC bilayers. The complete coating was already achieved at a PC : GP weight ratio of about 9:1. Image analysis profiles of digitalized TEM micrographs (PC : GP weight ratio 8:2) show that this film was formed by a multilayer structure. The periods of the average distance of the pattern ordering in layer structures (9-10 layers) were of about 2-3 nm and the thickness of the complete film was of about 25 nm. Higher amounts of glycoprotein resulted in a growth of this film, which exhibited at the highest proportion of this compound (50% in weight) a multifilm structure. An increasing resistance of liposomes to be affected by dodecyl maltoside both at subsolubilizing and solubilizing levels occurred as the proportion of the glycoprotein in the system rose, although this protective effect was more effective at low proportions of this compound (PC : GP weight ratios from 9:1 to 8:2). Thus, although a direct dependence was found between the growth of the enveloping structure and the resistance of the coated liposomes to be affected by the surfactant, the more effective protection occurred when this structure was a thin film formed by the assembly of various layers of GP of about 2-3 nm. Copyright 1999 John Wiley & Sons, Inc.

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

A dedicated dynamic light scattering (DLS) setup was employed to study the undulations of freely suspended planar lipid bilayers, the so-called black lipid membranes (BLM), over a previously inaccessible spread of frequencies (relaxation times ranging from 10(-2) to 10(-6) s) and wavevectors (250 cm(-1) < q < 38,000 cm(-1)). For a BLM consisting of 1,2-dielaidoyl-sn-3-glycero-phosphocholine (DEPC) doped with two different proportions of the cationic lipid analog dioctadecyl-dimethylammonium bromide (DODAB) we observed an increase of the lateral tension of the membrane with the DODAB concentration. The experimentally determined dispersion behavior of the transverse shear mode was in excellent agreement with the theoretical predictions of a first-order hydrodynamic theory. The symmetric adsorption of the crystalline bacterial cell surface layer (S-layer) proteins from Bacillus coagulans E38-66 to a weakly cationic BLM (1.5 mol % DODAB) causes a drastic reduction of the membrane tension well beyond the previous DODAB-induced tension increase. The likely reason for this behavior is an increase of molecular order along the lipid chains by the protein and/or partial protein penetration into the lipid headgroup region. S-layer protein adsorption to a highly cationic BLM (14 mol % DODAB) shows after 7 h incubation time an even stronger decrease of the membrane tension by a factor of five, but additionally a significant increase of the (previously negligible) surface viscosity, again in excellent agreement with the hydrodynamic theory. Further incubation (24 h) shows a drastic increase of the membrane bending energy by three orders of magnitude as a result of a large-scale, two-dimensional recrystallization of the S-layer proteins at both sides of the BLM. The results demonstrate the potential of the method for the assessment of the different stages of protein adsorption and recrystallization at a membrane surface by measurements of the collective membrane modes and their analysis in terms of a hydrodynamic theory.

Stabilizing effect of an S-layer on liposomes towards thermal or mechanical stress

Isolated subunits of the crystalline cell surface layer (S-layer) protein of Bacillus stearothermophilus PV72/p2 were recrystallized on positively charged unilamellar liposomes. Liposomes were composed of dipalmitoylphosphatidylcholine (DPPC), cholesterol and hexadecylamine (HDA) in a molar ratio of 10:5:4 and they were prepared by the dehydration-rehydration method followed by an extrusion procedure. The S-layer protein to DPPC ratio was 5.7 nmol/micromol which approximately corresponds to the theoretical value estimated by using the areas occupied by the S-layer lattice and the lipid membrane. Coating of the positively charged liposomes with S-layer protein resulted in inversion of the zeta-potential from +29.1 mV to -27.1 mV. Covalent crosslinking of the recrystallized S-layer protein was achieved with glutaraldehyde. Chemical analysis revealed that almost all amino groups (>95%) from HDA in the liposomal membrane were involved in the reaction. To study the influence of an S-layer lattice on the stability of the liposomes, the hydrophilic marker carboxyfluoresceine (CF) was encapsulated and its release was determined for plain and S-layer-coated liposomes in the course of mechanical and thermal challenges. In comparison to plain liposomes, S-layer-coated liposomes released only half the amount of enclosed CF upon exposure to shear forces or ultrasonication as mechanical stress factors. Furthermore, temperature shifts from 25 degrees C to 55 degrees C and vice versa induced considerably less CF release from S-layer-coated than from plain liposomes. A similar stabilizing effect of the S-layer lattice was observed after glutaraldehyde treatment of plain and S-layer-coated liposomes.

Bacterial S-layer protein coupling to lipids: x-ray reflectivity and grazing incidence diffraction studies

The coupling of bacterial surface (S)-layer proteins to lipid membranes is studied in molecular detail for proteins from Bacillus sphaericus CCM2177 and B. coagulans E38-66 recrystallized at dipalmitoylphosphatidylethanolamine (DPPE) monolayers on aqueous buffer. A comparison of the monolayer structure before and after protein recrystallization shows minimal reorganization of the lipid chains. By contrast, the lipid headgroups show major rearrangements. For the B. sphaericus CCM2177 protein underneath DPPE monolayers, x-ray reflectivity data suggest that amino acid side chains intercalate the lipid headgroups at least to the phosphate moieties, and probably further beyond. The number of electrons in the headgroup region increases by more than four per lipid. Analysis of the changes of the deduced electron density profiles in terms of a molecular interpretation shows that the phosphatidylethanolamine headgroups must reorient toward the surface normal to accommodate such changes. In terms of the protein structure (which is as yet unknown in three dimensions), the electron density profile reveals a thickness lz approximately 90 A of the recrystallized S-layer and shows water-filled cavities near its center. The protein volume fraction reaches maxima of >60% in two horizontal sections of the S-layer, close to the lipid monolayer and close to the free subphase. In between it drops to approximately 20%. Four S-layer protein monomers are located within the unit cell of a square lattice with a spacing of approximately 131 A.

Self-assembled alpha-hemolysin pores in an S-layer-supported lipid bilayer

The effects of a supporting proteinaceous surface-layer (S-layer) from Bacillus coagulans E38-66 on a 1,2-diphytanoyl-sn-glycero-3-phosphatidylcholine (DPhPC) bilayer were investigated. Comparative voltage clamp studies on plain and S-layer supported DPhPC bilayers revealed no significant difference in the capacitance. The conductance of the composite membrane decreased slightly upon recrystallization of the S-layer. Thus, the attached S-layer lattice did not interpenetrate or rupture the DPhPC bilayer. The self-assembly of a pore-forming protein into the S-layer supported lipid bilayer was examined. Staphylococcal alpha-hemolysin formed lytic pores when added to the lipid-exposed side. The assembly was slow compared to unsupported membranes, perhaps due to an altered fluidity of the lipid bilayer. No assembly could be detected upon adding alpha-hemolysin monomers to the S-layer-faced side of the composite membrane. Therefore, the intrinsic molecular sieving properties of the S-layer lattice do not allow passage of alpha-hemolysin monomers through the S-layer pores to the lipid bilayer. In comparison to plain lipid bilayers, the S-layer supported lipid membrane had a decreased tendency to rupture in the presence of alpha-hemolysin.

Voltage clamp studies on S-layer-supported tetraether lipid membranes

Isolated subunits from the cell surface proteins (S-layer) of Bacillus coagulans E38-66 have been recrystallized on a glycerol dialkyl nonitol tetraether lipid (GDNT)-monolayer and the electrophysical features of this biomimetic membrane have been investigated in comparison to unsupported GDNT-monolayers. The GDNT-monolayer, spread on a Langmuir-Blodgett trough, was clamped with the tip of a glass patch pipette. In order to investigate the barrier function and potential to incorporate functional molecules, voltage-clamp examinations on plain and S-layer-supported GDNT-monolayers were per-formed. Our results indicate the formation of a tight GDNT-monolayer sealing the tip of the glass pipette, and a decrease in conductance of the GDNT-monolayer upon recrystallization of the S-layer protein. Thus, the S-layer protein, apparently, did not penetrate or rupture the lipid monolayer. The valinomycin-mediated increase in conductance was less pronounced for the S-layer-supported than for the plain GDNT-monolayer, indicating differences in the accessibility and/or in the fluidity of the lipid membranes. Furthermore. in contrast to plain GDNT-monolayers. S-layer supported GDNT-monolayers with high valinomycin-mediated conductance persisted over long, periods of time, indicating enhanced stability. These composite S-layer/lipid films may constitute a new tool for electrophysical and electrophysiological studies on membrane-associated and membrane-integrated biomolecules.

Molecular biology of S-layers

In this chapter we report on the molecular biology of crystalline surface layers of different bacterial groups. The limited information indicates that there are many variations on a common theme. Sequence variety, antigenic diversity, gene expression, rearrangements, influence of environmental factors and applied aspects are addressed. There is considerable variety in the S-layer composition, which was elucidated by sequence analysis of the corresponding genes. In Corynebacterium glutamicum one major cell wall protein is responsible for the formation of a highly ordered, hexagonal array. In contrast, two abundant surface proteins from the S-layer of Bacillus anthracis. Each protein possesses three S-layer homology motifs and one protein could be a virulence factor. The antigenic diversity and ABC transporters are important features, which have been studied in methanogenic archaea. The expression of the S-layer components is controlled by three genes in the case of Thermus thermophilus. One has repressor activity on the S-layer gene promoter, the second codes for the S-layer protein. The rearrangement by reciprocal recombination was investigated in Campylobacter fetus. 7-8 S-layer proteins with a high degree of homology at the 5' and 3' ends were found. Environmental changes influence the surface properties of Bacillus stearothermophilus. Depending on oxygen supply, this species produces different S-layer proteins. Finally, the molecular bases for some applications are discussed. Recombinant S-layer fusion proteins have been designed for biotechnology.

Biotechnology and biomimetic with crystalline bacterial cell surface layers (S-layers)

Crystalline bacterial cell surface layers (S-layers) are the outermost cell envelope component of many eubacteria and archaeobacteria. S-layers are composed of a single protein or glycoprotein species and exhibit oblique, square or hexagonal lattice symmetry. Pores passing through these monomolecular arrays show identical size and morphology, and functional groups are aligned in well-defined positions and orientations. Due to these unique features, S-layers have broad application potential in biotechnology including functioning as biomimetic membranes. Presently, S-layers are used (i) for the production of isoporous ultrafiltration membranes with very well defined molecular sieving and adsorption properties, (ii) as matrices for the controlled immobilization of biologically active macromolecules (e.g., enzymes, antibodies, ligands) as required for biosensors, affinity membranes and affinity microparticles as well as for solid phase assays, (iii) as stabilizing structures for Langmuir-Blodgett films and liposomes and (iv) as carriers and adjuvants for weakly immunogenic antigens and haptens.

Liposomes coated with crystalline bacterial cells surface protein (S-layer) as immobilization structures for macromolecules

Isolated subunits from the crystalline cell surface layer (S-layer) of Bacillus coagulans E38-66 were recrystallized on positively charged liposomes. The liposomes were composed of dipalmitoylphosphatidylcholine/cholesterol and stearylamine. The natural arrangement of the S-layer subunits on the bacterial surface is as an oblique (p2) lattice. The subunits attached to positively charged liposomes by their inner face (which bears a net negative charge) in an orientation identical to the lattice on intact cells. The S-layer protein, once recrystallized on liposomes, was crosslinked with glutaraldehyde and subsequently used as a matrix for the covalent attachment of macromolecules. The high stability of S-layer-coated liposomes and the possibility for immobilizing biologically active molecules on the crystalline array may offer potential in various different liposome applications.

Large-scale recrystallization of the S-layer of Bacillus coagulans E38-66 at the air/water interface and on lipid films

S-layer protein isolated from Bacillus coagulans E38-66 could be recrystallized into large-scale coherent monolayers at an air/water interface and on phospholipid films spread on a Langmuir-Blodgett trough. Because of the asymmetry in the physiochemical surface properties of the S-layer protein, the subunits were associated with their more hydrophobic outer face with the air/water interface and oriented with their negatively charged inner face to the zwitterionic head groups of the dipalmitoylphosphatidylcholine and dipalmitoylphosphatidylethanolamine (DPPE) monolayer films. The dynamic crystal growth at both types of interfaces was first initiated at several distant nucleation points. The individual monocrystalline areas grew isotropically in all directions until the front edge of neighboring crystals was met. The recrystallized S-layer protein and the S-layer-DPPE layer could be chemically cross-linked from the subphase with glutaraldehyde.