Evidence for an S-layer protein pool in the peptidoglycan of Bacillus stearothermophilus

Spatial organization of Clostridium difficile S-layer biogenesis

Surface layers (S-layers) are protective protein coats which form around all archaea and most bacterial cells. Clostridium difficile is a Gram-positive bacterium with an S-layer covering its peptidoglycan cell wall. The S-layer in C. difficile is constructed mainly of S-layer protein A (SlpA), which is a key virulence factor and an absolute requirement for disease. S-layer biogenesis is a complex multi-step process, disruption of which has severe consequences for the bacterium. We examined the subcellular localization of SlpA secretion and S-layer growth; observing formation of S-layer at specific sites that coincide with cell wall synthesis, while the secretion of SlpA from the cell is relatively delocalized. We conclude that this delocalized secretion of SlpA leads to a pool of precursor in the cell wall which is available to repair openings in the S-layer formed during cell growth or following damage.

The Widely Conserved ebo Cluster Is Involved in Precursor Transport to the Periplasm during Scytonemin Synthesis in Nostoc punctiforme

Scytonemin is a dimeric indole-phenol sunscreen synthesized by some cyanobacteria under conditions of exposure to UVA radiation. While its biosynthetic pathway has been elucidated only partially, comparative genomics reveals that the scytonemin operon often contains a cluster of five highly conserved genes (ebo cluster) of unknown function that is widespread and conserved among several bacterial and algal phyla. We sought to elucidate the function of the ebo cluster in the cyanobacterium Nostoc punctiforme by constructing and analyzing in-frame deletion mutants (one for each ebo gene and one for the entire cluster). Under conditions of UVA induction, all ebo mutants were scytoneminless, and all accumulated a single compound, the scytonemin monomer, clearly implicating all ebo genes in scytonemin production. We showed that the scytonemin monomer also accumulated in an induced deletion mutant of scyE, a non-ebo scytonemin gene whose product is demonstrably targeted to the periplasm. Confocal autofluorescence microscopy revealed that the accumulation was confined to the cytoplasm in all ebo mutants but that that was not the case in the scyE deletion, with an intact ebo cluster, where the scytonemin monomer was also excreted to the periplasm. The results implicate the ebo cluster in the export of the scytonemin monomer to the periplasm for final oxidative dimerization by ScyE. By extension, the ebo gene cluster may play similar roles in metabolite translocation across many bacterial phyla. We discuss potential mechanisms for such a role on the basis of structural and phylogenetic considerations of the ebo proteins.IMPORTANCE Elucidating the biochemical and genetic basis of scytonemin constitutes an interesting challenge because of its unique structure and the unusual fact that it is partially synthesized in the periplasmic space. Our work points to the ebo gene cluster, associated with the scytonemin operon of cyanobacteria, as being responsible for the excretion of scytonemin intermediates from the cytoplasm into the periplasm during biosynthesis. Few conserved systems have been described that facilitate the membrane translocation of small molecules. Because the ebo cluster is well conserved among a large diversity of bacteria and algae and yet insights into its potential function are lacking, our findings suggest that translocation of small molecules across the plasma membrane may be its generic role across microbes.

In Vitro Characterization of the Two-Stage Non-Classical Reassembly Pathway of S-Layers

The recombinant bacterial surface layer (S-layer) protein rSbpA of Lysinibacillus sphaericus CCM 2177 is an ideal model system to study non-classical nucleation and growth of protein crystals at surfaces since the recrystallization process may be separated into two distinct steps: (i) adsorption of S-layer protein monomers on silicon surfaces is completed within 5 min and the amount of bound S-layer protein sufficient for the subsequent formation of a closed crystalline monolayer; (ii) the recrystallization process is triggered—after washing away the unbound S-layer protein—by the addition of a CaCl₂ containing buffer solution, and completed after approximately 2 h. The entire self-assembly process including the formation of amorphous clusters, the subsequent transformation into crystalline monomolecular arrays, and finally crystal growth into extended lattices was investigated by quartz crystal microbalance with dissipation (QCM-D) and atomic force microscopy (AFM). Moreover, contact angle measurements showed that the surface properties of S-layers change from hydrophilic to hydrophobic as the crystallization proceeds. This two-step approach is new in basic and application driven S-layer research and, most likely, will have advantages for functionalizing surfaces (e.g., by spray-coating) with tailor-made biological sensing layers.

Diversity in S-layers

Surface layers, referred simply as S-layers, are the two-dimensional crystalline arrays of protein or glycoprotein subunits on cell surface. They are one of the most common outermost envelope components observed in prokaryotic organisms (Archaea and Bacteria). Over the past decades, S-layers have become an issue of increasing interest due to their ubiquitousness, special features and functions. Substantial work in this field provides evidences of an enormous diversity in S-layers. This paper reviews and illustrates the diversity from several different aspects, involving the S-layer-carrying strains, the structure of S-layers, the S-layer proteins and genes, as well as the functions of S-layers.

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.

Novel protein a affinity matrix prepared from two-dimensional protein crystals

In this article, we describe a novel type of affinity matrix which was prepared by covalently binding Protein A to crystalline cell surface layers (S-layers) from the gram-positive Clostridium thermohydrosulfuricum L111-69. S-layers were used in the form of cell wall fragments, which were obtained by breaking whole cells by ultrasonification and removing the cell content and the plasma membrane. In these thimble shaped structures, revealing a size of 1 to 2 mum, the peptidoglycan-containing layer was covered on both faces with a hexagonally ordered S-layer lattice composed of identical glycoprotein subunits. After crosslinking the S-layer protein with glutaraldehyde, carboxyl groups from acidic amino acids were activated with carbodiimide and used for immobilization of Protein A. Quantitative determination confirmed that up to two Protein A molecules were bound per S-layer subunit leading to a dense monomolecular coverage of the immobilization matrix with the ligand.Affinity microparticles were capable of adsorbing lgG from solutions of purified preparations, from artificial lgG-albumin mixtures, and from serum. The amount of lgG bound to affinity microparticles corresponded to the theoretical saturation capacity. Under appropriate conditions, up to 95% of the adsorbed lgG could be eluted again. Affinity microparticles were found to have an extremely low Protein A leakage and a high stability toward mechanical forces. Because pores in the S-layer lattice revealed a size of 4 to 5 nm, immobilization of Protein A and adsorption of lgG was restricted to the outermost surface area. This excludes mass transfer problems usually encountered with affinity matrices prepared from amorphous polymers where more than 90% of the ligands are immobilized in the interior. (c) 1994 John Wiley & Sons, Inc.

Single-molecule force spectroscopy reveals the individual mechanical unfolding pathways of a surface layer protein

Surface layers (S-layers) represent an almost universal feature of archaeal cell envelopes and are probably the most abundant bacterial cell proteins. S-layers are monomolecular crystalline structures of single protein or glycoprotein monomers that completely cover the cell surface during all stages of the cell growth cycle, thereby performing their intrinsic function under a constant intra- and intermolecular mechanical stress. In gram-positive bacteria, the individual S-layer proteins are anchored by a specific binding mechanism to polysaccharides (secondary cell wall polymers) that are linked to the underlying peptidoglycan layer. In this work, atomic force microscopy-based single-molecule force spectroscopy and a polyprotein approach are used to study the individual mechanical unfolding pathways of an S-layer protein. We uncover complex unfolding pathways involving the consecutive unfolding of structural intermediates, where a mechanical stability of 87 pN is revealed. Different initial extensibilities allow the hypothesis that S-layer proteins adapt highly stable, mechanically resilient conformations that are not extensible under the presence of a pulling force. Interestingly, a change of the unfolding pathway is observed when individual S-layer proteins interact with secondary cell wall polymers, which is a direct signature of a conformational change induced by the ligand. Moreover, the mechanical stability increases up to 110 pN. This work demonstrates that single-molecule force spectroscopy offers a powerful tool to detect subtle changes in the structure of an individual protein upon binding of a ligand and constitutes the first conformational study of surface layer proteins at the single-molecule level.

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.

Analysis of high-level S-layer protein secretion inCaulobacter crescentus

Caulobacter crescentus exhibits a hexagonally arranged protein layer on its outermost surface. RsaA, the sole protein of this “S-layer”, is secreted by a type I (ABC) transporter. Few type I transporters show high-level secretion, and few bacterial S-layers have been carefully examined for the amount of protein synthesis capacity needed to maintain cell coverage. Here we determined RsaA levels by quantitative immunoblotting methods, learned that very stable mRNA is a key factor in high-level secretion, and found that the transporter was capable of still higher secretion. A propensity for RsaA to aggregate was a barrier to quantitation, but with the use of S-layer shedding mutants and methods to keep RsaA soluble, we learned that ~31% of cell protein is RsaA. When multiple copies of rsaA were introduced, the level increased to ~51% of cell protein, a higher level than we are aware of for any protein in any bacterium. Unexpectedly, in comparing normal and S-layer shedding strains, an assembled S-layer was not a significant barrier to elevated secretion. The rsaA mRNA half-life was determined by real-time PCR to be 36 min, ranking with the most stable known in bacteria. A modification of the 5′ region resulted in a shorter half-life and a reduction in maximum protein synthesis levels. If secretion was prevented by knockout of type I transporter genes, RsaA levels dropped to 10% or less of normal, but with no significant reduction in rsaA mRNA. Overall, normal levels of RsaA were unexpectedly high, and still higher levels were not limited by transporter capability, the presence of an assembled S-layer, or the capacity of the cell’s physiology to produce large amounts of one protein. The normal upper limit of RsaA production appears to be controlled only by the level of an unusually stable message. Significant down-regulation is possible and is accomplished posttranscriptionally.

The high-molecular-mass amylase (HMMA) of Geobacillus stearothermophilus ATCC 12980 interacts with the cell wall components by virtue of three specific binding regions

The complete nucleotide sequence encoding the high-molecular-mass amylase (HMMA) of Geobacillus stearothermophilus ATCC 12980 was established by PCR techniques. Based on the hmma gene sequence, the full-length rHMMA, four N- or C-terminal rHMMA truncations as well as three C-terminal rHMMA fragments were cloned and heterologously expressed in Escherichia coli. Purified rHMMA forms were used either for affinity studies with the recombinant (r) S-layer protein SbsC (rSbsC), peptidoglycan-containing sacculi (PGS) and pure peptidoglycan (PG) devoid of the secondary cell wall polymer (SCWP), or for surface plasmon resonance (SPR) studies using rSbsC and isolated SCWP. In the C-terminal part of the HMMA, three specific binding regions, one for each cell wall component (rSbsC, SCWP and PG), could be identified. The functionality of the PG-binding domain could be confirmed by replacing the main part of the SCWP-binding domain of an S-layer protein by the PG-binding domain of the HMMA. The present work describes a completely new and highly economic strategy for cell adhesion of an exoenzyme.

Molecular organization of selected prokaryotic S-layer proteins

Regular crystalline surface layers (S-layers) are widespread among prokaryotes and probably represent the earliest cell wall structures. S-layer genes have been found in approximately 400 different species of the prokaryotic domains bacteria and archaea. S-layers usually consist of a single (glyco-)protein species with molecular masses ranging from about 40 to 200 kDa that form lattices of oblique, tetragonal, or hexagonal architecture. The primary sequences of hyperthermophilic archaeal species exhibit some characteristic signatures. Further adaptations to their specific environments occur by various post-translational modifications, such as linkage of glycans, lipids, phosphate, and sulfate groups to the protein or by proteolytic processing. Specific domains direct the anchoring of the S-layer to the underlying cell wall components and transport across the cytoplasma membrane. In addition to their presumptive original role as protective coats in archaea and bacteria, they have adapted new functions, e.g., as molecular sieves, attachment sites for extracellular enzymes, and virulence factors.

Periplasm, periplasmic spaces, and their relation to bacterial wall structure: novel secretion of selected periplasmic proteins from Pseudomonas aeruginosa

A brief overview of thin sections of cryopreserved walls from select eubacteria will be presented to suggest that all bacteria have functional periplasms, but that these are not necessarily confined to a periplasmic space such as found in typical gram-negative bacteria. Pseudomonas aeruginosa contains many components in its periplasmic space, some of which are required for infection. Throughout its growth cycle, P. aeruginosa blebs-off membrane vesicles that can possess DNA, endotoxin, phospholipase, protease, hemolysin, alkaline phosphatase, and autolysin, each of which must have a molecular phase that resides in the periplasm. These membrane packets make good delivery systems to convey these components to other bacteria and, possibly, tissue. Aminoglycoside antibiotics, such as gentamicin, produce a serious perturbation on the bacterium's surface (separate from the ribosomal effect), which contributes to the killing of the microorganism. Antibiotics such as this increase the size and number of the membrane blebs, which could contribute to septic shock of patients under drug therapy.

S-layer protein production by Corynebacterium strains is dependent on the carbon source

Three strains of Corynebacterium producing various amounts of PS2 S-layer protein were studied. For all strains, more PS2 was produced if the bacteria were grown in minimal medium supplemented with lactate than if they were grown in minimal medium supplemented with glucose. The consumption of substrate and PS2 production was studied in cultures with mixed carbon sources. It was found that the inhibitory effect of glucose consumption was stronger than the stimulatory effect of lactate in one strain, but not in the other two strains. The regulation of gene expression involved in S-layer formation may involve metabolic pathways, which probably differ between strains. S-layer organization was also studied by freeze-fracture electron microscopy. It was found that low levels of PS2 production correlated with the partial covering of the cell surface by a crystalline array. Finally, it was found that PS2 production was mainly regulated by changes in gene expression and that secretion was probably not a limiting step in PS2 accumulation.

Structural research on surface layers: a focus on stability, surface layer homology domains, and surface layer-cell wall interactions

Surface layers (S-layers) from Bacteria and Archaea are built from protein molecules arrayed in a two-dimensional lattice, forming the outermost cell wall layer in many prokaryotes. In almost half a century of S-layer research a wealth of structural, biochemical, and genetic data have accumulated, but it has not been possible to correlate sequence data with the tertiary structure of S-layer proteins to date. In this paper, some highlights of structural aspects of archaeal and bacterial S-layers that allow us to draw some conclusions on molecular properties are reviewed. We focus on the structural requirements for the extraordinary stability of many S-layer proteins, the structural and functional aspects of the S-layer homology domain found in S-layers, extracellular enzymes and related functional proteins, and outer membrane proteins, and the molecular interactions of S-layer proteins with other cell wall components. Finally, the perspectives and requirements for structural research on S-layers, which indicate that the investigation of isolated protein domains will be a prerequisite for solving S-layer structures at atomic resolution, are discussed.

A novel dipstick developed for rapid Bet v 1-specific IgE detection: recombinant allergen immobilized via a monoclonal antibody to crystalline bacterial cell-surface layers

The incidence of allergy to airborne proteins derived from tree and grass pollen, feces of mites, spores of molds, and pet dander has been increasing over the last decades. Since precise diagnosis is a prerequisite for successful immunotherapy, there is a rising demand for rapid, reliable, and inexpensive screening methods such as dipstick assays. With the purified recombinant major birch-pollen allergen rBet v 1a as model protein, crystalline bacterial cell-surface layers (S-layers) were tested for their applicability as an immobilization matrix for dipstick development. For this purpose, S-layers were deposited on a mechanically stable microporous support, cross-linked with glutaraldehyde, and free carboxylic acid groups of the S-layer protein were activated with carbodiimide. In the present test system, rBet v 1a was immobilized via the monoclonal mouse antibody BIP 1, which, unlike the allergen, is too large to enter the pores of the S-layer lattice, and which therefore formed a closed monolayer on the outermost surface of the crystal lattice. Moreover, BIP 1 is known to modulate IgE binding to the allergen. After incubation of the dipsticks in serum, washing of the reaction zone under tap water, and binding of an anti-IgE alkaline phosphatase conjugate, 5-bromo-4-chloro-3-indolyl phosphate/nitro blue tetrazolium was used as substrate, forming an IgE concentration-dependent colored precipitate on the S-layer surface. The investigation of patient sera previously tested with the CAP system confirmed the specificity of the S-layer-based dipstick assay. Since the dipstick is easy to handle and the whole test procedure takes only 90 min, this test system should be applicable for rapid determination of specific IgE and for first screening in the doctor's practice.

Influence of the secondary cell wall polymer on the reassembly, recrystallization, and stability properties of the S-layer protein from Bacillus stearothermophilus PV72/p2

The high-molecular-weight secondary cell wall polymer (SCWP) from Bacillus stearothermophilus PV72/p2 is mainly composed of N-acetylglucosamine (GlcNAc) and N-acetylmannosamine (ManNAc) and is involved in anchoring the S-layer protein via its N-terminal region to the rigid cell wall layer. In addition to this binding function, the SCWP was found to inhibit the formation of self-assembly products during dialysis of the guanidine hydrochloride (GHCl)-extracted S-layer protein. The degree of assembly (DA; percent assembled from total S-layer protein) that could be achieved strongly depended on the amount of SCWP added to the GHCl-extracted S-layer protein and decreased from 90 to 10% when the concentration of the SCWP was increased from 10 to 120 microg/mg of S-layer protein. The SCWP kept the S-layer protein in the water-soluble state and favored its recrystallization on solid supports such as poly-L-lysine-coated electron microscopy grids. Derived from the orientation of the base vectors of the oblique S-layer lattice, the subunits had bound with their charge-neutral outer face, leaving the N-terminal region with the polymer binding domain exposed to the ambient environment. From cell wall fragments about half of the S-layer protein could be extracted with 1 M GlcNAc, indicating that the linkage type between the S-layer protein and the SCWP could be related to that of the lectin-polysaccharide type. Interestingly, GlcNAc had an effect on the in vitro self-assembly and recrystallization properties of the S-layer protein that was similar to that of the isolated SCWP. The SCWP generally enhanced the stability of the S-layer protein against endoproteinase Glu-C attack and specifically protected a potential cleavage site in position 138 of the mature S-layer protein.

Functions of S-layers

Although S-layers are being increasingly identified on Bacteria and Archaea, it is enigmatic that in most cases S-layer function continues to elude us. In a few instances, S-layers have been shown to be virulence factors on pathogens (e.g. Campylobacter fetus ssp. fetus and Aeromonas salmonicida), protective against Bdellovibrio, a depository for surface-exposed enzymes (e.g. Bacillus stearothermophilus), shape-determining agents (e.g. Thermoproteus tenax) and nucleation factors for fine-grain mineral development (e.g. Synechococcus GL 24). Yet, for the vast majority of S-layered bacteria, the natural function of these crystalline arrays continues to be evasive. The following review up-dates the functional basis of S-layers and describes such diverse topics as the effect of S-layers on the Gram stain, bacteriophage adsorption in lactobacilli, phagocytosis by human polymorphonuclear leukocytes, the adhesion of a high-molecular-mass amylase, outer membrane porosity, and the secretion of extracellular enzymes of Thermoanaerobacterium. In addition, the functional aspect of calcium on the Caulobacter S-layer is explained.

Evidence that the N-terminal part of the S-layer protein from Bacillus stearothermophilus PV72/p2 recognizes a secondary cell wall polymer

The S-layer of Bacillus stearothermophilus PV72/p2 shows oblique lattice symmetry and is composed of identical protein subunits with a molecular weight of 97,000. The isolated S-layer subunits could bind and recrystallize into the oblique lattice on native peptidoglycan-containing sacculi which consist of peptidoglycan of the A1gamma chemotype and a secondary cell wall polymer with an estimated molecular weight of 24,000. The secondary cell wall polymer could be completely extracted from peptidoglycan-containing sacculi with 48% HF, indicating the presence of phosphodiester linkages between the polymer chains and the peptidoglycan backbone. The cell wall polymer was composed mainly of GlcNAc and ManNAc in a molar ratio of 4:1, constituted about 20% of the peptidoglycan-containing sacculus dry weight, and was also detected in the fraction of the S-layer self-assembly products. Extraction experiments and recrystallization of the whole S-layer protein and proteolytic cleavage fragments confirmed that the secondary cell wall polymer is responsible for anchoring the S-layer subunits by the N-terminal part to the peptidoglycan-containing sacculi. In addition to this binding function, the cell wall polymer was found to influence the in vitro self-assembly of the guanidinium hydrochloride-extracted S-layer protein. Chemical modification studies further showed that the secondary cell wall polymer does not contribute significant free amino or carboxylate groups to the peptidoglycan-containing sacculi.

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.

In vivo expression of the Lactobacillus brevis S-layer gene

Lactobacillus brevis possesses a surface layer protein (SlpA) with tightly regulated synthesis. The slpA gene is expressed by two adjacent promoters, P1 and P2. The level of P2-derived transcripts was approximately 10 times higher than that of P1-derived transcripts throughout the entire growth of L. brevis. The half-lives of slpA transcripts were shown to be exceptionally long (14 min).

Evidence that an N-terminal S-layer protein fragment triggers the release of a cell-associated high-molecular-weight amylase in Bacillus stearothermophilus ATCC 12980

During growth on starch medium, the S-layer-carrying Bacillus stearothermophilus ATCC 12980 and an S-layer-deficient variant each secreted three amylases, with identical molecular weights of 58,000, 122,000, and 184,000, into the culture fluid. Only the high-molecular-weight amylase (hmwA) was also identified as cell associated. Extraction and reassociation experiments showed that the hmwA had a high-level affinity to the peptidoglycan-containing layer and to the S-layer surface, but the interactions with the peptidoglycan-containing layer were stronger than those with the S-layer surface. For the S-layer-deficient variant, no changes in the amount of cell-associated and free hmwA could be observed during growth on starch medium, while for the S-layer-carrying strain, cell association of the hmwA strongly depended on the growth phase of the cells. The maximum amount of cell-associated hmwA was observed 3 h after inoculation, which corresponded to early exponential growth. The steady decrease in cell-associated hmwA during continued growth correlated with the appearance and the increasing intensity of a protein with an apparent molecular weight of 60,000 on sodium dodecyl sulfate gels. This protein had a high-level affinity to the peptidoglycan-containing layer and was identified as an N-terminal S-layer protein fragment which did not result from proteolytic cleavage of the whole S-layer protein but seems to be a truncated copy of the S-layer protein which is coexpressed with the hmwA under certain culture conditions. During growth on starch medium, the N-terminal S-layer protein fragment was integrated into the S-layer lattice, which led to the loss of its regular structure over a wide range and to the loss of amylase binding sites. Results obtained in the present study provide evidence that the N-terminal part of the S-layer protein is responsible for the anchoring of the subunits to the peptidoglycan-containing layer, while the surface-located C-terminal half could function as a binding site for the hmwA.

Two-dimensional paracrystalline glycoprotein S-layers as a novel matrix for the immobilization of human IgG and their use as microparticles in immunoassays

In the present study, cup-shaped 1-3 microns large cell wall fragments from Thermoanaerobacter thermohydrosulfuricus L111-69 covered with a hexagonal S-layer lattice composed of glycoprotein subunits were shown to act as a matrix for the immobilization of human IgG. After cross-linking the S-layer glycoprotein lattice with glutaraldehyde (S-layer microparticles), IgG was either bound to carbodiimide activated carboxyl groups from acidic amino acids from the protein moiety or to the carbohydrate chains activated with cyanogen bromide or oxidized with periodate. After determining the binding capacity of the S-layer lattice for human IgG, the orientation of the immobilized antibody molecules was investigated using anti-human IgG peroxidase conjugates with different specificity. Attachment of S-layer microparticles with covalently bound human IgG to microplates precoated with anti-human IgG of different specificity led to clear correlations between the amount of applied human IgG and the absorption values in the immunoassays. The steepest absorption curves were obtained when human IgG was bound to the carbohydrate chains exposed on the surface of the S-layer lattice. This confirmed that the location and the accessibility of the immobilized antibodies on S-layer microparticles is of major importance for the response in immunoassays. In addition to the high reproducibility of the amount of IgG which could be bound to the S-layer lattice and the high reproducibility of the absorption curves in the immunoassays, one major advantage of using cup-shaped S-layer microparticles can be seen in the considerable increase of the actual surface available for binding processes and immunological reactions.

Immunoreactivity of allergen (Bet v 1) conjugated to crystalline bacterial cell surface layers (S-layers)

BACKGROUND

Crystalline cell surface layers (S-layers) from Gram-positive eubacteria had been demonstrated as carrier/adjuvants for chemically synthesized tumor-associated oligosaccharide haptens and capsular polysaccharide antigens of Streptococcus pneumoniae strains.

OBJECTIVES

The applicability of S-layers as vaccine carrier for treatment of Type I allergy was investigated.

STUDY DESIGN

Native or cross-linked S-layer self-assembly products and cell wall preparations from Bacillus sphaericus CCM 2177 and Thermoanaerobacter thermohydrosulfuricus L111-69 and L110-69 were used for immobilization of recombinant major birch pollen allergen Bet v 1.

RESULTS AND CONCLUSIONS

Depending on the carrier used, amounts of approximately 20-40 micrograms allergen per mg conjugate could be immobilized. By application of L-glutamic acid dimethyl ester as a spacer, this value could be increased approximately 10-fold. The functionality of the rBet v 1-conjugates was assessed in immunological systems. (i) The presence of intact B-cell epitopes was demonstrated in inhibition experiments using human Bet v 1-specific IgE. (ii) The rBet v 1-S-layer conjugates were immunogenic in mice. (iii) The proliferation of rBet v 1-specific T-cell clones suggested that the peptides created by processing of immobilized Bet v 1 were similar to those derived from natural allergen. (iv) Stimulation of human allergen-specific TH2 lymphocytes with S-layer-conjugated Bet v 1 led to a modulation of the cytokine production pattern from TH2 to TH0/TH1. This study indicates that S-layers may be suitable carriers for few immunotherapeutical vaccines for Type 1 hypersensitivity.

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.

The S-layer from Bacillus stearothermophilus DSM 2358 functions as an adhesion site for a high-molecular-weight amylase

The S-layer lattice from Bacillus stearothermophilus DSM 2358 completely covers the cell surface and exhibits oblique symmetry. During growth of B. stearothermophilus DSM 2358 on starch medium, three amylases with molecular weights of 58,000, 98,000, and 184,000 were secreted into the culture fluid, but only the high-molecular-weight enzyme was found to be cell associated. Studies of interactions between cell wall components and amylases revealed no affinity of the high-molecular-weight amylase to isolated peptidoglycan. On the other hand, this enzyme was always found to be associated with S-layer self-assembly products or S-layer fragments released during preparation of spheroplasts by treatment of whole cells with lysozyme. The molar ratio of S-layer subunits to the bound amylase was approximately 8:1, which corresponded to one enzyme molecule per four morphological subunits. Immunoblotting experiments with polyclonal antisera against the high-molecular-weight amylase revealed a strong immunological signal in response to the enzyme but no cross-reaction with the S-layer protein or the smaller amylases. Immunogold labeling of whole cells with anti-amylase antiserum showed that the high-molecular-weight amylase is located on the outer face of the S-layer lattice. Because extraction of the amylase was possible without disintegration of the S-layer lattice into its constituent subunits, it can be excluded that the enzyme is incorporated into the crystal lattice and participates in the self-assembly process. Affinity experiments strongly suggest the presence of a specific recognition mechanism between the amylase molecules and S-layer protein domains either exposed on the outermost surface or inside the pores. In summary, results obtained in this study confirmed that the S-layer protein from B. stearothermophilus DSM 2358 functions as an adhesion site for a high-molecular-weight amylase.

Isolation of two physiologically induced variant strains of Bacillus stearothermophilus NRS 2004/3a and characterization of their S-layer lattices

During growth of Bacillus stearothermophilus NRS 2004/3a in continuous culture on complex medium, the chemical properties of the S-layer glycoprotein and the characteristic oblique lattice were maintained only if glucose was used as the sole carbon source. With increased aeration, amino acids were also metabolized, accompanied by liberation of ammonium and by changes in the S-layer protein. Depending on the stage of fermentation at which oxygen limitation was relieved, two different variants, one with a more delicate oblique S-layer lattice (variant 3a/V1) and one with a square S-layer lattice (variant 3a/V2), were isolated. During the switch from the wild-type strain to a variant or from variant 3a/V2 to variant 3a/V1, monolayers of two types of S-layer lattices could be demonstrated on the surfaces of single cells. S-layer proteins from variants had different molecular sizes and a significantly lower carbohydrate content than S-layer proteins from the wild-type strain did. Although the S-layer lattices from the wild-type and variant strains showed quite different protein mass distributions in two- and three-dimensional reconstructions, neither the amino acid composition nor the pore size, as determined by permeability studies, was significantly changed. Peptide mapping and N-terminal sequencing results strongly indicated that the three S-layer proteins are encoded by different genes and are not derived from a universal precursor form.

Crystalline bacterial cell surface layers

Crystalline arrays of proteinaceous subunits forming surface layers (S-layers) are one of the most commonly observed prokaryotic cell envelope structures. They are ubiquitous amongst Gram-positive and Gram-negative archeaobacteria and eubacteria and, if present, account for the major protein species produced by the cells. S-layers can provide organisms with a selection advantage by providing various functions including protective coats, molecular sieves, ion traps and structures involved in cell surface interactions. S-layers were identified as contributing to virulence when present as a structural component of pathogens. In Gram-negative archaeobacteria they are involved in determining cell shape and cell division. The crystalline arrays reveal a broad-application potential in biotechnology, vaccine development and molecular nanotechnology.

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

In this paper, the importance of charged amino and carboxyl groups for the integrity of the cell surface layer (S-layer) lattice from Bacillus coagulans E38-66 and for the self-assembly of the isolated subunits was investigated. Amidination of the free amino groups which preserved their positive net charge had no influence on both. On the other hand, acetylation and succinylation, which converted the amino groups into either neutral or negatively charged groups, and amidation of carboxyl groups were accompanied by the disintegration or at least by the loss of the regular structure of the S-layer lattice. Treatment of S-layer monolayers with the zero-length cross-linker carbodiimide led to the introduction of peptide bonds between activated carboxyl groups and amino groups from adjacent subunits. This clearly indicated that in the native S-layer lattice the charged groups are located closely enough for direct electrostatic interactions. Under disrupting conditions in which the S-layer polypeptide chains were unfolded, 58% of the Asx and Glx residues could be amidated, indicating that they occur in the free carboxylic acid form. As derived from chemical modification of monolayer self-assembly products, about 90% of the lysine and 70% of the aspartic and glutamic acid residues are aligned on the surface of the S-layer protein domains. This corresponded to 45 amino groups and to 63 carboxyl groups per S-layer subunit. Labelling experiments with macromolecules with different sizes and charges and adsorption studies with ion-exchange particles revealed a surplus of free carboxyl groups on the inner and on the outer faces of the S-layer lattice. Since the carboxyl groups on the outer S-layer face were accessible only for protein molecules significantly smaller then the S-layer protomers or for positively charged, thin polymer chains extending from the surface of ion-exchange beads, the negatively charged sites must be located within indentations of the corrugated S-layer protein network. This was in contrast to the carboxyl groups on the inner S-layer face, which were found to be exposed on elevations of the S-layer protein domains (D. Pum, M. Sára, and U.B. Sleytr, J. Bacteriol. 171:5296-5303, 1989).