Functional expression of adhesive peptides as fusions to Escherichia coli flagellin

Navigating the landscape of PD-1/PD-L1 imaging tracers: from challenges to opportunities

Immunotherapy targeted to immune checkpoint inhibitors, such as the program cell death receptor (PD-1) and its ligand (PD-L1), has revolutionized cancer treatment. However, it is now well-known that PD-1/PD-L1 immunotherapy response is inconsistent among patients. The current challenge is to customize treatment regimens per patient, which could be possible if the PD-1/PD-L1 expression and dynamic landscape are known. With positron emission tomography (PET) imaging, it is possible to image these immune targets non-invasively and system-wide during therapy. A successful PET imaging tracer should meet specific criteria concerning target affinity, specificity, clearance rate and target-specific uptake, to name a few. The structural profile of such a tracer will define its properties and can be used to optimize tracers in development and design new ones. Currently, a range of PD-1/PD-L1-targeting PET tracers are available from different molecular categories that have shown impressive preclinical and clinical results, each with its own advantages and disadvantages. This review will provide an overview of current PET tracers targeting the PD-1/PD-L1 axis. Antibody, peptide, and antibody fragment tracers will be discussed with respect to their molecular characteristics and binding properties and ways to optimize them.

Escherichia coli O157:H7 F9 Fimbriae Recognize Plant Xyloglucan and Elicit a Response in Arabidopsis thaliana

Fresh produce is often a source of enterohaemorrhagic Escherichia coli (EHEC) outbreaks. Fimbriae are extracellular structures involved in cell-to-cell attachment and surface colonisation. F9 (Fml) fimbriae have been shown to be expressed at temperatures lower than 37 °C, implying a function beyond the mammalian host. We demonstrate that F9 fimbriae recognize plant cell wall hemicellulose, specifically galactosylated side chains of xyloglucan, using glycan arrays. E. coli expressing F9 fimbriae had a positive advantage for adherence to spinach hemicellulose extract and tissues, which have galactosylated oligosaccharides as recognized by LM24 and LM25 antibodies. As fimbriae are multimeric structures with a molecular pattern, we investigated whether F9 fimbriae could induce a transcriptional response in model plant Arabidopsis thaliana, compared with flagella and another fimbrial type, E. coli common pilus (ECP), using DNA microarrays. F9 induced the differential expression of 435 genes, including genes involved in the plant defence response. The expression of F9 at environmentally relevant temperatures and its recognition of plant xyloglucan adds to the suite of adhesins EHEC has available to exploit the plant niche.

Crosstalk between bacterial conjugation and motility is mediated by plasmid-borne regulators

Plasmid conjugation is a major horizontal gene transfer mechanism. The acquisition of a plasmid may cause a perturbation of the cell functions in addition to provide advantageous properties for the recipient cell, such as the gaining of antibiotic resistances. The interplay between plasmid and chromosomal functions has been studied using the IncHI1 plasmid R27. Plasmids of the incompatibility group HI1, isolated from several Gram-negative pathogens, are associated with the spread of multidrug resistance. Their conjugation is tightly regulated by temperature, being repressed at temperatures within the host (37°C). In this report, we described that at permissive temperature, when conjugation of plasmid R27 is prompted, a reduction in the motility of the cells is observed. This reduction is mediated by the plasmid-encoded regulators TrhR/TrhY, which together with HtdA form a plasmid-borne regulatory circuit controlling R27 conjugation. TrhR/TrhY, required to induce R27 conjugation, is responsible for the downregulation of the flagella synthesis and the consequent decrease in motility. TrhR/TrhY repress, direct or indirectly, the expression of the specific flagellar sigma subunit FliA and, consequently, the expression of all genes located bellow in the flagellar expression cascade.

Genetically Engineered Flagella Form Collagen-like Ordered Structures for Inducing Stem Cell Differentiation

Bacteria use flagella, the protein nanofibers on their surface, as a molecular machine to swim. Flagella are polymerized from monomers, flagellins, which can display a peptide by genetic means. However, flagella as genetically modifiable nanofibers have not been used in building bone extracellular matrix-like structures for inducing stem cell differentiation in non-osteogenic medium. Here we discovered that interactions between Ca2+ ions and flagella (displaying a collagen-like peptide (GPP)8 on every flagellin) resulted in ordered bundle-like structures, which were further mineralized with hydroxyapatite to form ordered fibrous matrix. The resultant matrix significantly induced the osteogenic differentiation of stem cells, much more efficiently than wild-type flagella and type I collagen. This work shows that flagella can be used as protein building blocks in generating biomimetic materials.

Enhanced Biofilm Formation and Membrane Vesicle Release by Escherichia coli Expressing a Commonly Occurring Plasmid Gene, kil

Escherichia coli is one of the most prevalent microorganisms forming biofilms on indwelling medical devices, as well as a representative model to study the biology and ecology of biofilms. Here, we report that a small plasmid gene, kil, enhances biofilm formation of E. coli. The kil gene is widely conserved among naturally occurring colicinogenic plasmids such as ColE1 plasmid, and is also present in some plasmid derivatives used as cloning vectors. First, we found that overexpression of the kil gene product dramatically increased biofilm mass enriched with extracellular DNA in the outer membrane-compromised strain RN102, a deep rough LPS mutant E. coli K-12 derivative. We also found that the kil-enhanced biofilm formation was further promoted by addition of physiologically relevant concentrations of Mg2+, not only in the case of RN102, but also with the parental strain BW25113, which retains intact core-oligosaccharide LPS. Biofilm formation by kil-expressing BW25113 strain (BW25113 kil+ ) was significantly inhibited by protease but not DNase I. In addition, a large amount of proteinous materials were released from the BW25113 kil+ cells. These materials contained soluble cytoplasmic and periplasmic proteins, and insoluble membrane vesicles (MVs). The kil-induced MVs were composed of not only outer membrane/periplasmic proteins, but also inner membrane/cytoplasmic proteins, indicating that MVs from both of the outer and inner membranes could be released into the extracellular milieu. Subcellular fractionation analysis revealed that the Kil proteins translocated to both the outer and inner membranes in whole cells of BW25113 kil+ . Furthermore, the BW25113 kil+ showed not only reduced viability in the stationary growth phase, but also increased susceptibility to killing by predator bacteria, Vibrio cholerae expressing the type VI secretion system, despite no obvious change in morphology and physiology of the bacterial membrane under regular culture conditions. Taken together, our findings suggest that there is risk of increasing biofilm formation and spreading of numerous MVs releasing various cellular components due to kil gene expression. From another point of view, our findings could also offer efficient MV production strategies using a conditional kil vector in biotechnological applications.

Bacterial Flagellins: Does Size Matter?

The bacterial flagellum is the principal organelle of motility in bacteria. Here, we address the question of size when applied to the chief flagellar protein flagellin and the flagellar filament. Surprisingly, nature furnishes multiple examples of 'giant flagellins' greater than a thousand amino acids in length, with large surface-exposed hypervariable domains. We review the contexts in which these giant flagellins occur, speculate as to their functions, and highlight the potential for biotechnology to build on what nature provides.

Discovery of a proteolytic flagellin family in diverse bacterial phyla that assembles enzymatically active flagella

Bacterial flagella are cell locomotion and occasional adhesion organelles composed primarily of the polymeric protein flagellin, but to date have not been associated with any enzymatic function. Here, we report the bioinformatics-driven discovery of a class of enzymatic flagellins that assemble to form proteolytically active flagella. Originating by a metallopeptidase insertion into the central flagellin hypervariable region, this flagellin family has expanded to at least 74 bacterial species. In the pathogen, Clostridium haemolyticum, metallopeptidase-containing flagellin (which we termed flagellinolysin) is the second most abundant protein in the flagella and is localized to the extracellular flagellar surface. Purified flagellar filaments and recombinant flagellin exhibit proteolytic activity, cleaving nearly 1000 different peptides. With ~ 20,000 flagellin copies per  ~ 10-μm flagella this assembles the largest proteolytic complex known. Flagellum-mediated extracellular proteolysis expands our understanding of the functional plasticity of bacterial flagella, revealing this family as enzymatic biopolymers that mediate interactions with diverse peptide substrates.

So far no enzymatic activity has been attributed to flagellin, the major component of bacterial flagella. Here the authors use bioinformatic analysis and identify a metallopeptidase insertion in flagellins from 74 bacterial species and show that recombinant flagellin and flagellar filaments have proteolytic activity.

RGD Peptide Cell-Surface Display Enhances the Targeting and Therapeutic Efficacy of Attenuated Salmonella-mediated Cancer Therapy

Bacteria-based anticancer therapies aim to overcome the limitations of current cancer therapy by actively targeting and efficiently removing cancer. To achieve this goal, new approaches that target and maintain bacterial drugs at sufficient concentrations during the therapeutic window are essential. Here, we examined the tumor tropism of attenuated Salmonella typhimurium displaying the RGD peptide sequence (ACDCRGDCFCG) on the external loop of outer membrane protein A (OmpA). RGD-displaying Salmonella strongly bound to cancer cells overexpressing αvβ3, but weakly bound to αvβ3-negative cancer cells, suggesting the feasibility of displaying a preferential homing peptide on the bacterial surface. In vivo studies revealed that RGD-displaying Salmonellae showed strong targeting efficiency, resulting in the regression in αvβ3-overexpressing cancer xenografts, and prolonged survival of mouse models of human breast cancer (MDA-MB-231) and human melanoma (MDA-MB-435). Thus, surface engineering of Salmonellae to display RGD peptides increases both their targeting efficiency and therapeutic effect.

Escherichia coli common pilus (ECP) targets arabinosyl residues in plant cell walls to mediate adhesion to fresh produce plants

Outbreaks of verotoxigenic Escherichia coli are often associated with fresh produce. However, the molecular basis to adherence is unknown beyond ionic lipid-flagellum interactions in plant cell membranes. We demonstrate that arabinans present in different constituents of plant cell walls are targeted for adherence by E. coli common pilus (ECP; or meningitis-associated and temperature-regulated (Mat) fimbriae) for E. coli serotypes O157:H7 and O18:K1:H7. l-Arabinose is a common constituent of plant cell wall that is rarely found in other organisms, whereas ECP is widespread in E. coli and other environmental enteric species. ECP bound to oligosaccharides of at least arabinotriose or longer in a glycan array, plant cell wall pectic polysaccharides, and plant glycoproteins. Recognition overlapped with the antibody LM13, which binds arabinanase-sensitive pectic epitopes, and showed a preferential affinity for (1→5)-α-linked l-arabinosyl residues and longer chains of arabinan as demonstrated with the use of arabinan-degrading enzymes. Functional adherence in planta was mediated by the adhesin EcpD in combination with the structural subunit, EcpA, and expression was demonstrated with an ecpR-GFP fusion and ECP antibodies. Spinach was found to be enriched for ECP/LM13 targets compared with lettuce. Specific recognition of arabinosyl residues may help explain the persistence of E. coli in the wider environment and association of verotoxigenic E. coli with some fresh produce plants by exploitation of a glycan found only in plant, not animal, cells.

Attachment of Escherichia coli to Listeria monocytogenes for pediocin-mediated killing

Listeria phage endolysin cell wall-binding domain (CBD) from the Listeria phage A500 was fused with flagellar subunit FliC in Escherichia coli, aiming at binding of E. coli cells to Listeria cells, followed by enhanced killing of Listeria by pediocin production. FliC::CBD chimeric flagella were expressed and detected by Western blot. However, only few chimeric flagella could be isolated from the recombinant cells compared with sufficient amount of wild-type flagella obtained from the host cells. Interestingly, wild-type flagella extract showed capacity of binding Listeria cells. Pediocin-secreting E. coli cells with Listeria-binding flagella killed approximately 40 % of the Listeria cells, whereas cell-free spent growth medium with the same pediocin concentration only inhibited Listeria growth. These results suggested that binding the Listeria to bacteriocin-secreting cells improves killing.

A proteinaceous fraction of wheat bran may interfere in the attachment of enterotoxigenic E. coli K88 (F4+) to porcine epithelial cells

Wheat bran (WB) from Triticum aestivum has many beneficial effects on human health. To the best of our knowledge, very little has been published about its ability to prevent pathogenic bacterial adhesion in the intestine. Here, a WB extract was fractionated using different strategies, and the obtained fractions were tested in different in vitro methodologies to evaluate their interference in the attachment of enterotoxigenic Escherichia coli (ETEC) K88 to intestinal porcine epithelial cells (IPEC-J2) with the aim of identifying the putative anti-adhesive molecules. It was found that a proteinaceous compound in the >300-kDa fraction mediates the recognition of ETEC K88 to IPEC-J2. Further fractionation of the >300-kDa sample by size-exclusion chromatography showed several proteins below 90 kDa, suggesting that the target protein belongs to a high-molecular-weight (MW) multi-component protein complex. The identification of some relevant excised bands was performed by mass spectrometry (MS) and mostly revealed the presence of various protease inhibitors (PIs) of low MW: Serpin-Z2B, Class II chitinase, endogenous alpha-amylase/subtilisin inhibitor and alpha-amylase/trypsin inhibitor CM3. Furthermore, an incubation of the WB extract with ETEC K88 allowed for the identification of a 7S storage protein globulin of wheat, Globulin 3 of 66 kDa, which may be one of the most firmly attached WB proteins to ETEC K88 cells. Further studies should be performed to gain an understanding of the molecular recognition of the blocking process that takes place. All gathered information can eventually pave the way for the development of novel anti-adhesion therapeutic agents to prevent bacterial pathogenesis.

Preparation of sticky Escherichia coli through surface display of an adhesive catecholamine moiety

Mussels attach to virtually all types of inorganic and organic surfaces in aqueous environments, and catecholamines composed of 3,4-dihydroxy-l-phenylalanine (DOPA), lysine, and histidine in mussel adhesive proteins play a key role in the robust adhesion. DOPA is an unusual catecholic amino acid, and its side chain is called catechol. In this study, we displayed the adhesive moiety of DOPA-histidine on Escherichia coli surfaces using outer membrane protein W as an anchoring motif for the first time. Localization of catecholamines on the cell surface was confirmed by Western blot and immunofluorescence microscopy. Furthermore, cell-to-cell cohesion (i.e., cellular aggregation) induced by the displayed catecholamine and synthesis of gold nanoparticles on the cell surface support functional display of adhesive catecholamines. The engineered E. coli exhibited significant adhesion onto various material surfaces, including silica and glass microparticles, gold, titanium, silicon, poly(ethylene terephthalate), poly(urethane), and poly(dimethylsiloxane). The uniqueness of this approach utilizing the engineered sticky E. coli is that no chemistry for cell attachment are necessary, and the ability of spontaneous E. coli attachment allows one to immobilize the cells on challenging material surfaces such as synthetic polymers. Therefore, we envision that mussel-inspired catecholamine yielded sticky E. coli that can be used as a new type of engineered microbe for various emerging fields, such as whole living cell attachment on versatile material surfaces, cell-to-cell communication systems, and many others.

Enhanced biofilm formation by Escherichia coli LPS mutants defective in Hep biosynthesis

Lipopolysaccharide (LPS) is the major component of the surface of Gram-negative bacteria and its polysaccharide portion is situated at the outermost region. We investigated the relationship between the polysaccharide portion of LPS and biofilm formation using a series of Escherichia coli mutants defective in genes earlier shown to affect the LPS sugar compositions. Biofilm formation by a deep rough LPS mutant, the hldE strain, was strongly enhanced in comparison with the parental strain and other LPS mutants. The hldE strain also showed a phenotype of increased auto-aggregation and stronger cell surface hydrophobicity compared to the wild-type. Similar results were obtained with another deep rough LPS mutant, the waaC strain whose LPS showed same molecular mass as that of the hldE strain. Confocal laser scanning microscopy (CLSM) analysis and biofilm formation assay using DNase I revealed that biofilm formation by the hldE strain was dependent on extracellular DNA. Furthermore, a loss of flagella and an increase in amount of outer membrane vesicles in case of the hldE strain were also observed by transmission electron microscopy and atomic force microscopy, respectively. In addition, we demonstrated that a mutation in the hldE locus, which alters the LPS structure, caused changes in both expression and properties of several surface bacterial factors involved in biofilm formation and virulence. We suggest that the implication of these results should be considered in the context of biofilm formation on abiotic surfaces, which is frequently associated with nosocominal infections such as the catheter-associated infections.

The fimbriae activator MatA switches off motility in Escherichia coli by repression of the flagellar master operon flhDC

Flagella provide advantages to Escherichia coli by facilitating taxis towards nutrients and away from unfavourable niches. On the other hand, flagellation is an energy sink to the bacterial cell, and flagella also stimulate host innate inflammatory responses against infecting bacteria. The flagellar assembly pathway is ordered and under a complex regulatory circuit that involves three classes of temporally regulated promoters as well as the flagellar master regulator FlhD(4)C(2). We report here that transcription of the flhDC operon from the class 1 promoter is under negative regulation by MatA, a key activator of the common mat (or ecp) fimbria operon that enhances biofilm formation by E. coli. Ectopic expression of MatA completely precluded motility and flagellar synthesis in the meningitis-associated E. coli isolate IHE 3034. Northern blotting, analysis of chromosomal promoter-lacZ fusions and electrophoretic mobility shift assays revealed an interaction between MatA and the flhDC promoter region that apparently repressed flagellum biosynthesis. However, inactivation of matA in the chromosome of IHE 3034 had only a minor effect on flagellation, which underlines the complexity of regulatory signals that promote flagellation in E. coli. We propose that the opposite regulatory actions of MatA on mat and on flhDC promoters advance the adaptation of E. coli from a planktonic to an adhesive lifestyle.

Adhesive polypeptides of Staphylococcus aureus identified using a novel secretion library technique in Escherichia coli

BACKGROUND

Bacterial adhesive proteins, called adhesins, are frequently the decisive factor in initiation of a bacterial infection. Characterization of such molecules is crucial for the understanding of bacterial pathogenesis, design of vaccines and development of antibacterial drugs. Because adhesins are frequently difficult to express, their characterization has often been hampered. Alternative expression methods developed for the analysis of adhesins, e.g. surface display techniques, suffer from various drawbacks and reports on high-level extracellular secretion of heterologous proteins in Gram-negative bacteria are scarce. These expression techniques are currently a field of active research. The purpose of the current study was to construct a convenient, new technique for identification of unknown bacterial adhesive polypeptides directly from the growth medium of the Escherichia coli host and to identify novel proteinaceous adhesins of the model organism Staphylococcus aureus.

RESULTS

Randomly fragmented chromosomal DNA of S. aureus was cloned into a unique restriction site of our expression vector, which facilitates secretion of foreign FLAG-tagged polypeptides into the growth medium of E. coli ΔfliCΔfliD, to generate a library of 1663 clones expressing FLAG-tagged polypeptides. Sequence and bioinformatics analyses showed that in our example, the library covered approximately 32% of the S. aureus proteome. Polypeptides from the growth medium of the library clones were screened for binding to a selection of S. aureus target molecules and adhesive fragments of known staphylococcal adhesins (e.g coagulase and fibronectin-binding protein A) as well as polypeptides of novel function (e.g. a universal stress protein and phosphoribosylamino-imidazole carboxylase ATPase subunit) were detected. The results were further validated using purified His-tagged recombinant proteins of the corresponding fragments in enzyme-linked immunoassay and surface plasmon resonance analysis.

CONCLUSIONS

A new technique for identification of unknown bacterial adhesive polypeptides was constructed. Application of the method on S. aureus allowed us to identify three known adhesins and in addition, five new polypeptides binding to human plasma and extracellular matrix proteins. The method, here used on S. aureus, is convenient due to the use of soluble proteins from the growth medium and can in principle be applied to any bacterial species of interest.

Pili and flagella biology, structure, and biotechnological applications

Bacteria and Archaea expose on their outer surfaces a variety of thread-like proteinaceous organelles with which they interact with their environments. These structures are repetitive assemblies of covalently or non-covalently linked protein subunits, organized into filamentous polymers known as pili ("hair"), flagella ("whips") or injectisomes ("needles"). They serve different roles in cell motility, adhesion and host invasion, protein and DNA secretion and uptake, conductance, or cellular encapsulation. Here we describe the functional, morphological and genetic diversity of these bacterial filamentous protein structures. The organized, multi-copy build-up and/or the natural function of pili and flagella have lead to their biotechnological application as display and secretion tools, as therapeutic targets or as molecular motors. We review the documented and potential technological exploitation of bacterial surface filaments in light of their structural and functional traits.

Similar and divergent effects of ppGpp and DksA deficiencies on transcription in Escherichia coli

The concerted action of ppGpp and DksA in transcription has been widely documented. In disparity with this model, phenotypic studies showed that ppGpp and DksA might also have independent and opposing roles in gene expression in Escherichia coli. In this study we used a transcriptomic approach to compare the global transcriptional patterns of gene expression in strains deficient in ppGpp (ppGpp(0)) and/or DksA (DeltadksA). Approximately 6 and 7% of all genes were significantly affected by more than twofold in ppGpp- and DksA-deficient strains, respectively, increasing to 13% of all genes in the ppGpp(0) DeltadksA strain. Although the data indicate that most of the affected genes were copositively or conegatively regulated by ppGpp and DksA, some genes that were independently and/or differentially regulated by the two factors were found. The large functional group of chemotaxis and flagellum synthesis genes were notably differentially affected, with all genes being upregulated in the DksA-deficient strain but 60% of them being downregulated in the ppGpp-deficient strain. Revealingly, mutations in the antipausing Gre factors suppress the upregulation observed in the DksA-deficient strain, emphasizing the importance of the secondary channel of the RNA polymerase for regulation and fine-tuning of gene expression in E. coli.

The SfaXII protein from newborn meningitis E. coli is involved in regulation of motility and type 1 fimbriae expression

The genomes of pathogenic Escherichia coli may contain several different fimbrial operons. How bacteria regulate and coordinate the choice of fimbrial expression under different circumstances remains largely unanswered. In this report we have investigated the role of the sfaX(II) gene associated to the Sfa(II) fimbrial determinant in the E. coli isolate IHE3034. sfaX(II) belongs to a subfamily of genes, the 17k Da genes, located near different fimbrial operons in uropathogenic and newborn meningitis E. coli (NMEC) strains. Using the NMEC isolate IHE3034 and non-pathogenic E. coli strains we found that the sfaX(II) gene had an inhibitory effect on type 1 fimbriae expression. Down-regulation of type 1 fimbriae was exerted at transcriptional level both by inhibiting expression from the fimA promoter and by reducing the frequency of OFF-to-ON switching. The effect of sfaX(II) on expression of the recombinase FimB that catalyzes OFF-to-ON switching might explain the described reduction in percentage of ON cells. Moreover, expression of the sfaX(II) gene strongly influenced motility and flagella production of the NMEC isolate IHE3034. We propose that the sfaX(II) gene, and presumably other members in the 17 kDa gene family, may play a role in the control of virulence related gene expression in pathogenic E. coli.

Bifunctional nanotube scaffolds for diverse ligands are purified simply from Escherichia coli strains coexpressing two functionalized flagellar genes

We functionalized Escherichia coli FliC flagellin proteins to form tailored nanotubes binding single types or pairs of ligands, including divalent cations, fluorescent antibodies, or biotin-avidin-linked moieties such as ferritins. The ratio of each tag in bifunctionalized flagella could be toggled extending their sophistication as nanoscaffolds. Tobacco Etch Virus (TEV) protease site-containing FliCs were cleaved by the cognate protease without filament disintegration, potentiating their use as removable nanolithography masks to deposit attached ligands by protease cleavage.

The development of a flagellin surface display expression system in a moderate thermophile, Bacillus halodurans Alk36

This study relates to the development of an alkaliphilic, thermotolerant, Gram-positive isolate, Bacillus halodurans Alk36, for the over-production and surface display of chimeric gene products. This bacterium continuously over-produces flagellin. To harness this ability, key genetic tools, such as gene targeted inactivation, were developed for this strain. The hag gene, which codes for flagellin, was inactivated on the chromosome giving rise to the B. halodurans BhFC01 mutant. Polylinkers were inserted as in-frame, chimeric, flagellin sandwich fusions to identify the permissive insertion sites corresponding to the variable regions of the flagellin protein. Flagellin expression and motility were evaluated for these constructs. Two sites were identified for possible peptide insertion in the flagellin gene, one of which produced functional flagella and was able to restore the motility phenotype to a non-motile mutant. Peptides encoding a poly-histidine peptide and the HIV-1 subtype C gp120 epitope were, respectively, incorporated into this site as in-frame fusions. The peptides were found to be successfully displayed on the cell surface and functional through metal binding and immunological studies, respectively.

Simultaneous display of multiple foreign peptides in the FliD capping and FliC filament proteins of the Escherichia coli flagellum

The bacterial flagellum is composed of more than 20 different proteins. The filament, which constitutes the major extracellular part of the flagellum, is built up of approximately 20,000 FliC molecules that assemble at the growing distal end of the filament. A capping structure composed of five FliD molecules located at the tip of the filament promotes polymerization of FliC. Lack of FliD leads to release of the subunits into the growth medium. We show here that FliD can be successfully used in bacterial surface display. We tested various insertion sites in the capping protein, and the optimal region for display was at the variable region in FliD. Deletion and/or insertion at other sites resulted in decreased formation of flagella. We further developed the technique into a multihybrid display system in which three foreign peptides are simultaneously expressed within the same flagellum, i.e., D repeats of FnBPA from Staphylococcus aureus at the tip and fragments of YadA from Yersinia enterocolitica as well as SlpA from Lactobacillus crispatus along the filament. This technology can have biotechnological applications, e.g., in simultaneous delivery of several effector molecules.

Display of active enzymes on the cell surface of Escherichia coli using PgsA anchor protein and their application to bioconversion

We have developed a novel Escherichia coli cell surface display system by employing PgsA as an anchoring motif. In our display system, C-terminal fusion to PgsA anchor protein from Bacillus subtilis was used. The enzymes selected for display were alpha-amylase (AmyA) from Streptococcus bovis 148 and lipase B (CALB) from Candida antarctica. The molecular mass values of AmyA and CALB are approximately 77 and 34 kDa, respectively. The enzymes were displayed on the surface as a fusion protein with a FLAG peptide tag at the C terminus. Both the PgsA-AmyA-FLAG and PgsA-CALB-FLAG fusion proteins were shown to be displayed by immunofluorescence labeling using anti-FLAG antibody. The displayed enzymes were active forms, and AmyA and CALB activities reached 990 U/g (dry cell weight) and 4.6 U/g (dry cell weight), respectively. AmyA-displaying E. coli cells grew utilizing cornstarch as the sole carbon source, while CALB-displaying E. coli cells catalyzed enantioselective transesterification, indicating that they are effective whole-cell biocatalysts. Since a target enzyme with a size of 77 kDa and an industrially useful lipase have been successfully displayed on the cell surface of E. coli for the first time, PgsA protein is probably a useful anchoring motif to display various enzymes.

Extracellular secretion of polypeptides using a modified Escherichia coli flagellar secretion apparatus

We developed a modified flagellar type III secretion apparatus to secrete heterologous polypeptides into the growth medium of Escherichia coli. The secretion was facilitated by fusing the 173-bp untranslated DNA fragment upstream of the gene fliC (encoding flagellin) as well as a transcriptional terminator from fliC, into the gene encoding the polypeptide of interest. The polypeptides secreted into the growth medium at concentrations ranging from 1 to 15 mg/l were from Campylobacter jejuni (262 residues in length), Streptococcus pneumoniae (434 residues), Staphylococcus aureus (115 residues), and N-terminal FliC hybrid proteins, for example, the eukaryotic green fluorescent protein (238 residues). The expressed proteins represented >50% of total secreted protein. Previously reported protein yields from extracellular secretion of foreign proteins in E. coli have been low, approximately 100 microg/l. The strengths of our method are the concentration and purity of the secreted proteins and its versatility with regard to the proteins' length and origin.

Microbial cell-surface display

Cell-surface display allows peptides and proteins to be displayed on the surface of microbial cells by fusing them with the anchoring motifs. The protein to be displayed - the passenger protein - can be fused to an anchoring motif - the carrier protein - by N-terminal fusion, C-terminal fusion or sandwich fusion. The characteristics of carrier protein, passenger protein and host cell, and fusion method all affect the efficiency of surface display of proteins. Microbial cell-surface display has many potential applications, including live vaccine development, peptide library screening, bioconversion using whole cell biocatalyst and bioadsorption.

Selection of peptide entry motifs by bacterial surface display

Surface display technologies have been established previously to select peptides and polypeptides that interact with purified immobilized ligands. In the present study, we designed and implemented a surface display-based technique to identify novel peptide motifs that mediate entry into eukaryotic cells. An Escherichia coli library expressing surface-displayed peptides was combined with eukaryotic cells and the gentamicin protection assay was performed to select recombinant E. coli, which were internalized into eukaryotic cells by virtue of the displayed peptides. To establish the proof of principle of this approach, the fibronectin-binding motifs of the fibronectin-binding protein A of Staphylococcus aureus were inserted into the E. coli FhuA protein. Surface expression of the fusion proteins was demonstrated by functional assays and by FACS analysis. The fibronectin-binding motifs were shown to mediate entry of the bacteria into non-phagocytic eukaryotic cells and brought about the preferential selection of these bacteria over E. coli expressing parental FhuA, with an enrichment of 100000-fold. Four entry sequences were selected and identified using an S. aureus library of peptides displayed in the FhuA protein on the surface of E. coli. These sequences included novel entry motifs as well as integrin-binding Arg-Gly-Asp (RGD) motifs and promoted a high degree of bacterial entry. Bacterial surface display is thus a powerful tool to effectively select and identify entry peptide motifs.

Domains in the S-layer protein CbsA of Lactobacillus crispatus involved in adherence to collagens, laminin and lipoteichoic acids and in self-assembly

The protein regions in the S-layer protein CbsA of Lactobacillus crispatus JCM 5810, needed for binding to collagens and laminin, anchoring to bacterial cell wall, as well as self-assembly, were mapped by deletion analysis of His-tagged peptides isolated from Escherichia coli and by heterologous expression on Lactobacillus casei. Mature CbsA is 410 amino acids long, and stepwise genetic truncation at both termini revealed that the region 32-271 carries the infor-mation for self-assembly of CbsA into a periodic structure. The lactobacillar S-layer proteins exhibit sequence variation in their assembly domain, but the border regions 30-34 and 269-274 in CbsA are conserved in valine-rich short sequences. Short deletions or substitutions at these regions affected the morphology of His-CbsA polymers, which varied from sheet-like to cylindrical tubular polymers, and further truncation beyond the DNA encoding residues 32 and 271 leads to a non-periodic aggregation. The self-assembly of the truncated peptides, as seen by electron microscopy, was correlated with their behaviour in a cross-linking study. The shorter peptides not forming a regular polymer were observed by the cross-linking study and mass spectrometry to form dimers, trimers and tetramers, whereas the other peptides were cross-linked to large multimers only. Binding of solubilized type I and IV collagens was observed with the His-CbsA peptides 1-274 and 31-287, but not with the smaller peptides regardless of their ability to form regular polymers. Strain JCM 5810 also adheres to immobilized laminin and, in order to analyse the possible laminin binding by CbsA, cbsA and its fragments were expressed on the surface of L. casei. Expression of the CbsA peptides 1-274, 1-287, 28-287 and 31-287 on L. casei conferred adhesiveness to both laminin and collagen immobilized on glass as well as to laminin- and collagen-containing regions in chicken colon and ileum. The C-terminal peptides 251-410 and 288-410 bound to L. crispatus JCM 5810 cells from which the S-layer had been depleted by chemical extraction, whereas no binding was seen with the His-CbsA peptides 1-250 or 1-269 or to cells with an intact S-layer. The His-CbsA peptides 251-410 and 288-410 bound to teichoic acids of several bacterial species. The results show that CbsA is an adhesive complex with an N-terminal assembly domain exhibiting affinity for pericellular tissue components and a cationic C-terminal domain binding to negatively charged cell wall components.

Display of proteins on bacteria

Display of heterologous proteins on the surface of microorganisms, enabled by means of recombinant DNA technology, has become an increasingly used strategy in various applications in microbiology, biotechnology and vaccinology. Gram-negative, Gram-positive bacteria, viruses and phages are all being investigated in such applications. This review will focus on the bacterial display systems and applications. Live bacterial vaccine delivery vehicles are being developed through the surface display of foreign antigens on the bacterial surfaces. In this field, 'second generation' vaccine delivery vehicles are at present being generated by the addition of mucosal targeting signals, through co-display of adhesins, in order to achieve targeting of the live bacteria to immunoreactive sites to thereby increase immune responses. Engineered bacteria are further being evaluated as novel microbial biocatalysts with heterologous enzymes immobilized as surface exposed on the bacterial cell surface. A discussion has started whether bacteria can find use as new types of whole-cell diagnostic devices since single-chain antibodies and other type of tailor-made binding proteins can be displayed on bacteria. Bacteria with increased binding capacity for certain metal ions can be created and potential environmental or biosensor applications for such recombinant bacteria as biosorbents are being discussed. Certain bacteria have also been employed for display of various poly-peptide libraries for use as devices in in vitro selection applications. Through various selection principles, individual clones with desired properties can be selected from such libraries. This article explains the basic principles of the different bacterial display systems, and discusses current uses and possible future trends of these emerging technologies.

Identification by flagellum display of an epithelial cell- and fibronectin-binding function in the SlpA surface protein of Lactobacillus brevis

Depletion of the SlpA protein from the bacterial surface greatly reduced the adhesion of Lactobacillus brevis ATCC 8287 to the human intestinal cell lines Caco-2 and Intestine 407, the endothelial cell line EA-hy926, and the urinary bladder cell line T24, as well as immobilized fibronectin. For functional analysis of the SlpA surface protein, different regions of the slpA gene were expressed as internal in-frame fusions in the variable region of the fliC(H7) gene of Escherichia coli. The resulting chimeric flagella carried inserts up to 275 amino acids long from the mature S-layer protein, which is 435 amino acids in size. The expression of the SlpA fragments on the chimeric flagella was assessed by immunoelectron microscopy and Western blotting using anti-SlpA antibodies, and their binding to human cells was assessed by indirect immunofluorescence. Chimeric flagella harboring inserts that represented the N-terminal part of the S-layer protein bound to the epithelial cell lines, whereas the C-terminal part of the S-layer protein did not confer binding on the flagella. The shortest S-layer peptide capable of detectable binding was 81 amino acid residues in size and represented residues 96 through 176 in the unprocessed S-layer protein. The bacteria and the chimeric flagella did not show detectable binding to erythrocytes, whereas the SlpA-expressing ATCC 8287 cells as well as the chimeric SlpA 96-245/FliC flagella bound to immobilized fibronectin. The N-terminal SlpA peptide 96-176 or 96-200 fused to FliC was not recognized in Western blotting or immunoelectron microscopy by a polyclonal serum raised against the S-layer protein; the antiserum, however, reacted in immunofluorescence with the ATCC 8287 cells. In contrast, an antiserum raised against the His-tagged peptide 96-245 of SlpA bound to the hybrid flagella with the N-terminal SlpA inserts but did not react with ATCC 8287 cells. The results identify the S-layer of L. brevis ATCC 8287 as an adhesin with affinity for human epithelial cells and fibronectin and locate the receptor-binding region within a fragment of 81 amino acids in the N-terminal part of the molecule, which in native S-layer seems inaccessible to antibodies.

matB, a common fimbrillin gene of Escherichia coli, expressed in a genetically conserved, virulent clonal group

A novel fimbrial type in Escherichia coli was identified and characterized. The expression of the fimbria was associated with the O18acK1H7 clonal group of E. coli, which cause newborn meningitis and septicemia when grown at low temperature; hence, it was named the Mat (meningitis associated and temperature regulated) fimbria. The fimbriae were purified from a fimA::cat sfaA::Gm fliC::St derivative of the O18K1H7 isolate E. coli IHE 3034. The purified Mat fimbrillin had an apparent molecular mass of 18 kDa and did not serologically cross-react with the type 1 or S fimbria of the same strain. The matB gene encoding the major fimbrillin was cloned from the genomic DNA of the fimA::cat sfaA::Gm fliC::St derivative of IHE 3034. The predicted MatB sequence was of 195 amino acids, contained a signal sequence of 22 residues, and did not show significant homology to any of the previously characterized fimbrial proteins. The DNA sequence of matB was 97.8% identical to a region from nucleotides 17882 to 18469 in the 6- to 8-min region of the E. coli K-12 chromosome, reported to encode a hypothetical protein. The 7-kb DNA fragment containing matB of IHE 3034 was found by restriction mapping and partial DNA sequencing to be highly similar to the corresponding region in the K-12 chromosome. Trans complementation of the matB::cat mutation in the IHE 3034 chromosome showed that matB in combination with matA or matC restored surface expression of the Mat fimbria. A total of 27 isolates representing K-12 strains and the major pathogroups of E. coli were analyzed for the presence of a matB homolog as well as for expression of the Mat fimbria. A conserved matB homolog was found in 25 isolates; however, expression of the Mat fimbriae was detected only in the O18acK1H7 isolates. Expression of the Mat fimbria was temperature regulated, with no or a very small amount of fimbriae or intracellular MatB fimbrillin being detected in cells cultivated at 37(o)C. Reverse transcriptase PCR and complementation assays with mat genes controlled by the inducible trc promoter indicated that regulation of Mat fimbria expression involved both transcriptional and posttranscriptional events.

Characterization of the collagen-binding S-layer protein CbsA of Lactobacillus crispatus

The cbsA gene of Lactobacillus crispatus strain JCM 5810, encoding a protein that mediates adhesiveness to collagens, was characterized and expressed in Escherichia coli. The cbsA open reading frame encoded a signal sequence of 30 amino acids and a mature polypeptide of 410 amino acids with typical features of a bacterial S-layer protein. The cbsA gene product was expressed as a His tag fusion protein, purified by affinity chromatography, and shown to bind solubilized as well as immobilized type I and IV collagens. Three other Lactobacillus S-layer proteins, SlpA, CbsB, and SlpnB, bound collagens only weakly, and sequence comparisons of CbsA with these S-layer proteins were used to select sites in cbsA where deletions and mutations were introduced. In addition, hybrid S-layer proteins that contained the N or the C terminus from CbsA, SlpA, or SlpnB as well as N- and C-terminally truncated peptides from CbsA were constructed by gene fusion. Analysis of these molecules revealed the major collagen-binding region within the N-terminal 287 residues and a weaker type I collagen-binding region in the C terminus of the CbsA molecule. The mutated or hybrid CbsA molecules and peptides that failed to polymerize into a periodic S-layer did not bind collagens, suggesting that the crystal structure with a regular array is optimal for expression of collagen binding by CbsA. Strain JCM 5810 was found to contain another S-layer gene termed cbsB that was 44% identical in sequence to cbsA. RNA analysis showed that cbsA, but not cbsB, was transcribed under laboratory conditions. S-layer-protein-expressing cells of strain JCM 5810 adhered to collagen-containing regions in the chicken colon, suggesting that CbsA-mediated collagen binding represents a true tissue adherence property of L. crispatus.

Construction of a multihybrid display system: flagellar filaments carrying two foreign adhesive peptides

A multivalent, bifunctional flagellum carrying two different adhesive peptides in separate flagellin subunits within a filament was constructed in Escherichia coli. The inserted peptides were the fibronectin-binding 115-mer D repeat region of Staphylococcus aureus and the 302-mer collagen-binding region of YadA of Yersinia enterocolitica. Western blotting, immunoelectron microscopy, and adhesion tests with hybrid flagella from an in trans-complemented DeltafliC E. coli strain showed that individual filaments consisted of both recombinant flagellins.

Peptide display on bacterial flagella: principles and applications

Expression of foreign peptides as fusions to bacterial cell surface proteins has gained increasing attention in basic, as well as applied research during the last decade. A wide range of heterologous peptides have been expressed, and the spectrum of available carrier proteins is also wide. The choice of carrier protein is frequently ruled by the application of the fusion protein constructed. This review is focused on flagella display, which is based on genetic fusion of foreign peptides into a surface-exposed, dispensable region of flagellin, the flagellar major subunit present in thousands of copies per filament. Expression of these constructs in flagellin-deficient host strains results in hybrid flagella carrying the heterologous peptides in thousands of intimately-associated copies. The first and still most frequent application of flagella display is the construction of novel recombinant vaccines. Flagella display has also been used in peptide display as an alternative to the phage-display technique. One application involves fusion into a disulfide loop of Escherichia coli thioredoxin that has been inserted into flagellin, this system facilitates expression of random peptides in a conformationally constrained manner readily accessible on the flagellar surface. The random peptide library has been applied in antibody epitope mapping and is suitable for biopanning procedures in the study of ligand-receptor interactions. Many bacterial adhesins are of complex nature and thereby difficult to analyse by conventional methods. Direct flagella display has proven to be applicable also in bacterial adhesion technology since large fragments, up to 302 amino acid residues in length, of bacterial adhesins can be functionally expressed as fusions to flagellin. Hybrid flagella are easily purified and can easily be analysed for binding to various targets, such as immobilized proteins, tissue sections, as well as cell cultures.

Functional mapping of the Yersinia enterocolitica adhesin YadA. Identification Of eight NSVAIG - S motifs in the amino-terminal half of the protein involved in collagen binding

The virulence plasmid-encoded YadA of Yersinia enterocolitica serotype O:3 is a 430-amino-acid outer membrane protein, synthesized with a 25-amino-acid signal peptide. YadA forms homotrimeric surface structures that function as adhesin between bacteria and collagen as well as other host proteins. The structure-function relationships of YadA were studied, and the collagen-binding determinants of YadA were located to its amino-terminal half. Collagen did not bind to any of the overlapping 16-mer YadA peptides, indicating that the collagen binding site of YadA is conformational. Epitope mapping of YadA identified 12 linear antigenic epitopes altogether. Seven epitopes were uniquely recognized by an anti-YadA antiserum able to inhibit collagen binding. Four of these epitopes shared a motif NSVAIG-S that is repeated eight times within the N-terminal half of YadA. Site-directed mutagenesis showed that these motifs are absolutely required for YadA-mediated collagen binding, revealing a novel type of collagen-binding mechanism.

Characterization of adhesive epitopes with the OmpS display system

OmpS is an outer membrane protein of Vibrio cholerae where it forms trimeric pores that function in the uptake of maltose and maltodextrins. Based on sequence similarity to LamB proteins, a model of OmpS folding in the outer membrane has been constructed. According to this model, OmpS contains 18 transmembrane beta-strands and nine surface-accessible loops. Adhesive epitopes can, when inserted into surface-accessible loop 4 (L4) and expressed in Escherichia coli, retain their functional characteristics. We inserted three D-repeats from the Staphylococcus aureus fibronectin-binding protein FnBPA into L4 of OmpS and showed that E. coli cells expressing these hybrids bind fibronectin. DNA fragments covering the N-terminal half of the globoside-binding P-fimbrial adhesin class II PapG of E. coli were cloned into the same surface accessible loop (L4) of OmpS. Fragments of papG encoding 53 or 186 amino acids from the N-terminal end of class II PapG adhesin were found to confer bacterial adhesiveness to globoside. Removal of 23 amino acids from the N-terminus of PapG did not affect receptor binding, but removal of 31 amino acids abolished it. The newly developed night sky image technique was also used to demonstrate the binding properties of membrane vesicles carrying the hybrid proteins. We raised antibodies against the purified hybrid protein containing 53 amino acids from PapG. This antiserum recognized the P-fimbriae on E. coli cells. These data provide evidence that the N-terminal first 53 amino acids of class II PapG contain the receptor-binding domain.

Surface display of functional fibronectin-binding domains on Staphylococcus carnosus

The surface expression in Staphylococcus carnosus of three different fibronectin binding domains (FNBDs), derived from fibronectin binding proteins of Streptococcus dysgalactiae and Staphylococcus aureus, has been investigated. Surface localization of the chimeric proteins containing the FNBDs was demonstrated. All three surface-displayed FNBDs were demonstrated to bind fibronectin in whole-cell enzyme-linked binding assays. Furthermore, for one of the constructs, intranasal immunizations with the recombinant bacteria resulted in improved antibody responses to a model immunogen present within the chimeric surface proteins. The implications of the results for the design of live bacterial vaccine delivery systems are discussed.

Amino acid residue Ala-62 in the FimH fimbrial adhesin is critical for the adhesiveness of meningitis-associated Escherichia coli to collagens

Adhesion of meningitis-associated Escherichia coli O18acK1H7 to collagens was characterized. The E. coli strain IHE 3034 adhered to type IV and type I collagens but not to type III collagen immobilized on glass. Collagens lack terminal mannosyl units, yet the bacterial adhesion was completely abolished in the presence of alpha-methyl-D-mannoside. A cat cassette was introduced into the filmA gene of IHE 3034, and the resulting mutant strain IHE 3034-2 failed to adhere to collagens. In contrast, insertion of a Gm cassette into the sfaA gene of IHE 3034, encoding the S-fimbrillin, had no significant effect on the adhesiveness. The fim cluster from IHE 3034 was cloned and expressed in trans in the fimA::cat mutant strain IHE 3034-2. The complemented strain IHE 3034-2(pRPO-1) exhibited adhesiveness to type IV and type I collagens, confirming the function of the type 1 fimbria in the adhesion. We have previously shown that the type 1 fimbria from E. coli K-12 strain PC31 does not confer bacterial adhesiveness to collagens. The fimH genes from E. coli IHE 3034 as well as from PC31 were expressed in the fimH-null strain MS4. The FimH from IHE 3034 potentiated collagen adherence, whereas the FimH from PC31 was inactive. Sequence comparison of fimH from IHE 3034 and PC31 revealed five amino-acid differences in the predicted mature FimH proteins: at residues 27, 62, 70, 78 and 201. Each of these residues in the IHE 3034-FimH were individually substituted to the corresponding amino acid in the PC31-FimH. The substitution S62-->A completely abolished collagen adhesiveness. The reverse substitution A62-->S in the PC31-FimH as well as in the FimH from another E. coli strain induced collagen adhesiveness to the level seen with IHE 3034-FimH. Out of nine fimH genes analysed from isolates of E. coli, collagen adhesiveness as well as alanine at position 62 in FimH were found only in two O18acK1H7 isolates with the isoenzyme profile ET type 1. Our results demonstrate that the amino-acid residue Ala-62 in the FimH lectin is critical for the adhesion to collagens by a highly virulent clonal group of E. coli.