Autodisplay: functional display of active beta-lactamase on the surface of Escherichia coli by the AIDA-I autotransporter

Amber Codon Mutational Scanning and Bioorthogonal PEGylation for Epitope Mapping of Antibody Binding Sites on Human Arginase-1

Epitope mapping is crucial for understanding immunological responses to protein therapeutics. Here, we combined genetic code expansion and bacterial surface display to incorporate S-allylcysteine (SAC) into human arginase-1 (hArg1) via Methanococcoides burtonii pyrrolysyl-tRNA synthetase. Using an amber codon deep mutational scanning and sequencing workflow, we mapped SAC incorporation efficiency across the hArg1 sequence, providing insights into structural and sequence dependencies of noncanonical amino acid incorporation. We used mutually bioorthogonal allyl/tetrazine and azide/DBCO chemistries to achieve site-specific PEGylation and fluorescent labeling of hArg1, revealing insights into SAC side chain reactivity and solvent accessibility of residues in hArg1. This system was further applied to determine the binding epitope of a monoclonal antibody on the surface of hArg1, providing high-resolution data on the impact of PEGylation residue position on antibody binding. Our method produces high dimensional data of noncanonical amino acid incorporation efficiency, site-specific functionalization enabled by mutually bioorthogonal chemistries, and epitope mapping of therapeutic proteins.

A multifarious bacterial surface display: potential platform for biotechnological applications

Bacterial-cell surface display represents a novel field of protein engineering, which is grounds for presenting recombinant proteins or peptides on the surface of host cells. This technique is primarily used for endowing cellular activity on the host cells and enables several biotechnological applications. In this review, we comprehensively summarize the speciality of bacterial surface display, specifically in gram-positive and gram-negative organisms and then we depict the practical cases to show the importance of bacterial cell surface display in biomedicine and bioremediation domains. We manifest that among other display systems such as phages and ribosomes, the cell surface display using bacterial cells can be used to avoid the loss of combinatorial protein libraries and also open the possibility of isolating target-binding variants using high-throughput selection platforms. Thus, it is becoming a robust tool for functionalizing microbes to serve as a potential implement for various bioengineering purposes.

Constructing Escherichia coli co-display systems for biodegradation of polyethylene terephthalate

BACKGROUND

The accumulation of fast-growing polyethylene terephthalate (PET) wastes has posed numerous threats to the environments and human health. Enzymatic degradation of PET is a promising approach for PET waste treatment. Currently, the efficiency of various PET biodegradation systems requires further improvements.

RESULTS

In this work, we engineered whole cell systems with co-display of strong adhesive proteins and the most active PETase for PET biodegradation in E. coli cells. Adhesive proteins of cp52k and mfp-3 and Fast-PETase were simultaneously displayed on the surfaces of E. coli cells, and the resulting cells displaying mfp-3 showed 50% increase of adhesion ability compared to those without adhesive proteins. Consequently, the degradation rate of E. coli cells co-displaying mfp-3 and Fast-PETase for amorphous PET exceeded 15% within 24 h, exhibiting fast and thorough PET degradation.

CONCLUSIONS

Through the engineering of co-display systems in E. coli cells, PET degradation efficiency was significantly increased compared to E. coli cells with sole display of Fast-PETase and free enzyme. This feasible E. coli co-display system could be served as a convenient tool for extending the treatment options for PET biodegradation.

Phylogenetic Classification and Functional Review of Autotransporters

Autotransporters are the core component of a molecular nano-machine that delivers cargo proteins across the outer membrane of Gram-negative bacteria. Part of the type V secretion system, this large family of proteins play a central role in controlling bacterial interactions with their environment by promoting adhesion to surfaces, biofilm formation, host colonization and invasion as well as cytotoxicity and immunomodulation. As such, autotransporters are key facilitators of fitness and pathogenesis and enable co-operation or competition with other bacteria. Recent years have witnessed a dramatic increase in the number of autotransporter sequences reported and a steady rise in functional studies, which further link these proteins to multiple virulence phenotypes. In this review we provide an overview of our current knowledge on classical autotransporter proteins, the archetype of this protein superfamily. We also carry out a phylogenetic analysis of their functional domains and present a new classification system for this exquisitely diverse group of bacterial proteins. The sixteen phylogenetic divisions identified establish sensible relationships between well characterized autotransporters and inform structural and functional predictions of uncharacterized proteins, which may guide future research aimed at addressing multiple unanswered aspects in this group of therapeutically important bacterial factors.

Autotransporter-based surface expression and complementation of split TreA fragments utilized for the detection of antibodies against bovine leukemia virus

Bovine Leukemia Virus (BLV) is an oncogenic virus which is the etiological agent of a neoplastic disease in infected cattle called enzootic bovine leukemia (EBL). The most common and sensitive diagnostic methods for EBL like enzyme-linked immunosorbent assay (ELISA) is time-consuming and requires manual handling which makes it unsuitable as an on-farm diagnostic test. Hence, there is a need for an alternative test with rapid detection and reduced manual labour. We have previously reported the use of E. coli periplasmic trehalase (TreA) in a split enzyme sensor diagnostic technology to detect immunoglobulins and antigen-specific antibodies. In the current study, a more sensitive detection was attempted by bacterial surface display of split TreA fragment by fusion with the autotransporter AIDA-I. The split TreA fragments fused to antigens require antigen-specific antibodies for complementation and to trigger trehalase activity. This surface complementation strategy was used to detect anti-BLV antibodies in clinical serum by incorporating the antigenic BLV capsid protein in the fusion proteins. To validate this assay, a panel of serum samples obtained from BLV positive and negative cattle were tested in comparison with ELISA results. Evaluation of this panel resulted in positive detection of all true positive samples. We further demonstrated that this assay can be enhanced by pre-adsorption of clinical serum samples using E. coli cells to increase the specificity and help reduce nonspecific binding. In conclusion, the p24 antigen specific BLV assay is a potential tool for simple and rapid diagnosis of BLV infection, which is compatible with both lab-based and a more user friendly on-farm format.

FimH-based display of functional eukaryotic proteins on bacteria surfaces

The demand for recombinant proteins for analytic and therapeutic purposes is increasing; however, most currently used bacterial production systems accumulate the recombinant proteins in the intracellular space, which requires denaturating procedures for harvesting and functional testing. We here present a novel FimH-based expression system that enables display of fully functional eukaryotic proteins while preventing technical difficulties in translocating, folding, stabilizing and isolating the displayed proteins. As examples, Gaussia Luciferase (GLuc), epidermal growth factor (EGF), transforming growth factor-α (TGF-α) and epiregulin (EPRG) were expressed as FimH fusion proteins on the surface of E. coli bacteria. The fusion proteins were functionally active and could be released from the bacterial surface by specific proteolytic cleavage into the culture supernatant allowing harvesting of the produced proteins. EGFR ligands, produced as FimH fusion proteins and released by proteolytic cleavage, bound to the EGF receptor (EGFR) on cancer cells inducing EGFR phosphorylation. In another application of the technology, GLuc-FimH expressed on the surface of bacteria was used to track tumor-infiltrating bacteria by bioluminescence imaging upon application to mice, thereby visualizing the colonization of transplanted tumors. The examples indicate that the FimH-fusion protein technology can be used in various applications that require functionally active proteins to be displayed on bacterial surfaces or released into the culture supernatant.

Orientation and density control of proteins on solid matters by outer membrane coating: Analytical and diagnostic applications

Autodisplay is an expression system for the display of recombinant proteins on the outer membrane (OM) of gram negative bacteria and has been developed for translocation studies, whole cell biocatalysis, bioremediation, inhibitor screening, and enzyme refolding. Recently, affinity proteins such as IgG-binding Z-domains and biotin-binding streptavidin have been autodisplayed on the OM of Escherichia coli for analytical and biomedical applications. The secretion mechanism of the autodisplay system was used and orientation and density control of these affinity proteins were determined. Affinity protein-autodisplaying E. coli cells have been used to coat solid supports in immunoassays. For this purpose, the OM of autodisplayed E. coli cells was separated and isolated by the aid of detergents. The structure of the resulting OM liposomes as well as their physico-chemical parameters, were analyzed. OM liposomes were used subsequently for coating various solid matters including microplates and biosensor transducer surfaces and the formation of OM layers were monitored. OM layer formation on solid matters was shown to increase the sensitivity of immunoassays and biosensors. In this review, analytical and diagnostic applications are described in particular concerning orientation and density control of autodisplayed affinity proteins.

The Electrosome: A Surface-Displayed Enzymatic Cascade in a Biofuel Cell's Anode and a High-Density Surface-Displayed Biocathodic Enzyme

The limitation of surface-display systems in biofuel cells to a single redox enzyme is a major drawback of hybrid biofuel cells, resulting in a low copy-number of enzymes per yeast cell and a limitation in displaying enzymatic cascades. Here we present the electrosome, a novel surface-display system based on the specific interaction between the cellulosomal scaffoldin protein and a cascade of redox enzymes that allows multiple electron-release by fuel oxidation. The electrosome is composed of two compartments: (i) a hybrid anode, which consists of dockerin-containing enzymes attached specifically to cohesin sites in the scaffoldin to assemble an ethanol oxidation cascade, and (ii) a hybrid cathode, which consists of a dockerin-containing oxygen-reducing enzyme attached in multiple copies to the cohesin-bearing scaffoldin. Each of the two compartments was designed, displayed, and tested separately. The new hybrid cell compartments displayed enhanced performance over traditional biofuel cells; in the anode, the cascade of ethanol oxidation demonstrated higher performance than a cell with just a single enzyme. In the cathode, a higher copy number per yeast cell of the oxygen-reducing enzyme copper oxidase has reduced the effect of competitive inhibition resulting from yeast oxygen consumption. This work paves the way for the assembly of more complex cascades using different enzymes and larger scaffoldins to further improve the performance of hybrid cells.

Combining DNA Vaccine and AIDA-1 in Attenuated Salmonella Activates Tumor-Specific CD4+ and CD8+ T-cell Responses

Stimulation of tumor-specific responses in both CD4⁺ and CD8⁺ T cells has been a challenge for effective tumor vaccines. We designed a vaccine vector containing the AIDA-1 autotransporter and DNA vaccine elements, generating a murine melanoma vaccine that was delivered by the attenuated Salmonella strain SL7207. Growth of murine subcutaneous melanoma was significantly inhibited by intranasal immunization with the Salmonella tumor vaccine. The vaccine activated tumor-specific CD4⁺ and CD8⁺ T-cell responses, with increased T-cell proliferation, tumor antigen-specific Th1 cytokine production, increased percentages of tetramer positive cells, and cytotoxicity. CD4⁺ or CD8⁺ T-cell depletion resulted in the loss of antitumor activity of the Salmonella tumor vaccine, suggesting that the efficacy of the vaccine was dependent on both CD4⁺ and CD8⁺ T cells. Lung metastasis of the tumor was also inhibited by vaccine treatment. Similarly, the percentages of tumor-specific Th1 cytokine production by CD4⁺ and CD8⁺ T cells in the spleen, tumor, and bronchoalveolar lavage were increased after vaccine treatment. Tumor-specific proliferation of CD4⁺ and CD8⁺ T cells was also promoted by the vaccine. Tetramer staining and cytotoxicity assay showed enhanced tumor-specific CD8⁺ T-cell response after vaccine treatment. Therefore, the Salmonella tumor vaccine could activate both tumor-specific CD4⁺ and CD8⁺ T-cell responses. This vaccine strategy may be widely applicable to the development of oral or nasal vaccines against tumors. Cancer Immunol Res; 5(6); 503-14. ©2017 AACR.

Poly-protein G-expressing bacteria enhance the sensitivity of immunoassays

The sensitivities of solid-phase immunoassays are limited by the quantity of detection antibodies bound to their antigens on the solid phase. Here, we developed a poly-protein G-expressing bacterium as an antibody-trapping microparticle to enhance the signals of immunoassays by increasing the accumulation of detection antibodies on the given antigen. Eight tandemly repeated fragment crystallisable (Fc) binding domains of protein G were stably expressed on the surface of Escherichia coli BL21 cells (termed BL21/8G). BL21/8G cells showed a higher avidity for trapping antibodies on their surface than monomeric protein G-expressing BL21 (BL21/1G) cells did. In the sandwich enzyme-linked immunosorbent assay (ELISA), simply mixing the detection antibody with BL21/8G provided a detection limit of 6 pg/mL for human interferon-α (IFN-α) and a limit of 30 pg/mL for polyethylene glycol (PEG)-conjugated IFN-α (Pegasys), which are better than that of the traditional ELISA (30 pg/mL for IFN-α and 100 pg/mL for Pegasys). Moreover, the sensitivity of the Western blot for low-abundance Pegasys (0.4 ng/well) was increased by 25 folds upon mixing of an anti-PEG antibody with BL21/8G cells. By simply being mixed with a detection antibody, the poly-protein G-expressing bacteria can provide a new method to sensitively detect low-abundance target molecules in solid-phase immunoassays.

Surface display of Aggregatibacter actinomycetemcomitans autotransporter Aae and dispersin B hybrid act as antibiofilm agents

Among the various proteins expressed by the periodontopathogen Aggregatibacter actinomycetemcomitans, two proteins play important roles for survival in the oral cavity. The autotransporter Aae facilitates the attachment of the pathogen to oral epithelial cells, which act as a reservoir, while the biofilm-degrading glycoside hydrolase dispersin B facilitates the movement of daughter cells from the mature biofilm to a new site. The objective of this study was to use the potential of these two proteins to control biofilms. To this end, we generated a hybrid construct between the Aae C-terminal translocating domain and dispersin B, and mobilized it into Escherichia coli Rosetta (DE3) pLysS cells. Immunofluorescence analysis of the modified E. coli cells confirmed the presence of dispersin B on the surface. Further, the membrane localization of the displayed dispersin B was confirmed with Western blot analysis. The integrity of the E. coli cells displaying the dispersin B was confirmed through FACS analysis. The hydrolytic activity of the surface-displayed dispersin B was confirmed by using 4-methylumbelliferyl-β-d-glucopyranoside as the substrate. The detachment ability of the dispersin B surface-displaying E. coli cells was shown using Staphylococcus epidermidis and Actinobacillus pleuropneumoniae biofilms in a microtiter assay. We concluded that the Aae β-domain is sufficient to translocate foreign enzymes in the native folded form and that the method of Aae-mediated translocation of surface displayed enzymes might be useful for control of biofilms.

Autodisplay of Human Hyaluronidase Hyal-1 on Escherichia coli and Identification of Plant-Derived Enzyme Inhibitors

Hyaluronan (HA) is the main component of the extracellular matrix (ECM). Depending on its chain size, it is generally accepted to exert diverse effects. High molecular weight HA is anti-angiogenic, immunosuppressive and anti-inflammatory, while lower fragments are angiogenic and inflammatory. Human hyaluronidase Hyal-1 (Hyal-1) is one of the main enzymes in the metabolism of HA. This makes Hyal-1 an interesting target. Not only for functional and mechanistic studies, but also for drug development. In this work, Hyal-1 was expressed on the surface of E. coli, by applying Autodisplay, to overcome formation of inactive “inclusion bodies”. With the cells displaying Hyal-1 an activity assay was performed using “stains-all” dye. Subsequently, the inhibitory effects of four saponins and 14 plant extracts on the activity of surface displayed Hyal-1 were evaluated. The determined IC₅₀ values were 177 µM for glycyrrhizic acid, 108 µM for gypsophila saponin 2, 371 µM for SA1657 and 296 µM for SA1641. Malvae sylvestris flos, Equiseti herba and Ononidis radix extracts showed IC₅₀ values between 1.4 and 1.7 mg/mL. In summary, Autodisplay enabled the expression of functional human target protein Hyal-1 in E. coli and facilitated an accelerated testing of potential inhibitors.

Functional display of heterotetrameric human protein kinase CK2 on Escherichia coli: a novel tool for drug discovery

Background

Human protein kinase CK2 represents a novel therapeutic target for neoplastic diseases. Inhibitors are in need to explore the druggability and the therapeutic options of this enzyme. A bottleneck in the search for new inhibitors is the availability of the target for testing. Therefore an assay was developed to provide easy access to CK2 for discovery of novel inhibitors.

Results

Autodisplay was used to present human CK2 on the surface of Escherichia coli. Heterotetrameric CK2 consists of two subunits, α and β, which were displayed individually on the surface. Co-display of CK2α and CK2β on the cell surface led to the formation of functional holoenzyme, as demonstrated by NaCl dependency of enzymatic activity, which differs from that of the catalytic subunit CK2α without β. In addition interaction of CK2α and CK2β at the cell surface was confirmed by co-immunoprecipitation assays. Surface displayed CK2 holoenzyme enabled an easy IC₅₀ value determination. The IC₅₀ values for the known CK2 inhibitors TBB and Silmitasertib were determined to be 50 and 3.3 nM, respectively.

Conclusion

Surface-displayed CK2α and CK2β assembled on the cell surface of E. coli to an active tetrameric holoenzyme. The whole-cell CK2 autodisplay assay as developed is suitable for inhibition studies. Furthermore, it can be used to determine quantitative CK2 inhibition data such as IC₅₀ values. In summary, this is the first report on the functional surface display of a heterotetrameric enzyme on E. coli.

An engineered autotransporter-based surface expression vector enables efficient display of Affibody molecules on OmpT-negative E. coli as well as protease-mediated secretion in OmpT-positive strains

Background

Cell display technologies (e.g. bacterial display) are attractive in directed evolution as they provide the option to use flow-cytometric cell sorting for selection from combinatorial libraries. The aim of this study was to engineer and investigate an expression vector system with dual functionalities: i) recombinant display of Affibody libraries on Escherichia coli for directed evolution and ii) small scale secreted production of candidate affinity proteins, allowing initial downstream characterizations prior to subcloning. Autotransporters form a class of surface proteins in Gram-negative bacteria that have potential for efficient translocation and tethering of recombinant passenger proteins to the outer membrane. We engineered a bacterial display vector based on the E. coli AIDA-I autotransporter for anchoring to the bacterial surface. Potential advantages of employing autotransporters combined with E. coli as host include: high surface expression level, high transformation frequency, alternative promoter systems available, efficient translocation to the outer membrane and tolerance for large multi-domain passenger proteins.

Results

The new vector was designed to comprise an expression cassette encoding for an Affibody molecule, three albumin binding domains for monitoring of surface expression levels, an Outer membrane Protease T (OmpT) recognition site for potential protease-mediated secretion of displayed affinity proteins and a histidine-tag for purification. A panel of vectors with different promoters were generated and evaluated, and suitable cultivation conditions were investigated. The results demonstrated a high surface expression level of the different evaluated Affibody molecules, high correlation between target binding and surface expression level, high signal-to-background ratio, efficient secretion and purification of binders in OmpT-positive hosts as well as tight regulation of surface expression for the titratable promoters. Importantly, a mock selection using FACS from a 1:100,000 background yielded around 20,000-fold enrichment in a single round and high viability of the isolated bacteria after sorting.

Conclusions

The new expression vectors are promising for combinatorial engineering of Affibody molecules and the strategy for small-scale production of soluble recombinant proteins has the potential to increase throughput of the entire discovery process.

Electronic supplementary material

The online version of this article (doi:10.1186/s12934-014-0179-z) contains supplementary material, which is available to authorized users.

Autodisplay of an archaeal γ-lactamase on the cell surface of Escherichia coli using Xcc_Est as an anchoring scaffold and its application for preparation of the enantiopure antiviral drug intermediate (-) vince lactam

At present, autotransporter protein mediated surface display has opened a new dimension in the development of whole-cell biocatalysts. Here, we report the identification of a novel autotransporter Xcc_Est from Xanthomonas campestris pv campestris 8004 by bioinformatic analysis and application of Xcc_Est as an anchoring motif for surface display of γ-lactamase (Gla) from thermophilic archaeon Sulfolobus solfataricus P2 in Escherichia coli. The localization of γ-lactamase in the cell envelope was monitored by Western blot, activity assay and flow cytometry analysis. Either the full-length or truncated Xcc_Est could efficiently transport γ-lactamase to the cell surface. Compared with the free enzyme, the displayed γ-lactamase exhibited optimum temperature of 30 °C other than 90 °C, with a substantial decrease of 60 °C. Under the preparation system, the engineered E. coli with autodisplayed γ-lactamase converted 100 g racemic vince lactam to produce 49.2 g (-) vince lactam at 30 °C within 4 h. By using chiral HPLC, the ee value of the produced (-) vince lactam was determined to be 99.5 %. The whole-cell biocatalyst exhibited excellent stability under the operational conditions. Our results indicate that the E. coli with surface displayed γ-lactamase is an efficient and economical whole cell biocatalyst for preparing the antiviral drug intermediate (-) vince lactam at mild temperature, eliminating expensive energy cost performed at high temperature.

The contribution of Pir protein family to yeast cell surface display

Proteins with internal repeats (Pir) in the Baker's yeast are located on the cell wall and include four highly homologous members. Recently, Pir proteins have become increasingly used as anchor proteins in yeast cell surface display systems. These display systems are classified into three types: N-terminal fusion, C-terminal fusion, and inserted fusion. In addition to the GPI (glycosylphosphatidyl inositol) and the FL/FS anchor proteins, these three Pir-based systems significantly increase the choices for target proteins to be displayed. Furthermore, Pir proteins can also be used as a fusion partner for target proteins to be effectively secreted into culture medium. Here, we summarize the development and application of Pir proteins as anchor proteins.

Expression of β-glucuronidase on the surface of bacteria enhances activation of glucuronide prodrugs

Extracellular activation of hydrophilic glucuronide prodrugs by β-glucuronidase (βG) was examined to increase the therapeutic efficacy of bacteria-directed enzyme prodrug therapy (BDEPT). βG was expressed on the surface of Escherichia coli by fusion to either the bacterial autotransporter protein Adhesin (membrane βG (mβG)/AIDA) or the lipoprotein (lpp) outermembrane protein A (mβG/lpp). Both mβG/AIDA and mβG/lpp were expressed on the bacterial surface, but only mβG/AIDA displayed enzymatic activity. The rate of substrate hydrolysis by mβG/AIDA-BL21cells was 2.6-fold greater than by pβG-BL21 cells, which express periplasmic βG. Human colon cancer HCT116 cells that were incubated with mβG/AIDA-BL21 bacteria were sensitive to a glucuronide prodrug (p-hydroxy aniline mustard β-D-glucuronide, HAMG) with an half maximal inhibitory concentration (IC50) value of 226.53±45.4 μM, similar to the IC50 value of the active drug (p-hydroxy aniline mustard, pHAM; 70.6±6.75 μM), indicating that mβG/AIDA on BL21 bacteria could rapidly and efficiently convert HAMG to an active anticancer agent. These results suggest that surface display of functional βG on bacteria can enhance the hydrolysis of glucuronide prodrugs and may increase the effectiveness of BDEPT.

Probing the applicability of autotransporter based surface display with the EstA autotransporter of Pseudomonas stutzeri A15

Background

Autotransporters represent a widespread family of secreted proteins in Gram-negative bacteria. Their seemingly easy secretion mechanism and modular structure make them interesting candidates for cell surface display of heterologous proteins. The most widely applied host organism for this purpose is Escherichia coli. Pseudomonas stutzeri A15 is an interesting candidate host for environmentally relevant biotechnological applications. With the recently characterized P. stutzeri A15 EstA autotransporter at hand, all tools for developing a surface display system for environmental use are available. More general, this system could serve as a case-study to test the broad applicability of autotransporter based surface display.

Results

Based on the P. stutzeri A15 EstA autotransporter β-domain, a surface display expression module was constructed for use in P. stutzeri A15. Proof of concept of this module was presented by successful surface display of the original EstA passenger domain, which retained its full esterase activity. Almost all of the tested heterologous passenger domains however were not exposed at the cell surface of P. stutzeri A15, as assessed by whole cell proteinase K treatment. Only for a beta-lactamase protein, cell surface display in P. stutzeri A15 was comparable to presentation of the original EstA passenger domain. Development of expression modules based on the full-length EstA autotransporter did not resolve these problems.

Conclusions

Since only one of the tested heterologous passenger proteins could be displayed at the cell surface of P. stutzeri A15 to a notable extent, our results indicate that the EstA autotransporter cannot be regarded as a broad spectrum cell surface display system in P. stutzeri A15.

XadM, a novel adhesin of Xanthomonas oryzae pv. oryzae, exhibits similarity to Rhs family proteins and is required for optimum attachment, biofilm formation, and virulence

By screening a transposon-induced mutant library of Xanthomonas oryzae pv. oryzae, the bacterial blight pathogen of rice, we have identified a novel 5.241-kb open reading frame (ORF) named xadM that is required for optimum virulence and colonization. This ORF encodes a protein, XadM, of 1,746 amino acids that exhibits significant similarity to Rhs family proteins. The XadM protein contains several repeat domains similar to a wall-associated surface protein of Bacillus subtilis, which has been proposed to be involved in carbohydrate binding. The role of XadM in X. oryzae pv. oryzae adhesion was demonstrated by the impaired ability of an xadM mutant strain to attach and form biofilms. Furthermore, we show that XadM is exposed on the cell surface and its expression is regulated by growth conditions and plays an important role in the early attachment and entry inside rice leaves. Interestingly, XadM homologs are present in several diverse bacteria, including many Xanthomonas spp. and animal-pathogenic bacteria belonging to Burkholderia spp. This is the first report of a role for XadM, an Rhs family protein, in adhesion and virulence of any pathogenic bacteria.

Cell surface display of a β-glucosidase employing the type V secretion system on ethanologenic Escherichia coli for the fermentation of cellobiose to ethanol

We used the autodisplay system AIDA-I, which belongs to the type V secretion system (TVSS), to display the β-glucosidase BglC from Thermobifida fusca on the outer membrane of the ethanologenic Escherichia coli strain MS04 (MG1655 ∆pflB, ∆adhE, ∆frdA, ∆xylFGH, ∆ldhA, PpflB::pdc (Zm)-adhB (Zm)). MS04 that was transformed with the plasmid pAIDABglCRHis showed cellobiase activity (171 U/g(CDW)) and fermented 40 g/l cellobiose in mineral medium in 60 h with an ethanol yield of 81 % of the theoretical maximum. Whole-cell protease treatment, SDS-PAGE, and Western-blot analysis demonstrated that BglC was attached to the external surface of the outer membrane of MS04. When attached to the cells, BglC showed 93.3 % relative activity in the presence of 40 g/l ethanol and retained 100 % of its activity following 2 days of incubation at 37 °C with the same ethanol concentration. This study shows the potential of the TVSS (AIDA-I) and BglC as tools for the production of lignocellulosic bio-commodities.

Autodisplay of functional CYP106A2 in Escherichia coli

Cytochrome P450 enzymes catalyse a wide variety of reactions, including the hydroxylation and epoxidation of CC bonds, and dealkylation reactions. There is high interest in these reactions for biotechnology and pharmaceutical processes. Many P450s require membrane surroundings and have substrates that do not cross biological membranes. To circumvent these obstacles, CYP106A2 from Bacillus megaterium was expressed on the outer membrane of Escherichia coli cells by Autodisplay. Exposure on the surface was confirmed by a protease accessibility test and flow cytometry after immunolabelling. HPLC assays showed that 0.5 ml of cells displaying the enzyme (OD₅₇₈ = 6) converted 9.13 μmol of deoxycorticosterone to 15β-OH-deoxycorticosterone within 1h. Imipramine and abietic acid were also accepted as substrates. The number of active enzyme molecules per cell was calculated to be 20,000. Surprisingly, surface-exposed CYP106A2 was active in E. coli BL21 without the external addition of the heme group. However, when CYP106A2 was expressed on the surface of an E. coli strain lacking the TolC channel protein (JW5503), enzymatic activity was almost completely abolished. The activity of CYP106A2 on the surface of E. coli JW5503 could be restored by the external addition of the heme group. This suggests, as has been reported before, that E. coli uses a TolC-dependent mechanism to export heme into the growth media, where it can be scavenged by a surface-displayed apoenzyme. Our results indicate that Autodisplay enables the functional surface display of P450 enzymes and provides a new platform to access their synthetic potential.

IcsA autotransporter passenger promotes increased fusion protein expression on the cell surface

Background

Autotransporters are attractive cell surface display vehicles as they lack complex adaptor proteins necessary for protein export. Recent reports have suggested that the native effector domain (α domain) and translocation domain (β domain) interact with each other to drive translocation of the effector domain to the outer membrane. In this report we compared the expression, surface localisation and folding of TEM-1 β-lactamase (Bla) and maltose binding protein (MalE or MBP) fused to either full length Shigella flexneri IcsA (IcsA) autotransporter or to the β domain alone (IcsA_β) to determine the contribution of the native IcsA α domain in presenting the fusion proteins on the surface of E. coli K-12 UT5600 (ΔompT).

Results

Expression of IcsA-Bla was greater than IcsA_β-Bla. High levels of IcsA-MalE were detected but IcsA_β-MalE was not expressed. All fusion proteins other than IcsA_β-MalE were localised to the outer membrane and were detected on the surface of UT5600 via immunofluorescence microscopy. All bacteria expressing IcsA-MalE were labelled with both α-IcsA and α-MBP. UT5600 expressing IcsA_β-MalE was not labelled with α-MBP. A third of UT5600 expressing IcsA-Bla were detectable with α-Bla but only 5% of UT5600 (IcsA_β-Bla) were labelled with α-Bla. The correct folding of the Bla moiety when fused to IcsA and IcsA_β was also retained as UT5600 expressing either fusion protein exhibited a decreased zone of inhibition in the presence of ampicillin. UT5600 expressing IcsA-Bla was more resistant compared to UT5600 expressing IcsA_β-Bla.

Conclusions

The export mechanism of autotransporters is not well understood but accumulating evidence suggest a critical role for the native effector or α domain in facilitating its own export via interactions with the translocation or β domain. This is the first report directly comparing expression of heterologous proteins fused to the full length IcsA autotransporter and fusion to the β domain alone. Protein expression and surface presentation of the fusion proteins were dramatically improved when fused to IcsA rather than IcsA_β. Future studies involved in designing autotransporters as cell surface display vehicles would benefit from including the native α domain. This work also provides further evidence for a key interaction between the autotransporter α and β domains.

Surface display of Aeromonas hydrophila GAPDH in attenuated Vibrio anguillarum to develop a Noval multivalent vector vaccine

Displaying foreign antigens on the surface of attenuated or avirulent bacteria is an important strategy to develop live multivalent vector vaccines. In our previous work, several efficient surface display systems have been established based on outer membrane anchoring elements, which could successfully display heterologous proteins in attenuated Vibrio anguillarum. In this work, the glyceraldehyde-3-phosphate dehydrogenase (GAPDH) from pathogenic Aeromonas hydrophila LSA34 was fused to seven display systems and introduced into attenuated V. anguillarum strain MVAV6203 (AV) to get seven GAPDH-display strains. The strain AV/pN-gapA showed the best display efficacy of GAPDH and was tested as the multivalent vaccine candidate. Further immune protection evaluation of AV/pN-gapA in turbot (Scophtalmus maximus) demonstrated that the attenuated V. anguillarum with surface-displayed GAPDH of A. hydrophila LSA34 effectively protected turbot from the infections of A. hydrophila and V. anguillarum and showed potential value for further multivalent vaccine development.

Development of a LytE-based high-density surface display system in Bacillus subtilis

The three N‐terminal, tandemly arranged LysM motifs from a Bacillus subtilis cell wall hydrolase, LytE, formed a cell wall‐binding module. This module, designated CWBM_LytE, was demonstrated to have tight cell wall‐binding capability and could recognize two classes of cell wall binding sites with fivefold difference in affinity. The lower‐affinity sites were approximately three times more abundant. Fusion proteins with β‐lactamase attached to either the N‐ or C‐terminal end of CWBM_LytE showed lower cell wall‐binding affinity. The number of the wall‐bound fusion proteins was less than that of CWBM_LytE. These effects were less dramatic with CWBM_LytE at the N‐terminal end of the fusion. Both CWBM_LytE and β‐lactamase were essentially functional whether they were at the N‐ or C‐terminal end of the fusion. In the optimal case, 1.2 × 10⁷ molecules could be displayed per cell. As cells overproducing CWBM_LytE and its fusions formed filamentous cells (with an average of nine individual cells per filamentous cell), 1.1 × 10⁸β‐lactamase molecules could be displayed per filamentous cell. Overproduced CWBM_LytE and its fusions were distributed on the entire cell surface. Surface exposure and accessibility of these proteins were confirmed by immunofluorescence microscopy.

Surface display of the receptor-binding domain of the F17a-G fimbrial adhesin through the autotransporter AIDA-I leads to permeability of bacterial cells

Surface exposure of antigens on bacterial cells can be critical for eliciting an effective antibody response. Therefore, we investigated the cellular localization of the fimbrial F17a-G receptor-binding domain, fused to the translocator domain of the AIDA-I autotransporter. Synthesis of the fusion protein, under the control of the L-arabinose-inducible PBAD promoter, was shown to permeabilize Escherichia coli K-12 and Salmonella enterica serovar Typhimurium cells. The presence of permeable cells interfered with several methods that are typically used to determine surface exposure of proteins, such as protease treatment and whole-cell ELISA. Double immunofluorescence microscopy, using a second antibody directed against beta-galactosidase, a bacterial protein expressed in the cytoplasm, allowed the simultaneous detection of antigen expression and permeability in individual cells.

Surface display of GFP by Pseudomonas syringae truncated ice nucleation protein in attenuated Vibrio anguillarum strain

Microbial cell surface display of foreign proteins has been widely developed for many potential applications in live vaccine construction, whole-cell biocatalysts, and bioadsorption. To investigate the feasibility of displaying heterologous proteins on the surface of attenuated Vibrio anguillarum strain for potential multivalent live vaccine development, different display systems were built upon a truncated ice nucleation protein (INP) from Pseudomonas syringae ICMP3023 whose N- and C-terminal domains were considered to be the putative membrane-anchoring motifs. Green fluorescent protein (GFP), as a reporter, was fused with the display systems in different forms of N-GFP, NC-GFP, and N-GFP-C. Analysis of the total expression level and surface localization of GFP demonstrated that the truncated P. syringae INP could be used to display foreign protein in V. anguillarum, while the system of N-GFP showed the higher levels of total expression and surface display based on unit cell density among the three and might be available for further carrier vaccine development.

Novel bacterial surface display systems based on outer membrane anchoring elements from the marine bacterium Vibrio anguillarum

Surface display of heterologous peptides and proteins such as receptors, antigens, and enzymes on live bacterial cells is of considerable value for various biotechnological and industrial applications. In this study, a series of novel cell surface display systems were examined by using Vibrio anguillarum outer membrane protein and outer membrane lipoprotein as anchoring motifs. These display systems consist of (i) the signal sequence and first 11 N-terminal amino acids of V. anguillarum outer membrane lipoprotein Wza, or the signal sequence and first 9 N-terminal amino acids of the mature major Escherichia coli lipoprotein Lpp, and (ii) transmembrane domains of V. anguillarum outer membrane proteins Omporf1, OmpU, or Omp26La. In order to assay the translocation efficiency of constructed display systems in bacteria, green fluorescent protein (GFP) was inserted to the systems and the results of GFP surface localization confirmed that four of the six surface display systems could successfully display GFP on the E. coli surface. For assaying its potential application in live bacteria carrier vaccines, an excellent display system Wza-Omporf1 was fused with the major capsid protein (MCP) of large yellow croaker iridovirus and introduced into attenuated V. anguillarum strain MVAV6203, and subsequent analysis of MCP surface localization proved that the novel display system Wza-Omporf1 could function as a strong tool in V. anguillarum carrier vaccine development.

The autodisplay story, from discovery to biotechnical and biomedical applications

Among the pathways used by gram-negative bacteria for protein secretion, the autotransporter pathway represents a solution of impressive simplicity. Proteins are transported, independent of their nature as recombinant or native passengers, as long as the coding nucleotide sequence is inserted in frame between those of an N-terminal signal peptide and a C-terminal domain, referred to as the beta-barrel of the outer membrane translocation unit. The immunoglobulin A1 (IgA1) protease from Neisseria gonorrhoeae was the first identified member of the autotransporter family of secreted proteins. The IgA1 protease was employed in initial experiments investigating autotransporter-mediated surface display of recombinant proteins and to investigate structural and functional requirements. Various other autotransporter proteins have since been described, and the autodisplay system was developed on the basis of the natural Escherichia coli autotransporter protein AIDA-I (adhesin involved in diffuse adherence). Autodisplay has been used for the surface display of random peptide libraries to successfully screen for novel enzyme inhibitors. The autodisplay system was also used for the surface display of functional enzymes, including esterases, oxidoreductases, and electron transfer proteins. Whole E. coli cells displaying enzymes have been utilized to efficiently synthesize industrially important rare organic compounds with specific chirality. Autodisplay of epitopes on the surface of attenuated Salmonella carriers has also provided a novel way to induce immune protection after oral vaccination. This review summarizes the structural and functional features of the autodisplay system, illustrating its discovery and most recent applications. Autodisplay facilitates the export of more than 100,000 recombinant molecules per single cell and permits the oligomerization of subunits on the cell surface as well as the incorporation of inorganic prosthetic groups after transport of apoproteins onto the bacterial surface without disturbing bacterial integrity or viability. We discuss future biotechnical and biomedical applications in the light of these achievements.

Presentation of functional organophosphorus hydrolase fusions on the surface ofEscherichia coliby the AIDA-I autotransporter pathway

We report, the surface presentation of organophosphorus hydrolase (OPH) and green fluorescent protein (GFP) fusions by employing the adhesin-involved-in-diffuse-adherence (AIDA-I) translocator domain as a transporter and anchoring motif. The surface location of the OPH-GFP fusion protein was confirmed by immunofluorescence microscopy, and protease accessibility, followed by Western blotting analysis. The investigation of growth kinetics and stability of resting cultures showed that the presence of the AIDA-I translocator domain in the outer membrane neither inhibits cell growth nor affects cell viability. Furthermore, the surface-exposed OPH-GFP was shown to have enzymatic activity and a functional fluorescence moiety. These results suggest that AIDA-I autotransporter is a useful tool to present heterologous macromolecule passenger proteins on the bacterial surface. Our strategy of linking GFP to OPH and the possibility to employ various bacterial species as host has enormous potential for enhancing field use.

Highly attenuated Bordetella pertussis strain BPZE1 as a potential live vehicle for delivery of heterologous vaccine candidates

Bordetella pertussis, the causative agent of whooping cough, is a promising and attractive candidate for vaccine delivery via the nasal route, provided that suitable attenuation of this pathogen has been obtained. Recently, the highly attenuated B. pertussis BPZE1 strain has been described as a potential live pertussis vaccine for humans. We investigated here the use of BPZE1 as a live vehicle for heterologous vaccine candidates. Previous studies have reported the filamentous hemagglutinin (FHA), a major B. pertussis adhesin, as a carrier to express foreign antigens in B. pertussis. In this study, we also examined the BrkA autotransporter as a surface display system. Three copies of the neutralizing peptide SP70 from enterovirus 71 (EV71) were fused to FHA or in the passenger domain of BrkA, and each chimera was expressed in BPZE1. The FHA-(SP70)3 and BrkA-(SP70)3 chimeras were successfully secreted and exposed at the bacterial surface, respectively. Nasal administration of the live recombinant strains triggered a strong and sustained systemic anti-SP70 antibody response in mice, although the titers and neutralizing activities against EV71 were significantly higher in the sera of mice immunized with the BrkA-(SP70)3-producing strain. These data indicate that the highly attenuated BPZE1 strain is a potential candidate for vaccine delivery via the nasal route with the BrkA autotransporter as an alternative to FHA for the presentation of the heterologous vaccine antigens.

Limited tolerance towards folded elements during secretion of the autotransporter Hbp

Many virulence factors secreted by pathogenic Gram-negative bacteria belong to the autotransporter (AT) family. ATs consist of a passenger domain, which is the actual secreted moiety, and a beta-domain that facilitates the transfer of the passenger domain across the outer membrane. Here, we analysed folding and translocation of the AT passenger, using Escherichia coli haemoglobin protease (Hbp) as a model protein. Dual cysteine mutagenesis, instigated by the unique crystal structure of the Hbp passenger, resulted in intramolecular disulphide bond formation dependent on the periplasmic enzyme DsbA. A small loop tied off by a disulphide bond did not interfere with secretion of Hbp. In contrast, a bond between different domains of the Hbp passenger completely blocked secretion resulting in degradation by the periplasmic protease DegP. In the absence of DegP, a translocation intermediate accumulated in the outer membrane. A similar jammed intermediate was formed upon insertion of a calmodulin folding moiety into Hbp. The data suggest that Hbp can fold in the periplasm but must retain a certain degree of flexibility and/or modest width to allow translocation across the outer membrane.

Lipoprotein PssN of Rhizobium leguminosarum bv. trifolii: subcellular localization and possible involvement in exopolysaccharide export

Surface expression of exopolysaccharides (EPS) in gram-negative bacteria depends on the activity of proteins found in the cytoplasmic membrane, the periplasmic space, and the outer membrane. pssTNOP genes identified in Rhizobium leguminosarum bv. trifolii strain TA1 encode proteins that might be components of the EPS polymerization and secretion system. In this study, we have characterized PssN protein. Employing pssN-phoA and pssN-lacZ gene fusions and in vivo acylation with [3H]palmitate, we demonstrated that PssN is a 43-kDa lipoprotein directed to the periplasm by an N-terminal signal sequence. Membrane detergent fractionation followed by sucrose gradient centrifugation showed that PssN is an outer membrane-associated protein. Indirect immunofluorescence with anti-PssN and fluorescein isothiocyanate-conjugated antibodies and protease digestion of spheroplasts and intact cells of TA1 provided evidence that PssN is oriented towards the periplasmic space. Chemical cross-linking of TA1 and E. coli cells overproducing PssN-His6 protein showed that PssN might exist as a homo-oligomer of at least two monomers. Investigation of the secondary structure of purified PssN-His6 protein by Fourier transform infrared spectroscopy revealed the predominant presence of beta-structure; however, alpha-helices were also detected. Influence of an increased amount of PssN protein on the TA1 phenotype was assessed and correlated with a moderate enhancement of EPS production.

The periplasmic folding of a cysteineless autotransporter passenger domain interferes with its outer membrane translocation

Autotransporters are single polypeptides consisting of an outer membrane translocation domain mediating the translocation of a passenger domain. The periplasmic folding state of the passenger domain is controversial. By comparisons of passenger domains differing in their folding properties, our results suggest that periplasmic folding of passenger domains interferes with translocation.

Surface display of proteins by gram-negative bacterial autotransporters

Expressing proteins of interest as fusions to proteins of the bacterial envelope is a powerful technique with many biotechnological and medical applications. Autotransporters have recently emerged as a good tool for bacterial surface display. These proteins are composed of an N-terminal signal peptide, followed by a passenger domain and a translocator domain that mediates the outer membrane translocation of the passenger. The natural passenger domain of autotransporters can be replaced by heterologous proteins that become displayed at the bacterial surface by the translocator domain. The simplicity and versatility of this system has made it very attractive and it has been used to display functional enzymes, vaccine antigens as well as polypeptides libraries. The recent advances in the study of the translocation mechanism of autotransporters have raised several controversial issues with implications for their use as display systems. These issues include the requirement for the displayed polypeptides to remain in a translocation-competent state in the periplasm, the requirement for specific signal sequences and "autochaperone" domains, and the influence of the genetic background of the expression host strain. It is therefore important to better understand the mechanism of translocation of autotransporters in order to employ them to their full potential. This review will focus on the recent advances in the study of the translocation mechanism of autotransporters and describe practical considerations regarding their use for bacterial surface display.

Delivery of heterologous protein antigens via hemolysin or autotransporter systems by an attenuated ler mutant of rabbit enteropathogenic Escherichia coli

In this report, we describe the use of an attenuated regulatory mutant of a rabbit enteropathogenic Escherichia coli (rEPEC) as a live vaccine vector to deliver heterologous protein antigens using two dedicated transport systems, a Salmonella autotransporter and the E. coli hemolysin apparatus. We previously reported that an isogeneic ler (LEE encoded regulator) mutant of rEPEC O103:H2 is attenuated and immunogenic in rabbits. We first evaluated the Salmonella autotransporter MisL containing the immunodominant B-cell epitope of the circumsporozoite protein from Plasmodium falciparum, (NANP)8, fused to the C-terminal translocator domain under the control of the constitutive Tac17 promoter. The rEPEC ler mutant was able to express and to translocate the (NANP)8 passenger peptide to the bacterial surface. We next investigated the delivery of Shiga toxin B subunit (Stx1B) from human enterohemorrhagic E. coli by the rEPEC ler mutant via the MisL autotransporter or the E. coli hemolysin secretion apparatus. The autotransporter and hemolysin plasmids expressed similar levels of Stx1B (30-40 ng/ml/OD600). Only 6% of Stx1B was found in the autotransporter supernatants; the rest was cell-associated, with a small fraction of the Stx1B surface-exposed as determined by immunofluorescence. In contrast, 88% of Stx1B was secreted into culture supernatants by the hemolysin secretion system. In an in vivo study, no significant protection was observed in rabbits inoculated with the ler mutant harboring the Stx1B-autotransporter plasmid following experimental challenge with RDEC-H19A, the prototype rEPEC containing an Stx-converting phage. In contrast, rabbits inoculated with the rEPEC ler mutant containing the Stx1B-hemolysin fusion were partially protected from RDEC-H19A infection as demonstrated by decreased weight loss (p<0.008) when compared to rabbits inoculated with the parent ler mutant. Our results suggest that attenuated rEPEC are capable of serving as vaccine vectors to express heterologous protein antigens from different cellular locations and deliver these antigens to the intestinal mucosa. With this system, secreted proteins may be more effective than cell-associated antigens in generating protection.

Autodisplay: efficient bacterial surface display of recombinant proteins

To display a protein or peptide with a distinct function at the surface of a living bacterial cell is a challenging exercise with constantly increasing impact in many areas of biochemistry and biotechnology. Among other systems in Gram-negative bacteria, the Autodisplay system provides striking advantages when used to express a recombinant protein at the surface of Escherichia coli or related bacteria. The Autodisplay system has been developed on the basis of and by exploiting the natural secretion mechanism of the AIDA-I autotransporter protein. It offers the expression of more than 10(5) recombinant molecules per single cell, permits the multimerization of subunits expressed from monomeric genes at the cell surface, and allows, after transport of an apoprotein to the cell surface, the incorporation of an inorganic prosthetic group without disturbing cell integrity or cell viability. Moreover, whole cells displaying recombinant proteins by Autodisplay can be subjected to high-throughput screening (HTS) methods such as ELISA or FACS, thus enabling the screening of surface display libraries and providing access to directed evolution of the recombinant protein displayed at the cell surface. In this review, the application of the Autodisplay system for the surface display of enzymes, enzyme inhibitors, epitopes, antigens, protein and peptide libraries is summarised and the perspectives of the system are discussed.

Extracellular accumulation of recombinant proteins fused to the carrier protein YebF in Escherichia coli

Bacterial protein secretion is important in the life cycles of most bacteria, in which it contributes to the formation of pili and flagella and makes available extracellular enzymes to digest polymers for nutritional purposes and toxins to kill host cells in infections of humans, animals and plants. It is generally accepted that nonpathogenic laboratory strains of Escherichia coli, particularly K12 strains, do not secrete proteins into the extracellular medium under routine growth conditions. In this study, we report that commonly used laboratory strains secrete YebF, a small (10.8 kDa in the native form), soluble endogenous protein into the medium, challenging the status quo view that laboratory strains do not secrete proteins to the medium. We further show that 'passenger' proteins linked to the carboxyl end of YebF are efficiently secreted. The function of YebF is unknown, but its use as a carrier for transgenic proteins provides a tool to circumvent toxicity and other contamination issues associated with protein production in E. coli.