Secretion of recombinant proteins via the chaperone/usher pathway in Escherichia coli

Oriented Insertion of ESR-Containing Hybrid Proteins in Proteoliposomes

Microbial rhodopsins comprise a diverse family of retinal-containing membrane proteins that convert absorbed light energy to transmembrane ion transport or sensory signals. Incorporation of these proteins in proteoliposomes allows their properties to be studied in a native-like environment; however, unidirectional protein orientation in the artificial membranes is rarely observed. We aimed to obtain proteoliposomes with unidirectional orientation using a proton-pumping retinal protein from Exiguobacterium sibiricum, ESR, as a model. Three ESR hybrids with soluble protein domains (mCherry or thioredoxin at the C-terminus and Caf1M chaperone at the N-terminus) were obtained and characterized. The photocycle of the hybrid proteins incorporated in proteoliposomes demonstrated a higher pK_a of the M state accumulation compared to that of the wild-type ESR. Large negative electrogenic phases and an increase in the relative amplitude of kinetic components in the microsecond time range in the kinetics of membrane potential generation of ESR-Cherry and ESR-Trx indicate a decrease in the efficiency of transmembrane proton transport. On the contrary, Caf-ESR demonstrates a native-like kinetics of membrane potential generation and the corresponding electrogenic stages. Our experiments show that the hybrid with Caf1M promotes the unidirectional orientation of ESR in proteoliposomes.

Unraveling the molecular determinants of the anti-phagocytic protein cloak of plague bacteria

The pathogenic bacterium Yersina pestis is protected from macrophage engulfment by a capsule like antigen, F1, formed of long polymers of the monomer protein, Caf1. However, despite the importance of this pathogen, the mechanism of protection was not understood. Here we demonstrate how F1 protects the bacteria from phagocytosis. First, we show that Escherichia coli expressing F1 showed greatly reduced adherence to macrophages. Furthermore, the few cells that did adhere remained on the macrophage surface and were not engulfed. We then inserted, by mutation, an “RGDS” integrin binding motif into Caf1. This did not change the number of cells adhering to macrophages but increased the fraction of adherent cells that were engulfed. Therefore, F1 protects in two separate ways, reducing cell adhesion, possibly by acting as a polymer brush, and hiding innate receptor binding sites needed for engulfment. F1 is very robust and we show that E. coli expressing weakened mutant polymers are engulfed like the RGDS mutant. This suggests that innate attachment sites on the native cell surface are exposed if F1 is weakened. Single-molecule force spectroscopy (SMFS) experiments revealed that wild-type F1 displays a very high mechanical stability of 400 pN. However, the mechanical resistance of the destabilised mutants, that were fully engulfed, was only 20% weaker. By only marginally exceeding the mechanical force applied to the Caf1 polymer during phagocytosis it may be that the exceptional tensile strength evolved to resist the forces applied at this stage of engulfment.

Macrophages, a type of white blood cell, form an important element of our immune defence. They interrogate other cells’ surfaces for molecular clues and ingest those presenting a threat in a process known as phagocytosis. Not surprisingly, pathogenic bacteria have developed ways to evade this fate. The plague bacterium, Yersinia pestis, produces the long polymeric F1 coat protein which enables it to avoid ingestion, but the mechanism was unclear. We show that equipping Escherichia coli cells with an F1 coat protected them from phagocytosis by two separate mechanisms, reducing contact with the macrophage surface and hiding the signals that tell the macrophages they are targets. F1 is also a very stable protein polymer and using single molecule force spectroscopy we showed it also has a very high resistance to pulling forces. Surprisingly, mutations which reduced this by only 20% caused adherent bacteria to be fully ingested, indicating that cells are subject to significant forces prior to recognition and ingestion. Thus, F1 has evolved three notable properties (i) physical; creation of a hydrated polymer brush to inhibit surface interactions, (ii) chemical; absence of molecular recognition clues needed for engulfment and (iii) mechanical; strength that maintains the camouflage layer during surface stretching.

Expression in Escherichia coli of fusion protein comprising α-conotoxin TxIB and preservation of selectivity to nicotinic acetylcholine receptors in the purified product

Nicotinic acetylcholine receptors (nAChRs) are ligand-gated ion channels, which are widely distributed in the central and peripheral nervous system. The α6β2* nAChR is an important subtype, which is closely associated with nicotine addiction and movement disorders etc. α-conotoxin TxIB with 16-amino acid residues specifically targets α6β2* nAChR with no obvious effect on other nAChR subtypes. However, chemical synthesis of TxIB is expensive, and the quantity of native TxIB extracted from cone snail is limited. In the present study, we attempted to obtain TxIB using biological method based on the recombinant expression in Escherichia coli (E. coli). The synthetic gene encoding mature peptide of TxIB was inserted in pET-31b(+) vector and transformed into E. coli strain BLR(DE3)pLysS for expression. The recombinant fusion protein KSI-TxIB-His₆ (KSI, ketosteroid isomerase) was expressed successfully as inclusion body in E. coli, which was purified by Ni-NTA affinity chromatography column and cleaved by cyanogen bromide (CNBr) to release recombinant α-conotoxin TxIB (rTxIB). Then, rTxIB was purified by reverse-phase high-performance liquid chromatography (RP-HPLC) and was identified by electrospray ionization mass spectrometry (ESI-MS). Pharmacological activity of rTxIB was assessed by electrophysiological approaches. The results indicated that it preserved about 50% of potency, but, was even more important, had the same selectivity as the natural conotoxin which may provide an alternative method for quantity production of small peptides with low cost on the premise of not changing their potency.

Purification and characterization of a recombinant Yersinia pestis V-F1 "Reversed" fusion protein for use as a new subunit vaccine against plague

We previously developed a unique recombinant protein vaccine against plague composed of a fusion between the Fraction 1 capsular antigen (F1) and the V antigen. To determine if overall expression, solubility, and recovery of the F1-V fusion protein could be enhanced, we modified the original fusion. Standard recombinant DNA techniques were used to reverse the gene order such that the V antigen coding sequence was fused at its C-terminus to the N-terminus of F1. The F1 secretion signal sequence (F1S) was subsequently fused to the N-terminus of V. This new fusion protein, designated F1S-V-F1, was then co-expressed with the Y. pestis Caf1M periplasmic chaperone protein in BL21-Star Escherichia coli. Recombinant strains expressing F1-V, F1S-F1-V, or F1S-V-F1 were compared by cell fractionation, SDS-PAGE, Western blotting, and suspension immunolabelling. F1S-V-F1 exhibited enhanced solubility and secretion when co-expressed with Caf1M resulting in a recombinant protein that is processed in a similar manner to the native F1 protein. Purification of F1S-V-F1 was accomplished by anion-exchange and hydrophobic interaction chromatography. The purification method produced greater than 1mg of purified soluble protein per liter of induced culture. F1S-V-F1 polymerization characteristics were comparable to the native F1. The purified F1S-V-F1 protein appeared equivalent to F1-V in its ability to be recognized by neutralizing antibodies.

Adhesive organelles of Gram-negative pathogens assembled with the classical chaperone/usher machinery: structure and function from a clinical standpoint

This review summarizes current knowledge on the structure, function, assembly and biomedical applications of the superfamily of adhesive fimbrial organelles exposed on the surface of Gram-negative pathogens with the classical chaperone/usher machinery. High-resolution three-dimensional (3D) structure studies of the minifibers assembling with the FGL (having a long F1-G1 loop) and FGS (having a short F1-G1 loop) chaperones show that they exploit the same principle of donor-strand complementation for polymerization of subunits. The 3D structure of adhesive subunits bound to host-cell receptors and the final architecture of adhesive fimbrial organelles reveal two functional families of the organelles, respectively, possessing polyadhesive and monoadhesive binding. The FGL and FGS chaperone-assembled polyadhesins are encoded exclusively by the gene clusters of the γ3- and κ-monophyletic groups, respectively, while gene clusters belonging to the γ1-, γ2-, γ4-, and π-fimbrial clades exclusively encode FGS chaperone-assembled monoadhesins. Novel approaches are suggested for a rational design of antimicrobials inhibiting the organelle assembly or inhibiting their binding to host-cell receptors. Vaccines are currently under development based on the recombinant subunits of adhesins.

FGL chaperone-assembled fimbrial polyadhesins: anti-immune armament of Gram-negative bacterial pathogens

This review summarizes the current knowledge on the structure, function, assembly, and biomedical applications of the family of adhesive fimbrial organelles assembled on the surface of Gram-negative pathogens via the FGL chaperone/usher pathway. Recent studies revealed the unique structural and functional properties of these organelles, distinguishing them from a related family, FGS chaperone-assembled adhesive pili. The FGL chaperone-assembled organelles consist of linear polymers of one or two types of protein subunits, each possessing one or two independent adhesive sites specific to different host cell receptors. This structural organization enables these fimbrial organelles to function as polyadhesins. Fimbrial polyadhesins may ensure polyvalent fastening of bacteria to the host cells, aggregating their receptors and triggering subversive signals that allow pathogens to evade immune defense. The FGL chaperone-assembled fimbrial polyadhesins are attractive targets for vaccine and drug design.

A comparison of immunogenicity and protective immunity against experimental plague by intranasal and/or combined with oral immunization of mice with attenuated Salmonella serovar Typhimurium expressing secreted Yersinia pestis F1 and V antigen

We investigated the relative immunogenicity and protective efficacy of recombinant X85MF1 and X85V strains of DeltacyaDeltacrpDeltaasd-attenuated Salmonella Typhimurium expressing, respectively, secreted Yersinia pestis F1 and V antigens, following intranasal (i.n.) or i.n. combined with oral immunization for a mouse model. A single i.n. dose of 10(8) CFU of X85MF1 or X85V induced appreciable serum F1- or V-specific IgG titres, although oral immunization did not. Mice i.n. immunized three times (i.n. x 3) with Salmonella achieved the most substantial F1/V-specific IgG titres, as compared with corresponding titres for an oral-primed, i.n.-boosted (twice; oral-i.n. x 2) immunization regimen. The level of V-specific IgG was significantly greater than that of F1-specific IgG (P<0.001). Analysis of the IgG antibodies subclasses revealed comparable levels of V-specific Th-2-type IgG1 and Th-1-type IgG2a, and a predominance of F1-specific Th-1-type IgG2a antibodies. In mice immunized intranasally, X85V stimulated a greater IL-10-secreting-cell response in the lungs than did X85MF1, but impaired the induction of gamma-interferon-secreting cells. A program of i.n. x 3 and/or oral-i.n. x 2 immunization with X85V provided levels of protection against a subsequent lethal challenge with Y. pestis, of, respectively, 60% and 20%, whereas 80% protection was provided following the same immunization but with X85MF1.

Development of expression vectors for Escherichia coli based on the pCR2 replicon

Background

Recent developments in metabolic engineering and the need for expanded compatibility required for co-expression studies, underscore the importance of developing new plasmid vectors with properties such as stability and compatibility.

Results

We utilized the pCR2 replicon of Corynebacterium renale, which harbours multiple plasmids, for constructing a range of expression vectors. Different antibiotic-resistance markers were introduced and the vectors were found to be 100% stable over a large number of generations in the absence of selection pressure. Compatibility of this plasmid was studied with different Escherichia coli plasmid replicons viz. pMB1 and p15A. It was observed that pCR2 was able to coexist with these E.coli plasmids for 60 generations in the absence of selection pressure. Soluble intracellular production was checked by expressing GFP under the lac promoter in an expression plasmid pCR2GFP. Also high level production of human IFNγ was obtained by cloning the h-IFNγ under a T7 promoter in the expression plasmid pCR2-IFNγ and using a dual plasmid heat shock system for expression. Repeated sub-culturing in the absence of selection pressure for six days did not lead to any fall in the production levels post induction, for both GFP and h-IFNγ, demonstrating that pCR2 is a useful plasmid in terms of stability and compatibility.

Conclusion

We have constructed a series of expression vectors based on the pCR2 replicon and demonstrated its high stability and sustained expression capacity, in the absence of selection pressure which will make it an efficient tool for metabolic engineering and co-expression studies, as well as for scale up of expression.

Enhanced immune response by amphotericin B following NS1 protein prime-oral recombinant Salmonella vaccine boost vaccination protects mice from dengue virus challenge

A recombinant vaccine strain SL3261/pLT105 of attenuated aroA Salmonella enterica serovar Typhimurium SL3261 strain expressing a secreted dengue virus type 2 non-structural NS1 and Yersinia pestis F1 (Caf1) fusion protein, rNS1:Caf1, was generated. Immunological evaluation was performed by prime-boost vaccine regimen. Oral immunization of mice with 1 x 10(9)cfu of SL3261/pLT105 only induced low levels of NS1-specific antibody response and protective immunity following dengue virus challenge. The parenteral NS1 protein priming-oral Salmonella boosting protocol enhanced both NS1-specific serum IgG response and protective efficacy as compared to mice immunized with each type vaccine alone. Addition of an antifungal antibiotic amphotericin B (AmB) to Salmonella vaccine further enhanced the synergic effects of prime-boost vaccine regimen on the elicited NS1-specific serum IgG response and the protective efficacy. Together, the results demonstrated that the rNS1:Caf1 producing Salmonella SL3261/pLT105 strain fails to provide effective protection as an oral vaccine alone despite co-administration of AmB as an adjuvant capable of enhancing the immune responses, and moreover, the protein priming-oral Salmonella vaccine boosting approach in combination with AmB as an immunization regimen may have the potential to be further explored as an alternative approach for dengue vaccine development.

Donor strand complementation mechanism in the biogenesis of non-pilus systems

The F1 antigen of Yersinia pestis belongs to a class of non-pilus adhesins assembled via a classical chaperone-usher pathway. Such pathways consist of PapD-like chaperones that bind subunits and pilot them to the outer membrane usher, where they are assembled into surface structures. In a recombinant Escherichia coli model system, chaperone-subunit (Caf1M:Caf1n) complexes accumulate in the periplasm. Three independent methods showed that these complexes are rod- or coil-shaped linear arrays of Caf1 subunits capped at one end by a single copy of Caf1M chaperone. Deletion and point mutagenesis identified an N-terminal donor strand region of Caf1 that was essential for polymerization in vitro, in the periplasm and at the cell surface, but not for chaperone-subunit interaction. Partial protease digestion of periplasmic complexes revealed that this region becomes buried upon formation of Caf1:Caf1 contacts. These results show that, despite the capsule-like appearance of F1 antigen, the basic structure is assembled as a linear array of subunits held together by intersubunit donor strand complementation. This example shows that strikingly different architectures can be achieved by the same general principle of donor strand complementation and suggests that a similar basic polymer organization will be shared by all surface structures assembled by classical chaperone-usher pathways.