Exploring the N-terminal role of a heterologous protein in secreting out ofEscherichia coli

Impact of a random TN5 mutation on endoglucanase secretion in ruminal cellulolytic Escherichia coli

OBJECTIVE

Most protein secretion systems are found in gram-negative bacteria, but the mechanism of endoglucanase (BcsZ) secretion in Escherichia coli (E. coli) remains unclear.

METHODS

In this study, we used JBZ-DH5α (which overexpresses BcsZ on the E. coli DH5α genome) as the initial strain. A mutant library was created by randomly inserting the TN5 transposon into the genome, and mutants with reduced transparent circles were identified on Congo red plates. The insertion sites of transposons in the genome were determined through whole-genome sequencing.

RESULTS

The results revealed that the genes rnc, lon, and suhB, which encode RNC-ribonuclease III (RNC), LON-protease (LON), and SuhB-inositol phosphatase (SuhB), respectively, were disrupted. BcsZ secretion decreased in E. coli DH5α when the lon, rnc, or suhB genes were deleted, but the overexpression of these genes restored their secretion levels.

CONCLUSION

These findings suggest that the lon, rnc, and suhB genes play a role in BcsZ secretion in E. coli, potentially enhancing our knowledge of BcsZ secretion and offering a strategy to increase protein secretion in E. coli as a cell factory.

Engineered probiotic Lactobacillus plantarum WCSF I for monitoring and treatment of Staphylococcus aureus infection

IMPORTANCE

Bacterial infection and the emergence of drug-resistant strains are major problems in clinical treatment. Staphylococcus aureus, which typically infects the skin and blood of animals, is also a potential intestinal pathogen that needs to be addressed by the emergence of a new treatment approach. Probiotic therapy is the most likely alternative to antibiotic therapy to solve the problem of bacterial drug resistance in clinical practice. In this study, the engineered Lactobacillus plantarum can not only sense the signal AIP to detect S. aureus but also kill S. aureus by secreting the lysostaphin enzyme. Our strategy employed an Agr quorum-sensing genetic circuit to simultaneously detect and treat pathogenic bacteria, which provided a theoretical possibility for solving practical clinical bacterial infection cases in the future.

Regulation of Cell Membrane Permeability Enhanced the Non-Classical Secretion of γ-Cyclodextrin Glycosyltransferase in Bacillus subtilis

γ-Cyclodextrin glycosyltransferase (γ-CGTase, EC 2.4.1.19) is an essential enzyme required in the production of γ-cyclodextrin, which shows huge prospects in the food, medicine, materials, and chemical industries. In this study, γ-CGTase from Bacillus sp. G-825-6 STB17 was successfully cloned and expressed in Bacillus subtilis WB600. The final extracellular activity of γ-CGTase can reach 45.34 U/mL with the deletion of the signal peptide, which was about 11.3 times of the initial level of γ-CGTase secreted by the general pathway. By monitoring the whole cultivation process, secretion was divided into two stages, which were dominated by cell membrane changes and apoptosis. The measurement of lactate dehydrogenase and the results of fluorescence microscopy demonstrated that the cell membrane permeability changed significantly in the middle stage of fermentation, proving that it played a crucial role in the non-classical secretion of γ-CGTase. Furthermore, the addition of Triton X-100, a non-ionic surfactant, remarkably enhanced the secretion level of γ-CGTase by 21.5%, which was caused by the increase in cell membrane permeability. This work is the first to obtain the extracellular expression of CGTase via the non-classical secretion pathway in B. subtilis and provides a new strategy to enhance extracellular expression by regulating the cell membranes.

Engineering multifunctional enzymes for agro-biomass utilization

Lignocellulosic biomass is a plentiful renewable resource that can be converted into a wide range of high-value-added industrial products. However, the complexity of its structural integrity is one of the major constraints and requires combinations of different fibrolytic enzymes for the cost-effective, industrially and environmentally feasible transformation. An interesting approach is constructing multifunctional enzymes, either in a single polypeptide or by joining multiple domains with linkers and performing diverse reactions simultaneously, in a single host. The production of such chimera proteins multiplies the advantages of different enzymatic reactions in a single setup, in lesser time, at lower production cost and with desirable and improved catalytic activities. This review embodies the various domain-tailoring and extracellular secretion strategies, possible solutions to their challenges, and efforts to experimentally connect different catalytic activities in a single host, as well as their applications.

Non-classical secretion of a type I L-asparaginase in Bacillus subtilis

L-asparaginase (EC 3.5.1.1) showed great commercial value owing to its effective treatment of acute lymphoblastic leukemia (ALL), lymphoid system malignancies and Hodgkin disease, and also to its use in the prevention of acrylamide formation in fried and baked foods. In this study, a type I L-asparaginase gene from Bacillus licheniformis Z-1 (BlAase) was cloned and expressed in Bacillus subtilis RIK 1285. Results showed that even without the mediation of any N-terminal signal peptides, BlAase can efficiently secrete into the medium. Further investigation indicated that the secretion of the BlAase was via neither Sec- nor Tat-dependent secretion pathway, and both the N- and C-terminal regions of the BlAase were essential for its expression and secretion, implying that BlAase might be secreted via a non-classical secretion pathway. To explore its secretion ability, BlAase was used as a signal peptide to direct the secretion of various heterologous proteins, where two of five proteins were successfully secreted with the mediation of BlAase. To the best of our knowledge, this is the first time to achieve extracellular expression of L-asparaginase via non-classical protein secretion pathway in B. subtilis, and provide a potential tool for secretion of recombinant proteins expressed in B. subtilis using BlAase as a signal peptide.

Identification of Vibrio parahaemolyticus and Vibrio spp. Specific Outer Membrane Proteins by Reverse Vaccinology and Surface Proteome

The discovery of outer membrane proteins (OMPs) with desirable specificity and surface availability is a fundamental challenge to develop accurate immunodiagnostic assay and multivalent vaccine of pathogenic Vibrio species in food and aquaculture. Herein 101 OMPs were systemically screened from 4,831 non-redundant proteins of Vibrio parahaemolyticus by bioinformatical predication of signaling peptides, transmembrane (TM) α-helix, and subcellular location. The sequence homology analysis with 32 species of Vibrio spp. and all the non-Vibrio strains revealed that 15 OMPs were conserved in at least 23 Vibrio species, including BamA (VP2310), GspD (VP0133), Tolc (VP0425), OmpK (VP2362), OmpW (VPA0096), LptD (VP0339), Pal (VP1061), flagellar L-ring protein (VP0782), flagellar protein MotY (VP2111), hypothetical protein (VP1713), fimbrial assembly protein (VP2746), VacJ lipoprotein (VP2214), agglutination protein (VP1634), and lipoprotein (VP1267), Chitobiase (VP0755); high adhesion probability of flgH, LptD, OmpK, and OmpW indicated they were potential multivalent Vibrio vaccine candidates. V. parahaemolyticus OMPs were found to share high homology with at least one or two Vibrio species, 19 OMPs including OmpA like protein (VPA073), CsuD (VPA1504), and MtrC (VP1220) were found relatively specific to V. parahaemolyticus. The surface proteomic study by enzymatical shaving the cells showed the capsular polysaccharides most likely limited the protease action, while the glycosidases improved the availability of OMPs to trypsin. The OmpA (VPA1186, VPA0248, VP0764), Omp (VPA0166), OmpU (VP2467), BamA (VP2310), TolC (VP0425), GspD (VP0133), OmpK (VP2362), lpp (VPA1469), Pal (VP1061), agglutination protein (VP1634), and putative iron (III) compound receptor (VPA1435) have better availability on the cell surface.

Principle and potential applications of the non-classical protein secretory pathway in bacteria

In addition to the extracellular proteins secreted by known secretory pathways, a number of cytoplasmic proteins without predicable or known signal sequences or secretory motifs have been found in the extracellular milieu, and were consequently classified as non-classically secreted proteins. Non-classical protein secretion is considered to be a general, conserved cellular phenomenon in both eukaryotes and prokaryotes. There are several research hotspots on the non-classical protein secretory pathway, and the most important two of them are the recognition principle of substrate proteins and possible secretory mechanisms. To date, researchers have made some progress in understanding the characteristics of these proteins. For example, it was discovered that many non-classically secreted proteins exist and are secreted in multimeric form. Some of these proteins prefer to be clustered and exported at the poles and the septum of the cell. The majority of these proteins play different functions when they are in the intra- and extracellular environments, and several of their functions are related to survival and pathogenicity. Furthermore, non-classically secreted proteins can be used as leading proteins to guide a POI (protein of interest) out of the cells, which provides a novel strategy for protein secretion with potential applications in the industry. Summarizing these findings, this review emphasizes the hot spots related to non-classically secreted proteins in bacteria, lists the most important hypotheses on the selection and secretion mechanisms of non-classically secreted proteins, and put forward their potential applications.

Factors Influencing Recombinant Protein Secretion Efficiency in Gram-Positive Bacteria: Signal Peptide and Beyond

Signal peptides are short peptides directing newly synthesized proteins toward the secretory pathway. These N-terminal signal sequences are ubiquitous to all prokaryotes and eukaryotes. Signal peptides play a significant role in recombinant protein production. Previous studies have demonstrated that the secretion amount of a given target protein varies significantly depending on the signal peptide that is fused to the protein. Signal peptide selection and signal peptide modification are the two main methods for the optimization of a recombinant protein secretion. However, the highly efficient signal peptide for a target protein with a specific bacterial expression host is not predictable so far. In this article, we collect several signal peptides that have previously performed well for recombinant protein secretion in gram-positive bacteria. We also discuss several factors influencing recombinant protein secretion efficiency in gram-positive bacteria. Signal peptides with a higher charge/length ratio in n-region, more consensus residues at the-3 and-1positions in c-region and a much higher proportion of coils are more likely to perform well in the secretion of recombinant proteins. These summaries can be utilized to the selection and directed modification of signal peptides for a given recombinant protein.

Developing Gram-negative bacteria for the secretion of heterologous proteins

Gram-negative bacteria are attractive hosts for recombinant protein production because they are fast growing, easy to manipulate, and genetically stable in large cultures. However, the utility of these microbes would expand if they also could secrete the product at commercial scales. Secretion of biotechnologically relevant proteins into the extracellular medium increases product purity from cell culture, decreases downstream processing requirements, and reduces overall cost. Thus, researchers are devoting significant attention to engineering Gram-negative bacteria to secrete recombinant proteins to the extracellular medium. Secretion from these bacteria operates through highly specialized systems, which are able to translocate proteins from the cytosol to the extracellular medium in either one or two steps. Building on past successes, researchers continue to increase the secretion efficiency and titer through these systems in an effort to make them viable for industrial production. Efforts include modifying the secretion tags required for recombinant protein secretion, developing methods to screen or select rapidly for clones with higher titer or efficiency, and improving reliability and robustness of high titer secretion through genetic manipulations. An additional focus is the expression of secretion machineries from pathogenic bacteria in the "workhorse" of biotechnology, Escherichia coli, to reduce handling of pathogenic strains. This review will cover recent advances toward the development of high-expressing, high-secreting Gram-negative production strains.

An isolated cellulolytic Escherichia coli from bovine rumen produces ethanol and hydrogen from corn straw

BACKGROUND

Lignocellulosic biomass is the most abundant resource on earth. Lignocellulose is mainly composed of cellulose, hemicelluloses, and lignin. The special construction of three kinds of constituents led to the prevention of effective degradation. The goal of this work was to investigate the great potentials of bovine rumen for novel cellulolytic bacterial isolation, which may be used for chemicals and biofuel production from lignocellulose.

RESULTS

A cellulolytic strain, ZH-4, was isolated from Inner Mongolia bovine rumen. This strain was identified as Escherichia coli by morphological, physiological, and biochemical characteristics and 16S rDNA gene sequencing. The extracellular enzyme activity analysis showed that this strain produces extracellular cellulases with an exoglucanase activity of 9.13 IU, an endoglucanase activity of 5.31 IU, and a β-glucosidase activity of 7.27 IU at the pH 6.8. This strain was found to produce 0.36 g/L ethanol and 4.71 mL/g hydrogen from corn straw with cellulose degradation ratio of 14.30% and hemicellulose degradation ratio of 11.39%.

CONCLUSIONS

It is the first time that a cellulolytic E. coli was isolated and characterized form the bovine rumen. This provided a great opportunity for researchers to investigate the evolution mechanisms of the microorganisms in the rumen and provided great chance to produce biofuels and chemicals directly from engineered E. coli using consolidated bioprocess.

Multimer recognition and secretion by the non-classical secretion pathway in Bacillus subtilis

Non-classical protein secretion in bacteria is a common phenomenon. However, the selection principle for non-classical secretion pathways remains unclear. Here, our experimental data, to our knowledge, are the first to show that folded multimeric proteins can be recognized and excreted by a non-classical secretion pathway in Bacillus subtilis. We explored the secretion pattern of a typical cytoplasmic protein D-psicose 3-epimerase from Ruminococcus sp. 5_1_39BFAA (RDPE), and showed that its non-classical secretion is not simply due to cell lysis. Analysis of truncation variants revealed that the C- and N-terminus, and two hydrophobic domains, are required for structural stability and non-classical secretion of RDPE. Alanine scanning mutagenesis of the hydrophobic segments of RDPE revealed that hydrophobic residues mediated the equilibrium between its folded and unfolded forms. Reporter mCherry and GFP fusions with RDPE regions show that its secretion requires an intact tetrameric protein complex. Using cross-linked tetramers, we show that folded tetrameric RDPE can be secreted as a single unit. Finally, we provide evidence that the non-classical secretion pathway has a strong preference for multimeric substrates, which accumulate at the poles and septum region. Altogether, these data show that a multimer recognition mechanism is likely applicable across the non-classical secretion pathway.