Novel bacterial membrane surface display system using cell wall-less L-forms of Proteus mirabilis and Escherichia coli

Comparative genomics hints at dispensability of multiple essential genes in two Escherichia coli L-form strains

Despite the critical role of bacterial cell walls in maintaining cell shapes, certain environmental stressors can induce the transition of many bacterial species into a wall-deficient state called L-form. Long-term induced Escherichia coli L-forms lose their rod shape and usually hold significant mutations that affect cell division and growth. Besides this, the genetic background of L-form bacteria is still poorly understood. In the present study, the genomes of two stable L-form strains of E. coli (NC-7 and LWF+) were sequenced and their gene mutation status was determined and compared with their parental strains. Comparative genomic analysis between two L-forms reveals both unique adaptions and common mutated genes, many of which belong to essential gene categories not involved in cell wall biosynthesis, indicating that L-form genetic adaptation impacts crucial metabolic pathways. Missense variants from L-forms and Lenski's long-term evolution experiment (LTEE) were analyzed in parallel using an optimized DeepSequence pipeline to investigate predicted mutation effects (α) on protein functions. We report that the two L-form strains analyzed display a frequency of 6-10% (0% for LTEE) in mutated essential genes where the missense variants have substantial impact on protein functions (α<0.5). This indicates the emergence of different survival strategies in L-forms through changes in essential genes during adaptions to cell wall deficiency. Collectively, our results shed light on the detailed genetic background of two E. coli L-forms and pave the way for further investigations of the gene functions in L-form bacterial models.

Enhancing Secretion of Endoglucanase in Zymomonas mobilis by Disturbing Peptidoglycan Synthesis

Zymomonas mobilis (Z. mobilis) is a potential candidate for consolidated bioprocessing (CBP) strain in lignocellulosic biorefinery. However, the low-level secretion of cellulases limits this CBP process, and the mechanism of protein secretion affected by cell wall peptidoglycan is also not well understood. Here we constructed several Penicillin Binding Proteins (PBPs)-deficient strains derivated from Z. mobilis S192 to perturb the cell wall peptidoglycan network and investigated the effects of peptidoglycan on the endoglucanase secretion. Results showed that extracellular recombinant endoglucanase production was significantly enhanced in PBPs mutant strains, notably, △1089/0959 (4.09-fold) and △0959 (5.76-fold) in comparison to parent strains. Besides, for PBPs-deficient strains, the growth performance was not significantly inhibited but with enhanced antibiotic sensitivity and reduced inhibitor tolerance, otherwise, cell morphology was altered obviously. The concentration of intracellular soluble peptidoglycan was increased, especially for single gene deletion. Outer membrane permeability of PBPs-deficient strains was also improved, notably, △1089/0959 (1.14-fold) and △0959 (1.07-fold), which might explain the increased endoglucanase extracellular secretion. Our finding indicated that PBPs-deficient Z. mobilis is capable of increasing endoglucanase extracellular secretion via cell wall peptidoglycan disturbance and it will provide a foundation for the development of CBP technology in Z. mobilis in the future. IMPORTANCE Cell wall peptidoglycan has the function to maintain cell robustness, and also acts as the barrier to secret recombinant proteins from the cytoplasm to extracellular space in Z. mobilis and other bacterias. Herein, we perturb the peptidoglycan synthesis network via knocking out PBPs (ZMO0197, ZMO0959, ZMO1089) in order to enhance recombinant endoglycanase extracellular secretion in Z. mobilis S912. This study can not only lay the foundation for understanding the regulatory network of cell wall synthesis but also provide guidance for the construction of CBP strains in Z. mobilis.

Perturbation of the peptidoglycan network and utilization of the signal recognition particle-dependent pathway enhances the extracellular production of a truncational mutant of CelA in Escherichia coli

Caldicellulosiruptor bescii is the most thermophilic, cellulolytic bacterium known and has the native ability to utilize unpretreated plant biomass. Cellulase A (CelA) is the most abundant enzyme in the exoproteome of C. bescii and is primarily responsible for its cellulolytic ability. CelA contains a family 9 glycoside hydrolase and a family 48 glycoside hydrolase connected by linker regions and three carbohydrate-binding domains. A truncated version of the enzyme (TM1) containing only the endoglucanase domain is thermostable and actively degrades crystalline cellulose. A catalytically active TM1 was successfully produced via the attachment of the PelB signal peptide (P-TM1), which mediates post-translational secretion via the SecB-dependent translocation pathway. We sought to enhance the extracellular secretion of TM1 using an alternative pathway, the signal recognition particle (SRP)-dependent translocation pathway. The co-translational extracellular secretion of TM1 via the SRP pathway (D-TM1) resulted in a specific activity that was 4.9 times higher than that associated with P-TM1 overexpression. In batch fermentations, the recombinant Escherichia coli overexpressing D-TM1 produced 1.86 ± 0.06 U/ml of TM1 in the culture medium, showing a specific activity of 1.25 ± 0.05 U/mg cell, 2.7- and 3.7-fold higher than the corresponding values of the strain overexpressing P-TM1. We suggest that the TM1 secretion system developed in this study can be applied to enhance the capacity of E. coli as a microbial cell factory for the extracellular secretion of this as well as a variety proteins important for commercial production.

Chemogenetic engineering of nitrobindin toward an artificial epoxygenase

Chemogenetic engineering turned the heme protein nitrobindin into an artificial epoxygenase: MnPPIX was introduced and subsequent protein engineering increased the activity in the epoxidation of styrene derivatives by overall 7-fold.

From Polyclonal Sera to Recombinant Antibodies: A Review of Immunological Detection of Gluten in Foodstuff

Gluten is the ethanol-soluble protein fraction of cereal endosperms like wheat, rye, and barley. It is widely used in the food industry because of the physical-chemical properties it gives to dough. Nevertheless, there are some gluten-related diseases that are presenting increasing prevalences, e.g., celiac disease, for which a strict gluten-free diet is the best treatment. Due to this situation, gluten labeling legislation has been developed in several countries around the world. This article reviews the gluten immune detection systems that have been applied to comply with such regulations. These systems have followed the development of antibody biotechnology, which comprise three major methodologies: polyclonal antibodies, monoclonal antibodies (mAbs) derived from hybridoma cells (some examples are 401.21, R5, G12, and α-20 antibodies), and the most recent methodology of recombinant antibodies. Initially, the main objective was the consecution of new high-affinity antibodies, resulting in low detection and quantification limits that are mainly achieved with the R5 mAb (the gold standard for gluten detection). Increasing knowledge about the causes of gluten-related diseases has increased the complexity of research in this field, with current efforts not only focusing on the development of more specific and sensitive systems for gluten but also the detection of protein motifs related to pathogenicity. New tools based on recombinant antibodies will provide adequate safety and traceability methodologies to meet the increasing market demand for gluten-free products.

Boosting Secretion of Extracellular Protein by Escherichia coli via Cell Wall Perturbation

Escherichia coli is one of the most widely used host microorganisms for recombinant protein expression and metabolic engineering, but it cannot efficiently secrete recombinant proteins to extracellular space. Here, extracellular protein secretion was enhanced in E. coli by deleting two d,d-carboxypeptidase genes (dacA and dacB, single and double deletions) to perturb the cell wall peptidoglycan network. Deletion of dacA and dacB enhanced the accumulation of intracellular soluble peptidoglycan in E. coli and affected cell morphology, resulting in a more irregular cell shape and the appearance of transparent bulges. Deletion of dacA and dacB appears to disrupt the normal rigid structure, presumably due to perturbation and destruction of the cell wall peptidoglycan network. The extracellular green fluorescent protein (GFP) fluorescence intensity of deletion mutants was increased by >2.0-fold compared with that of control cells, and that of the double deletion mutant was increased by 2.7-fold. Extracellular recombinant fibroblast growth factor receptor 2 (FGFR2) and collagen E4 secretion in deletion mutants was also enhanced compared with that in the control cells. Additionally, the extracellular recombinant amylase activity of single-deletion mutants BL21 ΔdacA pETDuet-amyk and BL21 ΔdacB pETDuet-amyk was increased 2.5- and 3.1-fold, respectively. The extracellular distribution of α-galactosidase by deletion mutants was also increased by >2.0-fold. Deletion of dacA and dacB increased outer membrane permeability, which could explain the enhanced extracellular protein secretion.IMPORTANCE Cell surface structure stabilization is important for extracellular secretion of proteins in Escherichia coli As the main constituent of the cell wall, peptidoglycan contributes to cell structure robustness and stability. Here, we perturbed the peptidoglycan network by deleting dacA and dacB genes encoding d,d-carboxypeptidase enzymes to improve extracellular protein secretion. This new strategy could enhance the capacity of E. coli as a microbial cell factory for extracellular secretion of proteins and chemicals.

Secretion of recombinant proteins from E. coli

The microorganism Escherichia coli is commonly used for recombinant protein production. Despite several advantageous characteristics like fast growth and high protein yields, its inability to easily secrete recombinant proteins into the extracellular medium remains a drawback for industrial production processes. To overcome this limitation, a multitude of approaches to enhance the extracellular yield and the secretion efficiency of recombinant proteins have been developed in recent years. Here, a comprehensive overview of secretion mechanisms for recombinant proteins from E. coli is given and divided into three main sections. First, the structure of the E. coli cell envelope and the known natural secretion systems are described. Second, the use and optimization of different one- or two-step secretion systems for recombinant protein production, as well as further permeabilization methods are discussed. Finally, the often-overlooked role of cell lysis in secretion studies and its analysis are addressed. So far, effective approaches for increasing the extracellular protein concentration to more than 10 g/L and almost 100% secretion efficiency exist, however, the large range of optimization methods and their combinations suggests that the potential for secretory protein production from E. coli has not yet been fully realized.

Development and characterization of membrane surface display system using molecular chaperon, prsA, of Bacillus subtilis

We report a new membrane surface display system based on molecular chaperon, prsA, of Bacillus subtilis. Clostridium thermocellum cellulase, celA, was fused to C-terminal end of PrsA. Cellulase activity of B. subtilis protoplast, which expressed PrsA-CelA was 15 times higher compared to control strain. More than 85% of total cellulase activity was observed in surface displayed format and less than 15% of total cellulase activity was found in supernatant. Flow cytometric analysis of protoplast of PrsA-CelA fusion expressing bacteria provided another proof of uniform expression of fusion protein onto cytoplasmic membrane of B. subtilis. Without lysozyme treatment, only part of cellulase activity (10%) was observed in whole cell fraction.

Recombinant protein secretion in Escherichia coli

The secretory production of recombinant proteins by the Gram-negative bacterium Escherichia coli has several advantages over intracellular production as inclusion bodies. In most cases, targeting protein to the periplasmic space or to the culture medium facilitates downstream processing, folding, and in vivo stability, enabling the production of soluble and biologically active proteins at a reduced process cost. This review presents several strategies that can be used for recombinant protein secretion in E. coli and discusses their advantages and limitations depending on the characteristics of the target protein to be produced.

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.

Cell-Surface display of heterologous proteins: From high-throughput screening to environmental applications

A variety of expression systems for the display of either short peptides or fully folded proteins on E.coli and, to a lesser extent, on Gram-positive bacteria have been developed. The expression of proteins on the surface of microbial cells has proved extremely important for numerous applications ranging from combinatorial library screening and protein engineering, to whole cell biocatalysts and adsorbants for bioremediation purposes.