Role of the ompT mutation in stimulated decrease in colony-forming ability due to intracellular protein aggregate formation in Escherichia coli strain BL21

Recently we found that the cells of Escherichia coli strain BL21 producing a fusion protein, GST-Sup35NM, show a much more rapid decrease in colony-forming ability in the stationary phase than control cells. In this study, it was found that an extract of the cells producing GST-Sup35NM forms fibrous protein polymers containing GST-Sup35NM. In the course of the study, we realized that strain BL21 carried the ompT mutation. We suspected that the deficiency in OmpT protease was responsible for the observed phenotype. To test this, we introduced the wild-type ompT gene into strain BL21, and found that the transformed cells recovered the wild-type phenotype. We concluded that OmpT protease, though known to localize on the cell surface, is involved in protein quality control within the cell.     

Production of a polymer-forming fusion protein in Escerichia coli strain BL21

In the course of studying [PSI(+)], a yeast prion, we found inadvertently that Escherichia coli strain BL21 overproducing a fusion protein, in which the prion-domain of Sup35 was connected to the C terminus of glutathione S-transferase, grew normally to the stationary phase and rapidly decreased in colony-forming ability thereafter. Evidence indicated that protein polymers consisting mainly of the fusion protein GST-Sup35NM (about 70% of the mass) and its N-terminal fragments were formed in extract prepared from the cells producing GST-Sup35NM. It was further found that cells of strain BL21 accumulated the protein polymers during prolonged cultivation. Based on these results, we contend that the initially observed defect in colony forming ability is the direct or indirect consequence of intracellular formation and accumulation of the protein polymers.     

Effects of mutations in yeast prion [PSI+] on amyloid toxicity manifested in Escherichia coli strain BL21

We previously showed that over production of a fusion protein in which the prion domain of Saccharomyces cerevisiae [PSI+] is connected to glutathione S-transferase (GST-Sup35NM) causes a marked decrease in the colony forming ability of Escherichia coli strain BL21 after reaching stationary phase. Evidence indicated that the observed toxicity was attributable to intracellular formation of fibrous aggregates of GST-Sup35NM. In this report, we describe the isolation of plasmids that encode mutant forms of GST-Sup35NM which do not confer the toxicity to E. coli strain BL21. Each of the four spontaneous mutant-forms of GST-Sup35NM obtained revealed amino acid substitutions. One substitution was located in the N domain, and the others in the M domain. Congo red binding assay indicated that none of these mutant protein underwent conformational alteration in vitro. From these results, we conclude that the M domain, in collaboration with the N domain, plays an essential role in aggregation of Sup35NM. In addition, our data demonstrate the usefulness of the E. coli expression system in studying aggregate-forming proteins.     

Effects of mutations in yeast prion [PSI+] on amyloid toxicity manifested in Escherichia coli strain BL21

We previously showed that over production of a fusion protein in which the prion domain of Saccharomyces cerevisiae [PSI⁺] is connected to glutathione S-transferase (GST-Sup35NM) causes a marked decrease in the colony forming ability of Escherichia coli strain BL21 after reaching stationary phase. Evidence indicated that the observed toxicity was attributable to intracellular formation of fibrous aggregates of GST-Sup35NM. In this report, we describe the isolation of plasmids that encode mutant forms of GST-Sup35NM which do not confer the toxicity to E. coli strain BL21. Each of the four spontaneous mutant-forms of GST-Sup35NM obtained revealed amino acid substitutions. One substitution was located in the N domain, and the others in the M domain. Congo red binding assay indicated that none of these mutant proteins underwent conformational alteration in vitro. From these results, we conclude that the M domain, in collaboration with the N domain, plays an essential role in aggregation of Sup35NM. In addition, our data demonstrate the usefulness of the E. coli expression system in studying aggregate-forming proteins.Key words: [PSI⁺], yeast prion, protein aggregation, mutant selection, predication of protein secondary structure     

Antimicrobial properties of arginine- and lysine-rich histones and involvement of bacterial outer membrane protease T in their differential mode of actions

There is growing evidence of the antimicrobial properties of histones and histone-derived peptides; however, most of them are specific to lysine (Lys)-rich histones (H1, H2A, and H2B). In the present study, we focused on arginine (Arg)-rich histones (H3 and H4) and investigated their antimicrobial properties in comparison with those of histone H2B. In a standard microdilution assay, calf thymus histones H2B, H3, and H4 showed growth inhibitory activity against the bacterial outer membrane protease T (OmpT) gene-expressing Escherichia coli strain JCM5491 with calculated 50% growth inhibitory concentrations of 3.8, 10, and 12.7 μM, respectively. A lysate prepared from the JCM5491 cells was capable of strongly, moderately, and slightly fragmenting histones H2B, H3, and H4, respectively. While the lysate prepared from the cells of the ompT-deleted E. coli strain BL21(DE3) did not digest these histones, the ompT-transformed BL21(DE3), termed BL21/OmpT⁺, cell lysate digested the histones more strongly than the JCM5491 cell lysate. Laser confocal and scanning electron microscopic analyses demonstrated that while histone H2B penetrated the cell membrane of JCM5491 or BL21/OmpT⁺ cells, histones H3 and H4 remained on the cell surface and subsequently disrupted the cell membrane structure with bleb formation in a manner similar to general antimicrobial peptides. The BL21(DE3) cells treated with each histone showed no bleb formation, but cell integrity was affected and the cell surface was corrugated. Consequently, it is suggested that OmpT is involved in the antimicrobial properties of Arg- and Lys-rich histones and that the modes of antimicrobial action of these histones are different.     

Enterohaemorrhagic Escherichia coli produces outer membrane vesicles as an active defence system against antimicrobial peptide LL‐37

Antimicrobial peptides (AMPs) are important components of the innate immune system. Enterohaemorrhagic Escherichia coli (EHEC), a food‐borne pathogen causing serious diarrheal diseases, must overcome attack by AMPs. Here, we show that resistance of EHEC against human cathelicidin LL‐37, a primary AMP, was enhanced by butyrate, which has been shown to act as a stimulant for the expression of virulence genes. The increase of resistance depended on the activation of the ompT gene, which encodes the outer membrane protease OmpT for LL‐37. The expression of the ompT gene was enhanced through the activation system for virulence genes. The increase in ompT expression did not result in an increase in OmpT protease in bacteria but in enhancement of the production of OmpT‐loaded outer membrane vesicles (OMVs), which primarily contributed to the increase in LL‐37‐resistance. Furthermore, a sublethal dosage of LL‐37 stimulated the production of OMVs. Finally, we showed that OMVs produced by OmpT‐positive strains protect the OmpT‐negative strain, which is susceptible to LL‐37 by itself more efficiently than OMVs from the ompT mutant. These results indicate that EHEC enhances the secretion of OmpT‐loaded OMVs in coordination with the activation of virulence genes during infection and blocks bacterial cell attack by LL‐37.     

The absence of a surface protease, OmpT, determines the intercellular spreading ability of Shigella: the relationship between the ompT and kcpA loci

A large plasmid-encoded protein, VirG, on the bacterial surface is essential for the spreading of Shigella by eliciting polar deposition of filamentous actin In the cytoplasm of epithelial cells. VirG expression from the large plasmid is diminished greatly when it is introduced into Escherichia coli K-12 from Shigella. In an attempt to identify factors affecting VirG expression, we found that the absence of the ompT gene, encoding outer membrane protease OmpT, restored full production of VirG protein to E. coli K-12. Conversely, upon introduction of the ompT gene of E. coii K-12 into Shigella, spreading ability was completely abolished, probably because of the proteolytic degradation of VirG protein by OmpT. Analysis of the DNA sequence of the ompT region indicated that the absence of the ompT gene occurred in Shigella and enteroinvasive E. coli strains, and that the absent DNA segment corresponded to a remnant lambdoid phage structure found in E. coli K-12, which encompasses a 21 kb DNA segment spanning from argU through to the ompT genes. Since ompT is located near purE in E. coli K-12 and a virulence locus for provoking keratocon-junctivitis in the eyes of guinea-pigs, named kcpA is located near purE in S. fiexnerl, and the two loci are involved in VirG expression, the KcpA～ mutants of S. flexneri 2a constructed were examined for correlation between acquisition of ompT and VirG degradation. Our data suggest that the previous recognition of a kcpA locus in S. flexneri is the result of transfer of the ompr gene from E. coli K-12, giving rise to a KcpA phenotype. These results indicate that the lack of OmpT protease confers upon Shigella the ability to spread into adjacent epithelial cells.     

Escherichia coli outer membrane protease OmpT confers resistance to urinary cationic peptides

Escherichia coli OmpT, located in the outer membrane, has been characterized as a plasminogen activator, with the ability to hydrolyze protamine and block its entry. In this investigation, a complex of low molecular weight cationic peptides purified from human urine by a combination of membrane ultrafiltration and weak cation exchange chromatography was characterized. The impact of OmpT on E. coli resistance to urinary cationic peptides was investigated by testing ompT knockout strains. The ompT mutants were more susceptible to urinary cationic peptides than ompT⁺ strains, and this difference was abolished by complementation of the mutants with pUC19 carrying the ompT gene. The urinary protease inhibitor ulinastatin greatly decreased the resistance of the ompT⁺ strains. Overall, the data indicate that OmpT may help E. coli persist longer in the urinary tract by enabling it to resist the antimicrobial activity of urinary cationic peptides.     

Location of the ompT gene relative to that of appY on the physical map of the Escherichia coli chromosome

Results concerning the precise location of the ompT gene (encoding the outer membrane protease OmpT) on the Escherichia coli chromosome were obtained which disagree with published restriction sites in the gene. It is shown that the gene, together with appY, is present on a 3.075 PstI fragment, encompassing positions 596–598 of the E. coli physical map.     

Identification of OmpT as the Protease That Hydrolyzes the Antimicrobial Peptide Protamine before It Enters Growing Cells of Escherichia coli

The influence of extracytoplasmic proteases on the resistance of Escherichia coli to the antimicrobial peptide protamine was investigated by testing strains with deletions in the protease genes degP, ptr, and ompT. Only ΔompT strains were hypersusceptible to protamine. This effect was abolished by plasmids carrying ompT. Both at low and at high Mg²⁺ concentrations, ompT⁺ strains cleared protamine from the medium within a few minutes. By contrast, at high Mg²⁺ concentrations, protamine remained present for at least 1 h in the medium of an ompT strain. These data indicate that OmpT is the protease that degrades protamine and that it exerts this function at the external face of the outer membrane.     

Enhanced fluorescent properties of an OmpT site deleted mutant of Green Fluorescent Protein

Background  

The green fluorescent protein has revolutionized many areas of cell biology and biotechnology since it is widely used in determining gene expression and for localization of protein expression. Expression of recombinant GFP in E. coli K12 host from pBAD24M-GFP construct upon arabinose induction was significantly lower than that seen in E. coli B cells with higher expression at 30°C as compared to 37°C in E. coli K12 hosts. Since OmpT levels are higher at 37°C than at 30°C, it prompted us to modify the OmpT proteolytic sites of GFP and examine such an effect on GFP expression and fluorescence. Upon modification of one of the two putative OmpT cleavage sites of GFP, we observed several folds enhanced fluorescence of GFP as compared to unmodified GFPuv (Wild Type-WT). The western blot studies of the WT and the SDM II GFP mutant using anti-GFP antibody showed prominent degradation of GFP with negligible degradation in case of SDM II GFP mutant while no such degradation of GFP was seen for both the clones when expressed in BL21 cells. The SDM II GFP mutant also showed enhanced GFP fluorescence in other E. coli K12 OmpT hosts like E. coli JM109 and LE 392 in comparison to WT GFPuv. Inclusion of an OmpT inhibitor, like zinc with WT GFP lysate expressed from an E. coli K12 host was found to reduce degradation of GFP fluorescence by two fold.     

ompT encodes the Escherichia coli outer membrane protease that cleaves T7 RNA polymerase during purification.

Bacteriophage T7 RNA polymerase is stable in Escherichia coli but very susceptible to cleavage by at least one endoprotease after cell lysis. The major source of this endoprotease activity was found to be localized to the outer membrane of the cell. A rapid whole-cell assay was developed to screen different strains for the presence of this proteolytic activity. Using this assay, we identified some common laboratory strains that totally lack the protease. Genetic and Southern analyses of these null strains allowed us to conclude that the protease that cleaves T7 RNA polymerase is OmpT (formerly termed protein a), a known outer membrane endoprotease, and that the null phenotype results from deletion of the OmpT structural gene. A recombinant plasmid carrying the ompT gene enables these deletion strains to synthesize OmpT and converts them to a protease-positive phenotype. The plasmid led to overproduction of OmpT protein and protease activity in the E. coli K-12 and B strains we used, but only weak expression in the E. coli C strain, C1757. This strain-dependent difference in ompT expression was investigated with respect to the known influence of envZ on OmpT synthesis. A small deletion in the ompT region of the plasmid greatly diminishes the amount of OmpT protein and plasmid-encoded protease present in outer membranes. Use of ompT deletion strains for production of T7 RNA polymerase from the cloned gene has made purification of intact T7 RNA polymerase routine. Such strains may be useful for purification of other proteins expressed in E. coli.     

Processing by OmpT of fusion proteins carrying the HlyA transport signal during secretion by the Escherichia coli hemolysin transport system.

Summary A fusion gene (ces-hlyA _s) was constructed by ligating the genetic information for the C-terminal 60 amino acids (hlyA _s) ofEscherichia coli hemolysin (H1yA) to the ces gene for a cholesterol esterase/lipase (CE) from aPseudomonas species. Part (about 30 %) of the expressed fusion protein CE-H1yA_s was secreted inE. coli carryinghlyB andhlyD genes. Following the insertion between the reporter gene andhlyA _s of a linker sequence that contains the information for potential cleavage sites for the outer membrane protease OmpT, two different fusion proteins (PhoA-H1yA_s and CE-HlyA_s) were shown to be cleaved by OmpT between the two parts during H1yB/H1yD-mediated secretion. Processed PhoA and CE accumulated in the supernatant. The efficiency of cleavage by OmpT was considerably improved by increasedompT gene dose. It was further shown that OmpT preferentially recognizes potential cleavage sites within the linker sequence.     

地衣芽孢杆菌E7氨肽酶的异源表达及其在高效蛋白水解中的应用

【目的】将地衣芽孢杆菌(Bacilluslicheniformis)E7氨肽酶基因pepN克隆到大肠杆菌(Escherichia coli) BL21中,实现氨肽酶Ec PepN的异源表达,研究重组酶的酶学性质及其与碱性蛋白酶协同作用,高效水解大豆蛋白和酪蛋白,产生小分子活性肽和游离氨基酸。【方法】以地衣芽孢杆菌E7基因组DNA为模板,将氨肽酶基因pepN克隆到载体pET28a中,构建重组表达载体pET28-pepN,转化到大肠杆菌BL21感受态细胞中,经DNA测序验证,获得重组菌E. coli BL21/pET28-pepN。利用镍离子亲和层析柱对重组酶进行分离纯化,研究纯酶的pH和温度稳定性、半衰期和NaCl的耐受性等酶学性质。以商品化氨肽酶与碱性蛋白酶协同作用为对照,重组酶Ec PepN与碱性蛋白酶协同水解大豆蛋白和酪蛋白,测定水解产物中小分子活性肽和游离氨基酸的组成。【结果】Ec PepN在大肠杆菌BL21中可溶性表达,SDS-PAGE分析表明纯化的重组酶在52kDa左右显示单一条带。在7种测定底物中,Ec PepN的最适底物为Ala-pNA。在最适条件(pH 9.0和50°C...     

Growth of <Emphasis Type="Italic">E. coli</Emphasis> BL21 in minimal media with different gluconeogenic carbon sources and salt contents

Escherichia coli strain BL21 is commonly used as a host strain for protein expression and purification. For structural analysis, proteins are frequently isotopically labeled with deuterium (²H), ¹³C, or ¹⁵N by growing E. coli cultures in a medium containing the appropriate isotope. When large quantities of fully deuterated proteins are required, E. coli is often grown in minimal media with deuterated succinate or acetate as the carbon source because these are less expensive. Despite the widespread use of BL21, we found no data on the effect of different minimal media and carbon sources on BL21 growth. In this study, we assessed the growth behavior of E. coli BL21 in minimal media with different gluconeogenic carbon sources. Though BL21 grew reasonably well on glycerol and pyruvate, it had a prolonged lag-phase on succinate (20 h), acetate (10 h), and fumarate (20 h), attributed to the physiological adaptation of E. coli cells. Wild-type strain NCM3722 (K12) grew well on all the substrates. We also examined the growth of E. coli BL21 in minimal media that differed in their salt composition but not in their source of carbon. The commonly used M9 medium did not support the optimum growth of E. coli BL21 in minimal medium. The addition of ferrous sulphate to M9 medium (otherwise lacking it) increased the growth rate of E. coli cultures and significantly increased their cell density in the stationary phase. An erratum to this article can be found at     

Sequence of the regulatory region of omp T, the gene specifying major outer membrane protein a (3b) of Escherichia coli K-12: implications for regulation and processing

Summary The DNA of the promoter region of ompT, including the putative start for the pro-OmpT protein (proprotein a), has been sequenced. Previous studies showed that trypsin inhibitors prevent the processing of pro-OmpT to OmpT protein which led to the prediction that the processing site would be a lysine or an arginine. The deduced amino acid sequence contains a lysine at amino acid 12 and an arginine at amino acid 17 from the N terminus. Chou-Fassman analysis would predict processing at the lysine (but not the arginine) to remove a 1389 dalton peptide, consistent with the fact that the estimated molecular masses of pro-OmpT and OmpT are 42 kd and 40 kd respectively. In addition, the predicted mRNA of the promoter region can form a stable secondary structure (-17.1 kcal) that sequesters the Shine-Dalgarno (SD) sequence as well as the initiator AUG codon. There is evidence that the perA (tpo, envZ) gene product is required for synthesis of OmpT protein (as well as several outer membrane and periplasmic proteins). The perA gene product could be activating translation of OmpT protein by disrupting the mRNA secondary structure that sequesters the SD sequence. OmpT protein synthesis is reduced at temperatures below 32°C and this may also be related to the greater stability of the sequestered SD sequence of the mRNA at low temperature.     

Location of the ompT gene relative to that of appY on the physical map of the Escherichia coli chromosome

Results concerning the precise location of the ompT gene (encoding the outer membrane protease OmpT) on the Escherichia coli chromosome were obtained which disagree with published restriction sites in the gene. It is shown that the gene, together with appY, is present on a 3.075 PstI fragment, encompassing positions 596–598 of the E. coli physical map.     

GPI anchoring facilitates propagation and spread of misfolded Sup35 aggregates in mammalian cells

Prion diseases differ from other amyloid‐associated protein misfolding diseases (e.g. Alzheimer's) because they are naturally transmitted between individuals and involve spread of protein aggregation between tissues. Factors underlying these features of prion diseases are poorly understood. Of all protein misfolding disorders, only prion diseases involve the misfolding of a glycosylphosphatidylinositol (GPI)‐anchored protein. To test whether GPI anchoring can modulate the propagation and spread of protein aggregates, a GPI‐anchored version of the amyloidogenic yeast protein Sup35NM (Sup35^GPI) was expressed in neuronal cells. Treatment of cells with Sup35NM fibrils induced the GPI anchor‐dependent formation of self‐propagating, detergent‐insoluble, protease‐resistant, prion‐like aggregates of Sup35^GPI. Live‐cell imaging showed intercellular spread of Sup35^GPI aggregation to involve contact between aggregate‐positive and aggregate‐negative cells and transfer of Sup35^GPI from aggregate‐positive cells. These data demonstrate GPI anchoring facilitates the propagation and spread of protein aggregation and thus may enhance the transmissibility and pathogenesis of prion diseases relative to other protein misfolding diseases.     

Secretion-dependent proteolysis of heterologous protein by recombinant Escherichia coli is connected to an increased activity of the energy-generating dissimilatory pathway

The synthesis of a proteolytically unstable protein, originally designed for periplasmic export in recombinant Escherichia coli BL21(DE3), a strain naturally deficient for the ATP-dependent protease Lon (or La) and the outer membrane protease OmpT, is associated with a severe growth inhibition. This inhibition is not observed in BL21(DE3) synthesizing a closely related but proteolytically stable protein that is sequestered into inclusion bodies. It is shown that the growth inhibition is mainly caused by a slower cell division rate and a reduced growth yield and not by a general loss of cell division competence. Cells proceed with their normal growth characteristics when exposed again to conditions that do not sustain the expression of the heterologous gene. The performance of cells synthesizing either the stable or the degraded protein was also studied in high cell density cultures by employing a new method to calculate the actual specific growth rate, the biomass yield coefficient, and the dissimilated fraction of the carbon substrate in real-time. It is shown that the growth inhibition of cells synthesizing the proteolytically degraded protein is connected to an increased dissimilation of the carbon substrate resulting in a concomitant reduction of the growth rate and the biomass yield coefficient with respect to the carbon source. It is postulated that the increased dissimilation of the carbon substrate by lon-deficient Bl21(DE3) cells synthesizing the proteolytically unstable protein may result from a higher energy demand required for the in vivo degradation of this protein by ATP-dependent proteases different from the protease Lon.