Components of the type six secretion system are substrates of Francisella tularensis Schu S4 DsbA-like FipB protein

Unveiling the versatility of the thioredoxin framework: Insights from the structural examination of Francisella tularensis DsbA1

In bacteria the formation of disulphide bonds is facilitated by a family of enzymes known as the disulphide bond forming (Dsb) proteins, which, despite low sequence homology, belong to the thioredoxin (TRX) superfamily. Among these enzymes is the disulphide bond-forming protein A (DsbA); a periplasmic thiol oxidase responsible for catalysing the oxidative folding of numerous cell envelope and secreted proteins. Pathogenic bacteria often contain diverse Dsb proteins with distinct functionalities commonly associated with pathogenesis. Here we investigate FtDsbA1, a DsbA homologue from the Gram-negative bacterium Francisella tularensis. Our study shows that FtDsbA1 shares a conserved TRX-like fold bridged by an alpha helical bundle showcased by all DsbA-like proteins. However, FtDsbA1 displays a highly unique variation on this structure, containing an extended and flexible N-terminus and secondary structural elements inserted within the core of the TRX fold itself, which together twist the overall DsbA-like architecture. Additionally, FtDsbA1 exhibits variations to the well conserved active site with an unusual dipeptide in the catalytic CXXC redox centre (CGKC), and a trans configuration for the conserved cis-proline loop, known for governing DsbA-substrate interactions. FtDsbA1's redox properties are comparable to other DsbA enzymes, however, consistent with its atypical structure, functional analysis reveals FtDsbA1 has a high degree of substrate specificity suggesting a specialised role within F. tularensis' oxidative folding pathway. Overall, this work underscores the remarkable malleability of the TRX catalytic core, a ubiquitous and ancestral protein fold. This not only contributes to broadening the structural and functional diversity seen within proteins utilising this core fold but will also enhance the accuracy of AI-driven protein structural prediction tools.

The Balance between Protealysin and Its Substrate, the Outer Membrane Protein OmpX, Regulates Serratia proteamaculans Invasion

Serratia are opportunistic bacteria, causing infections in plants, insects, animals and humans under certain conditions. The development of bacterial infection in the human body involves several stages of host-pathogen interaction, including entry into non-phagocytic cells to evade host immune cells. The facultative pathogen Serratia proteamaculans is capable of penetrating eukaryotic cells. These bacteria synthesize an actin-specific metalloprotease named protealysin. After transformation with a plasmid carrying the protealysin gene, noninvasive E. coli penetrate eukaryotic cells. This suggests that protealysin may play a key role in S. proteamaculans invasion. This review addresses the mechanisms underlying protealysin's involvement in bacterial invasion, highlighting the main findings as follows. Protealysin can be delivered into the eukaryotic cell by the type VI secretion system and/or by bacterial outer membrane vesicles. By cleaving actin in the host cell, protealysin can mediate the reversible actin rearrangements required for bacterial invasion. However, inactivation of the protealysin gene leads to an increase, rather than decrease, in the intensity of S. proteamaculans invasion. This indicates the presence of virulence factors among bacterial protealysin substrates. Indeed, protealysin cleaves the virulence factors, including the bacterial surface protein OmpX. OmpX increases the expression of the EGFR and β1 integrin, which are involved in S. proteamaculans invasion. It has been shown that an increase in the invasion of genetically modified S. proteamaculans may be the result of the accumulation of full-length OmpX on the bacterial surface, which is not cleaved by protealysin. Thus, the intensity of the S. proteamaculans invasion is determined by the balance between the active protealysin and its substrate OmpX.

Dual Role for DsbA in Attacking and Targeted Bacterial Cells during Type VI Secretion System-Mediated Competition

Incorporation of disulfide bonds into proteins can be critical for function or stability. In bacterial cells, the periplasmic enzyme DsbA is responsible for disulfide incorporation into many extra-cytoplasmic proteins. The type VI secretion system (T6SS) is a widely occurring nanomachine that delivers toxic effector proteins directly into rival bacterial cells, playing a key role in inter-bacterial competition. We report that two redundant DsbA proteins are required for virulence and for proper deployment of the T6SS in the opportunistic pathogen Serratia marcescens, with several T6SS components being subject to the action of DsbA in secreting cells. Importantly, we demonstrate that DsbA also plays a critical role in recipient target cells, being required for the toxicity of certain incoming effector proteins. Thus we reveal that target cell functions can be hijacked by T6SS effectors for effector activation, adding a further level of complexity to the T6SS-mediated inter-bacterial interactions which define varied microbial communities.

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Type VI secretion systems (T6SSs) are used by bacteria to attack competitors

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Disulfide bond formation by DsbA promotes assembly of an active T6SS in Serratia

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DsbA in the target cell is needed for activation of certain incoming T6SS effectors

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This work reveals that T6SS-delivered effectors can hijack target cell functions

Geraniol as a novel antivirulence agent against bacillary dysentery-causing Shigella sonnei

Antimicrobial resistance has emerged as a major challenge to modern medicine and it has become urgent to seek alternative approaches to treat infections caused by fast-evolving multi-resistant clones of bacillary dysentery-causing Shigella sonnei. Here, we show that geraniol, a natural substance present in the essential oils of plants such as rose and lemongrass, can reduce S. sonnei proliferation inside host cells and protect Galleria mellonella larvae from killing by S. sonnei infection. We present evidence that geraniol competitively inhibits the catalytic activity of the master virulence regulator, DsbA, a periplasmic disulphide bond oxidoreductase required for Shigella survival in the host cell cytosol. Our observations suggest that geraniol holds a great therapeutic potential that should be further exploited.

Impact of selected amino acids of HP0377 (Helicobacter pylori thiol oxidoreductase) on its functioning as a CcmG (cytochrome c maturation) protein and Dsb (disulfide bond) isomerase

Helicobacter pylori HP0377 is a thiol oxidoreductase, a member of the CcmG family involved in cytochrome biogenesis, as previously shown by in vitro experiments. In this report, we document that HP0377 also acts in vivo in the cytochrome assembly process in Bacillus subtilis, where it complements the lack of ResA. However, unlike other characterized proteins in this family, HP0377 is a dithiol reductase and isomerase. We elucidated how the amino acid composition of its active site modulates its functionality. We demonstrated that cis-proline (P156) is involved in its interaction with the redox partner (CcdA), as a P156T HP0377 variant is inactive in vivo and is present in the oxidized form in B. subtilis. Furthermore, we showed that engineering the HP0377 active motif by changing CSYC motif into CSYS or SSYC, clearly diminishes two activities (reduction and isomerization) of the protein. Whereas HP0377CSYA is inactive in reduction as well as in isomerization, HP0377CSYS retains reductive activity. Also, replacement of F95 by Q decreases its ability to regenerate scRNase and does not influence the reductive activity of HP0377CSYS towards apocytochrome c. HP0377 is also distinguished from other CcmGs as it forms a 2:1 complex with apocytochrome c. Phylogenetic analyses showed that, although HP0377 is capable of complementing ResA in Bacillus subtilis, its thioredoxin domain has a different origin, presumably common to DsbC.

Francisella tularensis D-Ala D-Ala Carboxypeptidase DacD Is Involved in Intracellular Replication and It Is Necessary for Bacterial Cell Wall Integrity

D-alanyl-D-alanine carboxypeptidase, product of dacD gene in Francisella, belongs to penicillin binding proteins (PBPs) and is involved in remodeling of newly synthetized peptidoglycan. In E. coli, PBPs are synthetized in various growth phases and they are able to substitute each other to a certain extent. The DacD protein was found to be accumulated in fraction enriched in membrane proteins from severely attenuated dsbA deletion mutant strain. It has been presumed that the DsbA is not a virulence factor by itself but that its substrates, whose correct folding and topology are dependent on the DsbA oxidoreductase and/or isomerase activities, are the primary virulence factors. Here we demonstrate that Francisella DacD is required for intracellular replication and virulence in mice. The dacD insertion mutant strain showed higher sensitivity to acidic pH, high temperature and high osmolarity when compared to the wild-type. Eventually, transmission electron microscopy revealed differences in mutant bacteria in both the size and defects in outer membrane underlying its SDS and serum sensitivity. Taken together these results suggest DacD plays an important role in Francisella pathogenicity.

The Multiple Localized Glyceraldehyde-3-Phosphate Dehydrogenase Contributes to the Attenuation of the Francisella tularensis dsbA Deletion Mutant

The DsbA homolog of Francisella tularensis was previously demonstrated to be required for intracellular replication and animal death. Disruption of the dsbA gene leads to a pleiotropic phenotype that could indirectly affect a number of different cellular pathways. To reveal the broad effects of DsbA, we compared fractions enriched in membrane proteins of the wild-type FSC200 strain with the dsbA deletion strain using a SILAC-based quantitative proteomic analysis. This analysis enabled identification of 63 proteins with significantly altered amounts in the dsbA mutant strain compared to the wild-type strain. These proteins comprise a quite heterogeneous group including hypothetical proteins, proteins associated with membrane structures, and potential secreted proteins. Many of them are known to be associated with F. tularensis virulence. Several proteins were selected for further studies focused on their potential role in tularemia's pathogenesis. Of them, only the gene encoding glyceraldehyde-3-phosphate dehydrogenase, an enzyme of glycolytic pathway, was found to be important for full virulence manifestations both in vivo and in vitro. We next created a viable mutant strain with deleted gapA gene and analyzed its phenotype. The gapA mutant is characterized by reduced virulence in mice, defective replication inside macrophages, and its ability to induce a protective immune response against systemic challenge with parental wild-type strain. We also demonstrate the multiple localization sites of this protein: In addition to within the cytosol, it was found on the cell surface, outside the cells, and in the culture medium. Recombinant GapA was successfully obtained, and it was shown that it binds host extracellular serum proteins like plasminogen, fibrinogen, and fibronectin.

The metabolic enzyme fructose-1,6-bisphosphate aldolase acts as a transcriptional regulator in pathogenic Francisella

The enzyme fructose-bisphosphate aldolase occupies a central position in glycolysis and gluconeogenesis pathways. Beyond its housekeeping role in metabolism, fructose-bisphosphate aldolase has been involved in additional functions and is considered as a potential target for drug development against pathogenic bacteria. Here, we address the role of fructose-bisphosphate aldolase in the bacterial pathogen Francisella novicida. We demonstrate that fructose-bisphosphate aldolase is important for bacterial multiplication in macrophages in the presence of gluconeogenic substrates. In addition, we unravel a direct role of this metabolic enzyme in transcription regulation of genes katG and rpoA, encoding catalase and an RNA polymerase subunit, respectively. We propose a model in which fructose-bisphosphate aldolase participates in the control of host redox homeostasis and the inflammatory immune response.The enzyme fructose-bisphosphate aldolase (FBA) plays central roles in glycolysis and gluconeogenesis. Here, Ziveri et al. show that FBA of the pathogen Francisella novicida acts, in addition, as a transcriptional regulator and is important for bacterial multiplication in macrophages.

Subversion of innate immune responses by Francisella involves the disruption of TRAF3 and TRAF6 signalling complexes

The success of pathogens depends on their ability to circumvent immune defences. Francisella tularensis is one of the most infectious bacteria known. The remarkable virulence of Francisella is believed to be due to its capacity to evade or subvert the immune system, but how remains obscure. Here, we show that Francisella triggers but concomitantly inhibits the Toll-like receptor, RIG-I-like receptor, and cytoplasmic DNA pathways. Francisella subverts these pathways at least in part by inhibiting K63-linked polyubiquitination and assembly of TRAF6 and TRAF3 complexes that control the transcriptional responses of pattern recognition receptors. We show that this mode of inhibition requires a functional type VI secretion system and/or the presence of live bacteria in the cytoplasm. The ability of Francisella to enter the cytosol while simultaneously inhibiting multiple pattern recognition receptor pathways may account for the notable capacity of this bacterium to invade and proliferate in the host without evoking a self-limiting innate immune response.

Bacterial thiol oxidoreductases - from basic research to new antibacterial strategies

The recent, rapid increase in bacterial antimicrobial resistance has become a major public health concern. One approach to generate new classes of antibacterials is targeting virulence rather than the viability of bacteria. Proteins of the Dsb system, which play a key role in the virulence of many pathogenic microorganisms, represent potential new drug targets. The first part of the article presents current knowledge of how the Dsb system impacts function of various protein secretion systems that influence the virulence of many pathogenic bacteria. Next, the review describes methods used to study the structure, biochemistry, and microbiology of the Dsb proteins and shows how these experiments broaden our knowledge about their function. The lessons gained from basic research have led to a specific search for inhibitors blocking the Dsb networks.

A mutagenesis-based approach identifies amino acids in the N-terminal part of Francisella tularensis IglE that critically control Type VI system-mediated secretion

The Gram-negative bacterium Francisella tularensis is the etiological agent of the zoonotic disease tularemia. Its life cycle is characterized by an ability to survive within phagocytic cells through phagosomal escape and replication in the cytosol, ultimately causing inflammasome activation and host cell death. Required for these processes is the Francisella Pathogenicity Island (FPI), which encodes a Type VI secretion system (T6SS) that is active during intracellular infection. In this study, we analyzed the role of the FPI-component IglE, a lipoprotein which we previously have shown to be secreted in a T6SS-dependent manner. We demonstrate that in F. tularensis LVS, IglE is an outer membrane protein. Upon infection of J774 cells, an ΔiglE mutant failed to escape from phagosomes, and subsequently, to multiply and cause cytopathogenicity. Moreover, ΔiglE was unable to activate the inflammasome, to inhibit LPS-stimulated secretion of TNF-α, and showed marked attenuation in the mouse model. In F. novicida, IglE was required for in vitro secretion of IglC and VgrG. A mutagenesis-based approach involving frameshift mutations and alanine substitution mutations within the first ∼ 38 residues of IglE revealed that drastic changes in the sequence of the extreme N-terminus (residues 2-6) were well tolerated and, intriguingly, caused hyper-secretion of IglE during intracellular infection, while even subtle mutations further downstream lead to impaired protein function. Taken together, this study highlights the importance of IglE in F. tularensis pathogenicity, and the contribution of the N-terminus for all of the above mentioned processes.