Crystal structure of cytotoxin protein suilysin from Streptococcus suis

Story of Pore-Forming Proteins from Deadly Disease-Causing Agents to Modern Applications with Evolutionary Significance

Animal venoms are a complex mixture of highly specialized toxic molecules. Among them, pore-forming proteins (PFPs) or toxins (PFTs) are one of the major disease-causing toxic elements. The ability of the PFPs in defense and toxicity through pore formation on the host cell surface makes them unique among the toxin proteins. These features made them attractive for academic and research purposes for years in the areas of microbiology as well as structural biology. All the PFPs share a common mechanism of action for the attack of host cells and pore formation in which the selected pore-forming motifs of the host cell membrane-bound protein molecules drive to the lipid bilayer of the cell membrane and eventually produces water-filled pores. But surprisingly their sequence similarity is very poor. Their existence can be seen both in a soluble state and also in transmembrane complexes in the cell membrane. PFPs are prevalent toxic factors that are predominately produced by all kingdoms of life such as virulence bacteria, nematodes, fungi, protozoan parasites, frogs, plants, and also from higher organisms. Nowadays, multiple approaches to applications of PFPs have been conducted by researchers both in basic as well as applied biological research. Although PFPs are very devastating for human health nowadays researchers have been successful in making these toxic proteins into therapeutics through the preparation of immunotoxins. We have discussed the structural, and functional mechanism of action, evolutionary significance through dendrogram, domain organization, and practical applications for various approaches. This review aims to emphasize the PFTs to summarize toxic proteins together for basic knowledge as well as to highlight the current challenges, and literature gap along with the perspective of promising biotechnological applications for their future research.

Canagliflozin Inhibited the Activity of Hemolysin and Reduced the Inflammatory Response Caused by Streptococcus suis

Highly virulent Streptococcus suis (S. suis) infections can cause Streptococcal toxic shock-like syndrome (STSLS) in pigs and humans, in which an excessive inflammatory response causes severe damage. Hemolysin (SLY) is a major virulence factor of S. suis serotype 2 that produces pores in the target cell membrane, leading to cytoplasmic K+ efflux and activation of the NLRP3 inflammasome, ultimately causing STSLS. The critical aspect of hemolysin in the pathogenesis of S. suis type 2 makes it an attractive target for the development of innovative anti-virulence drugs. Here, we use the S. suis toxin protein (SLY) as a target for virtual screening. A compound called canagliflozin, a hypoglycemic agent, was identified through screening. Canagliflozin significantly inhibits the hemolytic activity of hemolysin. The results combined with molecular dynamics simulation, surface plasmon resonance, and nano differential scanning fluorimetry show that canagliflozin inhibits the hemolytic activity of SLY by binding to SLY. In addition, canagliflozin markedly reduced the release of SC19-induced inflammatory factors at the cellular level and in mice. Importantly, the combination of canagliflozin and ampicillin had a 90% success rate in mice, significantly greater than the therapeutic effect of ampicillin. The findings suggest that canagliflozin may be a promising new drug candidate for S. suis infections.

Visualizing the Domino-Like Prepore-to-Pore Transition of Streptolysin O by High-Speed AFM

Pore-forming proteins (PFPs) are produced by various organisms, including pathogenic bacteria, and form pores within the target cell membrane. Streptolysin O (SLO) is a PFP produced by Streptococcus pyogenes and forms high-order oligomers on the membrane surface. In this prepore state, multiple α-helices in domain 3 of each subunit exist as unfolded structures and transiently interact with each other. They subsequently transition into transmembrane β-hairpins (TMHs) and form pores with diameters of 20-30 nm. However, in this pore formation process, the trigger of the transition in a subunit and collaboration between subunits remains elusive. Here, I observed the dynamic pore formation process using high-speed atomic force microscopy. During the oligomer transition process, each subunit was sequentially inserted into the membrane, propagating along the oligomer in a domino-like fashion (chain reaction). This process also occurred on hybrid oligomers containing wildtype and mutant subunits, which cannot insert into the membrane because of an introduced disulfide bond. Furthermore, propagation still occurred when an excessive force was added to hybrid oligomers in the prepore state. Based on the observed chain reactions, I estimate the free energies and forces that trigger the transition in a subunit. Furthermore, I hypothesize that the collaboration between subunits is related to the structure of their TMH regions and interactions between TMH-TMH and TMH-lipid molecules.

Virulence factors of Streptococcus anginosus - a molecular perspective

Streptococcus anginosus together with S. constellatus and S. intermedius constitute the Streptococcus anginosus group (SAG), until recently considered to be benign commensals of the human mucosa isolated predominantly from oral cavity, but also from upper respiratory, intestinal, and urogenital tracts. For years the virulence potential of SAG was underestimated, mainly due to complications in correct species identification and their assignment to the physiological microbiota. Still, SAG representatives have been associated with purulent infections at oral and non-oral sites resulting in abscesses formation and empyema. Also, life threatening blood infections caused by SAG have been reported. However, the understanding of SAG as potential pathogen is only fragmentary, albeit certain aspects of SAG infection seem sufficiently well described to deserve a systematic overview. In this review we summarize the current state of knowledge of the S. anginosus pathogenicity factors and their mechanisms of action.

Perforin-2 clockwise hand-over-hand pre-pore to pore transition mechanism

Perforin-2 (PFN2, MPEG1) is a pore-forming protein that acts as a first line of defense in the mammalian immune system, rapidly killing engulfed microbes within the phagolysosome in macrophages. PFN2 self-assembles into hexadecameric pre-pore rings that transition upon acidification into pores damaging target cell membranes. Here, using high-speed atomic force microscopy (HS-AFM) imaging and line-scanning and molecular dynamics simulation, we elucidate PFN2 pre-pore to pore transition pathways and dynamics. Upon acidification, the pre-pore rings (pre-pore-I) display frequent, 1.8 s-1, ring-opening dynamics that eventually, 0.2 s-1, initiate transition into an intermediate, short-lived, ~75 ms, pre-pore-II state, inducing a clockwise pre-pore-I to pre-pore-II propagation. Concomitantly, the first pre-pore-II subunit, undergoes a major conformational change to the pore state that propagates also clockwise at a rate ~15 s-1. Thus, the pre-pore to pore transition is a clockwise hand-over-hand mechanism that is accomplished within ~1.3 s. Our findings suggest a clockwise mechanism of membrane insertion that with variations may be general for the MACPF/CDC superfamily.

Bacterial pore-forming toxins

Pore-forming toxins (PFTs) are widely distributed in both Gram-negative and Gram-positive bacteria. PFTs can act as virulence factors that bacteria utilise in dissemination and host colonisation or, alternatively, they can be employed to compete with rival microbes in polymicrobial niches. PFTs transition from a soluble form to become membrane-embedded by undergoing large conformational changes. Once inserted, they perforate the membrane, causing uncontrolled efflux of ions and/or nutrients and dissipating the protonmotive force (PMF). In some instances, target cells intoxicated by PFTs display additional effects as part of the cellular response to pore formation. Significant progress has been made in the mechanistic description of pore formation for the different PFTs families, but in several cases a complete understanding of pore structure remains lacking. PFTs have evolved recognition mechanisms to bind specific receptors that define their host tropism, although this can be remarkably diverse even within the same family. Here we summarise the salient features of PFTs and highlight where additional research is necessary to fully understand the mechanism of pore formation by members of this diverse group of protein toxins.

Apigenin and Ampicillin as Combined Strategy to Treat Severe Streptococcus suis Infection

As an important zoonotic pathogen, Streptococcus suis (S. suis) can cause a variety of diseases both in human and animals, especially Streptococcal toxic shock-like syndrome (STSLS), which commonly appears in severe S. suis infection. STSLS is often accompanied by excessive production of inflammatory cytokines, which is the main cause of host death. Therefore, it is urgent to find a new strategy to relieve the damage caused by STSLS. In this study, we found, for the first time, that apigenin, as a flavonoid compound, could combine with ampicillin to treat severe S. suis infection. Studies found that apigenin did not affect the growth of S. suis and the secretion of suilysin (SLY), but it could significantly inhibit the hemolytic activity of SLY by directly binding to SLY and destroying its secondary structure. In cell assays, apigenin was found to have no significant toxic effects on effective concentrations, and have a good protective effect on S. suis-infected cells. More importantly, compared with the survival rate of S. suis-infected mice treated with only ampicillin, the survival rate of apigenin combined with an ampicillin-treated group significantly increased to 80%. In conclusion, all results indicate that apigenin in combination with conventional antibiotics can be a potential strategy for treating severe S. suis infection.

Acquiring high expression of suilysin enable non-epidemic Streptococccus suis to cause streptococcal toxic shock-like syndrome (STSLS) through NLRP3 inflammasome hyperactivation

The epidemic Streptococcus suis (S. suis) strain [Sequence type (ST) 7] was gradually evolving from the non-epidemic ST1 strain and got the ability for high expressing of suilysin (SLY). And the high expression of SLY was required for the epidemic strain to cause NLRP3 hyperactivation, which is essential for the induction of cytokines storm, dysfunction of multiple organs, and a high incidence of mortality, the characters of streptococcal toxic shock-like syndrome (STSLS). However, it remains to be elucidated whether acquiring high SLY expression due to genome evolution was sufficient for the non-epidemic strain to cause STSLS. Here, we found that the overexpression of SLY in ST1 strain (P1/7-SLY) could obviously increase the inflammasome activation, which was dependent on NLRP3 signalling. In contrast, the strain (P1/7-mSLY) overexpressing the mutant SLY (protein without hemolytic activity) could not significantly increase the inflammasome activation. Furthermore, similar to the epidemic strain, P1/7-SLY could cause STSLS in nlrp3^+/+ mice but not in nlrp3^−/− mice. In contrast, P1/7-mSLY could not cause STSLS in both nlrp3^+/+ mice and nlrp3^−/− mice. In summary, we demonstrate that genetic evolution enabling S. suis strain to express high level of SLY may be an essential and sufficient condition for NLRP3 inflammasome hyperactivation, which could further cause cytokines storm and STSLS.

The molecular mechanisms of listeriolysin O-induced lipid membrane damage

Listeria monocytogenes is an intracellular food-borne pathogen that causes listeriosis, a severe and potentially life-threatening disease. Listeria uses a number of virulence factors to proliferate and spread to various cells and tissues. In this process, three bacterial virulence factors, the pore-forming protein listeriolysin O and phospholipases PlcA and PlcB, play a crucial role. Listeriolysin O belongs to a family of cholesterol-dependent cytolysins that are mostly expressed by gram-positive bacteria. Its unique structural features in an otherwise conserved three-dimensional fold, such as the acidic triad and proline-glutamate-serine-threonine-like sequence, enable the regulation of its intracellular activity as well as distinct extracellular functions. The stability of listeriolysin O is pH- and temperature-dependent, and this provides another layer of control of its activity in cells. Moreover, many recent studies have demonstrated a unique mechanism of pore formation by listeriolysin O, i.e., the formation of arc-shaped oligomers that can subsequently fuse to form membrane defects of various shapes and sizes. During listerial invasion of host cells, these membrane defects can disrupt phagosome membranes, allowing bacteria to escape into the cytosol and rapidly multiply. The activity of listeriolysin O is profoundly dependent on the amount and accessibility of cholesterol in the lipid membrane, which can be modulated by the phospholipase PlcB. All these prominent features of listeriolysin O play a role during different stages of the L. monocytogenes life cycle by promoting the proliferation of the pathogen while mitigating excessive damage to its replicative niche in the cytosol of the host cell.

Mode of action and structural modelling of the interaction of formononetin with suilysin

AIMS

Suilysin is a critical pore-forming virulence factor of Streptococcus suis that has been demonstrated to substantially contribute to its pathogenicity. We have demonstrated that formononetin alleviates S. suis infection both in vivo and in vitro by targeting suilysin. However, the molecular mechanism of the effect is unclear. Our aim was to determine the molecular mechanism of the effect of formononetin on suilysin.

METHODS AND RESULTS

The mechanism of interaction between formononetin and suilysin was investigated by molecular modelling. The results indicated that formononetin was bound at the junction of domain two and domain four of suilysin. The binding free energy values indicated that the A415, Y412, E414, N413, T61, T62 and G416 residues are critical for this binding, this observation was confirmed by the changes in the flexibility of these residues and the distances between these residues and formononetin. The inhibitory effect of formononetin on the pore-forming activity of suilysin, binding constant and binding free energy were significantly decreased by site-specific mutagenesis of Y412 and N413. Finally, we analysed the spatial configuration of suilysin before and after formononetin binding, the results indicated that the binding changed the conformation of suilysin, especially the angle between domain two and domain four, resulting in the disruption of cholesterol binding to suilysin and in the loss of pore-forming activity.

CONCLUSIONS

Formononetin is located at the junction of domain two and domain four of suilysin, and Y412 and N413 play critical roles in the binding. Formononetin binding changes the angle between domain two and domain four of suilysin, resulting in the loss of the pore-inducing activity of suilysin.

SIGNIFICANCE AND IMPACT OF THE STUDY

This work will promote the application of formononetin to combat S. suis infections and may contribute to the development of new inhibitors or modification of existing inhibitors.

How protein engineering has revealed the molecular mechanisms of pore-forming toxins

Pore-forming proteins are found in prokaryotes, vertebrates, and invertebrates, and when involved in pathogenic processes they are classified as pore-forming toxins (PFTs). The use of gene engineering methods in combination with the information provided by the high-resolution crystal structures of the PFTs have allowed investigators to gain a deep understanding of their pore-forming mechanisms. In this chapter, we discuss how protein engineering has helped us and others to reveal the molecular mechanisms of pore formation by prokaryotic PFTs with an emphasis on our experiences with the cholesterol-dependent cytolysins (CDCs).

X-ray crystallography shines a light on pore-forming toxins

A common form of cellular attack by pathogenic bacteria is to secrete pore-forming toxins (PFTs). Capable of forming transmembrane pores in various biological membranes, PFTs have also been identified in a diverse range of other organisms such as sea anemones, earthworms and even mushrooms and trees. The mechanism of pore formation by PFTs is associated with substantial conformational changes in going from the water-soluble to transmembrane states of the protein. The determination of the crystal structures for numerous PFTs has shed much light on our understanding of these proteins. Other than elucidating the atomic structural details of PFTs and the conformational changes that must occur for pore formation, crystal structures have revealed structural homology that has led to the discovery of new PFTs and new PFT families. Here we review some key crystallographic results together with complimentary approaches for studying PFTs. We discuss how these studies have impacted our understanding of PFT function and guided research into biotechnical applications.

Structural Basis of the Pore-Forming Toxin/Membrane Interaction

With the rapid growth of antibiotic-resistant bacteria, it is urgent to develop alternative therapeutic strategies. Pore-forming toxins (PFTs) belong to the largest family of virulence factors of many pathogenic bacteria and constitute the most characterized classes of pore-forming proteins (PFPs). Recent studies revealed the structural basis of several PFTs, both as soluble monomers, and transmembrane oligomers. Upon interacting with host cells, the soluble monomer of bacterial PFTs assembles into transmembrane oligomeric complexes that insert into membranes and affect target cell-membrane permeability, leading to diverse cellular responses and outcomes. Herein we have reviewed the structural basis of pore formation and interaction of PFTs with the host cell membrane, which could add valuable contributions in comprehensive understanding of PFTs and searching for novel therapeutic strategies targeting PFTs and interaction with host receptors in the fight of bacterial antibiotic-resistance.

Inhibitory Effect of Piceatannol on Streptococcus suis Infection Both in vitro and in vivo

Suilysin (SLY) plays a critical role in Streptococcus suis infections making it an ideal target to the combat infection caused by this pathogen. In the present study, we found that piceatannol (PN), a natural compound, inhibits pore-formation by blocking the oligomerization of SLY without affecting the growth of S. suis and the expression of SLY. Furthermore, PN alleviated the J774 cell damage and the expression of the inflammatory cytokine tumor necrosis factor-α (TNF-α) and interleukin-1α (IL-1β) induced by S. suis in vitro. The computational biology and biochemistry results indicated that PN binds to the joint region of D2 and D4 in SLY, and Asn57, Pro58, Pro59, Glu76, Ile379, Glu380, and Glu418 were critical residues involved in the binding. The binding effect between PN and SLY hindered the SLY monomers from forming the oligomers, thereby weakening the hemolytic activity of SLY. This mechanism was also verified by hemolysis analysis and analysis of K_A formation after site-specific mutagenesis. Furthermore, PN protected mice from S. suis infections by reducing bacterial colony formation and the inflammatory response in target organs in vivo. These results indicate that PN is a feasible drug candidate to combat S. suis infections.

Structural basis for tuning activity and membrane specificity of bacterial cytolysins

Cholesterol-dependent cytolysins (CDCs) are pore-forming proteins that serve as major virulence factors for pathogenic bacteria. They target eukaryotic cells using different mechanisms, but all require the presence of cholesterol to pierce lipid bilayers. How CDCs use cholesterol to selectively lyse cells is essential for understanding virulence strategies of several pathogenic bacteria, and for repurposing CDCs to kill new cellular targets. Here we address that question by trapping an early state of pore formation for the CDC intermedilysin, bound to the human immune receptor CD59 in a nanodisc model membrane. Our cryo electron microscopy map reveals structural transitions required for oligomerization, which include the lateral movement of a key amphipathic helix. We demonstrate that the charge of this helix is crucial for tuning lytic activity of CDCs. Furthermore, we discover modifications that overcome the requirement of cholesterol for membrane rupture, which may facilitate engineering the target-cell specificity of pore-forming proteins.

All major cholesterol-dependent cytolysins use glycans as cellular receptors

Cholesterol-dependent cytolysins (CDCs) form pores in cholesterol-rich membranes, but cholesterol alone is insufficient to explain their cell and host tropism. Here, we show that all eight major CDCs have high-affinity lectin activity that identifies glycans as candidate cellular receptors. Streptolysin O, vaginolysin, and perfringolysin O bind multiple glycans, while pneumolysin, lectinolysin, and listeriolysin O recognize a single glycan class. Addition of exogenous carbohydrate receptors for each CDC inhibits toxin activity. We present a structure for suilysin domain 4 in complex with two distinct glycan receptors, P1 antigen and αGal/Galili. We report a wide range of binding affinities for cholesterol and for the cholesterol analog pregnenolone sulfate and show that CDCs bind glycans and cholesterol independently. Intermedilysin binds to the sialyl-TF O-glycan on its erythrocyte receptor, CD59. Removing sialyl-TF from CD59 reduces intermedilysin binding. Glycan-lectin interactions underpin the cellular tropism of CDCs and provide molecular targets to block their cytotoxic activity.

Complement C3aR/C5aR-binding protein Suilysin of Streptococcus suis contributes to monocyte chemotaxis

Streptococcus suis is an emerging swine and human pathogen causing severe infections and sudden death. During infection, complement C3a and C5a were reported to induce immune cells towards infection and injury sites via their corresponding receptors C3aR and C5aR. However, how S. suis evade immune surveillance mediated by C3aR and C5aR remains unclear. In this study, we analyze and construct an S. suis bacterial two-hybrid prey library containing 39 LPXTG motif anchored proteins and 18 secreted proteins. Two highly possible C3aR-binding proteins: thiol-activated toxin Suilysin, putative RTX family exoprotein A gene and three highly possible C5aR-binding proteins: thiol-activated toxin Suilysin, putative 5'-nucleotidase and subtilisin-like serine protease are identified through bacterial two-hybrid assay. Far-western blot assay confirms that a cholesterol-binding cytolysin Suilysin can interact with both C3aR and C5aR. Chemotaxis assays demonstrate that recombinant and natural Suilysin can inhibit monocyte chemotaxis mediated by C3a and C5a. These findings enlarge our knowledge of suilysin biological significance and provide a new perspective on S. suis complement evasion.

Inerolysin and vaginolysin, the cytolysins implicated in vaginal dysbiosis, differently impair molecular integrity of phospholipid membranes

The pore-forming toxins, inerolysin (INY) and vaginolysin (VLY), produced by vaginal bacteria Lactobacillus iners and Gardnerella vaginalis were studied using the artificial cholesterol-rich tethered bilayer membranes (tBLMs) by electrochemical techniques. The electrochemical impedance spectroscopy (EIS) of tBLMs attested for the toxin-induced impairment of the integrity of phospholipid membranes. This observation was in line with the atomic force microscopy data demonstrating formation of oligomeric protein assemblies in tBLMs. These assemblies exhibited different morphologies: VLY mostly formed complete rings, whereas INY produced arciform structures. We found that both EIS (membrane damage) and the surface plasmon resonance (protein binding) data obtained on tBLMs are in-line with the data obtained in human cell lysis experiments. EIS, however, is capable of capturing effects inaccessible for biological activity assays. Specifically, we found that the INY-induced damage of tBLMs is nearly a linear function of membrane cholesterol content, whereas VLY triggered significant damage only at high (50 mol%) cholesterol concentrations. The observed differences of INY and VLY activities on phospholipid membranes might have clinical importance: both toxin-producing bacteria have been found in healthy vagina and dysbiosis, suggesting the need for adaptation at different vaginal conditions. Our results broaden the possibilities of application of tBLMs in medical diagnostics.

An NLRP3 inflammasome-triggered cytokine storm contributes to Streptococcal toxic shock-like syndrome (STSLS)

Infection with the Streptococcus suis (S. suis) epidemic strain can cause Streptococcal toxic shock-like syndrome (STSLS), which is characterized by a cytokine storm, dysfunction of multiple organs and a high incidence of mortality despite adequate treatment. Despite some progress concerning the contribution of the inflammatory response to STSLS, the precise mechanism underlying STSLS development remains elusive. Here, we use a murine model to demonstrate that caspase-1 activity is critical for STSLS development. Furthermore, we show that inflammasome activation by S. suis is mainly dependent on NLRP3 but not on NLRP1, AIM2 or NLRC4. The important role of NLRP3 activation in STSLS is further confirmed in vivo with the NLRP3 inhibitor MCC950 and nlrp3-knockout mice. By comparison of WT strain with isogenic strains with mutation of various virulence genes for inflammasome activation, Suilysin is essential for inflammasome activation, which is dependent on the membrane perforation activity to cause cytosolic K+ efflux. Moreover, the mutant strain msly (P353L) expressing mutagenic SLY without hemolytic activity was unable to activate the inflammasome and does not cause STSLS. In summary, we demonstrate that the high membrane perforation activity of the epidemic strain induces a high level of NLRP3 inflammasome activation, which is essential for the development of the cytokine storm and multi-organ dysfunction in STSLS and suggests NLRP3 inflammasome as an attractive target for the treatment of STSLS.

Structure-function characterization of an insecticidal protein GNIP1Aa, a member of an MACPF and β-tripod families

The crystal structure of the Gram-negative insecticidal protein, GNIP1Aa, has been solved at 2.5-Å resolution. The protein consists of two structurally distinct domains, a MACPF (membrane attack complex/PerForin) and a previously uncharacterized type of domain. GNIP1Aa is unique in being a prokaryotic MACPF member to have both its structure and function identified. It was isolated from a Chromobacterium piscinae strain and is specifically toxic to Diabrotica virgifera virgifera larvae upon feeding. In members of the MACPF family, the MACPF domain has been shown to be important for protein oligomerization and formation of transmembrane pores, while accompanying domains define the specificity of the target of the toxicity. In GNIP1Aa the accompanying C-terminal domain has a unique fold composed of three pseudosymmetric subdomains with shared sequence similarity, a feature not obvious from the initial sequence examination. Our analysis places this domain into a protein family, named here β-tripod. Using mutagenesis, we identified functionally important regions in the β-tripod domain, which may be involved in target recognition.

Fine-tuning of the stability of β-strands by Y181 in perfringolysin O directs the prepore to pore transition

Perfringolysin O (PFO) is a toxic protein that forms β-barrel transmembrane pores upon binding to cholesterol-containing membranes. The formation of lytic pores requires conformational changes in PFO that lead to the conversion of water-soluble monomers into membrane-bound oligomers. Although the general outline of stepwise pore formation has been established, the underlying mechanistic details await clarification. To extend our understanding of the molecular mechanisms that control the pore formation, we compared the hydrogen-deuterium exchange patterns of PFO with its derivatives bearing mutations in the D3 domain. In the case of two of these mutations F318A, Y181A, known from previous work to lead to a decreased lytic activity, global destabilization of all protein domains was observed in their water-soluble forms. This was accompanied by local changes in D3 β-sheet, including unexpected stabilization of functionally important β1 strand in Y181A. In case of the double mutation (F318A/Y181A) that completely abolished the lytic activity, several local changes were retained, but the global destabilization effects of single mutations were reverted and hydrogen-deuterium exchange (HDX) pattern returned to PFO level. Strong structural perturbations were not observed in case of remaining variants in which other residues of the hydrophobic core of D3 domain were substituted by alanine. Our results indicate the existence in PFO of a well-tuned H-bonding network that maintains the stability of the D3 β-strands at appropriate level at each transformation step. F318 and Y181 moieties participate in this network and their role extends beyond their direct intermolecular interaction during oligomerization that was identified previously.

Cholesterol-dependent cytolysins: from water-soluble state to membrane pore

The cholesterol-dependent cytolysins (CDCs) are a family of bacterial toxins that are important virulence factors for a number of pathogenic Gram-positive bacterial species. CDCs are secreted as soluble, stable monomeric proteins that bind specifically to cholesterol-rich cell membranes, where they assemble into well-defined ring-shaped complexes of around 40 monomers. The complex then undergoes a concerted structural change, driving a large pore through the membrane, potentially lysing the target cell. Understanding the details of this process as the protein transitions from a discrete monomer to a complex, membrane-spanning protein machine is an ongoing challenge. While many of the details have been revealed, there are still questions that remain unanswered. In this review, we present an overview of some of the key features of the structure and function of the CDCs, including the structure of the secreted monomers, the process of interaction with target membranes, and the transition from bound monomers to complete pores. Future directions in CDC research and the potential of CDCs as research tools will also be discussed.

Fisetin Lowers Streptococcus suis serotype 2 Pathogenicity in Mice by Inhibiting the Hemolytic Activity of Suilysin

Streptococcus suis serotype 2 is a serious zoonotic pathogen and has attracted worldwide attention since the first human case was reported in Denmark in 1968. Some virulence factors have been reported to be involved in the pathogenesis of the infection caused by Streptococcus suis serotype 2, and then novel strategies to identify some anti-virulence compounds which can effectively inhibit the pathogenic bacterial infection have recently been reported. Suilysin is an essential virulence factor for Streptococcus suis serotype 2 since it creates pores in the target cells membranes, which aids bacterial colonization. The important role of suilysin in the virulence of Streptococcus suis serotype 2 renders it an ideal target for designing novel anti-virulence therapeutics. We find that fisetin, as a natural flavonoid, is a potent antagonist against suilysin-mediated hemolysis. The aim of this study is to evaluate the effect of fisetin on the hemolytic activity of suilysin from Streptococcus suis serotype 2. Fisetin is found to significantly inhibit the hemolytic activity of suilysin. Within the range of effective concentrations, fisetin does not influence the growth of Streptococcus suis serotype 2 and the expression of suilysin protein. In vitro, fisetin effectively inhibits the death of macrophages (J774A.1 and RAW264.7) infected with Streptococcus suis serotype 2 by weakening intracellular bacterial multiplication. Animal model experiment shows that fisetin effectively improves the survival rate of animals infected with Streptococcus suis serotype 2. Our findings suggest that fisetin could be used as an antitoxin against suilysin and be developed into a promising therapeutic candidate for treating Streptococcus suis serotype 2 infection.

Investigation of the inhibition effect and mechanism of myricetin to Suilysin by molecular modeling

In the present study, the inhibitory effect and mechanism of myricetin, a natural flavonoid compound, in relation to Suilysin (SLY) were investigated through molecular dynamics simulations, mutational analysis and fluorescence-quenching assays. Myricetin is a potential inhibitor that does not exhibit antimicrobial activity but has been shown to inhibit SLY cytotoxicity. Molecular dynamics simulations and mutational analysis revealed that myricetin binds directly to SLY in the gap between domains 2 and 3, an important region for oligomerization and pore formation. The results of principal component analysis (PCA) indicated that the binding of myricetin in this gap region restricts the conformational transition of SLY from a monomer to an oligomer, thereby counteracting the haemolytic activity of SLY. This mechanism was verified using a haemolysis assay. These results demonstrated that myricetin is a strong candidate as a novel therapeutic agent for the treatment of Streptococcus suis infections.

Mac Protein is not an Essential Virulence Factor for the Virulent Reference Strain Streptococcus suis P1/7

Streptococcus suis is a major pathogen of pigs and also an important zoonotic agent for humans. A S. suis protein containing Mac-1 domain (designated Mac) is a protective antigen, exclusively cleaves porcine IgM, and contributes to complement evasion with the presence of high titers of specific porcine anti-S. suis IgM, but its role in S. suis virulence has not been investigated in natural healthy host without specific IgM. In this study, a mac deletion mutant was constructed by homologous recombination in S. suis serotype 2 virulent reference strain P1/7. Deletion of mac did not significantly influence phagocytosis or intracellular survival within murine macrophages RAW264.7, or the oxidative-burst induction of RAW264.7 and murine neutrophils. Furthermore, the mutant is as virulent as the wild-type strain in pig, mouse, and zebrafish infection models. Our data suggest that Mac is not essential for S. suis virulence in strain P1/7 in natural healthy host without specific IgM, and the immunogenicity of Mac does not appear to correlate with its significance for virulence.

Suilysin-induced Platelet-Neutrophil Complexes Formation is Triggered by Pore Formation-dependent Calcium Influx

Platelet activation and platelet–neutrophil interactions have been found to be involved in inflammation, organ failure and soft-tissue necrosis in bacterial infections. Streptococcus suis, an emerging human pathogen, can cause streptococcal toxic-shock syndrome (STSS) similarly to Streptococcus pyogenes. Currently, S. suis–platelet interactions are poorly understood. Here, we found that suilysin (SLY), the S. suis cholesterol-dependent cytolysin (CDC), was the sole stimulus of S. suis that induced platelet-neutrophil complexes (PNC) formation. Furthermore, P-selectin released in α-granules mediated PNC formation. This process was triggered by the SLY-induced pore forming-dependent Ca²⁺ influx. Moreover, we demonstrated that the Ca²⁺ influx triggered an MLCK-dependent pathway playing critical roles in P-selectin activation and PNC formation, however, PLC-β-IP3/DAG-MLCK and Rho-ROCK-MLCK signalling were not involved. Additionally, the “outside-in” signalling had a smaller effect on the SLY-induced P-selectin release and PNC formation. Interestingly, other CDCs including pneumolysin and streptolysin O have also been found to induce PNC formation in a pore forming-dependent Ca²⁺ influx manner. It is possible that the bacterial CDC-mediated PNC formation is a similar response mechanism used by a wide range of bacteria. These findings may provide useful insight for discovering potential therapeutic targets for S. suis-associated STSS.

Effects of Suilysin on Streptococcus suis-Induced Platelet Aggregation

Blood platelets play important roles during pathological thrombocytopenia in streptococcal toxic shock syndrome (STSS). Streptococcus suis (S. suis) an emerging human pathogen, can cause STSS similarly to S. pyogenes. However, S. suis interactions with platelets are poorly understood. Here, we found that suilysin (SLY), different from other bacterial cholesterol-dependent cytolysins (CDCs), was the sole stimulus that induced platelet aggregation. Furthermore, the inside-out activation of GPIIb/IIIa of platelets mediated SLY-induced platelet aggregation. This process was triggered by Ca²⁺ influx that depend on the pore forming on platelets by SLY. Additionally, although SLY induced α-granule release occurred via the MLCK-dependent pathway, PLC-β-IP3/DAG-MLCK and Rho-ROCK-MLCK signaling were not involved in SLY-induced platelet aggregation. Interestingly, the pore dependent Ca²⁺ influx was also found to participate in the induction of platelet aggregation with pneumolysin (PLY) and streptolysin O (SLO), two other CDCs. It is possible that the CDC-mediated platelet aggregation we observed in S. suis is a similar response mechanism to that used by a wide range of bacteria. These findings might lead to the discovery of potential therapeutic targets for S. suis-associated STSS.

Suilysin Stimulates the Release of Heparin Binding Protein from Neutrophils and Increases Vascular Permeability in Mice

Most of the deaths that occurred during two large outbreaks of Streptococcus suis infections in 1998 and 2005 in China were caused by streptococcal toxic shock syndrome (STSS), which is characterized by increased vascular permeability. Heparin-binding protein (HBP) is thought to mediate the vascular leakage. The purpose of this study was to investigate the detailed mechanism underlying the release of HBP and the vascular leakage induced by S. suis. Significantly higher serum levels of HBP were detected in Chinese patients with STSS than in patients with meningitis or healthy controls. Suilysin (SLY) is an exotoxin secreted by the highly virulent strain 05ZYH33, and it stimulated the release of HBP from the polymorphonuclear neutrophils and mediated vascular leakage in mice. The release of HBP induced by SLY was caused by a calcium influx-dependent degranulation. Analyses using a pharmacological approach revealed that the release of HBP induced by SLY was related to Toll-like receptor 4, p38 mitogen-activated protein kinase, and the 1-phosphatidylinositol 3-kinase pathway. It was also dependent on a G protein-coupled seven-membrane spanning receptor. The results of this study provide new insights into the vascular leakage in STSS associated with non-Group A streptococci, which could lead to the discovery of potential therapeutic targets for STSS associated with S. suis.

Biological activities of suilysin: role in Streptococcus suis pathogenesis

Streptococcus suis is an important swine and zoonotic pathogen equipped with several virulence factors. The pore-forming toxins are the most abundant bacterial toxins and classified as critical virulence (associated) factors of several pathogens. The role of suilysin (SLY), a pore-forming cholesterol-dependent cytolysin of S. suis, as a true virulence factor is under debate. Most of the bacterial toxins have been reported to modulate the host immune system to facilitate invasion and subsequent replication of bacteria within respective host cells. SLY has been demonstrated to play an important role in the pathogenesis of S. suis infection and inflammatory response in vitro and in vivo. This review highlights the contributions of SLY to the pathogenicity of S. suis. It will address its role during the development of S. suis meningitis in pigs, as well as humans, and discuss SLY as a potential vaccine candidate.

Switch-like responses of two cholesterol sensors do not require protein oligomerization in membranes

Many cellular processes are sensitive to levels of cholesterol in specific membranes and show a strongly sigmoidal dependence on membrane composition. The sigmoidal responses of the cholesterol sensors involved in these processes could arise from several mechanisms, including positive cooperativity (protein effects) and limited cholesterol accessibility (membrane effects). Here, we describe a sigmoidal response that arises primarily from membrane effects due to sharp changes in the chemical activity of cholesterol. Our models for eukaryotic membrane-bound cholesterol sensors are soluble bacterial toxins that show an identical switch-like specificity for endoplasmic reticulum membrane cholesterol. We show that truncated versions of these toxins fail to form oligomers but still show sigmoidal binding to cholesterol-containing membranes. The nonlinear response emerges because interactions between bilayer lipids control cholesterol accessibility to toxins in a threshold-like fashion. Around these thresholds, the affinity of toxins for membrane cholesterol varies by >100-fold, generating highly cooperative lipid-dependent responses independently of protein-protein interactions. Such lipid-driven cooperativity may control the sensitivity of many cholesterol-dependent processes.

Giant MACPF/CDC pore forming toxins: A class of their own

Pore Forming Toxins (PFTs) represent a key mechanism for permitting the passage of proteins and small molecules across the lipid membrane. These proteins are typically produced as soluble monomers that self-assemble into ring-like oligomeric structures on the membrane surface. Following such assembly PFTs undergo a remarkable conformational change to insert into the lipid membrane. While many different protein families have independently evolved such ability, members of the Membrane Attack Complex PerForin/Cholesterol Dependent Cytolysin (MACPF/CDC) superfamily form distinctive giant β-barrel pores comprised of up to 50 monomers and up to 300Å in diameter. In this review we focus on recent advances in understanding the structure of these giant MACPF/CDC pores as well as the underlying molecular mechanisms leading to their formation. Commonalities and evolved variations of the pore forming mechanism across the superfamily are discussed. This article is part of a Special Issue entitled: Pore-Forming Toxins edited by Mauro Dalla Serra and Franco Gambale.

Isoleucine 61 is important for the hemolytic activity of pyolysin of Trueperella pyogenes

Pyolysin (PLO) is a hemolysin secreted by Trueperella pyogenes (T. pyogenes) and is important for the pathogenicity of T. pyogenes. Oligomerization of PLO monomers is a critical step in the process of hemolysis. However, the mechanisms of intermolecular interaction of PLO monomers are still not clearly illuminated. In this study, two monoclonal antibodies (mAbs) against PLO, named AP-1A3 and AP-4F12, respectively, were generated firstly, of which AP-1A3 showed no or undetectable hemolysis inhibition activity against recombinant PLO (rPLO), whereas AP-4F12 could markedly inhibit the hemolytic activity of rPLO. Epitope mapping revealed that AP-1A3 recognized amino acid residues ranging from 64 to 79 of mature PLO (91-106 including the signal peptide), whereas AP-4F12 recognized amino acid residues ranging from 58 to 75 (85-102 including the signal peptide). Comparison of the amino acid sequence of two epitopes revealed that six amino acid residues ranging from 58 to 63 of PLO were associated with the hemolytic activity of PLO. Alanine scan showed that substitution of each amino acid ranging from 58 to 62 with alanine had apparent impact on the hemolytic activity of rPLO, especially for the substitution of isoleucine 61 which caused almost complete loss of hemolytic activity of rPLO. Our findings identified a region in PLO and an amino acid in that region might play important role in the process of oligomerization of PLO monomers.

Crystal structure of Streptococcus pneumoniae pneumolysin provides key insights into early steps of pore formation

Pore-forming proteins are weapons often used by bacterial pathogens to breach the membrane barrier of target cells. Despite their critical role in infection important structural aspects of the mechanism of how these proteins assemble into pores remain unknown. Streptococcus pneumoniae is the world’s leading cause of pneumonia, meningitis, bacteremia and otitis media. Pneumolysin (PLY) is a major virulence factor of S. pneumoniae and a target for both small molecule drug development and vaccines. PLY is a member of the cholesterol-dependent cytolysins (CDCs), a family of pore-forming toxins that form gigantic pores in cell membranes. Here we present the structure of PLY determined by X-ray crystallography and, in solution, by small-angle X-ray scattering. The crystal structure reveals PLY assembles as a linear oligomer that provides key structural insights into the poorly understood early monomer-monomer interactions of CDCs at the membrane surface.

Toll-like receptor 4 confers inflammatory response to Suilysin

Streptococcus suis serotype 2 (SS2) is an emerging human pathogen worldwide. A large outbreak occurred in the summer of 2005 in China. Serum samples from this outbreak revealed that levels of the main proinflammatory cytokines were significantly higher in patients with streptococcal toxic-shock-like syndrome (STSLS) than in patients with meningitis only. However, the mechanism underlying the cytokine storm in STSLS caused by SS2 remained unclear. In this study, we found that suilysin (SLY) is the main protein inflammatory stimulus of SS2 and that native SLY (nSLY) stimulated cytokines independently of its haemolytic ability. Interestingly, a small amount of SLY (Å Mol/L) induced strong, long-term TNF-α release from human PBMCs. We also found that nSLY stimulated TNF-α in wild-type macrophages but not in macrophages from mice that carried a spontaneous mutation in TLR4 (P712H). We demonstrated for the first time that SLY stimulates immune cells through TLR4. In addition, the Myd88 adaptor-p38-MAPK pathway was involved in this process. The present study suggested that the TLR4-dependent inflammatory responses induced by SLY in host might contribute to the STSLS caused by SS2 and that p38-MAPK could be used as a target to control the release of excess TNF-α induced by SS2.

Dynamic Virus-Bacterium Interactions in a Porcine Precision-Cut Lung Slice Coinfection Model: Swine Influenza Virus Paves the Way for Streptococcus suis Infection in a Two-Step Process

Swine influenza virus (SIV) and Streptococcus suis are common pathogens of the respiratory tract in pigs, with both being associated with pneumonia. The interactions of both pathogens and their contribution to copathogenesis are only poorly understood. In the present study, we established a porcine precision-cut lung slice (PCLS) coinfection model and analyzed the effects of a primary SIV infection on secondary infection by S. suis at different time points. We found that SIV promoted adherence, colonization, and invasion of S. suis in a two-step process. First, in the initial stages, these effects were dependent on bacterial encapsulation, as shown by selective adherence of encapsulated, but not unencapsulated, S. suis to SIV-infected cells. Second, at a later stage of infection, SIV promoted S. suis adherence and invasion of deeper tissues by damaging ciliated epithelial cells. This effect was seen with a highly virulent SIV subtype H3N2 strain but not with a low-virulence subtype H1N1 strain, and it was independent of the bacterial capsule, since an unencapsulated S. suis mutant behaved in a way similar to that of the encapsulated wild-type strain. In conclusion, the PCLS coinfection model established here revealed novel insights into the dynamic interactions between SIV and S. suis during infection of the respiratory tract. It showed that at least two different mechanisms contribute to the beneficial effects of SIV for S. suis, including capsule-mediated bacterial attachment to SIV-infected cells and capsule-independent effects involving virus-mediated damage of ciliated epithelial cells.

The Unique Molecular Choreography of Giant Pore Formation by the Cholesterol-Dependent Cytolysins of Gram-Positive Bacteria

The mechanism by which the cholesterol-dependent cytolysins (CDCs) assemble their giant β-barrel pore in cholesterol-rich membranes has been the subject of intense study in the past two decades. A combination of structural, biophysical, and biochemical analyses has revealed deep insights into the series of complex and highly choreographed secondary and tertiary structural transitions that the CDCs undergo to assemble their β-barrel pore in eukaryotic membranes. Our knowledge of the molecular details of these dramatic structural changes in CDCs has transformed our understanding of how giant pore complexes are assembled and has been critical to our understanding of the mechanisms of other important classes of pore-forming toxins and proteins across the kingdoms of life. Finally, there are tantalizing hints that the CDC pore-forming mechanism is more sophisticated than previously imagined and that some CDCs are employed in pore-independent processes.

Stepwise visualization of membrane pore formation by suilysin, a bacterial cholesterol-dependent cytolysin

Membrane attack complex/perforin/cholesterol-dependent cytolysin (MACPF/CDC) proteins constitute a major superfamily of pore-forming proteins that act as bacterial virulence factors and effectors in immune defence. Upon binding to the membrane, they convert from the soluble monomeric form to oligomeric, membrane-inserted pores. Using real-time atomic force microscopy (AFM), electron microscopy (EM), and atomic structure fitting, we have mapped the structure and assembly pathways of a bacterial CDC in unprecedented detail and accuracy, focussing on suilysin from Streptococcus suis. We show that suilysin assembly is a noncooperative process that is terminated before the protein inserts into the membrane. The resulting ring-shaped pores and kinetically trapped arc-shaped assemblies are all seen to perforate the membrane, as also visible by the ejection of its lipids. Membrane insertion requires a concerted conformational change of the monomeric subunits, with a marked expansion in pore diameter due to large changes in subunit structure and packing.

A new model for pore formation by cholesterol-dependent cytolysins

Cholesterol Dependent Cytolysins (CDCs) are important bacterial virulence factors that form large (200–300 Å) membrane embedded pores in target cells. Currently, insights from X-ray crystallography, biophysical and single particle cryo-Electron Microscopy (cryo-EM) experiments suggest that soluble monomers first interact with the membrane surface via a C-terminal Immunoglobulin-like domain (Ig; Domain 4). Membrane bound oligomers then assemble into a prepore oligomeric form, following which the prepore assembly collapses towards the membrane surface, with concomitant release and insertion of the membrane spanning subunits. During this rearrangement it is proposed that Domain 2, a region comprising three β-strands that links the pore forming region (Domains 1 and 3) and the Ig domain, must undergo a significant yet currently undetermined, conformational change. Here we address this problem through a systematic molecular modeling and structural bioinformatics approach. Our work shows that simple rigid body rotations may account for the observed collapse of the prepore towards the membrane surface. Support for this idea comes from analysis of published cryo-EM maps of the pneumolysin pore, available crystal structures and molecular dynamics simulations. The latter data in particular reveal that Domains 1, 2 and 4 are able to undergo significant rotational movements with respect to each other. Together, our data provide new and testable insights into the mechanism of pore formation by CDCs.

Pore formation is central to the ability of cholesterol dependent cytolysins (CDCs) to act as important bacterial virulence factors. Secreted by numerous pathogens the toxins assemble into a circular ring and then perforate the target membrane to form the largest self-assembling proteinaceous pores known. In this paper we investigated computationally the conformational properties of the CDC molecule and deduced a new structural model of pore formation and membrane insertion that reconciles all experimental data. The mechanism of membrane perforation by CDCs put forward here reveals concerted and unsuspected domains motion of large amplitude, which conflicts with the currently proposed model. The work presented here procures a plausible structural mechanism of CDC oligomeric transition and furthers our understanding of pore formation by these important toxins.

Crystal structure of listeriolysin O reveals molecular details of oligomerization and pore formation

Listeriolysin O (LLO) is an essential virulence factor of Listeria monocytogenes that causes listeriosis. Listeria monocytogenes owes its ability to live within cells to the pH- and temperature-dependent pore-forming activity of LLO, which is unique among cholesterol-dependent cytolysins. LLO enables the bacteria to cross the phagosomal membrane and is also involved in activation of cellular processes, including the modulation of gene expression or intracellular Ca(2+) oscillations. Neither the pore-forming mechanism nor the mechanisms triggering the signalling processes in the host cell are known in detail. Here, we report the crystal structure of LLO, in which we identified regions important for oligomerization and pore formation. Mutants were characterized by determining their haemolytic and Ca(2+) uptake activity. We analysed the pore formation of LLO and its variants on erythrocyte ghosts by electron microscopy and show that pore formation requires precise interface interactions during toxin oligomerization on the membrane.

Streptococcus suis infection: an emerging/reemerging challenge of bacterial infectious diseases?

Streptococcus suis (S. suis) is a family of pathogenic gram-positive bacterial strains that represents a primary health problem in the swine industry worldwide. S. suis is also an emerging zoonotic pathogen that causes severe human infections clinically featuring with varied diseases/syndromes (such as meningitis, septicemia, and arthritis). Over the past few decades, continued efforts have made significant progress toward better understanding this zoonotic infectious entity, contributing in part to the elucidation of the molecular mechanism underlying its high pathogenicity. This review is aimed at presenting an updated overview of this pathogen from the perspective of molecular epidemiology, clinical diagnosis and typing, virulence mechanism, and protective antigens contributing to its zoonosis.

The β-galactosidase (BgaC) of the zoonotic pathogen Streptococcus suis is a surface protein without the involvement of bacterial virulence

Streptococcal pathogens have evolved to express exoglycosidases, one of which is BgaC β-galactosidase, to deglycosidate host surface glycolconjucates with exposure of the polysaccharide receptor for bacterial adherence. The paradigm BgaC protein is the bgaC product of Streptococcus, a bacterial surface-exposed β-galactosidase. Here we report the functional definition of the BgaC homologue from an epidemic Chinese strain 05ZYH33 of the zoonotic pathogen Streptococcus suis. Bioinformatics analyses revealed that S. suis BgaC shared the conserved active sites (W240, W243 and Y454). The recombinant BgaC protein of S. suis was purified to homogeneity. Enzymatic assays confirmed its activity of β-galactosidase. Also, the hydrolysis activity was found to be region-specific and sugar-specific for the Gal β-1,3-GlcNAc moiety of oligosaccharides. Flow cytometry analyses combined with immune electron microscopy demonstrated that S. suis BgaC is an atypical surface-anchored protein in that it lacks the “LPXTG” motif for typical surface proteins. Integrative evidence from cell lines and mice-based experiments showed that an inactivation of bgaC does not significantly impair the ability of neither adherence nor anti-phagocytosis, and consequently failed to attenuate bacterial virulence, which is somewhat similar to the scenario seen with S. pneumoniae. Therefore we concluded that S. suis BgaC is an atypical surface-exposed protein without the involvement of bacterial virulence.

Structural studies of Streptococcus pyogenes streptolysin O provide insights into the early steps of membrane penetration

Cholesterol-dependent cytolysins (CDCs) are a large family of bacterial toxins that exhibit a dependence on the presence of membrane cholesterol in forming large pores in cell membranes. Significant changes in the three-dimensional structure of these toxins are necessary to convert the soluble monomeric protein into a membrane pore. We have determined the crystal structure of the archetypical member of the CDC family, streptolysin O (SLO), a virulence factor from Streptococcus pyogenes. The overall fold is similar to previously reported CDC structures, although the C-terminal domain is in a different orientation with respect to the rest of the molecule. Surprisingly, a signature stretch of CDC sequence called the undecapeptide motif, a key region involved in membrane recognition, adopts a very different structure in SLO to that of the well-characterized CDC perfringolysin O (PFO), although the sequences in this region are identical. An analysis reveals that, in PFO, there are complementary interactions between the motif and the rest of domain 4 that are lost in SLO. Molecular dynamics simulations suggest that the loss of a salt bridge in SLO and a cation-pi interaction are determining factors in the extended conformation of the motif, which in turn appears to result in a greater flexibility of the neighboring L1 loop that houses a cholesterol-sensing motif. These differences may explain the differing abilities of SLO and PFO to efficiently penetrate target cell membranes in the first step of toxin insertion into the membrane.

Prediction of B-Cell Epitopes in Listeriolysin O, a Cholesterol Dependent Cytolysin Secreted by Listeria monocytogenes

Listeria monocytogenes is a gram-positive, foodborne bacterium responsible for disease in humans and animals. Listeriolysin O (LLO) is a required virulence factor for the pathogenic effects of L. monocytogenes. Bioinformatics revealed conserved putative epitopes of LLO that could be used to develop monoclonal antibodies against LLO. Continuous and discontinuous epitopes were located by using four different B-cell prediction algorithms. Three-dimensional molecular models were generated to more precisely characterize the predicted antigenicity of LLO. Domain 4 was predicted to contain five of eleven continuous epitopes. A large portion of domain 4 was also predicted to comprise discontinuous immunogenic epitopes. Domain 4 of LLO may serve as an immunogen for eliciting monoclonal antibodies that can be used to study the pathogenesis of L. monocytogenes as well as develop an inexpensive assay.

Perfringolysin O structure and mechanism of pore formation as a paradigm for cholesterol-dependent cytolysins

Cholesterol-dependent cytolysins (CDCs) constitute a family of pore forming toxins secreted by Gram-positive bacteria. These toxins form transmembrane pores by inserting a large β-barrel into cholesterol-containing membrane bilayers. Binding of water-soluble CDCs to the membrane triggers the formation of oligomers containing 35-50 monomers. The coordinated insertion of more than seventy β-hairpins into the membrane requires multiple structural conformational changes. Perfringolysin O (PFO), secreted by Clostridium perfringens, has become the prototype for the CDCs. In this chapter, we will describe current knowledge on the mechanism of PFO cytolysis, with special focus on cholesterol recognition, oligomerization, and the conformational changes involved in pore formation.