Interaction of theta-toxin (perfringolysin O), a cholesterol-binding cytolysin, with liposomal membranes: change in the aromatic side chains upon binding and insertion

Bacillus cereus cereolysin O induces pyroptosis in an undecapeptide-dependent manner

Bacillus cereus is a clinically significant foodborne pathogen that causes severe gastrointestinal and non-gastrointestinal disease. Cereolysin O (CLO) is a putative virulence factor of B. cereus, and its function remains to be investigated. In this study, we examined the biological activity of CLO from a deep sea B. cereus isolate. CLO was highly toxic to mammalian cells and triggered pyroptosis through NLRP3 inflammasome-mediated caspase 1 and gasdermin D activation. CLO-induced cell death involved ROS accumulation and K+ efflux, and was blocked by serum lipids. CLO bound specifically to cholesterol, and this binding was essential to CLO cytotoxicity. The structural integrity of the three tryptophan residues in the C-terminal undecapeptide was vital for CLO to interact with membrane lipids and cause membrane perforation. Taken together, these results provided new insights into the molecular mechanism of B. cereus CLO-mediated cytotoxicity.

Refinement of Singer-Nicolson fluid-mosaic model by microscopy imaging: Lipid rafts and actin-induced membrane compartmentalization

This year celebrates the 50th anniversary of the Singer-Nicolson fluid mosaic model for biological membranes. The next level of sophistication we have achieved for understanding plasma membrane (PM) structures, dynamics, and functions during these 50 years includes the PM interactions with cortical actin filaments and the partial demixing of membrane constituent molecules in the PM, particularly raft domains. Here, first, we summarize our current knowledge of these two structures and emphasize that they are interrelated. Second, we review the structure, molecular dynamics, and function of raft domains, with main focuses on raftophilic glycosylphosphatidylinositol-anchored proteins (GPI-APs) and their signal transduction mechanisms. We pay special attention to the results obtained by single-molecule imaging techniques and other advanced microscopy methods. We also clarify the limitations of present optical microscopy methods for visualizing raft domains, but emphasize that single-molecule imaging techniques can "detect" raft domains associated with molecules of interest in the PM.

Membrane stabilization versus perturbation by aromatic monoamine-modified γ-PGA for cryopreservation of human RBCs with high intracellular trehalose

As a nonreducing disaccharide, trehalose can be used as a biocompatible cryoprotectant for solvent-free cell cryopreservation, but the membrane-impermeability limits its cryoprotective efficiency. Herein, a series of aromatic monoamines with a 1-4 methylene spacer were grafted onto γ-poly(glutamic acid) (γ-PGA) for promoting intracellular trehalose uptake in human red blood cells (hRBCs) via membrane perturbation. The self-assembled nanoparticles of the obtained amphiphilic γ-PGA could be adsorbed on the cell membrane by the hydrophobic interaction to disturb the lipid arrangement and increase the membrane permeability of trehalose under hypertonic conditions. Results suggested that the intracellular trehalose could be enhanced progressively with the methylene spacer length, significantly increasing to 75.1 ± 0.7 mM by incubating hRBCs in 0.8 M trehalose containing phenylbutylamine-grafted γ-PGA at 4 °C for 24 h. Meanwhile, the other three polymers exhibited membrane stabilization in addition to improved intracellular trehalose, maintaining the membrane integrity during cryopreservation to achieve high cryosurvival. Molecular dynamics simulation further confirmed that defects could be formed by interaction of the above four amphiphilic polymers on the modeled phospholipid bilayer. It was believed that glycerol-free cryopreservation of human cells could be realized by using trehalose as the biocompatible cryoprotectant, and membrane stabilization can be a compensatory approach to membrane perturbation during impermeable biomolecule delivery.

Structural basis of Toxoplasma gondii perforin-like protein 1 membrane interaction and activity during egress

Intracellular pathogens must egress from the host cell to continue their infectious cycle. Apicomplexans are a phylum of intracellular protozoans that have evolved members of the membrane attack complex and perforin (MACPF) family of pore forming proteins to disrupt cellular membranes for traversing cells during tissue migration or egress from a replicative vacuole following intracellular reproduction. Previous work showed that the apicomplexan Toxoplasma gondii secretes a perforin-like protein (TgPLP1) that contains a C-terminal Domain (CTD) which is necessary for efficient parasite egress. However, the structural basis for CTD membrane binding and egress competency remained unknown. Here, we present evidence that TgPLP1 CTD prefers binding lipids that are abundant in the inner leaflet of the lipid bilayer. Additionally, solving the high-resolution crystal structure of the TgPLP1 APCβ domain within the CTD reveals an unusual double-layered β-prism fold that resembles only one other protein of known structure. Three direct repeat sequences comprise subdomains, with each constituting a wall of the β-prism fold. One subdomain features a protruding hydrophobic loop with an exposed tryptophan at its tip. Spectrophotometric measurements of intrinsic tryptophan fluorescence are consistent with insertion of the hydrophobic loop into a target membrane. Using CRISPR/Cas9 gene editing we show that parasite strains bearing mutations in the hydrophobic loop, including alanine substitution of the tip tryptophan, are equally deficient in egress as a strain lacking TgPLP1 altogether. Taken together our findings suggest a crucial role for the hydrophobic loop in anchoring TgPLP1 to the membrane to support its cytolytic activity and egress function.

The intracellular parasite Toxoplasma gondii infects many hosts including humans. Infected people with a weak immune system can suffer severe disease when the parasite replicates uncontrolled via repeated cycles of cell invasion, intracellular growth, and exit, resulting in cell death. Previous studies showed that T. gondii encodes a pore-forming protein, TgPLP1, which contains an unusual domain that is crucial for efficient exit from both the parasite containing vacuole and the host cell. However, how TgPLP1 recognizes and binds to the appropriate membrane is unclear. Here we use a combination of biochemistry, structural biology, and parasitology to identify a preference of TgPLP1 for specific lipids and show that a loop within the structure of the membrane-binding domain inserts into the target membrane and is necessary for exit from the parasite containing vacuole. Our study sheds light into the determinants of membrane binding in TgPLP1 and may inform the overall mechanism of pore formation in similar systems.

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.

Pathways for creation and annihilation of nanoscale biomembrane domains reveal alpha and beta-toxin nanopore formation processes

Raft-like functional domains with putative sizes of 20–200 nm and which are evolving dynamically are believed to be the most crucial regions in cellular membranes which determine cell signaling and various functions of cells.

R468A mutation in perfringolysin O destabilizes toxin structure and induces membrane fusion

Perfringolysin O (PFO) belongs to the family of cholesterol-dependent cytolysins. Upon binding to a cholesterol-containing membrane, PFO undergoes a series of structural changes that result in the formation of a β-barrel pore and cell lysis. Recognition and binding to cholesterol are mediated by the D4 domain, one of four domains of PFO. The D4 domain contains a conserved tryptophan-rich loop named undecapeptide (E₄₅₈CTGLAWEWWR₄₆₈) in which arginine 468 is essential for retaining allosteric coupling between D4 and other domains during interaction of PFO with the membrane. In this report we studied the impact of R468A mutation on the whole protein structure using hydrogen-deuterium exchange coupled with mass spectrometry. We found that in aqueous solution, compared to wild type (PFO), PFO^R468A showed increased deuterium uptake due to exposure of internal toxin regions to the solvent. This change reflected an overall structural destabilization of PFO^R468A in solution. Conversely, upon binding to cholesterol-containing membranes, PFO^R468A revealed a profound decrease of hydrogen-deuterium exchange when compared to PFO. This block of deuterium uptake resulted from PFO^R468A-induced aggregation and fusion of liposomes, as found by dynamic light scattering, microscopic observations and FRET measurements. In the result of liposome aggregation and fusion, the entire PFO^R468A molecule became shielded from aqueous solution and thereby was protected against proteolytic digestion and deuteration. We have established that structural changes induced by the R468A mutation lead to exposure of an additional cholesterol-independent liposome-binding site in PFO that confers its fusogenic property, altering the mode of the toxin action.

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.

Perfringolysin O: The Underrated Clostridium perfringens Toxin?

The anaerobic bacterium Clostridium perfringens expresses multiple toxins that promote disease development in both humans and animals. One such toxin is perfringolysin O (PFO, classically referred to as θ toxin), a pore-forming cholesterol-dependent cytolysin (CDC). PFO is secreted as a water-soluble monomer that recognizes and binds membranes via cholesterol. Membrane-bound monomers undergo structural changes that culminate in the formation of an oligomerized prepore complex on the membrane surface. The prepore then undergoes conversion into the bilayer-spanning pore measuring approximately 250–300 Å in diameter. PFO is expressed in nearly all identified C. perfringens strains and harbors interesting traits that suggest a potential undefined role for PFO in disease development. Research has demonstrated a role for PFO in gas gangrene progression and bovine necrohemorrhagic enteritis, but there is limited data available to determine if PFO also functions in additional disease presentations caused by C. perfringens. This review summarizes the known structural and functional characteristics of PFO, while highlighting recent insights into the potential contributions of PFO to disease pathogenesis.

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.

Crucial role of perfringolysin O D1 domain in orchestrating structural transitions leading to membrane-perforating pores: a hydrogen-deuterium exchange study

Perfringolysin O (PFO) is a toxic protein that binds to cholesterol-containing membranes, oligomerizes, and forms a β-barrel transmembrane pore, leading to cell lysis. Previous studies have uncovered the sequence of events in this multistage structural transition to a considerable detail, but the underlying molecular mechanisms are not yet fully understood. By measuring hydrogen-deuterium exchange patterns of peptide bond amide protons monitored by mass spectrometry (MS), we have mapped structural changes in PFO and its variant bearing a point mutation during incorporation to the lipid environment. We have defined all regions that undergo structural changes caused by the interaction with the lipid environment both in wild-type PFO, thus providing new experimental constraints for molecular modeling of the pore formation process, and in a point mutant, W165T, for which the pore formation process is known to be inefficient. We have demonstrated that point mutation W165T causes destabilization of protein solution structure, strongest for domain D1, which interrupts the pathway of structural transitions in other domains necessary for proper oligomerization in the membrane. In PFO, the strongest changes accompanying binding to the membrane focus in D1; the C-terminal part of D4; and strands β1, β4, and β5 of D3. These changes were much weaker for PFO(W165T) lipo where substantial stabilization was observed only in D4 domain. In this study, the application of hydrogen-deuterium exchange analysis monitored by MS provided new insight into conformational changes of PFO associated with the membrane binding, oligomerization, and lytic pore formation.

Transmembrane protein (perfringolysin o) association with ordered membrane domains (rafts) depends upon the raft-associating properties of protein-bound sterol

Because transmembrane (TM) protein localization, or nonlocalization, in ordered membrane domains (rafts) is a key to understanding membrane domain function, it is important to define the origin of protein-raft interaction. One hypothesis is that a tight noncovalent attachment of TM proteins to lipids that have a strong affinity for ordered domains can be sufficient to induce raft-protein interaction. The sterol-binding protein perfringolysin O (PFO) was used to test this hypothesis. PFO binds both to sterols that tend to localize in ordered domains (e.g., cholesterol), and to those that do not (e.g., coprostanol), but it does not bind to epicholesterol, a raft-promoting 3α-OH sterol. Using a fluorescence resonance energy transfer assay in model membrane vesicles containing coexisting ordered and disordered lipid domains, both TM and non-TM forms of PFO were found to concentrate in ordered domains in vesicles containing high and low-Tm lipids plus cholesterol or 1:1 (mol/mol) cholesterol/epicholesterol, whereas they concentrate in disordered domains in vesicles containing high-Tm and low-Tm lipids plus 1:1 (mol/mol) coprostanol/epicholesterol. Combined with previous studies this behavior indicates that TM protein association with ordered domains is dependent upon both the association of the protein-bound sterol with ordered domains and hydrophobic match between TM segments and rafts.

The influence of natural lipid asymmetry upon the conformation of a membrane-inserted protein (perfringolysin O)

Eukaryotic membrane proteins generally reside in membrane bilayers that have lipid asymmetry. However, in vitro studies of the impact of lipids upon membrane proteins are generally carried out in model membrane vesicles that lack lipid asymmetry. Our recently developed method to prepare lipid vesicles with asymmetry similar to that in plasma membranes and with controlled amounts of cholesterol was used to investigate the influence of lipid composition and lipid asymmetry upon the conformational behavior of the pore-forming, cholesterol-dependent cytolysin perfringolysin O (PFO). PFO conformational behavior in asymmetric vesicles was found to be distinct both from that in symmetric vesicles with the same lipid composition as the asymmetric vesicles and from that in vesicles containing either only the inner leaflet lipids from the asymmetric vesicles or only the outer leaflet lipids from the asymmetric vesicles. The presence of phosphatidylcholine in the outer leaflet increased the cholesterol concentration required to induce PFO binding, whereas phosphatidylethanolamine and phosphatidylserine in the inner leaflet of asymmetric vesicles stabilized the formation of a novel deeply inserted conformation that does not form pores, even though it contains transmembrane segments. This conformation may represent an important intermediate stage in PFO pore formation. These studies show that lipid asymmetry can strongly influence the behavior of membrane-inserted proteins.

The cholesterol-dependent cytolysin signature motif: a critical element in the allosteric pathway that couples membrane binding to pore assembly

The cholesterol-dependent cytolysins (CDCs) constitute a family of pore-forming toxins that contribute to the pathogenesis of a large number of Gram-positive bacterial pathogens.The most highly conserved region in the primary structure of the CDCs is the signature undecapeptide sequence (ECTGLAWEWWR). The CDC pore forming mechanism is highly sensitive to changes in its structure, yet its contribution to the molecular mechanism of the CDCs has remained enigmatic. Using a combination of fluorescence spectroscopic methods we provide evidence that shows the undecapeptide motif of the archetype CDC, perfringolysin O (PFO), is a key structural element in the allosteric coupling of the cholesterol-mediated membrane binding in domain 4 (D4) to distal structural changes in domain 3 (D3) that are required for the formation of the oligomeric pore complex. Loss of the undecapeptide function prevents all measurable D3 structural transitions, the intermolecular interaction of membrane bound monomers and the assembly of the oligomeric pore complex. We further show that this pathway does not exist in intermedilysin (ILY), a CDC that exhibits a divergent undecapeptide and that has evolved to use human CD59 rather than cholesterol as its receptor. These studies show for the first time that the undecapeptide of the cholesterol-binding CDCs forms a critical element of the allosteric pathway that controls the assembly of the pore complex.

The CDCs are a large family of pathogenesis-associated pore-forming toxins that are expressed by many Gram-positive pathogens. The conserved undecapeptide motif of the CDCs has been regarded as the signature peptide sequence for these toxins, yet its function has remained obscure. The studies herein show that the undecapeptide forms a critical structural element in the allosteric pathway that couples membrane binding to cholesterol to the initiation of distal structural changes, which are required for the assembly of the pore forming complex. These studies provide the first insight into the function of this highly conserved sequence and show that through evolution this pathway is missing in the CD59-binding CDCs.

Monomer-monomer interactions propagate structural transitions necessary for pore formation by the cholesterol-dependent cytolysins

The assembly of the cholesterol-dependent cytolysin (CDC) oligomeric pore complex requires a complex choreography of secondary and tertiary structural changes in domain 3 (D3) of the CDC monomer structure. A point mutation was identified in the archetype CDC, perfringolysin O, that blocks detectable D3 structural changes and traps the membrane-bound monomers in an early and reversible stage of oligomer assembly. Using this and other mutants we show that specific D3 structural changes are propagated from one membrane-bound monomer to another. Propagation of these structural changes results in the exposure of a β-strand in D3 that allows it to pair and form edge-on interactions with a second β-strand of a free membrane-bound monomer. Pairing of these strands establishes the final geometry of the pore complex and is necessary to drive the formation of the β-barrel pore. These studies provide new insights into how structural information is propagated between membrane-bound monomers of a self-assembling system and the interactions that establish the geometry of the final pore complex.

Membrane assembly of the cholesterol-dependent cytolysin pore complex

The cholesterol-dependent cytolysins (CDCs) are a large family of pore-forming toxins that are produced, secreted and contribute to the pathogenesis of many species of Gram-positive bacteria. The assembly of the CDC pore-forming complex has been under intense study for the past 20 years. These studies have revealed a molecular mechanism of pore formation that exhibits many novel features. The CDCs form large β-barrel pore complexes that are assembled from 35 to 40 soluble CDC monomers. Pore formation is dependent on the presence of membrane cholesterol, which functions as the receptor for most CDCs. Cholesterol binding initiates significant secondary and tertiary structural changes in the monomers, which lead to the assembly of a large membrane embedded β-barrel pore complex. This review will focus on the molecular mechanism of assembly of the CDC membrane pore complex and how these studies have led to insights into the mechanism of pore formation for other pore-forming proteins. This article is part of a Special Issue entitled: Protein Folding in Membranes.

Characterization of recombinant Streptococcus mitis-derived human platelet aggregation factor

We previously purified Streptococcus mitis-derived human platelet aggregation factor (Sm-hPAF) from the culture supernatant of S. mitis strain Nm-65, isolated from the tooth surface of a patient with Kawasaki disease. Here we produced recombinant Sm-hPAF protein (rSm-hPAF) in Escherichia coli, to determine whether rSm-hPAF conserves its platelet aggregation activity. rSm-hPAF precursor (665 amino acids) shows up to 36-56% identity with the family of cholesterol-dependent cytolysins (CDCs), and rSm-hPAF displayed potent hemolytic activity toward mammalian erythrocytes, including human erythrocytes with platelet aggregation activity. The 162-amino acid amino-terminal domain of rSm-hPAF was found in no other CDCs except lectinolysin; this domain is homologous to a portion of pneumococcal fucolectin-related protein. Interestingly, suilysin (SLY) and pneumolysin (PLY) of CDCs also exhibit substantial human platelet aggregation activity, similar to rSm-hPAF, and the platelet aggregation by rSm-hPAF, SLY, and PLY was morphologically confirmed using light and electron microscopy.

Perfringolysin O association with ordered lipid domains: implications for transmembrane protein raft affinity

Upon interaction with cholesterol, perfringolysin O (PFO) inserts into membranes and forms a rigid transmembrane (TM) β-barrel. PFO is believed to interact with liquid ordered lipid domains (lipid rafts). Because the origin of TM protein affinity for rafts is poorly understood, we investigated PFO raft affinity in vesicles having coexisting ordered and disordered lipid domains. Fluorescence resonance energy transfer (FRET) from PFO Trp to domain-localized acceptors indicated that PFO generally has a raft affinity between that of LW peptide (low raft affinity) and cholera toxin B (high raft affinity) in vesicles containing ordered domains rich in brain sphingomyelin or distearoylphosphatidylcholine. FRET also showed that ceramide, which increases exposure of cholesterol to water and thus displaces it from rafts, does not displace PFO from ordered domains. This can be explained by shielding of PFO-bound cholesterol from water. Finally, FRET showed that PFO affinity for ordered domains was higher in its non-TM (prepore) form than in its TM form, demonstrating that the TM portion of PFO interacts unfavorably with rafts. Microscopy studies in giant unilamellar vesicles confirmed that PFO exhibits intermediate raft affinity, and showed that TM PFO (but not non-TM PFO) concentrated at the edges of liquid ordered domains. These studies suggest that a combination of binding to raft-associating molecules and having a rigid TM structure that is unable to pack well in a highly ordered lipid environment can control TM protein domain localization. To accommodate these constraints, raft-associated TM proteins in cells may tend to locate within liquid disordered shells encapsulated within ordered domains.

Accessibility of cholesterol in endoplasmic reticulum membranes and activation of SREBP-2 switch abruptly at a common cholesterol threshold

Recent studies have shown that cooperative interactions in endoplasmic reticulum (ER) membranes between Scap, cholesterol, and Insig result in switch-like control over activation of SREBP-2 transcription factors. This allows cells to rapidly adjust rates of cholesterol synthesis and uptake in response to even slight deviations from physiological set-point levels, thereby ensuring cholesterol homeostasis. In the present study we directly probe for the accessibility of cholesterol in purified ER membranes. Using a soluble cholesterol-binding bacterial toxin, perfringolysin O, we show that cholesterol accessibility increases abruptly at ∼5 mol % ER cholesterol, the same concentration at which SREBP-2 activation is halted. This switch-like change in cholesterol accessibility is observed not only in purified ER membranes but also in liposomes made from ER lipid extracts. The accessibility of cholesterol in membranes is related to its chemical activity. Complex formation between cholesterol and some ER phospholipids can result in sharp changes in cholesterol chemical activity and its accessibility to perfringolysin O or membrane sensors like Scap. The control of the availability of the cholesterol ligand to participate in cooperative Scap/cholesterol/Insig interactions further sharpens the sensitive switch that exerts precise control over cholesterol levels in cell membranes.

The cholesterol-dependent cytolysin family of gram-positive bacterial toxins

The cholesterol-dependent cytolysins (CDCs) are a family of beta-barrel pore-forming toxins secreted by Gram-positive bacteria. These toxins are produced as water-soluble monomeric proteins that after binding to the target cell oligomerize on the membrane surface forming a ring-like pre-pore complex, and finally insert a large beta-barrel into the membrane (about 250 A in diameter). Formation of such a large transmembrane structure requires multiple and coordinated conformational changes. The presence of cholesterol in the target membrane is absolutely required for pore-formation, and therefore it was long thought that cholesterol was the cellular receptor for these toxins. However, not all the CDCs require cholesterol for binding. Intermedilysin, secreted by Streptoccocus intermedius only binds to membranes containing a protein receptor, but forms pores only if the membrane contains sufficient cholesterol. In contrast, perfringolysin O, secreted by Clostridium perfringens, only binds to membranes containing substantial amounts of cholesterol. The mechanisms by which cholesterol regulates the cytolytic activity of the CDCs are not understood at the molecular level. The C-terminus of perfringolysin O is involved in cholesterol recognition, and changes in the conformation of the loops located at the distal tip of this domain affect the toxin-membrane interactions. At the same time, the distribution of cholesterol in the membrane can modulate toxin binding. Recent studies support the concept that there is a dynamic interplay between the cholesterol-binding domain of the CDCs and the excess of cholesterol molecules in the target membrane.

Cholesterol-binding toxins and anti-cholesterol antibodies as structural probes for cholesterol localization

Cholesterol is one of the major constituents of mammalian cell membranes. It plays an indispensable role in regulating the structure and function of cell membranes and affects the pathology of various diseases. In recent decades much attention has been paid to the existence of membrane microdomains, generally termed lipid "rafts", and cholesterol, along with sphingolipids, is thought to play a critical role in raft structural organization and function. Cholesterol-binding probes are likely to provide useful tools for analyzing the distribution and dynamics of membrane cholesterol, as a structural element of raft microdomains, and elsewhere within the cell. Among the probes, non-toxic derivatives of perfringolysin O, a cholesterol-binding cytolysin, bind cholesterol in a concentration-dependent fashion with a strict threshold. They selectively recognize cholesterol in cholesterol-enriched membranes, and have been used in many studies to detect microdomains in plasma and intracellular membranes. Anti-cholesterol antibodies that recognize cholesterol in domain structures have been developed in recent years. In this chapter, we describe the characteristics of these cholesterol-binding proteins and their applications to studies on membrane cholesterol localization.

Pore formation: an ancient yet complex form of attack

Bacteria, as well as higher organisms such as sea anemones or earthworms, have developed sophisticated virulence factors such as the pore-forming toxins (PFTs) to mount their attack against the host. One of the most fascinating aspects of PFTs is that they can adopt a water-soluble form at the beginning of their lifetime and become an integral transmembrane protein in the membrane of the target cells. There is a growing understanding of the sequence of events and the various conformational changes undergone by these toxins in order to bind to the host cell surface, to penetrate the cell membranes and to achieve pore formation. These points will be addressed in this review.

Structural elements of the cholesterol-dependent cytolysins that are responsible for their cholesterol-sensitive membrane interactions

The pore-forming mechanism of the cholesterol-dependent cytolysins (CDCs) exhibits an absolute requirement for membrane cholesterol. The structural elements of the CDCs that mediate this interaction are not well understood. Three short hydrophobic loops (L1-L3) and a highly conserved undecapeptide sequence at the tip of domain 4 of the CDC structure are known to anchor the CDC to the membrane. It has been thought that the undecapeptide directly mediates the interaction of the CDCs with a cholesterol-rich cell surface. Herein we show that the L1-L3 loops, not the undecapeptide, are responsible for mediating the specific interaction of the CDCs with cholesterol-rich membranes. The membrane insertion of the undecapeptide was uncoupled from membrane binding by the covalent modification of the undecapeptide cysteine thiol. Modification of the cysteine prevented prepore to pore conversion, but did not affect membrane binding, thus demonstrating that undecapeptide membrane insertion follows that of the L1-L3 loops. These studies provide an example of a structural motif that specifically mediates the interaction of a bacterial toxin with a cholesterol-rich membrane.

Perfringolysin O, a cholesterol-binding cytolysin, as a probe for lipid rafts

Gaining an understanding of the structural and functional roles of cholesterol in membrane lipid rafts is a critical issue in studies on cellular signaling and because of the possible involvement of lipid rafts in various diseases. We have focused on the potential of perfringolysin O (theta-toxin), a cholesterol-binding cytolysin produced by Clostridium perfringens, as a probe for studies on membrane cholesterol. We prepared a protease-nicked and biotinylated derivative of perfringolysin O (BCtheta) that binds selectively to cholesterol in cholesterol-rich microdomains of cell membranes without causing membrane lesions. Since the domains fulfill the criteria of lipid rafts, BCtheta can be used to detect cholesterol-rich lipid rafts. This is in marked contrast to filipin, another cholesterol-binding reagent, which binds indiscriminately to cell cholesterol. Using BCtheta, we are now searching for molecules that localize specifically in cholesterol-rich lipid rafts. Recently, we demonstrated that the C-terminal domain of perfringolysin O, domain 4 (D4), possesses the same binding characteristics as BCtheta. BIAcore analysis showed that D4 binds specifically to cholesterol with the same binding affinity as the full-size toxin. Cell-bound D4 is recovered predominantly from detergent-insoluble, low-density membrane fractions where raft markers, such as cholesterol, flotillin and Src family kinases, are enriched, indicating that D4 also binds selectively to lipid rafts. Furthermore, a green fluorescent protein-D4 fusion protein (GFP-D4) was revealed to be useful for real-time monitoring of cholesterol in lipid rafts in the plasma membrane. In addition, the expression of GFP-D4 in the cytoplasm might allow the investigations of intracellular trafficking of lipid rafts. The simultaneous visualization of lipid rafts in plasma membranes and inside cells might help in gaining a total understanding of the dynamic behavior of lipid rafts.

Conformational changes that effect oligomerization and initiate pore formation are triggered throughout perfringolysin O upon binding to cholesterol

Pore formation by the cholesterol-dependent cytolysins (CDCs) requires the presence of cholesterol in the target membrane. Cholesterol was long thought to be the cellular receptor for these toxins, but not all CDCs require cholesterol for binding. Intermedilysin, secreted by Streptococcus intermedius, only binds to membranes containing the human protein CD59 but forms pores only if the membrane contains sufficient cholesterol. In contrast, perfringolysin O (PFO), secreted by Clostridium perfringens, only binds to membranes containing substantial amounts of cholesterol. Given that different steps in the assembly of various CDC pores require cholesterol, here we have analyzed to what extent cholesterol molecules, by themselves, can modulate the conformational changes associated with PFO oligomerization and pore formation. PFO binds to cholesterol when dispersed in aqueous solution, and this binding triggers the distant rearrangement of a beta-strand that exposes an oligomerization interface. Moreover, upon binding to cholesterol, PFO forms a prepore complex, unfolds two amphipathic transmembrane beta-hairpins, and positions their nonpolar surfaces so they associate with the hydrophobic cholesterol surface. The interaction of PFO with cholesterol is therefore sufficient to initiate an irreversible sequence of coupled conformational changes that extend throughout the toxin molecule.

Structures of perfringolysin O suggest a pathway for activation of cholesterol-dependent cytolysins

Cholesterol-dependent cytolysins (CDCs), a large family of bacterial toxins, are secreted as water-soluble monomers and yet are capable of generating oligomeric pores in membranes. Previous work has demonstrated that large scale structural rearrangements occur during this transition but the detailed mechanism by which these changes take place remains a puzzle. Despite evidence of structural and functional couplings between domains 3 and 4, the crystal structure of the CDC, perfringolysin O (PFO), shows the two domains do not make direct contact. Here, we present crystal structures of PFO that demonstrate movements of domain 4 are sufficient to trigger conformational changes that are transmitted through the molecule to the distant domain 3. These coupled movements result in a loss of many contacts between domain 3 and rest of the molecule that would eventually lead to the exposure of transmembrane regions in preparation for membrane insertion. The structures reveal a detailed molecular pathway that may be the basis for the allosteric transition that occurs on initial membrane binding leading to the exposure of membrane-spanning regions in a domain distant from the initial site of interaction.

Specific protein-membrane contacts are required for prepore and pore assembly by a cholesterol-dependent cytolysin

Three short hydrophobic loops and a conserved undecapeptide at the tip of domain 4 (D4) of the cholesterol-dependent cytolysins (CDCs) mediate the binding of the CDC monomers to cholesterol-rich cell membranes. But intermedilysin (ILY), from Streptococcus intermedius, does not bind to cholesterol-rich membranes unless they contain the human protein CD59. This observation suggested that the D4 loops, which include loops L1-L3 and the undecapeptide, of ILY were no longer required for its cell binding. However, we show here that membrane insertion of the D4 loops is required for the cytolysis by ILY. Receptor binding triggers changes in the structure of ILY that are necessary for oligomerization, but membrane insertion of the D4 loops is critical for oligomer assembly and pore formation. Defects that prevent membrane insertion of the undecapeptide also block assembly of the prepore oligomer, while defects in the membrane insertion of the L1-L3 loops prevent the conversion of the prepore oligomer to the pore complex. These studies reveal that pore formation by ILY, and probably other CDCs, is affected by an intricate and coupled sequence of interactions between domain 4 and the membrane.

Structural basis of pore formation by the bacterial toxin pneumolysin

The bacterial toxin pneumolysin is released as a soluble monomer that kills target cells by assembling into large oligomeric rings and forming pores in cholesterol-containing membranes. Using cryo-EM and image processing, we have determined the structures of membrane-surface bound (prepore) and inserted-pore oligomer forms, providing a direct observation of the conformational transition into the pore form of a cholesterol-dependent cytolysin. In the pore structure, the domains of the monomer separate and double over into an arch, forming a wall sealing the bilayer around the pore. This transformation is accomplished by substantial refolding of two of the four protein domains along with deformation of the membrane. Extension of protein density into the bilayer supports earlier predictions that the protein inserts beta hairpins into the membrane. With an oligomer size of up to 44 subunits in the pore, this assembly creates a transmembrane channel 260 A in diameter lined by 176 beta strands.

Insights into the action of the superfamily of cholesterol-dependent cytolysins from studies of intermedilysin

The cholesterol-dependent cytolysins (CDCs), a superfamily of pore-forming toxins, are characterized by a conserved undecapeptide motif that is believed to be critical for membrane recognition by means of cholesterol. Intermedilysin (ILY), an unusual member of the CDCs, exhibits specificity for human cells and contains nonconservative substitutions in the motif. We show that the cellular specificity of ILY is based on its ability to specifically bind to human cells and does not involve some other feature of the CDC mechanism. Furthermore, cellular recognition by ILY appears to be encoded in domain 4 alone but does not involve the variant undecapeptide of ILY. We show that the undecapeptide is involved in the prepore-to-pore conversion of ILY and so demonstrate a direct connection between the structure of the undecapeptide and the prepore-to-pore transition. We have determined the crystal structure of ILY, which, when compared to the known structure of a prototypical CDC, suggests that the basic aspects of its 3D structure are likely to be conserved in all CDCs.

Axon-dominant localization of cell-surface cholesterol in cultured hippocampal neurons and its disappearance in Niemann-Pick type C model cells

There is growing evidence showing the important role of cholesterol in maintaining neuronal function. In particular, much attention has been paid to the role of the cholesterol-rich microdomains called lipid rafts. However, the cholesterol distribution on neurons is not clear. Here, we investigated localization of cholesterol in cultured rat hippocampal neurons, using filipin and a novel cholesterol-binding reagent BCtheta. In our culture system, BCtheta detects only cell-surface cholesterol, whereas filipin stains both intracellular and cell-surface cholesterol. BCtheta staining appeared visible in a maturation-dependent manner and showed axon-dominant distribution of cell-surface cholesterol in fully matured neurons. A part of this cholesterol on axons was resistant to detergents at 4 degrees C, and thus might be involved in lipid rafts. Interestingly, Niemann-Pick type C model neurons induced by class 2 amphiphiles lost the cell-surface but not the intracellular cholesterol staining. Niemann-Pick type C disease is caused by the disruption of intracellular cholesterol transport and is known to induce neurodegeneration in brains accompanied by formation of neurofibrillary tangles. Our observations suggest the important role of cell-surface cholesterol in maintaining a functional axonal membrane and indicate that the observed defect in axonal surface cholesterol might lead to neurodegeneration.

The role of cholesterol in the activity of pneumolysin, a bacterial protein toxin

The mechanism via which pneumolysin (PLY), a toxin and major virulence factor of the bacterium Streptococcus pneumoniae, binds to its putative receptor, cholesterol, is still poorly understood. We present results from a series of biophysical studies that shed light on the interaction of PLY with cholesterol in solution and in lipid bilayers. PLY lyses cells whose walls contain cholesterol. Using standard hemolytic assays we have demonstrated that the hemolytic activity of PLY is inhibited by cholesterol, partially by ergosterol but not by lanosterol and that the functional stoichiometry of the cholesterol-PLY complex is 1:1. Tryptophan (Trp) fluorescence data recorded during PLY-cholesterol titration studies confirm this ratio, reveal a significant blue shift in the Trp fluorescence peak with increasing cholesterol concentrations indicative of increasing nonpolarity in the Trp environment, consistent with cholesterol binding by the tryptophans, and provide a measure of the affinity of cholesterol binding: K(d) = 400 +/- 100 nM. Finally, we have performed specular neutron reflectivity studies to observe the effect of PLY upon lipid bilayer structure.

Redefining cholesterol's role in the mechanism of the cholesterol-dependent cytolysins

The cholesterol-dependent cytolysins (CDCs) constitute a large family of pore-forming toxins that function exclusively on cholesterol-containing membranes. A detailed analysis of the various stages in the cytolytic mechanism of three members of the CDC family revealed that significant depletion of cholesterol from the erythrocyte membrane stalls these toxins in the prepore complex. Therefore, the depletion of membrane cholesterol prevents the insertion of the transmembrane beta-barrel and pore formation. These unprecedented findings provide a paradigm for the involvement of cholesterol in the CDC cytolytic mechanism and that of other pore-forming toxins whose activity is enhanced by the presence of membrane cholesterol.

Assembly and topography of the prepore complex in cholesterol-dependent cytolysins

Cholesterol-dependent cytolysins are a family of poreforming proteins that have been shown to be virulence factors for a large number of pathogenic bacteria. The mechanism of pore formation for these toxins involves a complex series of events that are known to include binding, oligomerization, and insertion of a transmembrane beta-barrel. Several features of this mechanism remain poorly understood and controversial. Whereas a prepore mechanism has been proposed for perfringolysin O, a very different mechanism has been proposed for the homologous member of the family, streptolysin O. To distinguish between the two models, a novel approach that directly measures the dimension of transmembranes pores was used. Pore formation itself was examined for both cytolysins by encapsulating fluorescein-labeled peptides and proteins of different sizes into liposomes. When these liposomes were re-suspended in a solution containing anti-fluorescein antibodies, toxin-mediated pore formation was monitored directly by the quenching of fluorescein emission as the encapsulated molecules were released, and the dyes were bound by the antibodies. The analysis of pore formation determined using this approach reveals that only large pores are produced by perfringolysin O and streptolysin O during insertion (and not small pores that grow in size). These results are consistent only with the formation of a prepore complex intermediate prior to insertion of the transmembrane beta-barrel into the bilayer. Fluorescence quenching experiments also revealed that PFO in the prepore complex contacts the membrane via domain 4, and that the individual transmembrane beta-hairpins in domain 3 are not exposed to the nonpolar core of the bilayer at this intermediate stage.

Biotinylated theta-toxin derivative as a probe to examine intracellular cholesterol-rich domains in normal and Niemann-Pick type C1 cells

BCtheta is a proteolytically nicked and biotinylated derivative of a cholesterol binding protein perfringolysin O (theta-toxin), and has been used to detect cholesterol-rich domains at the plasma membrane (PM). Here we show that by modifying the cell fixation condition, BCtheta can also be used to detect cholesterol-rich domains intracellularly. When cells were processed for PM cholesterol staining, the difference in BCtheta signals between the CT43 (CT) cell, a mutant Chinese hamster ovary cell line lacking the Niemann-Pick type C1 (NPC1) protein, and its parental cell 25RA (RA) was minimal. However, when cells were fixed with 4% paraformaldehyde, they became permeable to BCtheta. Under this condition, BCtheta mainly stained cholesterol-rich domains inside the cells, with the signal being much stronger in CT cells than in RA cells. The sensitivity of BCtheta staining was superior to that of filipin staining. The staining of cholesterol-rich domain(s) inside RA cells was sensitive to beta-cyclodextrin treatment, while most of the staining inside CT cells was relatively resistant to cyclodextrin treatment. Clear differences in intracellular BCtheta staining were also seen between the normal and mutant NPC1 fibroblasts of human or mouse origin. Thus, BCtheta is a powerful tool for visually monitoring cholesterol-rich domains inside normal and NPC cells.

The C-terminal domain of perfringolysin O is an essential cholesterol-binding unit targeting to cholesterol-rich microdomains

There is much evidence to indicate that cholesterol forms lateral membrane microdomains (rafts), and to suggest their important role in cellular signaling. However, no probe has been produced to analyze cholesterol behavior, especially cholesterol movement in rafts, in real time. To obtain a potent tool for analyzing cholesterol dynamics in rafts, we prepared and characterized several truncated fragments of theta-toxin (perfringolysin O), a cholesterol-binding cytolysin, whose chemically modified form has been recently shown to bind selectively to rafts. BIAcore and structural analyses demonstrate that the C-terminal domain (domain 4) of the toxin is the smallest functional unit that has the same cholesterol-binding activity as the full-size toxin with structural stability. Cell membrane-bound recombinant domain 4 was detected in the floating low-density fractions and was found to be cofractionated with the raft-associated protein Lck, indicating that recombinant domain 4 also binds selectively to cholesterol-rich rafts. Furthermore, an enhanced green fluorescent protein-domain 4 fusion protein stains membrane surfaces in a cholesterol-dependent manner in living cells. Therefore, domain 4 of theta-toxin is an essential cholesterol-binding unit targeting to cholesterol in membrane rafts, providing a very useful tool for further studies on lipid rafts on cell surfaces and inside cells.

The Listeria monocytogenes hemolysin has an acidic pH optimum to compartmentalize activity and prevent damage to infected host cells

Listeria monocytogenes is a facultative intracellular bacterial pathogen that escapes from a phagosome and grows in the host cell cytosol. The pore-forming cholesterol-dependent cytolysin, listeriolysin O (LLO), mediates bacterial escape from vesicles and is approximately 10-fold more active at an acidic than neutral pH. By swapping dissimilar residues from a pH-insensitive orthologue, perfringolysin O (PFO), we identified leucine 461 as unique to pathogenic Listeria and responsible for the acidic pH optimum of LLO. Conversion of leucine 461 to the threonine present in PFO increased the hemolytic activity of LLO almost 10-fold at a neutral pH. L. monocytogenes synthesizing LLO L461T, expressed from its endogenous site on the bacterial chromosome, resulted in a 100-fold virulence defect in the mouse listeriosis model. These bacteria escaped from acidic phagosomes and initially grew normally in cells and spread cell to cell, but prematurely permeabilized the host membrane and killed the cell. These data show that the acidic pH optimum of LLO results from an adaptive mutation that acts to limit cytolytic activity to acidic vesicles and prevent damage in the host cytosol, a strategy also used by host cells to compartmentalize lysosomal hydrolases.

The family of thiol-activated, cholesterol-binding cytolysins

Several species of both pathogenic and non-pathogenic grampositive bacteria within the genera Streptococcus, Clostridium and Bacillus secrete cytolytic proteins that belong to a single, highly homologous family. The most widely known members of this family are streptolysin O, listeriolysin, perfringolysin, and pneumolysin. These toxins specifically require membrane cholesterol but, apparently, do not depend on any other specific cell surface receptor, so that they are able to lyse the cytoplasmic membranes of virtually any animal cell. Upon binding as monomers, they oligomerize to form large pores with up to 30 nm internal diameter. These are the largest pores known, permitting permeation not only of ions and small metabolites but also of macromolecules. The latter property renders these toxins useful tools in cell biology. While several of these cytolysins have been shown to be determinants of bacterial pathogenicity, their biological roles may vary, as do the lifestyles of the bacteria secreting them. A unique function is surely fulfilled by listeriolysin O, which helps the intracellular pathogen Listeria monocytogenes escape from phagolysosomes and then spread to adjacent host cells.

The cholesterol-dependent cytolysins

In view of the recent studies on the CDCs, a reasonable schematic of the stages leading to membrane insertion of the CDCs can be assembled. As shown in Fig. 3, we propose that the CDC first binds to the membrane as a monomer. These monomers then diffuse laterally on the membrane surface to encounter other monomers or incomplete oligomeric complexes. Presumably, once the requisite oligomer size is reached, the prepore complex is converted into the pore complex and a large membrane channel is formed. During the conversion of the prepore complex to the pore complex, we predict that the TMHs of the subunits in the prepore complex insert into the bilayer in a concerted fashion to form the large transmembrane beta-barrel, although this still remains to be confirmed experimentally. Many intriguing problems concerning the cytolytic mechanism of the CDCs remain unsolved. The nature of the initial interaction of the CDC monomer with the membrane is currently one of the most controversial questions concerning the CDC mechanism. Is cholesterol involved in this interaction, as previously assumed, or do specific receptors exist for these toxins that remain to be discovered? Also, the trigger for membrane insertion and the regions of these toxins that facilitate the [figure: see text] interaction of the monomers during prepore complex formation are unknown. In addition, the temporal sequence of the multiple structural changes that accompany the conversion of the soluble CDC monomer into a membrane-inserted oligomer have yet to be defined or characterized kinetically.

Listeria pathogenesis and molecular virulence determinants

The gram-positive bacterium Listeria monocytogenes is the causative agent of listeriosis, a highly fatal opportunistic foodborne infection. Pregnant women, neonates, the elderly, and debilitated or immunocompromised patients in general are predominantly affected, although the disease can also develop in normal individuals. Clinical manifestations of invasive listeriosis are usually severe and include abortion, sepsis, and meningoencephalitis. Listeriosis can also manifest as a febrile gastroenteritis syndrome. In addition to humans, L. monocytogenes affects many vertebrate species, including birds. Listeria ivanovii, a second pathogenic species of the genus, is specific for ruminants. Our current view of the pathophysiology of listeriosis derives largely from studies with the mouse infection model. Pathogenic listeriae enter the host primarily through the intestine. The liver is thought to be their first target organ after intestinal translocation. In the liver, listeriae actively multiply until the infection is controlled by a cell-mediated immune response. This initial, subclinical step of listeriosis is thought to be common due to the frequent presence of pathogenic L. monocytogenes in food. In normal individuals, the continual exposure to listerial antigens probably contributes to the maintenance of anti-Listeria memory T cells. However, in debilitated and immunocompromised patients, the unrestricted proliferation of listeriae in the liver may result in prolonged low-level bacteremia, leading to invasion of the preferred secondary target organs (the brain and the gravid uterus) and to overt clinical disease. L. monocytogenes and L. ivanovii are facultative intracellular parasites able to survive in macrophages and to invade a variety of normally nonphagocytic cells, such as epithelial cells, hepatocytes, and endothelial cells. In all these cell types, pathogenic listeriae go through an intracellular life cycle involving early escape from the phagocytic vacuole, rapid intracytoplasmic multiplication, bacterially induced actin-based motility, and direct spread to neighboring cells, in which they reinitiate the cycle. In this way, listeriae disseminate in host tissues sheltered from the humoral arm of the immune system. Over the last 15 years, a number of virulence factors involved in key steps of this intracellular life cycle have been identified. This review describes in detail the molecular determinants of Listeria virulence and their mechanism of action and summarizes the current knowledge on the pathophysiology of listeriosis and the cell biology and host cell responses to Listeria infection. This article provides an updated perspective of the development of our understanding of Listeria pathogenesis from the first molecular genetic analyses of virulence mechanisms reported in 1985 until the start of the genomic era of Listeria research.

Pneumococcal virulence factors: structure and function

The overall goal for this review is to summarize the current body of knowledge about the structure and function of major known antigens of Streptococcus pneumoniae, a major gram-positive bacterial pathogen of humans. This information is then related to the role of these proteins in pneumococcal pathogenesis and in the development of new vaccines and/or other antimicrobial agents. S. pneumoniae is the most common cause of fatal community-acquired pneumonia in the elderly and is also one of the most common causes of middle ear infections and meningitis in children. The present vaccine for the pneumococcus consists of a mixture of 23 different capsular polysaccharides. While this vaccine is very effective in young adults, who are normally at low risk of serious disease, it is only about 60% effective in the elderly. In children younger than 2 years the vaccine is ineffective and is not recommended due to the inability of this age group to mount an antibody response to the pneumococcal polysaccharides. Antimicrobial drugs such as penicillin have diminished the risk from pneumococcal disease. Several pneumococcal proteins including pneumococcal surface proteins A and C, hyaluronate lyase, pneumolysin, autolysin, pneumococcal surface antigen A, choline binding protein A, and two neuraminidase enzymes are being investigated as potential vaccine or drug targets. Essentially all of these antigens have been or are being investigated on a structural level in addition to being characterized biochemically. Recently, three-dimensional structures for hyaluronate lyase and pneumococcal surface antigen A became available from X-ray crystallography determinations. Also, modeling studies based on biophysical measurements provided more information about the structures of pneumolysin and pneumococcal surface protein A. Structural and biochemical studies of these pneumococcal virulence factors have facilitated the development of novel antibiotics or protein antigen-based vaccines as an alternative to polysaccharide-based vaccines for the treatment of pneumococcal disease.

Mechanism of membrane insertion of a multimeric beta-barrel protein: perfringolysin O creates a pore using ordered and coupled conformational changes

Perfringolysin O, a bacterial cytolytic toxin, forms unusually large pores in cholesterol-containing membranes by the spontaneous insertion of two of its four domains into the bilayer. By monitoring the kinetics of domain-specific conformational changes and pore formation using fluorescence spectroscopy, the temporal sequence of domain-membrane interactions has been established. One membrane-exposed domain does not penetrate deeply into the bilayer and is not part of the actual pore, but is responsible for membrane recognition. This domain must bind to the membrane before insertion of the other domain into the bilayer is initiated. The two domains are conformationally coupled, even though they are spatially separated. Thus, cytolytic pore formation is accomplished by a novel mechanism of ordered conformational changes and interdomain communication.

Structure and molecular mechanism of a functional form of pneumolysin: a cholesterol-dependent cytolysin from Streptococcus pneumoniae

One of the key steps in understanding human disease arising from gram-positive bacteria lies in the mechanisms of the cholesterol-dependent cytolysins (CDCs). Pneumolysin (PLY), a CDC from Streptococcus pneumoniae, is of special importance due to the severe impacts of pneumococcal infections on mortality and morbidity worldwide. We have overexpressed, purified, and characterized PLY in its fully functional complex form with the enzyme bound to its receptor activator on target cells, cholesterol. The circular dichroism studies of PLY in solution with an excess of cholesterol show a change in the far UV spectrum consistent with a decrease in the beta-sheet and an increase in the random coil structures of the enzyme. Pore formation in membranes leading to cell lysis is the functional target for this cytolysin. The sedimentation velocity and equilibrium analyses of the cholesterol-bound enzyme show hydrodynamic properties different from those of the cholesterol-free form. The soluble form of the cholesterol-free enzyme exists in solution as a mixture of monomers and dimers, whereas the cholesterol-bound form exists only as a monomer. A mechanism of formation of PLY pores in the lipid bilayer of the target cells is discussed.

Studies on the structure and mechanism of a bacterial protein toxin by analytical ultracentrifugation and small-angle neutron scattering

Pneumolysin, an important virulence factor of the human pathogen Streptococcus pneumoniae, is a pore-forming toxin which also possesses the ability to activate the complement system directly. Pneumolysin binds to cholesterol in cell membrane surfaces as a prelude to pore formation, which involves the oligomerization of the protein. Two important aspects of the pore-forming activity of pneumolysin are therefore the effect of the toxin on bilayer membrane structure and the nature of the self-association into oligomers undergone by it. We have used analytical ultracentrifugation (AUC) to investigate oligomerization and small-angle neutron scattering (SANS) to investigate the changes in membrane structure accompanying pore formation. Pneumolysin self-associates in solution to form oligomeric structures apparently similar to those which appear on the membrane coincident with pore formation. It has previously been demonstrated by us using site-specific chemical derivatization of the protein that the self-interaction preceding oligomerization involves its C-terminal domain. The AUC experiments described here involved pneumolysin toxoids harbouring mutations in different domains, and support our previous conclusions that self-interaction via the C-terminal domain leads to oligomerization and that this may be related to the mechanism by which pneumolysin activates the complement system.SANS data at a variety of neutron contrasts were obtained from liposomes used as model cell membranes in the absence of pneumolysin, and following the addition of toxin at a number of concentrations. These experiments were designed to allow visualization of the effect that pneumolysin has on bilayer membrane structure resulting from oligomerization into a pore-forming complex. The structure of the liposomal membrane alone and following addition of pneumolysin was calculated by the fitting of scattering equations directly to the scattering curves. The fitting equations describe scattering from simple three-dimensional scattering volume models for the structures present in the sample, whose dimensions were varied iteratively within the fitting program. The overall trend was a thinning of the liposome surface on toxin attack, which was countered by the formation of localized structures thicker than the liposome bilayer itself, in a manner dependent on pneumolysin concentration. At the neutron contrast match point of the liposomes, pneumolysin oligomers were observed. Inactive toxin appeared to bind to the liposome but not to cause membrane alteration; subsequent activation of pneumolysin in situ brought about changes in liposome structure similar to those seen in the presence of active toxin. We propose that the changes in membrane structure on toxin attack which we have observed are related to the mechanism by which pneumolysin forms pores and provide an important perspective on protein/membrane interactions in general. We discuss these results in the light of published data concerning the interaction of gramicidin with bilayers and the hydrophobic mismatch effect.