Fatty Acids Covalently Bound to α-Hemolysin ofEscherichia coliAre Involved in the Molten Globule Conformation: Implication of Disordered Regions in Binding Promiscuity

Biophysical Analysis to Assess the Interaction of CRAC and CARC Motif Peptides of Alpha Hemolysin of Escherichia coli with Membranes

Alpha hemolysin of Escherichia coli (HlyA) is a pore-forming protein, which is a prototype of the "Repeat in Toxins" (RTX) family. It was demonstrated that HlyA-cholesterol interaction facilitates the insertion of the toxin into membranes. Putative cholesterol-binding sites, called cholesterol recognition/amino acid consensus (CRAC), and CARC (analogous to CRAC but with the opposite orientation) were identified in the HlyA sequence. In this context, two peptides were synthesized, one derived from a CARC site from the insertion domain of the toxin (residues 341-353) (PEP 1) and the other one from a CRAC site from the domain between the acylated lysines (residues 639-644) (PEP 2), to study their role in the interaction of HlyA with membranes. The interaction of peptides with membranes of different lipid compositions (pure POPC and POPC/Cho of 4:1 and 2:1 molar ratios) was analyzed by surface plasmon resonance and molecular dynamics simulations. Results demonstrate that both peptides interact preferentially with Cho-containing membranes, although PEP 2 presents a lower K_D than PEP 1. Molecular dynamics simulation results indicate that the insertion and interaction of PEP 2 with Cho-containing membranes are more prominent than those caused by PEP 1. The hemolytic activity of HlyA in the presence of peptides indicates that PEP 2 was the only one that inhibits HlyA activity, interfering in the binding between the toxin and cholesterol.

Role of Tryptophan Residues in the Toxicity and Photosensitized Inactivation of Escherichia coli α-Hemolysin

α-Hemolysin (HlyA) is an extracellular protein toxin secreted by uropathogenic strains of Escherichia coli that inserts into membranes of eukaryotic cells. The main goal of this work was to investigate the involvement of tryptophan (W) residues in the hemolytic activity of HlyA. We investigated the hemolytic activity of six single-point mutant proteins, in which one of the four Ws was replaced by cysteine (C) or leucine (L). We also analyzed the photoinactivation of HlyA with pterin (Ptr), an endogenous photosensitizer, as a method of unspecific oxidation of W and tyrosine (Y) residues. HlyA photoinactivation was analyzed by ultraviolet-visible spectrophotometry, hemolytic activity measurement, fluorescence spectroscopy, and electrophoretic analysis. The results indicate that Ws are important in the hemolytic process. Specifically, the chemical structure of the amino acid at position 578 is important for the acylation of HlyA at residue K563. Furthermore, the exposure of HlyA to ultraviolet radiation, with energy similar to that experienced under sun exposure, in the presence of Ptr induces the inactivation of the toxin, causing chemical changes in, at least, W and Y, the rate of damage to W residues being faster than that observed for Y residues. This work not only deepens our understanding of the structure-function relationship of the toxin but also introduces the possibility of using photoinactivation of HlyA for potential applications such as obtaining innocuous molecules for vaccine production and the elimination of the toxin from contaminated surfaces and drinking water.

Regulation of extracellular ATP of human erythrocytes treated with α-hemolysin. Effects of cell volume, morphology, rheology and hemolysis

Alpha-hemolysin (HlyA) of uropathogenic strains of Escherichia coli irreversibly binds to human erythrocytes (RBCs) and triggers activation of ATP release and metabolic changes ultimately leading to hemolysis. We studied the regulation of extracellular ATP (ATPe) of RBCs exposed to HlyA. Luminometry was used to assess ATP release and ATPe hydrolysis, whereas changes in cell volume and morphology were determined by electrical impedance, ektacytometry and aggregometry. Exposure of RBCs to HlyA induced a strong increase of [ATPe] (3-36-fold) and hemolysis (1-44-fold), partially compensated by [ATPe] hydrolysis by ectoATPases and intracellular ATPases released by dead cells. Carbenoxolone, a pannexin 1 inhibitor, partially inhibited ATP release (43-67%). The un-acylated toxin ProHlyA and the deletion analog HlyA∆914-936 were unable to induce ATP release or hemolysis. For HlyA treated RBCs, a data driven mathematical model showed that simultaneous lytic and non-lytic release mainly governed ATPe kinetics, while ATPe hydrolysis became important after prolonged toxin exposure. HlyA induced a 1.5-fold swelling, while blocking this swelling reduced ATP release by 77%. Blocking ATPe activation of purinergic P2X receptors reduced swelling by 60-80%. HlyA-RBCs showed an acute 1.3-2.2-fold increase of Ca²⁺i, increased crenation and externalization of phosphatidylserine. Perfusion of HlyA-RBCs through adhesion platforms showed strong adhesion to activated HMEC cells, followed by rapid detachment. HlyA exposed RBCs exhibited increased sphericity under osmotic stress, reduced elongation under shear stress, and very low aggregation in viscous media. Overall results showed that HlyA-RBCs displayed activated ATP release, high but weak adhesivity, low deformability and aggregability and high sphericity.

Interaction of acylated and unacylated forms of E. coli alpha-hemolysin with lipid monolayers: a PM-IRRAS study

Uropathogenic strains of Escherichia coli produce virulence factors, such as the protein toxin alpha-hemolysin (HlyA), that enable the bacteria to colonize the host and establish an infection. HlyA is synthetized as a protoxin (ProHlyA) that is transformed into the active form in the bacterial cytosol by the covalent linkage of two fatty-acyl moieties to the polypeptide chain before the secretion of HlyA into the extracellular medium. The aim of this work was to investigate the effect of the fatty acylation of HlyA on protein conformation and protein-membrane interactions. Polarization-modulated infrared reflection-absorption spectroscopy (PM-IRRAS) experiments were performed at the air-water interface, and lipid monolayers mimicking the outer leaflet of red-blood-cell membranes were used as model systems for the study of protein-membrane interaction. According to surface-pressure measurements, incorporation of the acylated protein into the lipid films was faster than that of the nonacylated form. PM-IRRAS measurements revealed that the adsorption of the proteins to the lipid monolayers induced disorder in the lipid acyl chains and also changed the elastic properties of the films independently of protein acylation. No significant difference was observed between HlyA and ProHlyA in the interaction with the model lipid monolayers; but when these proteins became adsorbed on a bare air-water interface, they adopted different secondary structures. The assumption of the correct protein conformation at a hydrophobic-hydrophilic interface could constitute a critical condition for biologic activity.

Hemolysin of uropathogenic Escherichia coli: A cloak or a dagger?

Hemolysin from uropathogenic Escherichia coli (UPEC) is a hemolytic and cytotoxic protein active against a broad range of species and cell types. Expression of hemolysin correlates with severity of infection, as up to 78% of UPEC isolates from pyelonephritis cases express hemolysin. Despite decades of research on hemolysin activity, the mechanism of intoxication and the function of hemolysin in UPEC infection remain elusive. Early in vitro research established the role of hemolysin as a lytic protein at high doses. It is hypothesized that hemolysin is secreted at sublytic doses in vivo and recent research has focused on understanding the more subtle effects of hemolysin both in vitro and in elegant infection models in vivo, including inoculation by micropuncture of individual kidney nephrons. As the field continues to evolve, comparisons of hemolysin function in isolates from a range of UTI infections will be important for delineating the role of this toxin. This article is part of a Special Issue entitled: Pore-Forming Toxins edited by Mauro Dalla Serra and Franco Gambale.

Calcium, acylation, and molecular confinement favor folding of Bordetella pertussis adenylate cyclase CyaA toxin into a monomeric and cytotoxic form

The adenylate cyclase (CyaA) toxin, a multidomain protein of 1706 amino acids, is one of the major virulence factors produced by Bordetella pertussis, the causative agent of whooping cough. CyaA is able to invade eukaryotic target cells in which it produces high levels of cAMP, thus altering the cellular physiology. Although CyaA has been extensively studied by various cellular and molecular approaches, the structural and functional states of the toxin remain poorly characterized. Indeed, CyaA is a large protein and exhibits a pronounced hydrophobic character, making it prone to aggregation into multimeric forms. As a result, CyaA has usually been extracted and stored in denaturing conditions. Here, we define the experimental conditions allowing CyaA folding into a monomeric and functional species. We found that CyaA forms mainly multimers when refolded by dialysis, dilution, or buffer exchange. However, a significant fraction of monomeric, folded protein could be obtained by exploiting molecular confinement on size exclusion chromatography. Folding of CyaA into a monomeric form was found to be critically dependent upon the presence of calcium and post-translational acylation of the protein. We further show that the monomeric preparation displayed hemolytic and cytotoxic activities suggesting that the monomer is the genuine, physiologically active form of the toxin. We hypothesize that the structural role of the post-translational acylation in CyaA folding may apply to other RTX toxins.

Equilibrium folding of pro-HlyA from Escherichia coli reveals a stable calcium ion dependent folding intermediate

HlyA from Escherichia coli is a member of the repeats in toxin (RTX) protein family, produced by a wide range of Gram-negative bacteria and secreted by a dedicated Type 1 Secretion System (T1SS). RTX proteins are thought to be secreted in an unfolded conformation and to fold upon secretion by Ca(2+) binding. However, the exact mechanism of secretion, ion binding and folding to the correct native state remains largely unknown. In this study we provide an easy protocol for high-level pro-HlyA purification from E. coli. Equilibrium folding studies, using intrinsic tryptophan fluorescence, revealed the well-known fact that Ca(2+) is essential for stability as well as correct folding of the whole protein. In the absence of Ca(2+), pro-HlyA adopts a non-native conformation. Such molecules could however be rescued by Ca(2+) addition, indicating that these are not dead-end species and that Ca(2+) drives pro-HlyA folding. More importantly, pro-HlyA unfolded via a two-state mechanism, whereas folding was a three-state process. The latter is indicative of the presence of a stable folding intermediate. Analysis of deletion and Trp mutants revealed that the first folding transition, at 6-7M urea, relates to Ca(2+) dependent structural changes at the extreme C-terminus of pro-HlyA, sensed exclusively by Trp914. Since all Trp residues of HlyA are located outside the RTX domain, our results demonstrate that Ca(2+) induced folding is not restricted to the RTX domain. Taken together, Ca(2+) binding to the pro-HlyA RTX domain is required to drive the folding of the entire protein to its native conformation.

Thermodynamic properties of the effector domains of MARTX toxins suggest their unfolding for translocation across the host membrane

MARTX (multifunctional autoprocessing repeats-in-toxin) family toxins are produced by Vibrio cholerae, Vibrio vulnificus, Aeromonas hydrophila and other Gram-negative bacteria. Effector domains of MARTX toxins cross the cytoplasmic membrane of a host cell through a putative pore formed by the toxin's glycine-rich repeats. The structure of the pore is unknown and the translocation mechanism of the effector domains is poorly understood. We examined the thermodynamic stability of the effector domains of V. cholerae and A. hydrophila MARTX toxins to elucidate the mechanism of their translocation. We found that all but one domain in each toxin are thermodynamically unstable and several acquire a molten globule state near human physiological temperatures. Fusion of the most stable cysteine protease domain to the adjacent effector domain reduces its thermodynamic stability ∼ 1.4-fold (from D G H 2 O 21.8 to 16.1 kJ mol(-1) ). Precipitation of several individual domains due to thermal denaturation is reduced upon their fusion into multi-domain constructs. We speculate that low thermostability of the MARTX effector domains correlates with that of many other membrane-penetrating toxins and implies their unfolding for cell entry. This study extends the list of thermolabile bacterial toxins, suggesting that this quality is essential and could be susceptible for selective targeting of pathogenic toxins.

Novel evidence for the specific interaction between cholesterol and α-haemolysin of Escherichia coli

The present study shows, for the first time, the interaction of HlyA with cholesterol. This interaction seems to favour a conformational state of the protein that allows its correct insertion into the membrane and its further oligomerization to form pores.

The RTX pore-forming toxin α-hemolysin of uropathogenic Escherichia coli: progress and perspectives

Members of the RTX family of protein toxins are functionally conserved among an assortment of bacterial pathogens. By disrupting host cell integrity through their pore-forming and cytolytic activities, this class of toxins allows pathogens to effectively tamper with normal host cell processes, promoting pathogenesis. Here, we focus on the biology of RTX toxins by describing salient properties of a prototype member, α-hemolysin, which is often encoded by strains of uropathogenic Escherichia coli. It has long been appreciated that RTX toxins can have distinct effects on host cells aside from outright lysis. Recently, advances in modeling and analysis of host-pathogen interactions have led to novel findings concerning the consequences of pore formation during host-pathogen interactions. We discuss current progress on longstanding questions concerning cell specificity and pore formation, new areas of investigation that involve toxin-mediated perturbations of host cell signaling cascades and perspectives on the future of RTX toxin investigation.

Membrane association and destabilization by Aggregatibacter actinomycetemcomitans leukotoxin requires changes in secondary structures

Aggregatibacter actinomycetemcomitans is a common inhabitant of the upper aerodigestive tract of humans and non-human primates and is associated with disseminated infections, including lung and brain abscesses, pediatric infective endocarditis, and localized aggressive periodontitis. Aggregatibacter actinomycetemcomitans secretes a repeats-in-toxin protein, leukotoxin, which exclusively kills lymphocyte function-associated antigen-1-bearing cells. The toxin's pathological mechanism is not fully understood; however, experimental evidence indicates that it involves the association with and subsequent destabilization of the target cell's plasma membrane. We have long hypothesized that leukotoxin secondary structure is strongly correlated with membrane association and destabilization. In this study, we tested this hypothesis by analysing lipid-induced changes in leukotoxin conformation. Upon incubation of leukotoxin with lipids that favor leukotoxin-membrane association, we observed an increase in leukotoxin α-helical content that was not observed with lipids that favor membrane destabilization. The change in leukotoxin conformation after incubation with these lipids suggests that membrane binding and membrane destabilization have distinct secondary structural requirements, suggesting that they are independent events. These studies provide insight into the mechanism of cell damage that leads to disease progression by A. actinomycetemcomitans.

Role of Salmonella Pathogenicity Island 1 protein IacP in Salmonella enterica serovar typhimurium pathogenesis

Gram-negative bacteria, including Salmonella enterica serovar Typhimurium, exploit type III secretion systems (T3SSs) through which virulence proteins are delivered into the host cytosol to reinforce invasive and replicative niches in their host. Although many secreted effector proteins and membrane-bound structural proteins in the T3SS have been characterized, the functions of many cytoplasmic proteins still remain unknown. In this study, we found that IacP, encoded by Salmonella pathogenicity island 1, was important for nonphagocytic cell invasion and bacterial virulence. When the iacP gene was deleted from several Salmonella serovar Typhimurium strains, the invasion into INT-407 epithelial cells was significantly decreased compared to that of their parental strains, and retarded rearrangements of actin fibers were observed for the iacP mutant-infected cells. Although IacP had no effect on the secretion of type III translocon proteins, the levels of secretion of the effector proteins SopB, SopA, and SopD into the culture medium were decreased in the iacP mutant. In a mouse infection model, mice infected with the iacP mutant exhibited alleviated pathological signs in the intestine and survived longer than did wild-type-infected mice. Taken together, IacP plays a key role in Salmonella virulence by regulating the translocation of T3SS effector proteins.

Mutations affecting the extreme C terminus of Escherichia coli haemolysin A reduce haemolytic activity by altering the folding of the toxin

Escherichia coli haemolysin A (HlyA), an RTX toxin, is secreted probably as an unfolded intermediate, by the type I (ABC transporter-dependent) pathway, utilizing a C-terminal secretion signal. However, the mechanism of translocation and post-translocation folding is not understood. We identified a mutation (hlyA99) at the extreme C terminus, which is dominant in competition experiments, blocking secretion of the wild-type toxin co-expressed in the same cell. This suggests that unlike recessive mutations which affect recognition of the translocation machinery, the hlyA99 mutation interferes with some later step in secretion. Indeed, the mutation reduced haemolytic activity of the toxin and the activity of β-lactamase when the latter was fused to a C-terminal 23 kDa fragment of HlyA carrying the hlyA99 mutation. A second mutant (hlyAdel6), lacking the six C-terminal residues of HlyA, also showed reduced haemolytic activity and neither mutant protein regained normal haemolytic activity in in vitro unfolding/refolding experiments. Tryptophan fluorescence spectroscopy indicated differences in structure between the secreted forms of wild-type HlyA and the HlyA Del6 mutant. These results suggested that the mutations affected the correct folding of both HlyA and the β-lactamase fusion. Thus, we propose a dual function for the HlyA C terminus involving an important role in post-translocation folding as well as targeting HlyA for secretion.

Relevance of fatty acid covalently bound to Escherichia coli alpha-hemolysin and membrane microdomains in the oligomerization process

alpha-Hemolysin (HlyA) is an exotoxin secreted by some pathogenic strains of Escherichia coli that causes lysis of several mammalian cells, including erythrocytes of different species. HlyA is synthesized as a protoxin, pro-HlyA, which is activated by acylation at two internal lysines Lys-563 and Lys-689. It has been proposed that pore formation is the mechanism of cytolytic activity for this toxin, as shown in experiments with whole cells, planar lipid membranes, and liposomes, but these experiments have yielded conflicting results about the structure of the pore. In this study, HlyA cysteine replacement mutant proteins of amino acids have been labeled with Alexa-488 and Alexa-546. Fluorescence resonance energy transfer measurements, employing labeled toxin bound to sheep ghost erythrocytes, have demonstrated that HlyA oligomerizes on erythrocyte membranes. As the cytotoxic activity is absolutely dependent on acylation, we have studied the role of acylation in the oligomerization, demonstrating that fatty acids are essential in this process. On the other hand, fluorescence resonance energy transfer and the hemolytic activity decrease when the erythrocyte ghosts are cholesterol-depleted, hence indicating the role of membrane microdomains in the clustering of HlyA. Simultaneously, HlyA was found in detergent-resistant membranes. Pro-HlyA has also been found in detergent-resistant membranes, thus demonstrating that the importance of acyl chains in toxin oligomerization is the promotion of protein-protein interaction. These results change the concept of the main role assigned to acyl chain in the targeting of proteins to membrane microdomains.