Pore-forming peptide of Entamoeba histolytica. Significance of positively charged amino acid residues for its mode of action

Taxon-Specific Proteins of the Pathogenic Entamoeba Species E. histolytica and E. nuttalli

The human protozoan parasite Entamoeba histolytica can live in the human intestine for months or years without generating any symptoms in the host. For unknown reasons, amoebae can suddenly destroy the intestinal mucosa and become invasive. This can lead to amoebic colitis or extraintestinal amoebiasis whereby the amoebae spread to other organs via the blood vessels, most commonly the liver where abscesses develop. Entamoeba nuttalli is the closest genetic relative of E. histolytica and is found in wild macaques. Another close relative is E. dispar, which asyptomatically infects the human intestine. Although all three species are closely related, only E. histolytica and E. nuttalli are able to penetrate their host’s intestinal epithelium. Lineage-specific genes and gene families may hold the key to understanding differences in virulence among species. Here we discuss those genes found in E. histolytica that have relatives in only one or neither of its sister species, with particular focus on the peptidase, AIG, Ariel, and BspA families.

Entamoeba histolytica and pathogenesis: A calcium connection

Calcium signaling plays a key role in many essential processes in almost all eukaryotic systems. It is believed that it may also be an important signaling system of the protist parasite Entamoeba histolytica. Motility, adhesion, cytolysis, and phagocytosis/trogocytosis are important steps in invasion and pathogenesis of E. histolytica, and Ca²⁺ signaling is thought to be associated with these processes leading to tissue invasion. There are a large number of Ca²⁺-binding proteins (CaBPs) in E. histolytica, and a number of these proteins appear to be associated with different steps in pathogenesis. The genome encodes 27 EF-hand–containing CaBPs in addition to a number of other Ca²⁺-binding domain/motif-containing proteins, which suggest intricate calcium signaling network in this parasite. Unlike other eukaryotes, a typical calmodulin-like protein has not been seen in E. histolytica. Though none of the CaBPs display sequence similarity with a typical calmodulin, extensive structural similarity has been seen in spite of lack of significant functional overlap with that of typical calmodulins. One of the unique features observed in E. histolytica is the identification of CaBPs (EhCaBP1, EhCaBP3) that have the ability to directly bind actin and modulate actin dynamics. Direct interaction of CaBPs with actin has not been seen in any other system. Pseudopod formation and phagocytosis are some of the processes that require actin dynamics, and some of the amoebic CaBPs (EhC2Pk, EhCaBP1, EhCaBP3, EhCaBP5) participate in this process. None of these E. histolytica CaBPs have any homolog in organisms other than different species of Entamoeba, suggesting a novel Ca²⁺ signaling pathway that has evolved in this genus.

Structure and function of a unique pore-forming protein from a pathogenic acanthamoeba

Human pathogens often produce soluble protein toxins that generate pores inside membranes, resulting in the death of target cells and tissue damage. In pathogenic amoebae, this has been exemplified with amoebapores of the enteric protozoan parasite Entamoeba histolytica. Here we characterize acanthaporin, to our knowledge the first pore-forming toxin to be described from acanthamoebae, which are free-living, bacteria-feeding, unicellular organisms that are opportunistic pathogens of increasing importance and cause severe and often fatal diseases. We isolated acanthaporin from extracts of virulent Acanthamoeba culbertsoni by tracking its pore-forming activity, molecularly cloned the gene of its precursor and recombinantly expressed the mature protein in bacteria. Acanthaporin was cytotoxic for human neuronal cells and exerted antimicrobial activity against a variety of bacterial strains by permeabilizing their membranes. The tertiary structures of acanthaporin's active monomeric form and inactive dimeric form, both solved by NMR spectroscopy, revealed a currently unknown protein fold and a pH-dependent trigger mechanism of activation.

Mechanisms of adherence, cytotoxicity and phagocytosis modulate the pathogenesis of Entamoeba histolytica

The unicellular parasite Entamoeba histolytica, the causative agent of the human disease amebiasis, has traditionally been distinguished from its nonpathogenic cousin Entamoeba dispar by its propensity for the ingestion of erythrocytes. This classic feature, along with the parasite’s ability to cause extensive host cell death, are critical mechanisms of pathogenesis during human infection. Recent advances have led to a greater understanding of the molecular components that allow E. histolytica to kill and phagocytose extracellular targets during human infection and include detailed studies of the role of the parasite’s cysteine proteinases and other effectors of cytotoxicity, as well as the mechanisms of ligand recognition, signaling and intracellular trafficking during phagocytosis.

The immunological functions of saposins

Saposins or sphingolipid activator proteins (SAPs) are small, nonenzymatic glycoproteins that are ubiquitously present in lysosomes. SAPs comprise the five molecules saposins A-D and the GM2 activator protein. Saposins are essential for sphingolipid degradation and membrane digestion. On the one hand, they bind the respective hydrolases required to catabolize sphingolipid molecules; on the other hand, saposins can interact with intralysosomal membrane structures to render lipids accessible to their degrading enzymes. Thus, saposins bridge the physicochemical gap between lipid substrate and hydrophilic hydrolases. Accordingly, defects in saposin function can lead to lysosomal lipid accumulation. In addition to their specific functions in sphingolipid metabolism, saposins have membrane-perturbing properties. At the low pH of lysosomes, saposins get protonated and exhibit a high binding affinity for anionic phospholipids. Based on their universal principle to interact with membrane bilayers, we present the immunological functions of saposins with regard to lipid antigen presentation to CD1-restricted T cells, processing of apoptotic bodies for antigen delivery and cross-priming, as well as their potential antimicrobial impact.

Structure and content of the Entamoeba histolytica genome

The intestinal parasite Entamoeba histolytica is one of the first protists for which a draft genome sequence has been published. Although the genome is still incomplete, it is unlikely that many genes are missing from the list of those already identified. In this chapter we summarise the features of the genome as they are currently understood and provide previously unpublished analyses of many of the genes.

Saposin-like proteins from the intestine of the blood-feeding hookworm, Ancylostoma caninum

Hookworms feed on blood, utilizing haemoglobin for nutrition, growth and reproduction. The haemoglobin digestion cascade has been partially elucidated, but the process immediately preceding this event, haemolysis, has received considerably less attention. We have cloned and expressed Ancylostoma caninum mRNAs encoding 2 proteins belonging to the saposin-like protein (SAPLIP) family, termed Ac-slp-1 and Ac-slp-2. The open reading frames of SLP-1 and SLP-2 were used to identify expressed sequence tags encoding SAPLIPs from the 4 major clades of animal parasitic nematodes. Both Ac-slp-1 and slp-2 mRNAs were shown to be expressed in all life stages assessed, with slp-1 predominantly being expressed in third-stage larvae (L3) before and after activation with dog serum. Recombinant SLP-1 and SLP-2 were expressed in insect cells and used to raise specific antisera in mice. These antisera were used as probes in fluorescence microscopy to localize the anatomic expression sites of both proteins to small, punctate organelles or vesicles within the intestinal cells of adult worms; weak staining was detected on the microvillar brush border of the intestine. Using transmission electron microscopy, both proteins were localized to similar vesicles in the intestinal cells of the L3. Recombinant proteins contained C-terminal purification tags that potentially precluded dimerization and possibly interfered with the subsequent detection of haemolytic activity. Their expression in the gut of the L3 and adult stages suggests a role for these hookworm SAPLIPs in the lysis of host cells during tissue migration and/or feeding.

A short guided tour through functional and structural features of saposin-like proteins

SAPLIPs (saposin-like proteins) are a diverse family of lipid-interacting proteins that have various and only partly understood, but nevertheless essential, cellular functions. Their existence is conserved in phylogenetically most distant organisms, such as primitive protozoa and mammals. Owing to their remarkable sequence variability, a common mechanism for their actions is not known. Some shared principles beyond their diversity have become evident by analysis of known three-dimensional structures. Whereas lipid interaction is the basis for their functions, the special cellular tasks are often defined by interaction partners other than lipids. Based on recent findings, this review summarizes phylogenetic relations, function and structural features of the members of this family.

Solution Structure of the Pore-forming Protein of Entamoeba histolytica

Amoebapore A is a 77-residue protein from the protozoan parasite and human pathogen Entamoeba histolytica. Amoebapores lyse both bacteria and eukaryotic cells by pore formation and play a pivotal role in the destruction of host tissues during amoebiasis, one of the most life-threatening parasitic diseases. Amoebapore A belongs to the superfamily of saposin-like proteins that are characterized by a conserved disulfide bond pattern and a fold consisting of five helices. Membrane-permeabilizing effector molecules of mammalian lymphocytes such as porcine NK-lysin and the human granulysin share these structural attributes. Several mechanisms have been proposed to explain how saposin-like proteins form membrane pores. All mechanisms indicate that the surface charge distribution of these proteins is the basis of their membrane binding capacity and pore formation. Here, we have solved the structure of amoebapore A by NMR spectroscopy. We demonstrate that the specific activation step of amoebapore A depends on a pH-dependent dimerization event and is modulated by a surface-exposed histidine residue. Thus, histidine-mediated dimerization is the molecular switch for pore formation and reveals a novel activation mechanism of pore-forming toxins.

Amoebapores and NK-lysin, members of a class of structurally distinct antimicrobial and cytolytic peptides from protozoa and mammals: a comparative functional analysis

Amoebapores, the pore-forming polypeptides of the protozoan parasite Entamoeba histolytica, and NK-lysin, an effector molecule of porcine NK (natural killer) and cytotoxic T cells, belong to the same protein family, the saposin-like proteins. As both types of protein are implicated in the killing of microbes in vivo, it appears that phylogenetically diverse organisms such as amoebae and mammals use similar effector molecules to fulfil a comparable task. However, structural features have led to the assumption that the proteins display their activities according to different modes of action. To address this question, we analysed the antibacterial, cytotoxic and pore-forming activities of these proteins in parallel and in comprehensive detail. Interestingly, the comparison of activities revealed significant differences. Whereas NK-lysin, recombinantly expressed, is efficient at a broad range of pH values, the amoebapores exhibited a pronounced pH dependence of all their activities, with markedly decreased activity at pH values above 6. Moreover, increasing salinity affects amoebapores more drastically than NK-lysin. All of the proteins compared were found to be potently active against Gram-positive bacteria, but only NK-lysin was equally efficient against Gram-negative bacteria. However, the amoebapores displayed five times higher pore-forming activity than NK-lysin, which is in accordance with the more hydrophobic character of the amoebapores compared with the essentially cationic NK-lysin.

Novelties in the field of antimicrobial compounds for the treatment of lower respiratory tract infections

Emergence of antimicrobial resistance is a growing problem and a public health threat. New drugs must be designed with emerging needs in mind: specific resistant and hard-to-treat organisms. But the difficulty to find real new drugs is a major problem. Only the oxazolidinones, the cationic peptides and the lipopeptide antibiotics can be truly regarded as structurally novel drugs, although the peptide deformylase inhibitors and, possibly, the pleuromutilins can be considered a potential advancement in the field. Obviously, these antibiotics must be reserved only to cases of documented ineffectiveness of the common antimicrobial agents.

Interaction of amoebapores and NK-lysin with symmetric phospholipid and asymmetric lipopolysaccharide/phospholipid bilayers

Amoebapores from protozoan parasite Entamoeba histolytica and NK-lysin of porcine cytotoxic lymphocytes belong to the same family of saposin-like proteins. In addition to the structural similarity, amoebapores and NK-lysin are both highly effective against prokaryotic and eukaryotic target cells in that they permeabilize the target cell membranes. Here, we have investigated in detail the protein/lipid interaction for the three isoforms of amoebapore and NK-lysin. Results obtained from electrical measurements on planar bilayer membranes, including reconstitution models of the lipid matrix of the outer membrane of Escherichia coli and phospholipid membranes, fluorescence energy transfer spectroscopy with liposomes, and monolayer measurements on a Langmuir trough, provided information on lipid preferences, pH dependences, and membrane interaction mechanisms. The three amoebapores led to the formation of transient pores with similar characteristics in conductance, sublevels, and lifetime for the different isoforms. The conductance of the pores was dependent on the polarity of the applied clamp voltage, and the distribution of the sublevels was affected by the value of the clamp voltage. The size of the pores and distribution of conductance sublevels differed between symmetric phospholipid and asymmetric lipopolysaccharide/phospholipid bilayers. Notably, NK-lysin caused the formation of well-defined pores, which were lipid- and voltage-dependent, and their characteristics differed from those induced by amoebapores; e.g., the protein concentration necessary to induce pore formation was 20 times higher. The biophysical data give important information on the mode of action of these small effector proteins, which may further lead to a better understanding of peptide-membrane interactions in general.

Molecular basis for membrane selectivity of NK-2, a potent peptide antibiotic derived from NK-lysin

Increasing resistance of pathogenic bacteria against antibiotics is a severe problem in health care. Natural antimicrobial peptides and derivatives thereof have emerged as promising candidates for "new antibiotics". In contrast to classical antibiotics, these peptides act by direct physical destabilization of the target cell membrane. Nevertheless, they exhibit a high specificity for bacteria over mammalian cells. However, the precise mechanism of action and the molecular basis for membrane selectivity are still a matter of debate. We have designed a new peptide antibiotic (NK-2) with enhanced antimicrobial activity based on an effector protein of mammalian immune cells (NK-lysin). Here we describe the interaction of this alpha-helical synthetic peptide with membrane mimetic systems, designed to mimic the lipid compositions of mammalian and bacterial cytoplasmic membranes. Utilizing fluorescence and biosensor assays, we could show that on one hand, NK-2 strongly interacts with negatively charged membranes; on the other hand, NK-2 is able to discriminate, without the necessity of negative charges, between the zwitterionic phospholipids phosphatidylethanolamine (PE) and phosphatidylcholine (PC), the major constituents of the outer leaflet of the cytoplasmic membranes of bacteria and mammalian cells, respectively.

Hemolytic activity and developmental expression of pore-forming peptide, clonorin

Peptides pore-forming in cell membrane have been identified from a wide range of animals. A putative pore-forming peptide deduced from a cDNA clone of Clonorchis sinensis (clonorin) was predicted to consist of four amphipathic alpha-helices. Clonorin contained six invariably conserved cysteine residues, identified to form three disulfide bonds. These predicted structural features are highly homologous with pore-forming peptides, the amoebapores. Recombinant clonorin showed hemolytic activity toward rabbit erythrocytes. The hemolytic activity of C. sinensis extract increased dose-dependently and was inhibited by anti-clonorin immune sera. The clonorin was expressed developmentally in juvenile and adult flukes and localized in the intestinal epithelium of adult flukes. It is proposed that, through lysing host cellular components, clonorin could enhance proteolytic digestion in the intestine of C. sinensis.

Antiphospholipid syndrome associated with intestinal amoebiasis

We present a case of intestinal amoebiasis with subsequent development of antiphospholipid syndrome, manifested by deep vein thrombosis and pulmonary emboli. Anticardiolipin antibodies (aCL) of IgM type at medium titer and aCL IgG antibody at low titer were determined during the days after the onset of infection. To our knowledge this is the first case of antiphospholipid syndrome associated with amoebiasis to be presented in the literature.

Novel putative saposin-like proteins of Entamoeba histolytica different from amoebapores

Amoebapores, the pore-forming proteins of Entamoeba histolytica, have been shown to play a pivotal role in the pathogenicity of the protozoan parasite. They belong to the functionally diverse family of saposin-like proteins (SAPLIPs) characterized by a conserved pattern of cysteine residues and the ability to interact with lipids. Here, we report the identification of genomic sequences encoding presumably novel SAPLIPs in E. histolytica and classify them in the structural and functional context provided by known family members. The genes of altogether 15 SAPLIPs are transcribed in the axenically cultured trophozoites as evidenced by reverse transcriptase-polymerase chain reaction. Interestingly, a remarkable sequence variety with a strong resemblance to that of known, functionally diverse SAPLIPs is present in this archaic, unicellular organism.

Membrane activity of (Cys48Ser) lung surfactant protein B increases with dimerisation

One of the possible functions of lung surfactant protein B (SP-B), an hydrophobic membrane-associated saposin-like protein, is to reduce the alveolar surface tension by promoting insertion of phospholipids into the air/liquid interface of the lung. SP-B is a covalent homodimer; Cys48 of two polypeptides form an intermolecular disulphide bond. In order to test whether dimerisation of SP-B is important for surfactant function, transgenic mice which express (Cys48Ser) human SP-B in a mouse SP-B null background were generated. In previous studies (Cys48Ser)SP-B showed a concentration-dependent in vitro activity, suggesting that it may form non-covalent dimers. Here (Cys48Ser)SP-B isolated from bronchoalveolar lavage of transgenic mice was studied at different concentrations by circular dichroism (CD) spectroscopy, pulsating bubble surfactometry, mass spectrometry and reversed-phase HPLC. The results indicate that (Cys48Ser)SP-B, both in a phospholipid environment and in organic solvents, is largely monomeric and exhibits low activity at concentrations lower than 1 -2 microM, while at higher concentrations it forms non-covalent dimers, which are nearly functionally equivalent to native SP-B in vitro. Furthermore, electrospray mass spectrometry showed that more dimers were found relative to the monomer when the polarity of the solvent was decreased, and when the concentration of SP-B increased. (Cys48Ser)SP-B also eluted earlier than native SP-B in reversed-phase HPLC. Taken together, these results indicate that a polar surface is buried upon dimerisation, thereby promoting formation of interchain ion pairs between Glu51-Arg52' and Glu51'-Arg52.

Granulysin: a novel antimicrobial

Granulysin is a novel lytic molecule produced by human cytolytic T-lymphocytes (CTLs) and natural killer (NK) cells. It is active against a broad range of microbes, including Gram-positive and -negative bacteria, parasites and Mycobacterium tuberculosis. It is functionally related to other antibacterial peptides, like defensins and magainins, but is structurally distinct. It has structural similarity to porcine NK-lysin and to amoebapores made by Entamoeba histolytica. Synthetic peptides derived from granulysin have differential activity against eukaryotic cells and bacteria. Selective bactericidal peptides may have therapeutic roles as novel antibiotics.

Membrane-permeabilizing polypeptides of amoebae – constituents of an archaic antimicrobial system

Amoebae may be viewed as primitive, actively phagocytosing eukaryotic cells, many of which use bacteria as a major nutrient source. At a very archaic level, amoebae exert mechanisms which kill bacteria comparable to those found in phagocytic cells of higher organisms. Accordingly, it is tempting to suggest that the ancestors of effector cells of the innate immune system were bacteria-feeding amoebae and that their molecular armament is ancient. Here, we summarize the characteristics of antimicrobial and cytolytic 77-residue polypeptides from the protozoon Entamoeba histolytica for which correlates were found in effector cells of the mammalian immune systems. Based on the current knowledge about these small membrane-destabilizing proteins of phylogenetically extremely diverse origin, we discuss similarities and differences in their structure and activities.

Granulysin, a T cell product, kills bacteria by altering membrane permeability

Granulysin, a protein located in the acidic granules of human NK cells and cytotoxic T cells, has antimicrobial activity against a broad spectrum of microbial pathogens. A predicted model generated from the nuclear magnetic resonance structure of a related protein, NK lysin, suggested that granulysin contains a four alpha helical bundle motif, with the alpha helices enriched for positively charged amino acids, including arginine and lysine residues. Denaturation of the polypeptide reduced the alpha helical content from 49 to 18% resulted in complete inhibition of antimicrobial activity. Chemical modification of the arginine, but not the lysine, residues also blocked the antimicrobial activity and interfered with the ability of granulysin to adhere to Escherichia coli and Mycobacterium tuberculosis. Granulysin increased the permeability of bacterial membranes, as judged by its ability to allow access of cytosolic ss-galactosidase to its impermeant substrate. By electron microscopy, granulysin triggered fluid accumulation in the periplasm of M. tuberculosis, consistent with osmotic perturbation. These data suggest that the ability of granulysin to kill microbial pathogens is dependent on direct interaction with the microbial cell wall and/or membrane, leading to increased permeability and lysis.

Arbitrarily primed PCR fingerprinting of RNA and DNA in Entamoeba histolytica

Differences were detected in the gene expression of strains of E. histolytica using RNA (RAP-PCR) and DNA fingerprinting (RAPD). Analysis of the electrophoretic profiles of the gels revealed some polymorphic markers that could be used in the individual characterization of the strains. The 260 bands generated by using five different primers for RAP-PCR, as well as RAPD, were employed in the construction of dendograms. The dendogram obtained based on the RAPD products permitted the distinction of symptomatic and asymptomatic isolates, as well the correlation between the polymorphism exhibited and the virulence of the strains. The dendogram obtained for the RAP-PCR products did not show a correlation with the virulence of the strains but revealed a high degree of intraspecific transcriptional variability that could be related to other biological features, whether or not these are involved in the pathogenesis of amebiasis.

Bactericidal and tumoricidal activities of synthetic peptides derived from granulysin

Granulysin, a 9-kDa protein localized to human CTL and NK cell granules, is cytolytic against tumor cells and microbes. Molecular modeling predicts that granulysin is composed of five alpha-helices separated by short loop regions. In this report, synthetic peptides corresponding to the linear granulysin sequence were characterized for lytic activity. Peptides corresponding to the central region of granulysin lyse bacteria, human cells, and synthetic liposomes, while peptides corresponding to the amino or carboxyl regions are not lytic. Peptides corresponding to either helix 2 or helix 3 lyse bacteria, while lysis of human cells and liposomes is dependent on the helix 3 sequence. Peptides in which positively charged arginine residues are substituted with neutral glutamine exhibit reduced lysis of all three targets. While reduction of recombinant 9-kDa granulysin increases lysis of Jurkat cells, reduction of cysteine-containing granulysin peptides decreases lysis of Jurkat cells. In contrast, lysis of bacteria by recombinant granulysin or by cysteine-containing granulysin peptides is unaffected by reducing conditions. Jurkat cells transfected with either CrmA or Bcl-2 are protected from lysis by recombinant granulysin or the peptides. Differential activity of granulysin peptides against tumor cells and bacteria may be exploited to develop specific antibiotics without toxicity for mammalian cells.

Pathogenesis of intestinal amebiasis: from molecules to disease

In spite of a wealth of knowledge on the biochemistry and cellular and molecular biology of Entamoeba histolytica, little has been done to apply these advances to our understanding of the lesions observed in patients with intestinal amebiasis. In this review, the pathological and histological findings in acute amebic colitis are related to the molecular mechanisms of E. histolytica pathogenicity described to date. Infection of the human colon by E. histolytica produces focal ulceration of the intestinal mucosa, resulting in dysentery (diarrhea with blood and mucus). Although a complete picture has not yet been achieved, the basic mechanisms involved in the production of focal lytic lesions include complex multifactorial processes in which lectins facilitate adhesion, proteases degrade extracellular matrix components, porins help nourish the parasite and may also kill incoming polymorphonuclear leukocytes and macrophages, and motility is used by the parasite to invade deeper layers of the colon. In addition, E. histolytica has developed mechanisms to modulate the immune response during acute infection. Nevertheless, much still needs to be unraveled to understand how this microscopic parasite has earned its well-deserved histolytic name.

Pathogenesis of intestinal amebiasis: from molecules to disease

In spite of a wealth of knowledge on the biochemistry and cellular and molecular biology of Entamoeba histolytica, little has been done to apply these advances to our understanding of the lesions observed in patients with intestinal amebiasis. In this review, the pathological and histological findings in acute amebic colitis are related to the molecular mechanisms of E. histolytica pathogenicity described to date. Infection of the human colon by E. histolytica produces focal ulceration of the intestinal mucosa, resulting in dysentery (diarrhea with blood and mucus). Although a complete picture has not yet been achieved, the basic mechanisms involved in the production of focal lytic lesions include complex multifactorial processes in which lectins facilitate adhesion, proteases degrade extracellular matrix components, porins help nourish the parasite and may also kill incoming polymorphonuclear leukocytes and macrophages, and motility is used by the parasite to invade deeper layers of the colon. In addition, E. histolytica has developed mechanisms to modulate the immune response during acute infection. Nevertheless, much still needs to be unraveled to understand how this microscopic parasite has earned its well-deserved histolytic name.

Pore-forming peptides of Entamoeba dispar. Similarity and divergence to amoebapores in structure, expression and activity

Amoebapore, a 77-residue peptide with pore-forming activity from the human pathogen Entamoeba histolytica, is implicated in the killing of phagocytosed bacteria and in the cytolytic reaction of the amoeba against host cells. Previously, we structurally and functionally characterized three amoebapore isoforms in E. histolytica but recognized only one homolog in the closely related but non-pathogenic species Entamoeba dispar. Here, we identified two novel amoebapore homologs from E. dispar by molecular cloning. Despite strong resemblance of the primary structures of the homologs, molecular modeling predicts a species-specific variance between the peptide structures. Parallel isolation from trophozoite extracts of the two species revealed a lower amount of pore-forming peptides in E. dispar and substantially higher activity of the major isoform from E. histolytica towards natural membranes than that from E. dispar. Differences in abundance and activity of the lytic polypeptides may have an impact on the pathogenicity of amoebae.

Comparative modeling of amoebapores and granulysin based on the NK-lysin structure-structural and functional implications

Amoebapores, the pore-forming polypeptides of the protozoan parasite Entamoeba histolytica, and effector proteins of porcine and human lymphocytes, namely NK-lysin and granulysin, reveal a substantial sequence similarity despite their enormous evolutionary distance. Moreover, all these polypeptides display antibacterial activity and are in higher concentrations cytolytic to eukaryotic cells. The recently solved NMR structure of NK-lysin enabled us to build the three dimensional structures of amoebapores and granulysin by comparative modeling. The generated models revealed the expected similarities, but also fundamental differences with respect to charge distribution, hydrophobicity and core packing. The combination of these structural properties and known biochemical data provides insight in the different membrane-interacting mechanisms of the proteins. For amoebapores, exposed hydrophobic grooves and a locally loosely packed protein core may allow a rearrangement of the protein and therefore may account for its ability to penetrate the target membrane and to form defined ion channels in planar lipid bilayers. In contrast, the structural features of NK-lysin and granulysin appear to be suitable for a membrane-perturbing mode of action rather than for channel formation.

Antimicrobial and cytolytic polypeptides of amoeboid protozoa--effector molecules of primitive phagocytes

Amoebae are primitive, actively phagocytosing eukaryotic cells, many of which use bacteria as a major nutrient source. One may suppose that amoebae possess an array of potent antimicrobial molecules acting in synergy to combat bacterial growth inside their phagosomes. Lysosome-like granular vesicles of Entamoeba histolytica contain a family of 77-residue peptides with a compact alpha-helical, disulfide-bonded fold. These polypeptides, named amoebapores, exhibit antibacterial and cytolytic activity by forming pores in membranes of various origin. It is of particular interest that amoebapores are structurally and functionally most similar to polypeptides of mammalian cytotoxic lymphocytes. In addition, amoebic granules contain bacteriolytic proteins with lysozyme-like properties. Some amoebic polypeptides may represent archaic analogs of effector molecules from invertebrates and vertebrates.

Royal Society of Tropical Medicine and Hygiene Meeting at Manson House, London, 19 February 1998. Amoebic disease. Entamoeba histolytica and E. dispar: comparison of molecules considered important for host tissue destruction

Entamoeba histolytica and E. dispar are genetically distinct but closely related protozoan species. Both colonize the human gut but only E. histolytica is able to invade tissues and cause disease. Comparison of the 2 species may help to elucidate the specific mechanisms involved in the pathogenicity of E. histolytica. During the last few years, various amoeba molecules considered to be important for pathogenic tissue invasion have been identified and characterized, such as a galactose-inhibitable surface lectin, pore-forming peptides and cysteine proteinases. This review summarizes present knowledge about the structure and function of these molecules, with emphasis on the differences between E. histolytica and E. dispar.

Interaction of alpha-helical peptides with phospholipid membrane: effects of chain length and hydrophobicity of peptides

To investigate the interaction of amphiphilic alpha-helical peptides with phospholipid membranes, we synthesized Ac-(Leu-Ala-Arg-Leu)3-NHCH3 (4[3]) and three derivatives, in which the chain length and the size of the hydrophobic region of the peptides were different from each other. These peptides formed an alpha-helical structure in the presence of vesicles. In the membrane-perturbation measurement, only 43 showed a strong membrane-perturbation activity below phase-transition temperature (25 degrees C), but above phase-transition temperature (50 degrees C), most peptides showed similar strong activities. On the other hand, in membrane-fusion measurement the long peptides, e.g., Ac-(Leu-Ala-Arg-Leu)3-(Leu-Arg-Ala-Leu)3-NHCH3, had strong activities at low peptide concentrations at 25 degrees C. The present study indicated a parallel relationship did not always exist between membrane fusion and perturbation caused by peptides.

Progress towards development of a vaccine for amebiasis

The application of molecular biologic techniques over the past decade has seen a tremendous growth in our knowledge of the biology of Entamoeba histolytica, the causative agent of amebic dysentery and amebic liver abscess. This approach has also led to the identification and structural characterization of three amebic antigens, the serine-rich Entamoeba histolytica protein (SREHP), the 170-kDa subunit of the Gal/GalNAc binding lectin, and the 29-kDa cysteine-rich protein, which all show promise as recombinant antigen-based vaccines to prevent amebiasis. In recent studies, an immunogenic dodecapeptide derived from the SREHP molecule has been genetically fused to the B subunit of cholera toxin, to create a recombinant protein capable of inducing both antiamebic and anti-cholera toxin antibodies when administered by the oral route. Continued progress in this area will bring us closer to the goal of a cost-effective oral combination "enteric pathogen" vaccine, capable of inducing protective mucosal immune responses to several clinically important enteric diseases, including amebiasis.

Haemolytic activity of stonustoxin from stonefish (Synanceja horrida) venom: pore formation and the role of cationic amino acid residues

Stonustoxin (SNTX) is a two-subunit protein toxin purified from the venom of the stonefish (Synanceja horrida), which induces potent haemolytic activity. We examined the pore-forming property of this non-enzymic protein by an osmotic protection assay. SNTX-induced haemolysis was completely prevented by osmotic protectants of adequate size [poly(ethylene) glycol 3000; molecular diameter approx. 3.2 nm]. Uncharged molecules of smaller size, such as raffinose and poly(ethylene) glycol 1000-2000, failed to protect against cell lysis. These findings indicate that SNTX induces the formation of hydrophilic pores in the cell membrane, which results in the lysis of erythrocytes. Since cationic residues contribute significantly to the cytolytic activity of several other pore-forming toxins, we examined the role of positively charged lysine and arginine residues in the haemolytic activity of SNTX. SNTX lost its haemolytic activity when the positively charged side chains of lysine residues were neutralized or converted into negatively charged side chains upon carbamylation or succinylation respectively. The haemolytic activity of SNTX was also inhibited by the modification of positively charged arginine residues using 2,3-butanedione. The loss of haemolysis showed strong correlation with the number of Lys or Arg residues modified. CD analyses, however, showed that the conformation of SNTX was not significantly affected by these chemical modifications. Further, the haemolytic activity of SNTX was competitively inhibited by various negatively charged lipids, such as phosphatidylserine, cardiolipin and monosialogangliosides. These results indicate that SNTX induces potent haemolytic activity through the formation of pores in the cell membrane, and that cationic residues play a crucial role in its cytolytic mechanism.

Amoebapores

The enormous cytolytic potential of Entamoeba histolytica appeals to parasitologists and immunologists because it kills target cells in a contact-dependent reaction resembling that of cytotoxic lymphocytes. In this review, Matthias Leippe summarizes what is currently known about a family of pore-forming peptides termed 'amoebapores', to which the cytolytic effect has been attributed, and describes the structural and functional properties of these potent factors, as well as their structure-activity relationships. Finally, a comparison is made with effector molecules of the mammalian defensive system.

Shortened amoebapore analogs with enhanced antibacterial and cytolytic activity

Amoebapores are cytolytic peptides of Entamoeba histolytica which function by the formation of ion channels in target cell membranes. Three isoforms (amoebapore A, B, and C) exist in amoebic cytoplasmic granules. They are composed of 77 amino acid residues arranged in four alpha-helical domains. In order to analyze the structure-function relationships, 15 synthetic peptides of 24-25 residues were constructed based on the assumption that the third helix is the membrane-penetrating domain and on the previous finding that positively charged residues are significant for activity. Activity of these short versions of amoebapores was determined towards artificial and natural targets, such as liposomes, bacteria, erythrocytes and a human tumor cell line. It was found that some of the novel peptides were highly active and showed a broader activity spectrum compared to the parent molecules.

Spontaneous release of cysteine proteinases but not of pore-forming peptides by viable Entamoeba histolytica

Invasive properties of pathogenic Entamoeba histolytica have been postulated to depend on the secretion or release of cysteine proteinases and pore-forming peptides (amoebapores) by trophozoites. To establish whether such toxic molecules are released by viable trophozoites or upon cellular disintegration, amoebae were maintained in various culture media, and activities in supernatants were monitored over time in correlation to cellular integrity. By measuring the release of the cytoplasmic marker enzyme NADP(+)-alcohol dehydrogenase, it became apparent that release of amoebapore was accompanied by cellular disintegration. In contrast, considerable quantities of cysteine proteinases were found to be present in culture supernatants also when amoebae remained intact. Treatment of amoebae with concanavalin A, bacterial lipopolysaccharides or the calcium ionophore A23187 did not result in amoebapore secretion suggesting that here target cell contact is required as an essential stimulus.