Glycosylation reactions in Plasmodium falciparum, Toxoplasma gondii, and Trypanosoma brucei brucei probed by the use of synthetic peptides

Post-Translational Modifications of Proteins of Malaria Parasites during the Life Cycle

Post-translational modifications (PTMs) are essential for regulating protein functions, influencing various fundamental processes in eukaryotes. These include, but are not limited to, cell signaling, protein trafficking, the epigenetic control of gene expression, and control of the cell cycle, as well as cell proliferation, differentiation, and interactions between cells. In this review, we discuss protein PTMs that play a key role in the malaria parasite biology and its pathogenesis. Phosphorylation, acetylation, methylation, lipidation and lipoxidation, glycosylation, ubiquitination and sumoylation, nitrosylation and glutathionylation, all of which occur in malarial parasites, are reviewed. We provide information regarding the biological significance of these modifications along all phases of the complex life cycle of Plasmodium spp. Importantly, not only the parasite, but also the host and vector protein PTMs are often crucial for parasite growth and development. In addition to metabolic regulations, protein PTMs can result in epitopes that are able to elicit both innate and adaptive immune responses of the host or vector. We discuss some existing and prospective results from antimalarial drug discovery trials that target various PTM-related processes in the parasite or host.

VAR2CSA Antibodies in Non-Pregnant Populations

The Plasmodium falciparum protein VAR2CSA is a critical mediator of placental malaria, and VAR2CSA antibodies (IgGs) are important to protect pregnant women. Although infrequently detected outside pregnancy, VAR2CSA IgGs were reported in men and children from Colombia and Brazil and in select African populations. These findings raise questions about the specificity of VAR2CSA IgGs and the mechanisms by which they are acquired outside pregnancy. Here we review the data on VAR2CSA IgGs in men and children from different malaria-endemic regions. We discuss experimental factors that may affect interpretation of the serological data and consider the biological relevance of VAR2CSA IgGs in non-pregnant populations. We propose potential mechanisms for the acquisition of VARCSA IgGs outside of pregnancy. We identify knowledge gaps and research priorities.

Cryptosporidium hominis gene catalog: a resource for the selection of novel Cryptosporidium vaccine candidates

Human cryptosporidiosis, caused primarily by Cryptosporidium hominis and a subset of Cryptosporidium parvum, is a major cause of moderate-to-severe diarrhea in children under 5 years of age in developing countries and can lead to nutritional stunting and death. Cryptosporidiosis is particularly severe and potentially lethal in immunocompromised hosts. Biological and technical challenges have impeded traditional vaccinology approaches to identify novel targets for the development of vaccines against C. hominis, the predominant species associated with human disease. We deemed that the existence of genomic resources for multiple species in the genus, including a much-improved genome assembly and annotation for C. hominis, makes a reverse vaccinology approach feasible. To this end, we sought to generate a searchable online resource, termed C. hominis gene catalog, which registers all C. hominis genes and their properties relevant for the identification and prioritization of candidate vaccine antigens, including physical attributes, properties related to antigenic potential and expression data. Using bioinformatic approaches, we identified ∼400 C. hominis genes containing properties typical of surface-exposed antigens, such as predicted glycosylphosphatidylinositol (GPI)-anchor motifs, multiple transmembrane motifs and/or signal peptides targeting the encoded protein to the secretory pathway. This set can be narrowed further, e.g. by focusing on potential GPI-anchored proteins lacking homologs in the human genome, but with homologs in the other Cryptosporidium species for which genomic data are available, and with low amino acid polymorphism. Additional selection criteria related to recombinant expression and purification include minimizing predicted post-translation modifications and potential disulfide bonds. Forty proteins satisfying these criteria were selected from 3745 proteins in the updated C. hominis annotation. The immunogenic potential of a few of these is currently being tested.

Database URL:http://cryptogc.igs.umaryland.edu

Sugar activation and glycosylation in Plasmodium

Glycoconjugates are important mediators of host-pathogen interactions and are usually very abundant in the surface of many protozoan parasites. However, in the particular case of Plasmodium species, previous works show that glycosylphosphatidylinositol anchor modifications, and to an unknown extent, a severely truncated N-glycosylation are the only glycosylation processes taking place in the parasite. Nevertheless, a detailed analysis of the parasite genome and the recent identification of the sugar nucleotide precursors biosynthesized by Plasmodium falciparum support a picture in which several overlooked, albeit not very prominent glycosylations may be occurring during the parasite life cycle. In this work, the authors review recent developments in the characterization of the biosynthesis of glycosylation precursors in the parasite, focusing on the outline of the possible fates of these precursors.

Analysis of the glycoproteome of Toxoplasma gondii using lectin affinity chromatography and tandem mass spectrometry

Glycoproteins are involved in many important molecular recognition processes including invasion, adhesion, differentiation, and development. To identify the glycoproteins of Toxoplasma gondii, a proteomic analysis was undertaken. T. gondii proteins were prepared and fractioned using lectin affinity chromatography. The proteins in each fraction were then separated using SDS-PAGE and identified by tryptic in gel digestion followed by tandem mass spectrometry. Utilizing these methods 132 proteins were identified. Among the identified proteins were 17 surface proteins, 9 microneme proteins, 15 rhoptry proteins, 11 heat shock proteins (HSP), and 32 hypothetical proteins. Several proteins had 1-5 transmembrane domains (TMD) with some being as large as 608.3 kDa. Both lectin-fluorescence labeling and lectin blotting were employed to confirm the presence of carbohydrates on the surface or cytoplasm of T. gondii parasites. PCR demonstrated that selected hypothetical proteins were expressed in T. gondii tachyzoites. This data provides a large-scale analysis of the T. gondii glycoproteome. Studies of the function of glycosylation of these proteins may help elucidate mechanism(s) involved in invasion improving drug therapy as well as identify glycoproteins that may prove to be useful as vaccine candidates.

VAR2CSA domains expressed in Escherichia coli induce cross-reactive antibodies to native protein

The variant surface antigen VAR2CSA is a pregnancy malaria vaccine candidate, but its size and polymorphism are obstacles to development. We expressed 3D7-type VAR2CSA domains in Escherichia coli as insoluble His-tagged proteins (Duffy binding-like [DBL] domains DBL1, DBL3, DBL4, and DBL5) that were denatured and refolded or as soluble glutathione S-transferase-tagged protein (DBL6). Anti-DBL5 antiserum cross-reacted with surface proteins of chondroitin sulfate A (CSA)-binding laboratory strains (3D7-CSA and FCR3-CSA) and a clinical pregnancy malaria isolate, whereas anti-DBL6 antiserum reacted only to 3D7 surface protein. This is the first report that E. coli-expressed VAR2CSA domains induce antibody to native VAR2CSA.

N-linked glycosylation of proteins in the protozoan parasite Toxoplasma gondii

Toxoplasma gondii is an obligate intracellular parasite of animal cells. Infection of humans is common and may result in devastating disease, especially in immunocompromised individuals. Despite previous reports that N-glycosylation of proteins may be a rare post-translational modification in this and related organisms, we demonstrate that it is actually quite prevalent in Toxoplasma. N-Glycosylation is completely inhibited by treatment of parasites with tunicamycin, but this does not appear to exert its major effect on the parasites until they have egressed from their host cells. Although the tunicamycin-treated parasites appear structurally normal at this time they are not motile and mostly incapable of invading new host cells. The few tunicamycin-treated parasites that do invade are severely affected in their ability to replicate and accumulate with a distended endoplasmic reticulum, deformed nuclei, and without recognizable late secretory organelles. We provide experimental evidence that indicate that Toxoplasma N-glycans differ structurally from those in other eukaryotes.

Refolding, purification, and crystallization of apical membrane antigen 1 from Plasmodium falciparum

Extracellular domains of malaria antigens almost invariably contain disulphide linkages but lack N- and O-linked glycosylation. The best practical approach to generating recombinant extracellular Plasmodium proteins is not established and the problems encountered when using a bacterial expression/refolding approach are discussed in detail. Limited proteolysis experiments were used to identify a relatively non-flexible core region of the Plasmodium falciparum protein apical membrane antigen 1 (AMA1), and refolding/purification was used to generate two fragments of AMA1. Several chromatographically distinct AMA1 variants were identified that are presumably differentially refolded proteins. One of these AMA1 preparations proved to be crystallizable and generated two crystal forms that diffracted X-rays to 2 A resolution.

Golgi duplication in Trypanosoma brucei

Duplication of the single Golgi apparatus in the protozoan parasite Trypanosoma brucei has been followed by tagging a putative Golgi enzyme and a matrix protein with variants of GFP. Video microscopy shows that the new Golgi appears de novo, near to the old Golgi, about two hours into the cell cycle and grows over a two-hour period until it is the same size as the old Golgi. Duplication of the endoplasmic reticulum (ER) export site follows exactly the same time course. Photobleaching experiments show that the new Golgi is not the exclusive product of the new ER export site. Rather, it is supplied, at least in part, by material directly from the old Golgi. Pharmacological experiments show that the site of the new Golgi and ER export is determined by the location of the new basal body.

O-glycosylation in Toxoplasma gondii: identification and analysis of a family of UDP-GalNAc:polypeptide N-acetylgalactosaminyltransferases

The initiation of mucin-type O-glycosylation is catalysed by a family of UDP-GalNAc:polypeptide N-acetylgalactosaminyltransferases (EC 2.4.1.41). These enzymes are responsible for the transfer of N-acetylgalactosamine from the nucleotide sugar donor, UDP-GalNAc, to the hydroxyl group on specific serine or threonine residues in acceptor proteins. By screening a Toxoplasma gondii cDNA library, three distinct isoforms of the ppGalNAc-T gene family were cloned. Two additional isoforms were identified and partially cloned following analysis of the T. gondii genome sequence database. All of the cloned and identified ppGalNAc-T's are type II membrane proteins that share up to 50% amino acid sequence identity within the conserved catalytic domain. They each contain an N-terminal cytoplasmic domain, a hydrophobic transmembrane domain, and a lumenal domain; the latter consists of stem, catalytic, and lectin-like domains. Moreover, each of this ppGalNAc-T's contains important sequence motifs that are typical for this class of glycosyltransferases. These include a glycosyltransferase 1 motif containing the DXH sequence, a Gal/GalNAc-T motif, and the CLD and QXW sequence motifs located in alpha-, beta-, and gamma-repeats present within the lectin-like domain. The coding regions of T. gondii ppGalNAc-T1, -T2, and -T3 reside in multiple exons ranging in number from 6 to 10. Our results demonstrate that mucin-type O-glycosylation in T. gondii is catalysed by a multimember gene family, which is evolutionarily conserved from single-celled eukaryotes through nematodes and insects up to mammals. Taken together, this information creates the basis for future studies of the function of the ppGalNAc-T gene family in the pathobiology of this apicomplexan parasite.

cDNA cloning and expression of UDP-N-acetyl-D-galactosamine:polypeptide N-acetylgalactosaminyltransferase T1 from Toxoplasma gondii

We report the cloning, expression, and characterization of the first UDP-GalNAc:polypetide N-acetylgalactosaminyltransferase (ppGalNAc-T) from the human disease-causing parasite, Toxoplasma gondii. This enzyme is also the first characterized ppGalNAc-T of protozoan origin. This type of enzyme catalyzes the initial step of mucin-type O-glycosylation, that is, the transfer of GalNAc in O-glycosidic linkage to serine and threonine residues in polypeptides. We used polymerase chain reaction amplification with degenerate primers and hybridization screening of a T. gondii cDNA library to identify this enzyme. The resulting 84-kDa type II membrane protein contains a 49-amino acid N-terminal cytoplasmic domain, a 22-amino acid hydrophobic transmembrane domain, and a 680-amino acid C-terminal lumenal domain. Conceptual translation of the cDNA sequence reveals a relatively long (i.e. 135 amino acids) stem region and the presence of several important sequence motifs. The latter include a glycosyltransferase 1 (GT1) motif containing a DXH sequence, a Gal/GalNAc-T motif, and a region homologous to ricin lectin. Northern blot analysis identified a single 5.5-kb ppGalNAc-T transcript. Comparison of the cDNA and genomic DNA sequences reveals that this transferase is encoded by 10 exons in a 10 kb region. When the recombinant construct was expressed in stably transfected Drosophila melanogaster S2 cells, the purified protein exhibited transferase activity in vitro. The identification of this enzyme in T. gondii demonstrates that this human parasite has its own enzymatic machinery for the O-glycosylation of toxoplasmal proteins.

Plasmodium falciparum merozoite surface protein 1

In addition to the major carbohydrate moieties of the glycosylphosphatidylinositol (GPI) anchor, we report that Plasmodium falciparum merozoite surface protein 1 (MSP-1) bears O-GlcNAc modifications predominantly in beta-anomeric configuration, in both the C- and N-terminal portions of the protein. Subcellular fractionation of parasitized erythrocytes in the late trophozoite/schizont stage reveals that GPI-anchored C-terminal fragments of MSP-1 are recovered in Triton X-100 resistant, low-density membrane fractions. Our results suggest that O-GlcNAc-modified MSP-1 N-terminal fragments tend to localize within the parasitophorous vacuolar membrane while GPI-anchored MSP-1 C-terminal fragments associate with low-density, Triton X-100 resistant membrane domains (rafts), redistribute in the parasitized erythrocyte and are eventually shed as membrane vesicles that also contain the endogenous, GPI-linked CD59.

Toxoplasma gondii: conserved protein machinery in an unusual secretory pathway?

The protozoan parasite Toxoplasma gondii has a very specialized secretory apparatus. In this review we will discuss how different methodological approaches, including morphological studies, biochemistry and genetics, are revealing a novel secretory organization, whose function is performed, mainly, by a highly conserved protein machinery found in mammalian cells.

The nuclear envelope serves as an intermediary between the ER and Golgi complex in the intracellular parasite Toxoplasma gondii

Morphological examination of the highly polarized protozoan parasite Toxoplasma gondii suggests that secretory traffic in this organism progresses from the endoplasmic reticulum to the Golgi apparatus using the nuclear envelope as an intermediate compartment. While the endoplasmic reticulum is predominantly located near the basal end of the parasite, the Golgi is invariably adjacent to the apical end of the nucleus, and the space between the Golgi and nuclear envelope is filled with numerous coatomer-coated vesicles. Staining with antiserum raised against recombinant T. gondii beta-COP confirms its association with the apical juxtanuclear region. Perturbation of protein secretion using brefeldin A, microtubule inhibitors or dithiothreitol disrupts the Golgi, causing swelling of the nuclear envelope, particularly at its basal end. Prolonged drug treatment leads to gross distention of the endoplasmic reticulum, filling the basal end of the parasite. Cloning and sequencing of the T. gondii homolog of the chaperonin protein BiP identifies the carboxy-terminal amino acid sequence HDEL as this organism's endoplasmic reticulum-retention signal. Appending the HDEL motif to a recombinant secretory protein (a chimera between the parasite's major surface protein fusion, P30, and the Green Fluorescent Protein) causes this secretory reporter to be retained intracellularly. P30-GFP-HDEL fluorescence was most intense within the nuclear envelope, particularly at the apical end. These data support a model of secretion in which protein traffic from the endoplasmic reticulum to Golgi occurs via the apical end of the nuclear envelope.

Plasmodium falciparum: heterologous synthesis of the transmission-blocking vaccine candidate Pfs48/45 in recombinant vaccinia virus-infected cells

With the aim of developing transmission-blocking vaccines based on the sexual stage-specific surface antigen Pfs48/45 of the human malaria parasite Plasmodium falciparum, the gene encoding Pfs48/45 was incorporated into the genome of a recombinant vaccinia virus. In virus-infected mammalian tissue culture cells, recombinant Pfs48/45 antigen (rPfs48/45) is posttranslational modified to produce a highly N-glycosylated polypeptide. The rPfs48/45 protein was radiolabeled with ethanolamine, consisting of a further posttranslational modification in the form of a glycosylphosphatidylinositol anchor at its carboxy-terminal end. The rPfs48/45 was not detected on the surface of the infected cells; instead, it remained within the secretion pathway of mammalian cells irrespective of the duration of infection or culture temperature. Studies with monoclonal antibodies specific for disulfide band-dependent epitopes of Pfs48/45 revealed that recombinant Pfs48/45 is not folded in its authentic conformation even if N-glycosylation was chemically inhibited. Infection of mice and rabbits with recombinant virus elicited Pfs48/45-specific antibodies; however, the antisera failed to block parasite transmission in a standard mosquito membrane-feeding assay.

Pathways and strategies for developing a malaria blood-stage vaccine

In the past 10 years, our knowledge of the malaria parasite has increased enormously: identification and analysis of parasite antigens, demonstration of protection of monkeys and mice following immunization with these antigens, and better understanding of the mechanisms of immunity to malaria and the pathogenesis of disease in malaria. Powerful new adjuvants have been developed, some of which--it is hoped--will be suitable for human use. Recently, a successful human trial of a vaccine aimed at sporozoites (the stage inoculated by mosquitoes) was completed. However, it is the red blood cell stage of the parasite that causes disease, and it is against this stage--in which the parasite grows at an exponential rate--that it has proven very difficult to induce a protective immune response by vaccination. This review focuses on recent exciting developments toward a blood-stage vaccine. We analyze the major obstacles to vaccine development and outline a strategy involving public- and industry-funded research that should result in development of a vaccine.

Targeting the secretory pathway of Toxoplasma gondii

Little is known about the extent of conservation in the organization of the secretory pathway in organisms as different as prokaryotes, eukaryotes, and humans. The protozoan parasite Toxoplasma gondii allows easy genetic manipulations, and numerous vectors for selection of transgenic parasites have been developed. One approach to study the molecular mechanism of protein sorting and trafficking is the expression of foreign proteins. Here we describe the design and application of a vector that targets proteins to the secretory pathway of T. gondii and yields high-level expression of Escherichia coli reporter proteins. The general strategies and potential problems in expressing foreign proteins in T. gondii are discussed.

Rab GTPases and the unusual secretory pathway of plasmodium

A detailed analysis of some of the unusual features of secretory protein trafficking in Plasmodium has been hindered by the paucity of markers available for identifying the different compartments of the parasite's secretory apparatus. Gary Ward, Lew Tilney and Gordon Langsley here outline what is currently known about the secretory pathways of Plasmodium falciparum, and discuss how the recent description of a family of parasite rab genes is being used to generate a set of compartment-specific markers. They illustrate this point by describing studies with PfRab6, an established Golgi marker in other eukaryotic cells, which argue in favor of a functional Golgi in Plasmodium spp.

Dynamic O-linked glycosylation of nuclear and cytoskeletal proteins

Modification of Ser and Thr residues by attachment of O-linked N-acetylglucos-amine [Ser(Thr)-O-GlcNAcylation] to eukaryotic nuclear and cytosolic proteins is as dynamic and possibly as abundant as Ser(Thr) phosphorylation. Known O-GlcNAcylated proteins include cytoskeletal proteins and their regulatory proteins; viral proteins; nuclear-pore, heat-shock, tumor-suppressor, and nuclearoncogene proteins; RNA polymerase II catalytic subunit; and a multitude of transcription factors. Although functionally diverse, all of these proteins are also phosphoproteins. Most O-GlcNAcylated proteins form highly regulated multimeric associations that are dependent upon their posttranslational modifications. Evidence is mounting that O-GlcNAcylation is an important regulatory modification that may have a reciprocal relationship with O-phosphorylation and may modulate many biological processes in eukaryotes.

Identification of a family of Rab G-proteins in Plasmodium falciparum and a detailed characterisation of pfrab6

As a first step towards developing a set of compartment-specific probes for studying protein trafficking in the malaria-infected erythrocyte, we describe here a family of Plasmodium falciparum Rab proteins. We characterise in detail P. falciparum Rab6 (PfRab6) a marker which in other cells is specific for the Golgi/trans Golgi network. Although PfRab6 mRNA is expressed throughout the intraerythrocytic cycle, maximal expression occurs at the trophozoite stage. Immunofluorescence microscopy shows that the distribution of PfRab6 changes during the final stages of parasite maturation, coalescing into multiple foci, each of which is associated with the nucleus of a forming daughter parasite.

Protective immune responses to apical membrane antigen 1 of Plasmodium chabaudi involve recognition of strain-specific epitopes

Apical membrane antigen 1 (AMA-1), an asexual blood-stage antigen of Plasmodium falciparum, is an important candidate for testing as a component of a malaria vaccine. This study investigates the nature of diversity in the Plasmodium chabaudi adami homolog of AMA-1 and the impact of that diversity on the efficacy of the recombinant antigen as a vaccine against challenge with a heterologous strain of P. chabaudi. The nucleotide sequence of the AMA-1 gene from strain DS differs from the published 556KA sequence at 79 sites. The large number of mutations, the nonrandom distribution of both synonymous and nonsynonymous mutations, and the nature of both the codon changes and the resulting amino acid substitutions suggest that positive selection operates on the AMA-1 gene in regions coding for antigenic sites. Protective immune responses induced by AMA-1 were strain specific. Immunization of mice with the refolded ectodomain of DS AMA-1 provided partial protection against challenge with virulent DS (homologous) parasites but failed to protect against challenge with avirulent 556KA (heterologous) parasites. Passive immunization of mice with rabbit antibodies raised against the same antigen had little effect on heterologous challenge but provided significant protection against the homologous DS parasites.

N-linked glycoproteins are related to schizogony of the intraerythrocytic stage in Plasmodium falciparum

Although the existence of O-linked oligosaccharide residues in glycoproteins of Plasmodium falciparum has been shown, the existence of N-linked glycoproteins is still a matter of controversy and skepticism. This report demonstrates the unequivocal presence of N-linked glycoproteins in P. falciparum, principally in the ring and young trophozoite stages of the intraerythrocytic cycle. These glycoproteins lose their capacity to bind to concanavalin A-Sepharose after treatment of cultures with tunicamycin under conditions that do not affect protein synthesis. When the glycoproteins were treated with N-Glycanase(R), oligosaccharides were released. It was possible to identify an N-linked glycoprotein of >200 kDa in the ring stage and also N-linked glycoproteins in the range of 200-30 kDa in the trophozoite stage. Treatment of trophozoites with 12 microM tunicamycin inhibited differentiation to the schizont stage. To our knowledge, this is the first report in the literature unequivocally showing N-linked glycoproteins in trophozoites of P. falciparum as well as their importance for the differentiation of the intraerythrocytic stages of this parasite.

The secretary pathway of plasmodium falciparum regulates transport of p82/RAP1 to the rhoptries

The rhoptries of Plasmodium falciparum are formed during a restricted period in the asexual erythrocytic cycle. The steps required for rhoptry biogenesis and the pathway for targeting proteins to the rhoptries have not been elucidated. Using the maturation of the Rhoptry-Associated Protein 1 (RAP-1) gene product to study these steps, it is reported here that a secretory pathway controls transport of protein complexes containing RAP-1 products to the rhoptries. Both brefeldin A (BFA) and low temperature reversibly block the processing of an 86-kDa precursor (Pr86) to the mature 82-kDa RAP-1 product (p82). Furthermore, the points of action of BFA and low temperature appear to overlap since their sequential application reversibly prevents Pr86 processing. Treatment of intact cells with N-ethylmaleimide, which prevents the fusion of transport vesicles with Golgi membranes in other eukaryotic cells, irreversibly blocks processing of Pr86. The role of the secretory pathway in targeting p82 protein complexes to the rhoptries product of RAP-1. These in vitro results also reveal that the RAP-1 product contains a cleavable N-terminal signal peptide and appears to be initially synthesized as an 84-kDa protein. The above data indicate that transport of p82 to the rhoptries is regulated by the secretory pathway and that the RAP-1 primary translation product differs in apparent molecular weight from the in vivo precursor Pr86. Our results suggest that rhoptry biogenesis is controlled in part by the secretory pathway and that the RAP-1 gene product acquires a previously undetected protein modification during its maturation.

Protein trafficking in kinetoplastid protozoa

The kinetoplastid protozoa infect hosts ranging from invertebrates to plants and mammals, causing diseases of medical and economic importance. They are the earliest-branching organisms in eucaryotic evolution to have either mitochondria or peroxisome-like microbodies. Investigation of their protein trafficking enables us to identify characteristics that have been conserved throughout eucaryotic evolution and also reveals how far variations, or alternative mechanisms, are possible. Protein trafficking in kinetoplastids is in many respects similar to that in higher eucaryotes, including mammals and yeasts. Differences in signal sequence specificities exist, however, for all subcellular locations so far examined in detail--microbodies, mitochondria, and endoplasmic reticulum--with signals being more degenerate, or shorter, than those of their higher eucaryotic counterparts. Some components of the normal array of trafficking mechanisms may be missing in most (if not all) kinetoplastids: examples are clathrin-coated vesicles, recycling receptors, and mannose 6-phosphate-mediated lysosomal targeting. Other aspects and structures are unique to the kinetoplastids or are as yet unexplained. Some of these peculiarities may eventually prove to be weak points that can be used as targets for chemotherapy; others may turn out to be much more widespread than currently suspected.