Localization of ferrochelatase in Plasmodium falciparum

E3 ubiquitin ligase RNF123-deficient mice exhibit reduced parasitemia and mortality in rodent malaria (Plasmodium yoelii 17XL) infection

Increased levels of several human ubiquitin ligases, including ring finger protein 123 (RNF123), in red blood cells with Plasmodium falciparum infection, have been reported. RNF123 is an E3 ubiquitin ligase that is highly expressed in erythroid cells. However, the function of the RNF123 gene and the relationship between the RNF123 gene and malarial parasite has not been clarified in vivo. In this study, we generated RNF123-deficient mice using the CRISPR/Cas9 system, and analyzed malaria susceptibility and erythrocyte morphology. The levels of parasitemia 5 days post-infection and mortality 21 days post-infection with the lethal type of rodent malaria (Plasmodium yoelii 17XL) in RNF123-deficient mice was significantly lower than that in wild-type mice. In contrast, red blood cell morphology in RNF123-deficient mice was almost normal. These results suggest that erythrocytic RNF123 plays a role in susceptibility to rodent malaria infection, but does not play a role in erythrocyte morphology.

Reactive Oxygen Species as the Brainbox in Malaria Treatment

Several measures are in place to combat the worldwide spread of malaria, especially in regions of high endemicity. In part, most common antimalarials, such as quinolines and artemisinin and its derivatives, deploy an ROS-mediated approach to kill malaria parasites. Although some antimalarials may share similar targets and mechanisms of action, varying levels of reactive oxygen species (ROS) generation may account for their varying pharmacological activities. Regardless of the numerous approaches employed currently and in development to treat malaria, concerningly, there has been increasing development of resistance by Plasmodium falciparum, which can be connected to the ability of the parasites to manage the oxidative stress from ROS produced under steady or treatment states. ROS generation has remained the mainstay in enforcing the antiparasitic activity of most conventional antimalarials. However, a combination of conventional drugs with ROS-generating ability and newer drugs that exploit vital metabolic pathways, such antioxidant machinery, could be the way forward in effective malaria control.

Malaria parasites both repress host CXCL10 and use it as a cue for growth acceleration

Pathogens are thought to use host molecular cues to control when to initiate life-cycle transitions, but these signals are mostly unknown, particularly for the parasitic disease malaria caused by Plasmodium falciparum. The chemokine CXCL10 is present at high levels in fatal cases of cerebral malaria patients, but is reduced in patients who survive and do not have complications. Here we show a Pf 'decision-sensing-system' controlled by CXCL10 concentration. High CXCL10 expression prompts P. falciparum to initiate a survival strategy via growth acceleration. Remarkably, P. falciparum inhibits CXCL10 synthesis in monocytes by disrupting the association of host ribosomes with CXCL10 transcripts. The underlying inhibition cascade involves RNA cargo delivery into monocytes that triggers RIG-I, which leads to HUR1 binding to an AU-rich domain of the CXCL10 3'UTR. These data indicate that when the parasite can no longer keep CXCL10 at low levels, it can exploit the chemokine as a cue to shift tactics and escape.

Host Porphobilinogen Deaminase Deficiency Confers Malaria Resistance in Plasmodium chabaudi but Not in Plasmodium berghei or Plasmodium falciparum During Intraerythrocytic Growth

An important component in host resistance to malaria infection are inherited mutations that give rise to abnormalities and deficiencies in erythrocyte proteins and enzymes. Understanding how such mutations confer protection against the disease may be useful for developing new treatment strategies. A mouse ENU-induced mutagenesis screen for novel malaria resistance-conferring mutations identified a novel non-sense mutation in the gene encoding porphobilinogen deaminase (PBGD) in mice, denoted here as PbgdMRI58155. Heterozygote PbgdMRI58155 mice exhibited ~50% reduction in cellular PBGD activity in both mature erythrocytes and reticulocytes, although enzyme activity was ~10 times higher in reticulocytes than erythrocytes. When challenged with blood-stage P. chabaudi, which preferentially infects erythrocytes, heterozygote mice showed a modest but significant resistance to infection, including reduced parasite growth. A series of assays conducted to investigate the mechanism of resistance indicated that mutant erythrocyte invasion by P. chabaudi was normal, but that following intraerythrocytic establishment a significantly greater proportions of parasites died and therefore, affected their ability to propagate. The Plasmodium resistance phenotype was not recapitulated in Pbgd-deficient mice infected with P. berghei, which prefers reticulocytes, or when P. falciparum was cultured in erythrocytes from patients with acute intermittent porphyria (AIP), which had modest (20-50%) reduced levels of PBGD. Furthermore, the growth of Pbgd-null P. falciparum and Pbgd-null P. berghei parasites, which grew at the same rate as their wild-type counterparts in normal cells, were not affected by the PBGD-deficient background of the AIP erythrocytes or Pbgd-deficient mice. Our results confirm the dispensability of parasite PBGD for P. berghei infection and intraerythrocytic growth of P. falciparum, but for the first time identify a requirement for host erythrocyte PBGD by P. chabaudi during in vivo blood stage infection.

Apicomplexan-like parasites are polyphyletic and widely but selectively dependent on cryptic plastid organelles

The phylum Apicomplexa comprises human pathogens such as Plasmodium but is also an under-explored hotspot of evolutionary diversity central to understanding the origins of parasitism and non-photosynthetic plastids. We generated single-cell transcriptomes for all major apicomplexan groups lacking large-scale sequence data. Phylogenetic analysis reveals that apicomplexan-like parasites are polyphyletic and their similar morphologies emerged convergently at least three times. Gregarines and eugregarines are monophyletic, against most expectations, and rhytidocystids and Eleutheroschizon are sister lineages to medically important taxa. Although previously unrecognized, plastids in deep-branching apicomplexans are common, and they contain some of the most divergent and AT-rich genomes ever found. In eugregarines, however, plastids are either abnormally reduced or absent, thus increasing known plastid losses in eukaryotes from two to four. Environmental sequences of ten novel plastid lineages and structural innovations in plastid proteins confirm that plastids in apicomplexans and their relatives are widespread and share a common, photosynthetic origin.

Microscopic parasites known collectively as apicomplexans are responsible for several infectious diseases in humans including malaria and toxoplasmosis. The cells of the malaria parasite and many other apicomplexans contain compartments known as cryptic chloroplasts that produce molecules the parasites need to survive. Cryptic chloroplasts are similar to the chloroplasts found in plant cells, but unlike plants the compartments in apicomplexans are unable to harvest energy from sunlight.

Since the cells of humans and other animals do not contain chloroplasts, cryptic chloroplasts are a potential target for new drugs to treat diseases caused by apicomplexans. However, it remains unclear how widespread cryptic chloroplasts are in these parasites, largely because few apicomplexans have been successfully grown in the laboratory.

To address this question, Janouškovec et al. used an approach called single-cell transcriptomics to study ten different apicomplexans. This provided new data about the genetic make-up of each parasite that the team analysed to find out how they are related to one another. The analysis revealed that, unexpectedly, apicomplexan parasites do not share a close common ancestor and are therefore not a natural grouping from an evolutionary perspective. Instead, their similar physical appearances and lifestyles evolved independently on at least three separate occasions.

Further analysis demonstrated that cryptic chloroplasts are common in apicomplexan parasites, including in lineages where they were not previously known to exist. However, at least three lineages of apicomplexans have independently lost their cryptic chloroplasts.

The findings of Janouškovec et al. shed new light on the importance of chloroplasts in the evolution of life and may help develop new treatments for diseases caused by apicomplexan parasites. Several drugs targeting the cryptic chloroplasts in malaria parasites are currently in clinical trials, and this work suggests that these drugs may also have the potential to be used against other apicomplexan parasites in the future.

Griseofulvin impairs intraerythrocytic growth of Plasmodium falciparum through ferrochelatase inhibition but lacks activity in an experimental human infection study

Griseofulvin, an orally active antifungal drug used to treat dermatophyte infections, has a secondary effect of inducing cytochrome P450-mediated production of N-methyl protoporphyrin IX (N-MPP). N-MPP is a potent competitive inhibitor of the heme biosynthetic-enzyme ferrochelatase, and inhibits the growth of cultured erythrocyte stage Plasmodium falciparum. Novel drugs against Plasmodium are needed to achieve malaria elimination. Thus, we investigated whether griseofulvin shows anti-plasmodial activity. We observed that the intraerythrocytic growth of P. falciparum is inhibited in red blood cells pretreated with griseofulvin in vitro. Treatment with 100 μM griseofulvin was sufficient to prevent parasite growth and induce the production of N-MPP. Inclusion of the ferrochelatase substrate PPIX blocked the inhibitory activity of griseofulvin, suggesting that griseofulvin exerts its activity through the N-MPP-dependent inhibition of ferrochelatase. In an ex-vivo study, red blood cells from griseofulvin-treated subjects were refractory to the growth of cultured P. falciparum. However, in a clinical trial griseofulvin failed to show either therapeutic or prophylactic effect in subjects infected with blood stage P. falciparum. Although the development of griseofulvin as an antimalarial is not warranted, it represents a novel inhibitor of P. falciparum growth and acts via the N-MPP-dependent inhibition of ferrochelatase.

The influence of host genetics on erythrocytes and malaria infection: is there therapeutic potential?

As parasites, Plasmodium species depend upon their host for survival. During the blood stage of their life-cycle parasites invade and reside within erythrocytes, commandeering host proteins and resources towards their own ends, and dramatically transforming the host cell. Parasites aptly avoid immune detection by minimizing the exposure of parasite proteins and removing themselves from circulation through cytoadherence. Erythrocytic disorders brought on by host genetic mutations can interfere with one or more of these processes, thereby providing a measure of protection against malaria to the host. This review summarizes recent findings regarding the mechanistic aspects of this protection, as mediated through the parasites interaction with abnormal erythrocytes. These novel findings include the reliance of the parasite on the host enzyme ferrochelatase, and the discovery of basigin and CD55 as obligate erythrocyte receptors for parasite invasion. The elucidation of these naturally occurring malaria resistance mechanisms is increasing the understanding of the host-parasite interaction, and as discussed below, is providing new insights into the development of therapies to prevent this disease.

Plasmodium falciparum mitochondria import tRNAs along with an active phenylalanyl-tRNA synthetase

The Plasmodium falciparum protein translation enzymes aminoacyl-tRNA synthetases (aaRSs) are an emergent family of drug targets. The aaRS ensemble catalyses transfer of amino acids to cognate tRNAs, thus providing charged tRNAs for ribosomal consumption. P. falciparum proteome expression relies on a total of 36 aaRSs for the three translationally independent compartments of cytoplasm, apicoplast and mitochondria. In the present study, we show that, of this set of 36, a single genomic copy of mitochondrial phenylalanyl-tRNA synthetase (mFRS) is targeted to the parasite mitochondria, and that the mFRS gene is exclusive to malaria parasites within the apicomplexan phyla. Our protein cellular localization studies based on immunofluorescence data show that, along with mFRS, P. falciparum harbours two more phenylalanyl-tRNA synthetase (FRS) assemblies that are localized to its apicoplast and cytoplasm. The 'extra' mFRS is found in mitochondria of all asexual blood stage parasites and is competent in aminoacylation. We show further that the parasite mitochondria import tRNAs from the cytoplasmic tRNA pool. Hence drug targeting of FRSs presents a unique opportunity to potentially stall protein production in all three parasite translational compartments.

Red cells from ferrochelatase-deficient erythropoietic protoporphyria patients are resistant to growth of malarial parasites

Many red cell polymorphisms are a result of selective pressure by the malarial parasite. Here, we add another red cell disease to the panoply of erythrocytic changes that give rise to resistance to malaria. Erythrocytes from individuals with erythropoietic protoporphyria (EPP) have low levels of the final enzyme in the heme biosynthetic pathway, ferrochelatase. Cells from these patients are resistant to the growth of Plasmodium falciparum malarial parasites. This phenomenon is due to the absence of ferrochelatase and not an accumulation of substrate, as demonstrated by the normal growth of P falciparum parasites in the EPP phenocopy, X-linked dominant protoporphyria, which has elevated substrate, and normal ferrochelatase levels. This observation was replicated in a mouse strain with a hypomorphic mutation in the murine ferrochelatase gene. The parasite enzyme is not essential for parasite growth as Plasmodium berghei parasites carrying a complete deletion of the ferrochelatase gene grow normally in erythrocytes, which confirms previous studies. That ferrochelatase is essential to parasite growth was confirmed by showing that inhibition of ferrochelatase using the specific competitive inhibitor, N-methylprotoporphyrin, produced a potent growth inhibition effect against cultures of P falciparum. This raises the possibility of targeting human ferrochelatase in a host-directed antimalarial strategy.

Radicicol confers mid-schizont arrest by inhibiting mitochondrial replication in Plasmodium falciparum

Radicicol, an antifungal antibiotic, was previously identified as a compound having antimalarial activity. However, its mechanism of action in Plasmodium falciparum was not elucidated. While characterizing its antimalarial function, we observed that radicicol manifested two distinct developmental defects in cultured P. falciparum in a concentration-dependent manner. At a low concentration of radicicol, a significant percentage of drug-treated parasites were arrested at the schizont stage, while at a higher concentration, the parasites were unable to multiply from schizont to ring. Also, the newly formed rings and trophozoites were extremely delayed in development, eventually leading to cell death. We intended to characterize the potential molecular target of radicicol at its sublethal doses. Our results demonstrated that radicicol specifically impaired mitochondrial replication. This decrement was associated with a severalfold increment of the topoisomerase VIB transcript as well as protein in treated cells over that of untreated parasites. Topoisomerase VIB was found to be localized in the organelle fraction. Our docking study revealed that radicicol fits into the Bergerat fold of Pf topoisomerase VIB present in its ATPase domain. Altogether, these data allow us to conclude that P. falciparum topoisomerase VIB might be one of the targets of radicicol causing inhibition of mitochondrial replication. Hence, radicicol can be suitably employed to explore the mitochondrial physiology of malaria parasites.

An FtsH protease is recruited to the mitochondrion of Plasmodium falciparum

The two organelles, apicoplast and mitochondrion, of the malaria parasite Plasmodium falciparum have unique morphology in liver and blood stages; they undergo complex branching and looping prior to division and segregation into daughter merozoites. Little is known about the molecular processes and proteins involved in organelle biogenesis in the parasite. We report the identification of an AAA+/FtsH protease homolog (PfFtsH1) that exhibits ATP- and Zn(2+)-dependent protease activity. PfFtsH1 undergoes processing, forms oligomeric assemblies, and is associated with the membrane fraction of the parasite cell. Generation of a transfectant parasite line with hemagglutinin-tagged PfFtsH1, and immunofluorescence assay with anti-PfFtsH1 Ab demonstrated that the protein localises to P. falciparum mitochondria. Phylogenetic analysis and the single transmembrane region identifiable in PfFtsH1 suggest that it is an i-AAA like inner mitochondrial membrane protein. Expression of PfFtsH1 in Escherichia coli converted a fraction of bacterial cells into division-defective filamentous forms implying a sequestering effect of the Plasmodium factor on the bacterial homolog, indicative of functional conservation with EcFtsH. These results identify a membrane-associated mitochondrial AAA+/FtsH protease as a candidate regulatory protein for organelle biogenesis in P. falciparum.

Mitochondrial localization of ferrochelatase in a red alga Cyanidioschyzon merolae

Ferrochelatase (FECH) is an essential enzyme for the final step of heme biosynthesis. In green plants, its activity has been reported in both plastids and mitochondria. However, the precise subcellular localization of FECH remains uncertain. In this study, we analyzed the localization of FECH in the unicellular red alga, Cyanidioschyzon merolae. Immunoblot and enzyme activity analyses of subcellular fractions localized little FECH in the plastid. In addition, immunofluorescence microscopy identified that both intrinsic and hemagglutinin (HA)-tagged FECH are localized in the mitochondrion. We therefore conclude that FECH is localized in the mitochondrion in C. merolae.

Targeting apicoplasts in malaria parasites

INTRODUCTION

The relict plastid, or apicoplast, is a characteristic feature of Plasmodium spp. and reflects the unusual evolutionary origins of these parasites. The essential role this organelle plays in the life of the parasite, and its unusual, non-mammalian metabolism, make the apicoplast an excellent drug target.

AREAS COVERED

This review focuses on the biological role of the apicoplast in the erythrocytic life cycle and what that reveals about existing drug targets. We also discuss the future of the apicoplast in the development of anti-malarials, emphasizing those pathways with greatest potential as a source of novel drug targets and emphasizing the need to understand in vitro drug responses to optimize eventual use of these drugs to treat malaria.

EXPERT OPINION

More than a decade of research on the apicoplast has confirmed the promise of this organelle as a source of drug targets. It is now possible to rationally assess the value of existing drugs and new drug targets, and to understand the role these drugs can play in the arsenal of anti-malarial treatments.

Novel antimalarial drug targets: hope for new antimalarial drugs

Malaria is a major global threat, that results in more than 2 million deaths each year. The treatment of malaria is becoming extremely difficult due to the emergence of drug-resistant parasites, the absence of an effective vaccine, and the spread of insecticide-resistant vectors. Thus, malarial therapy needs new chemotherapeutic approaches leading to the search for new drug targets. Here, we discuss different approaches to identifying novel antimalarial drug targets. We have also given due attention to the existing validated targets with a view to develop novel, rationally designed lead molecules. Some of the important parasite proteins are claimed to be the targets; however, further in vitro or in vivo structure-function studies of such proteins are crucial to validate these proteins as suitable targets. The interactome analysis among apicoplast, mitochondrion and genomic DNA will also be useful in identifying vital pathways or proteins regulating critical pathways for parasite growth and survival, and could be attractive targets. Molecules responsible for parasite invasion to host erythrocytes and ion channels of infected erythrocytes, essential for intra-erythrocyte survival and stage progression of parasites are also becoming attractive targets. This review will discuss and highlight the current understanding regarding the potential antimalarial drug targets, which could be utilized to develop novel antimalarials.

Quantitative time-course profiling of parasite and host cell proteins in the human malaria parasite Plasmodium falciparum

Studies of the Plasmodium falciparum transcriptome have shown that the tightly controlled progression of the parasite through the intra-erythrocytic developmental cycle (IDC) is accompanied by a continuous gene expression cascade in which most expressed genes exhibit a single transcriptional peak. Because the biochemical and cellular functions of most genes are mediated by the encoded proteins, understanding the relationship between mRNA and protein levels is crucial for inferring biological activity from transcriptional gene expression data. Although studies on other organisms show that <50% of protein abundance variation may be attributable to corresponding mRNA levels, the situation in Plasmodium is further complicated by the dynamic nature of the cyclic gene expression cascade. In this study, we simultaneously determined mRNA and protein abundance profiles for P. falciparum parasites during the IDC at 2-hour resolution based on oligonucleotide microarrays and two-dimensional differential gel electrophoresis protein gels. We find that most proteins are represented by more than one isoform, presumably because of post-translational modifications. Like transcripts, most proteins exhibit cyclic abundance profiles with one peak during the IDC, whereas the presence of functionally related proteins is highly correlated. In contrast, the abundance of most parasite proteins peaks significantly later (median 11 h) than the corresponding transcripts and often decreases slowly in the second half of the IDC. Computational modeling indicates that the considerable and varied incongruence between transcript and protein abundance may largely be caused by the dynamics of translation and protein degradation. Furthermore, we present cyclic abundance profiles also for parasite-associated human proteins and confirm the presence of five human proteins with a potential role in antioxidant defense within the parasites. Together, our data provide fundamental insights into transcript-protein relationships in P. falciparum that are important for the correct interpretation of transcriptional data and that may facilitate the improvement and development of malaria diagnostics and drug therapy.

Nuclear-encoded DnaJ homologue of Plasmodium falciparum interacts with replication ori of the apicoplast genome

The apicoplast of Plasmodium falciparum carries a 35 kb circular genome (plDNA) that replicates at the late trophozoite stage of the parasite intraerythocytic cycle. plDNA replication proceeds predominantly via a d-loop/bi-directional ori mechanism with replication ori localized within inverted repeat region. Although replication of the apicoplast genome is a validated drug target, the proteins involved in the replication process are only partially characterized. We analysed DNA-protein interactions at a plDNA replication ori region and report the identification of a nuclear-encoded DnaJ homologue that binds directly to ori elements of the plDNA molecule. PfDnaJ(A) interacted with the minor groove of the DNA double-helix and recognized a 13 bp sequence within the ori. Inhibition of binding with anti-PfDnaJ(A) antibodies confirmed identity of the protein in DNA-binding experiments with organellar protein fractions. The DNA-binding domain of the approximately 69 kDa PfDnaJ(A) lay within the N-terminal 38 kDa region that carries DnaJ signature motifs. In contrast to PfDnaJ(A) in parasite organellar fractions, the recombinant protein interacted with DNA in a sequence non-specific manner. Our results suggest a role for PfDnaJ(A) in replication/repair of the apicoplast genome.

Rewiring and regulation of cross-compartmentalized metabolism in protists

Plastid acquisition, endosymbiotic associations, lateral gene transfer, organelle degeneracy or even organelle loss influence metabolic capabilities in many different protists. Thus, metabolic diversity is sculpted through the gain of new metabolic functions and moderation or loss of pathways that are often essential in the majority of eukaryotes. What is perhaps less apparent to the casual observer is that the sub-compartmentalization of ubiquitous pathways has been repeatedly remodelled during eukaryotic evolution, and the textbook pictures of intermediary metabolism established for animals, yeast and plants are not conserved in many protists. Moreover, metabolic remodelling can strongly influence the regulatory mechanisms that control carbon flux through the major metabolic pathways. Here, we provide an overview of how core metabolism has been reorganized in various unicellular eukaryotes, focusing in particular on one near universal catabolic pathway (glycolysis) and one ancient anabolic pathway (isoprenoid biosynthesis). For the example of isoprenoid biosynthesis, the compartmentalization of this process in protists often appears to have been influenced by plastid acquisition and loss, whereas for glycolysis several unexpected modes of compartmentalization have emerged. Significantly, the example of trypanosomatid glycolysis illustrates nicely how mathematical modelling and systems biology can be used to uncover or understand novel modes of pathway regulation.

The evolution, metabolism and functions of the apicoplast

The malaria parasite, Plasmodium falciparum, harbours a relict plastid known as the ‘apicoplast’. The discovery of the apicoplast ushered in an exciting new prospect for drug development against the parasite. The eubacterial ancestry of the organelle offers a wealth of opportunities for the development of therapeutic interventions. Morphological, biochemical and bioinformatic studies of the apicoplast have further reinforced its ‘plant-like’ characteristics and potential as a drug target. However, we are still not sure why the apicoplast is essential for the parasite's survival. This review explores the origins and metabolic functions of the apicoplast. In an attempt to decipher the role of the organelle within the parasite we also take a closer look at the transporters decorating the plastid to better understand the metabolic exchanges between the apicoplast and the rest of the parasite cell.

Characterization of coproporphyrinogen III oxidase in Plasmodium falciparum cytosol

A unique hybrid pathway has been proposed for de novo heme biosynthesis in Plasmodium falciparum involving three different compartments of the parasite, namely mitochondrion, apicoplast and cytosol. While parasite mitochondrion and apicoplast have been shown to harbor key enzymes of the pathway, there has been no experimental evidence for the involvement of parasite cytosol in heme biosynthesis. In this study, a recombinant P. falciparum coproporphyrinogen III oxidase (rPfCPO) was produced in E. coli and confirmed to be active under aerobic conditions. rPfCPO behaved as a monomer of 61kDa molecular mass in gel filtration analysis. Immunofluorescence studies using antibodies to rPfCPO suggested that the enzyme was present in the parasite cytosol. These results were confirmed by detection of enzyme activity only in the parasite soluble fraction. Western blot analysis with anti-rPfCPO antibodies also revealed a 58kDa protein only in this fraction and not in the membrane fraction. The cytosolic presence of PfCPO provides evidence for a hybrid heme-biosynthetic pathway in the malarial parasite.

Evolving insights into protein trafficking to the multiple compartments of the apicomplexan plastid

The apicoplast is a relict plastid found in many medically important apicomplexan parasites, such as Plasmodium and Toxoplasma. Phylogenetic analysis and the presence of four bounding membranes indicate that the apicoplast arose from a secondary endosymbiosis. Here we review what has been discovered about the complex journey proteins take to reach compartments of the apicoplast. The targeting sequences for luminal proteins are well-defined, but those routing proteins to other compartments are only beginning to be studied. Recent work suggests that the trafficking mechanisms involve a variety of molecules of different phylogenetic origins. We highlight some remaining questions regarding protein trafficking to this divergent organelle.

The malarial parasite Plasmodium falciparum imports the human protein peroxiredoxin 2 for peroxide detoxification

Coevolution of the malarial parasite and its human host has resulted in a complex network of interactions contributing to the homeodynamics of the host-parasite unit. As a rapidly growing and multiplying organism, Plasmodium falciparum depends on an adequate antioxidant defense system that is efficient despite the absence of genuine catalase and glutathione peroxidase. Using different experimental approaches, we demonstrate that P. falciparum imports the human redox-active protein peroxiredoxin 2 (hPrx-2, hTPx1) into its cytosol. As shown by confocal microscopy and immunogold electron microscopy, hPrx-2 is also present in the Maurer's clefts, organelles that are described as being involved in parasite protein export. Enzyme kinetic analyses prove that hPrx-2 accepts Plasmodium cytosolic thioredoxin 1 as a reducing substrate. hPrx-2 accounts for roughly 50% of thioredoxin peroxidase activity in parasite extracts, thus indicating a functional role of hPrx-2 as an enzymatic scavenger of peroxides in the parasite. Under chloroquine treatment, a drug promoting oxidative stress, the abundance of hPrx-2 in the parasite increases significantly. P. falciparum has adapted to adopt the hPrx-2, thereby using the host protein for its own purposes.

Mitochondrial localization of functional ferrochelatase from Plasmodium falciparum

In the malarial parasite, enzymes of heme-biosynthetic pathway are distributed in different cellular compartments. The site of localization of ferrochelatase in the malarial parasite is crucial, since it will decide the ultimate site of heme synthesis. Earlier results have differed in terms of localization, being the mitochondrion or apicoplast and the functional enzyme has not been cloned, expressed and characterized. The present study reveals that Plasmodium falciparum ferrochelatase (PfFC) gene encodes multiple transcripts of which the one encoding the full length functional protein (PfFC) has been cloned and the recombinant protein over-expressed and purified from E. coli cells. The enzyme shows maximum activity with iron, while zinc is a poor substrate. Immunofluorescence studies with antibodies to functional ferrochelatase reveal that the native enzyme is localized to the mitochondrion of the parasite indicating that this organelle is the ultimate site of heme synthesis.

Localisation of Plasmodium falciparum uroporphyrinogen III decarboxylase of the heme-biosynthetic pathway in the apicoplast and characterisation of its catalytic properties

Uroporphyrinogen decarboxylase (UROD) is a key enzyme in the heme-biosynthetic pathway and in Plasmodium falciparum it occupies a strategic position in the proposed hybrid pathway for heme biosynthesis involving shuttling of intermediates between different subcellular compartments in the parasite. In the present study, we demonstrate that an N-terminally truncated recombinant P. falciparum UROD (r(Delta)PfUROD) over-expressed and purified from Escherichia coli cells, as well as the native enzyme from the parasite were catalytically less efficient compared with the host enzyme, although they were similar in other enzyme parameters. Molecular modeling of PfUROD based on the known crystal structure of the human enzyme indicated that the protein manifests a distorted triose phosphate isomerase (TIM) barrel fold which is conserved in all the known structures of UROD. The parasite enzyme shares all the conserved or invariant amino acid residues at the active and substrate binding sites, but is rich in lysine residues compared with the host enzyme. Mutation of specific lysine residues corresponding to residues at the dimer interface in human UROD enhanced the catalytic efficiency of the enzyme and dimer stability indicating that the lysine rich nature and weak dimer interface of the wild-type PfUROD could be responsible for its low catalytic efficiency. PfUROD was localised to the apicoplast, indicating the requirement of additional mechanisms for transport of the product coproporphyrinogen to other subcellular sites for its further conversion and ultimate heme formation.

Apicoplast and mitochondrion in gametocytogenesis of Plasmodium falciparum

Live cell imaging of human malaria parasites Plasmodium falciparum during gametocytogenesis revealed that the apicoplast does not grow, whereas the mitochondrion undergoes remarkable morphological development. A close connection of the two organelles is consistently maintained. The apicoplast and mitochondrion are not components of the male gametes, suggesting maternal inheritance.

Plastid-derived genes in the nonphotosynthetic alveolate Oxyrrhis marina

Reconstructing the history of plastid acquisition and loss in the alveolate protists is a difficult problem because our knowledge of the distribution of plastids in extant lineages is incomplete due to the possible presence of cryptic, nonphotosynthetic plastids in several lineages. The discovery of the apicoplast in apicomplexan parasites has drawn attention to this problem and, more specifically, to the question of whether many other nonphotosynthetic lineages also contain cryptic plastids or are derived from plastid-containing ancestors. Oxyrrhis marina is one such organism: It is a heterotrophic, early-branching member of the dinoflagellate lineage for which there is no evidence of a plastid. To investigate the possibility that O. marina is derived from a photosynthetic ancestor, we have generated and analyzed a large-scale EST database and searched for evidence of plastid-derived genes. Here, we describe 8 genes whose phylogeny shows them to be derived from plastid-targeted homologues. These genes encode proteins from several pathways known to be localized in the plastids of other algae, including synthesis of tetrapyrroles, isoprenoids, and amino acids, as well as carbon metabolism and oxygen detoxification. The 5' end of 5 cDNAs were also characterized using cap-dependent or spliced leader-mediated reverse transcriptase-polymerase chain reaction, revealing that at least 4 of these genes have retained leaders that are similar in nature to the plastid-targeting signals of other secondary plastids, suggesting that these proteins may be targeted to a cryptic organelle. At least 2 genes do not encode such leaders, and their products may presently function in the cytosol. Altogether, the presence of plastid-derived genes in O. marina shows that its ancestors contained a plastid, and the pathways represented by the genes and presence of targeting signals on at least some of the genes further suggests that a relict organelle may still exist to fulfill plastid metabolic functions.

Unique properties of Plasmodium falciparum porphobilinogen deaminase

The hybrid pathway for heme biosynthesis in the malarial parasite proposes the involvement of parasite genome-coded enzymes of the pathway localized in different compartments such as apicoplast, mitochondria, and cytosol. However, knowledge on the functionality and localization of many of these enzymes is not available. In this study, we demonstrate that porphobilinogen deaminase encoded by the Plasmodium falciparum genome (PfPBGD) has several unique biochemical properties. Studies carried out with PfPBGD partially purified from parasite membrane fraction, as well as recombinant PfPBGD lacking N-terminal 64 amino acids expressed and purified from Escherichia coli cells (DeltaPfPBGD), indicate that both the proteins are catalytically active. Surprisingly, PfPBGD catalyzes the conversion of porphobilinogen to uroporphyrinogen III (UROGEN III), indicating that it also possesses uroporphyrinogen III synthase (UROS) activity, catalyzing the next step. This obviates the necessity to have a separate gene for UROS that has not been so far annotated in the parasite genome. Interestingly, DeltaPfP-BGD gives rise to UROGEN III even after heat treatment, although UROS from other sources is known to be heat-sensitive. Based on the analysis of active site residues, a DeltaPfPBGDL116K mutant enzyme was created and the specific activity of this recombinant mutant enzyme is 5-fold higher than DeltaPfPBGD. More interestingly, DeltaPfPBGDL116K catalyzes the formation of uroporphyrinogen I (UROGEN I) in addition to UROGEN III, indicating that with increased PBGD activity the UROS activity of PBGD may perhaps become rate-limiting, thus leading to non-enzymatic cyclization of preuroporphyrinogen to UROGEN I. PfPBGD is localized to the apicoplast and is catalytically very inefficient compared with the host red cell enzyme.

Nuclear gyrB encodes a functional subunit of the Plasmodium falciparum gyrase that is involved in apicoplast DNA replication

The DNA replication machinery of the Plasmodium falciparum apicoplast is a validated drug target. Nuclear-encoded gyrase subunits are predicted to play a critical role in maintaining DNA topology during the D-loop/bi-directional ori replication process of the parasite. We show the presence of P. falciparum gyrase subunits in parasite lysates by using antibodies generated against recombinant gyrase A and B. The ATPase activity of PfGyrB was inhibited by novobiocin that also caused parasite death in culture. Reduction of apicoplast/nuclear DNA ratio in the presence of novobiocin indicated that the drug targets apicoplast DNA replication. Molecular modeling of gyrase A and B subunits revealed extensive fold conservation with the Escherichia coli counterparts as well as the presence of a long disordered loop adjacent to the ATPase domain of PfGyrB. Our results have implications for development of PfGyrB as a drug target against malaria.

Metabolic maps and functions of the Plasmodium mitochondrion

The mitochondrion of Plasmodium species is a validated drug target. However, very little is known about the functions of this organelle. In this review, we utilize data available from the Plasmodium falciparum genome sequencing project to piece together putative metabolic pathways that occur in the parasite, comparing this with the existing biochemical and cell biological knowledge. The Plasmodium mitochondrion contains both conserved and unusual features, including an active electron transport chain and many of the necessary enzymes for coenzyme Q and iron-sulphur cluster biosynthesis. It also plays an important role in pyrimidine metabolism. The mitochondrion participates in an unusual hybrid haem biosynthesis pathway, with enzymes localizing in both the mitochondrion and plastid organelles. The function of the tricarboxylic acid cycle in the mitochondrion is unclear. We discuss directions for future research into this fascinating, yet enigmatic, organelle.

A post-genomic view of the mitochondrion in malaria parasites

Mitochondria in Plasmodium parasites have many characteristics that distinguish them from mammalian mitochondria. Selective targeting of malaria parasite mitochondrial physiology has been exploited in successful antimalarial chemotherapy. At present, our understanding of the functions served by the parasite mitochondrion is somewhat limited, but the availability of the genomic sequences makes it possible to develop a framework of possible mitochondrial functions by providing information on genes encoding mitochondrially targeted proteins. This review aims to provide an overview of mitochondrial physiology in this post-genomic era. Although in many cases direct experimental proof for their mitochondrial functions may not be available at present, descriptions of these potential mitochondrial proteins can provide a basis for experimental approaches.

Mosaic origin of the heme biosynthesis pathway in photosynthetic eukaryotes

Heme biosynthesis represents one of the most essential metabolic pathways in living organisms, providing the precursors for cytochrome prosthetic groups, photosynthetic pigments, and vitamin B(12). Using genomic data, we have compared the heme pathway in the diatom Thalassiosira pseudonana and the red alga Cyanidioschyzon merolae to those of green algae and higher plants, as well as to those of heterotrophic eukaryotes (fungi, apicomplexans, and animals). Phylogenetic analyses showed the mosaic character of this pathway in photosynthetic eukaryotes. Although most of the algal and plant enzymes showed the expected plastid (cyanobacterial) origin, at least one of them (porphobilinogen deaminase) appears to have a mitochondrial (alpha-proteobacterial) origin. Another enzyme, glutamyl-tRNA synthase, obviously originated in the eukaryotic nucleus. Because all the plastid-targeted sequences consistently form a well-supported cluster, this suggests that genes were either transferred from the primary endosymbiont (cyanobacteria) to the primary host nucleus shortly after the primary endosymbiotic event or replaced with genes from other sources at an equally early time, i.e., before the formation of three primary plastid lineages. The one striking exception to this pattern is ferrochelatase, the enzyme catalyzing the first committed step to heme and bilin pigments. In this case, two red algal sequences do not cluster either with the other plastid sequences or with cyanobacterial sequences and appear to have a proteobacterial origin like that of the apicomplexan parasites Plasmodium and Toxoplasma. Although the heterokonts also acquired their plastid via secondary endosymbiosis from a red alga, the diatom has a typical plastid-cyanobacterial ferrochelatase. We have not found any remnants of the plastidlike heme pathway in the nonphotosynthetic heterokonts Phytophthora ramorum and Phytophthora sojae.

The plastid-derived organelle ofprotozoan human parasites asa target of established and emerging drugs

Human diseases like malaria, toxoplasmosis or cryptosporidiosis are caused by intracellular protozoan parasites of the phylum Apicomplexa and are still a major health problem worldwide. In the case of Plasmodium falciparum, the causative agent of tropical malaria, resistance against previously highly effective drugs is widespread and requires the continued development of new and affordable drugs. Most apicomplexan parasites possess a single plastid-derived organelle called apicoplast, which offers the great opportunity to tailor highly specific inhibitors against vital metabolic pathways resident in this compartment. This is due to the fact that several of these pathways, being of bacterial or algal origin, are absent in the mammalian host. In fact, the targets of several antibiotics already in use for years against some of these diseases can now be traced to the apicoplast and by knowing the molecular entities which are affected by these substances, improved drugs or drug combinations can be envisaged to emerge from this knowledge. Likewise, apicoplast-resident pathways like fatty acid or isoprenoid biosynthesis have already been proven to be the likely targets of the next drug generation. In this review the current knowledge on the different targets and available inhibitors (both established and experimental) will be summarised and an overview of the clinical efficacy of drugs that inhibit functions in the apicoplast and which have been tested in humans so far will be given.

The apicoplast of Plasmodium falciparum is translationally active

Apicoplast, the plastid-like organelle of apicomplexan parasites, has generated interest as a putative drug target. Although transcripts for genes encoded by the 35 kb circular plastid DNA have been detected, the actual presence of their protein products has only been postulated. We provide evidence for translation of the tufA gene encoded by the Plasmodium falciparum apicoplast genome. Translation elongation factor Tu (EF-Tu), the product of tufA, was localized within the organelle. TufA was found to express maximally in the trophozoite stage of the intraerythrocytic cycle. Additionally, the drug thiostrepton that has a binding site in apicoplast LSU rRNA, reduced P. falciparum apicoplast EF-Tu levels thus strengthening the view that translation in the apicoplast is the site of action of this drug.