Effect of heat-shock on Plasmodium falciparum viability, growth and expression of the heat-shock protein 'PFHSP70-I' gene

Identification of Co-Existing Mutations and Gene Expression Trends Associated With K13-Mediated Artemisinin Resistance in Plasmodium falciparum

Plasmodium falciparum infects millions and kills thousands of people annually the world over. With the emergence of artemisinin and/or multidrug resistant strains of the pathogen, it has become even more challenging to control and eliminate the disease. Multiomics studies of the parasite have started to provide a glimpse into the confounding genetics and mechanisms of artemisinin resistance and identified mutations in Kelch13 (K13) as a molecular marker of resistance. Over the years, thousands of genomes and transcriptomes of artemisinin-resistant/sensitive isolates have been documented, supplementing the search for new genes/pathways to target artemisinin-resistant isolates. This meta-analysis seeks to recap the genetic landscape and the transcriptional deregulation that demarcate artemisinin resistance in the field. To explore the genetic territory of artemisinin resistance, we use genomic single-nucleotide polymorphism (SNP) datasets from 2,517 isolates from 15 countries from the MalariaGEN Network (The Pf3K project, pilot data release 4, 2015) to dissect the prevalence, geographical distribution, and co-existing patterns of genetic markers associated with/enabling artemisinin resistance. We have identified several mutations which co-exist with the established markers of artemisinin resistance. Interestingly, K13-resistant parasites harbor α-ß hydrolase and putative HECT domain-containing protein genes with the maximum number of SNPs. We have also explored the multiple, publicly available transcriptomic datasets to identify genes from key biological pathways whose consistent deregulation may be contributing to the biology of resistant parasites. Surprisingly, glycolytic and pentose phosphate pathways were consistently downregulated in artemisinin-resistant parasites. Thus, this meta-analysis highlights the genetic and transcriptomic features of resistant parasites to propel further exploratory studies in the community to tackle artemisinin resistance.

The Role of Hsp70s in the Development and Pathogenicity of Plasmodium falciparum

The main agent of human malaria, the protozoa, Plasmodium falciparum is known to infect liver cells, subsequently invading the host erythrocyte, leading to the manifestation of clinical outcomes of the disease. As part of its survival in the human host, P. falciparum employs several heat shock protein (Hsp) families whose primary purpose is to ensure cytoprotection through their molecular chaperone role. The parasite expresses six Hsp70s that localise to various subcellular organelles of the parasite, with one, PfHsp70-x, being exported to the infected human erythrocyte. The role of these Hsp70s in the survival and pathogenicity of malaria has received immense research attention. Several studies have reported on their structure-function features, network partnerships, and elucidation of their potential substrates. Apart from their role in cytoprotection and pathogenicity, Hsp70s are implicated in antimalarial drug resistance. As such, they are deemed potential antimalarial drug candidates, especially suited for co-targeting in combination therapies. In addition, Hsp70 is implicated in host immune modulation. The current report highlights the various structure-function features of these proteins, their roles in the development of malaria, current and prospective efforts being employed towards targeting them in malaria intervention efforts.

Transcriptional variation in malaria parasites: why and how

Transcriptional differences enable the generation of alternative phenotypes from the same genome. In malaria parasites, transcriptional plasticity plays a major role in the process of adaptation to fluctuations in the environment. Multiple studies with culture-adapted parasites and field isolates are starting to unravel the different transcriptional alternatives available to Plasmodium falciparum and the underlying molecular mechanisms. Here we discuss how epigenetic variation, directed transcriptional responses and also genetic changes that affect transcript levels can all contribute to transcriptional variation and, ultimately, parasite survival. Some transcriptional changes are driven by stochastic events. These changes can occur spontaneously, resulting in heterogeneity within parasite populations that provides the grounds for adaptation by dynamic natural selection. However, transcriptional changes can also occur in response to external cues. A better understanding of the mechanisms that the parasite has evolved to alter its transcriptome may ultimately contribute to the design of strategies to combat malaria to which the parasite cannot adapt.

Temperature Dependence of Plasmodium falciparum Erythrocytic Stage Development

Plasmodium falciparum infection causes febrile illness and severe disease with multiple organ failure and death when treatment is delayed. Antipyretic treatment is standard, and inducing hypothermia has been proposed to protect the brain in cerebral malaria. Here, we investigated the temperature dependence of asexual-stage parasite development and parasite multiplication in vitro. Plasmodium falciparum laboratory strain TM267 was incubated for 2 hours (short exposure) or 48 hours (continuous exposure) at different temperatures (32°C, 34°C, 35°C, 38°C, 39°C, and 40°C). The starting parasite developmental stage (ring, trophozoite, or schizont) varied between experiments. The parasite multiplication rate (PMR) was reduced under both hyper- and hypothermic conditions; after continuous exposure, the mean PMR ± SD was 9.1 ± 1.2 at 37°C compared with 2.4 ± 1.8 at 32°C, 2.3 ± 0.4 at 34°C, and 0.4 ± 0.1 at 40°C (P < 0.01). Changes in PMR were not significant after 2-hour exposure at temperatures ranging from 32°C to 40°C. Morphological changes in parasite cytoplasm and nucleus could be observed after long exposure to low or high temperature. After 48-hour incubation, rosette formation (≥ 2 uninfected red blood cells bound to infected red blood cells) was decreased at 34°C or 39°C compared with that at 37°C. In conclusion, both hyper- and hypothermia reduce PMR and delay erythrocytic stage development of P. falciparum, subsequently reducing rosette formation.

Establishing Computational Approaches Towards Identifying Malarial Allosteric Modulators: A Case Study of Plasmodium falciparum Hsp70s

Combating malaria is almost a never-ending battle, as Plasmodium parasites develop resistance to the drugs used against them, as observed recently in artemisinin-based combination therapies. The main concern now is if the resistant parasite strains spread from Southeast Asia to Africa, the continent hosting most malaria cases. To prevent catastrophic results, we need to find non-conventional approaches. Allosteric drug targeting sites and modulators might be a new hope for malarial treatments. Heat shock proteins (HSPs) are potential malarial drug targets and have complex allosteric control mechanisms. Yet, studies on designing allosteric modulators against them are limited. Here, we identified allosteric modulators (SANC190 and SANC651) against P. falciparum Hsp70-1 and Hsp70-x, affecting the conformational dynamics of the proteins, delicately balanced by the endogenous ligands. Previously, we established a pipeline to identify allosteric sites and modulators. This study also further investigated alternative approaches to speed up the process by comparing all atom molecular dynamics simulations and dynamic residue network analysis with the coarse-grained (CG) versions of the calculations. Betweenness centrality (BC) profiles for PfHsp70-1 and PfHsp70-x derived from CG simulations not only revealed similar trends but also pointed to the same functional regions and specific residues corresponding to BC profile peaks.

Sunlight inhibits growth and induces markers of programmed cell death in Plasmodium falciparum in vitro

Background

Plasmodium falciparum is responsible for the majority of global malaria deaths. During the pathogenic blood stages of infection, a rapid increase in parasitaemia threatens the survival of the host before transmission of slow-maturing sexual parasites to the mosquito vector to continue the life cycle. Programmed cell death (PCD) may provide the parasite with the means to control its burden on the host and thereby ensure its own survival. Various environmental stress factors encountered during malaria may induce PCD in P. falciparum. This study is the first to characterize parasite cell death in response to natural sunlight.

Methods

The 3D7 strain of P. falciparum was cultured in vitro in donor erythrocytes. Synchronized and mixed-stage parasitized cultures were exposed to sunlight for 1 h and compared to cultures maintained in the dark, 24 h later. Mixed-stage parasites were also subjected to a second one-hour exposure at 24 h and assessed at 48 h. Parasitaemia was measured daily by flow cytometry. Biochemical markers of cell death were assessed, including DNA fragmentation, mitochondrial membrane polarization and phosphatidylserine externalization.

Results

Sunlight inhibited P. falciparum growth in vitro. Late-stage parasites were more severely affected than early stages. However, some late-stage parasites survived exposure to sunlight to form new rings 24 h later, as would be expected during PCD whereby only a portion of the population dies. DNA fragmentation was observed at 24 and 48 h and preceded mitochondrial hyperpolarization in mixed-stage parasites at 48 h. Mitochondrial hyperpolarization likely resulted from increased oxidative stress. Although data suggested increased phosphatidylserine externalization in mixed-stage parasites, results were not statistically significant.

Conclusion

The combination of biochemical markers and the survival of some parasites, despite exposure to a lethal stimulus, support the occurrence of PCD in P. falciparum.

Plasmodium falciparum Hop (PfHop) Interacts with the Hsp70 Chaperone in a Nucleotide-Dependent Fashion and Exhibits Ligand Selectivity

Heat shock proteins (Hsps) play an important role in the development and pathogenicity of malaria parasites. One of the most prominent functions of Hsps is to facilitate the folding of other proteins. Hsps are thought to play a crucial role when malaria parasites invade their host cells and during their subsequent development in hepatocytes and red blood cells. It is thought that Hsps maintain proteostasis under the unfavourable conditions that malaria parasites encounter in the host environment. Although heat shock protein 70 (Hsp70) is capable of independent folding of some proteins, its functional cooperation with heat shock protein 90 (Hsp90) facilitates folding of some proteins such as kinases and steroid hormone receptors into their fully functional forms. The cooperation of Hsp70 and Hsp90 occurs through an adaptor protein called Hsp70-Hsp90 organising protein (Hop). We previously characterised the Hop protein from Plasmodium falciparum (PfHop). We observed that the protein co-localised with the cytosol-localised chaperones, PfHsp70-1 and PfHsp90 at the blood stages of the malaria parasite. In the current study, we demonstrated that PfHop is a stress-inducible protein. We further explored the direct interaction between PfHop and PfHsp70-1 using far Western and surface plasmon resonance (SPR) analyses. The interaction of the two proteins was further validated by co-immunoprecipitation studies. We observed that PfHop and PfHsp70-1 associate in the absence and presence of either ATP or ADP. However, ADP appears to promote the association of the two proteins better than ATP. In addition, we investigated the specific interaction between PfHop TPR subdomains and PfHsp70-1/ PfHsp90, using a split-GFP approach. This method allowed us to observe that TPR1 and TPR2B subdomains of PfHop bind preferentially to the C-terminus of PfHsp70-1 compared to PfHsp90. Conversely, the TPR2A motif preferentially interacted with the C-terminus of PfHsp90. Finally, we observed that recombinant PfHop occasionally eluted as a protein species of twice its predicted size, suggesting that it may occur as a dimer. We conducted SPR analysis which suggested that PfHop is capable of self-association in presence or absence of ATP/ADP. Overall, our findings suggest that PfHop is a stress-inducible protein that directly associates with PfHsp70-1 and PfHsp90. In addition, the protein is capable of self-association. The findings suggest that PfHop serves as a module that brings these two prominent chaperones (PfHsp70-1 and PfHsp90) into a functional complex. Since PfHsp70-1 and PfHsp90 are essential for parasite growth, findings from this study are important towards the development of possible antimalarial inhibitors targeting the cooperation of these two chaperones.

Plasmodial HSP70s are functionally adapted to the malaria parasite life cycle

The human malaria parasite, Plasmodium falciparum, encodes a minimal complement of six heat shock protein 70s (PfHSP70s), some of which are highly expressed and are thought to play an important role in the survival and pathology of the parasite. In addition to canonical features of molecular chaperones, these HSP70s possess properties that reflect functional adaptation to a parasitic life style, including resistance to thermal insult during fever periods and host–parasite interactions. The parasite even exports an HSP70 to the host cell where it is likely to be involved in host cell modification. This review focuses on the features of the PfHSP70s, particularly with respect to their adaptation to the malaria parasite life cycle.

Plasmodial Hsp40 and Hsp70 chaperones: current and future perspectives

Plasmodium falciparumdisplays a large and remarkable variety of heat shock protein 40 family members (PfHsp40s). The majority of the PfHsp40s are poorly characterized, and although the functions of some of them have been suggested, their exact mechanism of action is still elusive and their interacting partners and client proteins are unknown. TheP. falciparumheat shock protein 70 family members (PfHsp70s) have been more extensively characterized than the PfHsp40s, with certain members shown to function as molecular chaperones. However, little is known about the PfHsp70-PfHsp40 chaperone partnerships. There is mounting evidence that these chaperones are important not only in protein homoeostasis and cytoprotection, but also in protein trafficking across the parasitophorous vacuole (PV) and into the infected erythrocyte. We propose that certain members of these chaperone families work together to maintain exported proteins in an unfolded state until they reach their final destination. In this review, we critically evaluate what is known and not known about PfHsp40s and PfHsp70s.

Turning up the heat: heat stress induces markers of programmed cell death in Plasmodium falciparum in vitro

Malaria is characterised by cyclical febrile episodes that result from the rupture of mature schizont-infected erythrocytes releasing merozoites. In patients infected with Plasmodium falciparum, fever may reach peak temperatures as high as 41 °C. Febrile episodes typically have a deleterious effect on parasites and probably benefit the host by aiding parasite clearance; however, the parasite may also gain advantage from limiting its burden on the host and prolonging infection to ensure development and transmission of slow-maturing gametocytes. Programmed cell death (PCD) may provide the parasite with a mechanism of self-limitation, although the occurrence and phenotype of PCD in the erythrocytic stages remain controversial due to conflicting data. This study aimed to characterise the cell death phenotype of P. falciparum in response to in vitro heat stress. A variety of biochemical markers of PCD, including DNA fragmentation, mitochondrial dysregulation and phosphatidylserine externalisation, as well as morphological studies of Giemsa-stained thin smears and real-time microscopy were utilised to characterise the phenotype. Heat stress decreased P. falciparum growth and development in vitro. Late-stage parasites were more susceptible, although early stages were more affected than expected. Early-stage parasites exposed to 41 °C exhibited markers of an apoptosis-like PCD phenotype, including DNA fragmentation and mitochondrial depolarisation. Heat-stressed late-stage parasites showed no significant DNA fragmentation or mitochondrial dysregulation; however, cytoplasmic vacuolisation was suggestive of an autophagy-like form of PCD. Our results therefore showed that biochemical and morphological markers of PCD varied with intra-erythrocytic parasite development and that P. falciparum exhibited facets of both apoptosis- and autophagy-like phenotypes after exposure to febrile temperatures, which may reflect a unique PCD phenotype.

The effect of mimicking febrile temperature and drug stress on malarial development

Background

Malaria remains one of the most important tropical diseases of human with 1–2 million deaths annually especially caused by P. falciparum. During malarial life cycle, they exposed to many environmentally stresses including wide temperature fluctuation and pharmacological active molecules. These trigger malarial evolutionarily adaptive responses. The effect of febrile temperature on malarial growth, development and drug susceptibility by mimicking patient in treatment failure before and after drug uptake was examined.

Methods

Sensitivities of P. falciparum to antimalarial drug (chloroquine, mefloquine, quinine and artesunate) were investigated based on the incorporation of [³H] hypoxanthine into parasite nucleic acids or radioisotopic technique. The number of parasites was examined under microscope following Giemsa staining and the parasite development at the end of each phase was counted and comparison of parasite number was made. The proteome was separated, blotted and hybridized with anti-Hsp70s primary antibody. The hybridized proteins were separately digested with trypsin and identified by MALDI-TOF peptide mass fingerprint.

Results

The results show that febrile temperature is capable of markedly inhibiting the growth of field isolate P. falciparum but not to K1 and 3D7 standard strains. K1 and 3D7 grown under heat shock developed greater and the reinfection rate was increased up to 2-folds when compared to that of non-heat shock group. The IC₅₀ value of K1 toward chloroquine, mefloquine and quinine under heat shock was higher than that of K1 under non-heat shock which is opposite to that of 3D7. Heat shock caused death in field isolated parasite. It was also found that the febrile temperature coped with chloroquine uptake had no effect to the development, drug sensitivity and the parasite number of K1 strain. In the opposite way, heat shock and chloroquine shows extremely effect toward 3D7 and field isolate PF91 as shown by higher number of dead parasites compared to that of control group. After culture under high temperature with artesunate, the total parasite number of all strains including K1, 3D7 and PF91 was extremely decreased and the parasite was not found at the end. Additionally, the expression of pfHsp70s was found in all strains and conditions as shown in 120 kDa hybridized band. However, the proteome extracted from K1 grown under heat shock with chloroquine, anti-pfHsp70 interacted with additional three bands identified by MALDI-TOF as elongation factor-1α (83 kDa), pfHsp86 (60 kDa) and phosphoethanolamine N-methyltransferase (43 kDa).

Conclusion

In conclusion, febrile temperature was capable of markedly inhibiting the growth of field isolate P. falciparum while the development, reinfection rate and drug (chloroquine, mefloquine and quinine) resistant level of standard strain K1 was enhanced. However, the febrile temperature coped with chloroquine had no effect to the development, drug sensitivity and the parasite number of K1 strain. In the opposite way, heat shock and chloroquine showed extremely effect toward 3D7 and field isolate PF91 as shown by some died parasites. Heat shock protein 70 (pfHSP70) of strain K1 under heat shock with chloroquine might involved in many pathways in order to sustain the parasite.

The Plasmodium falciparum heat shock protein 40, Pfj4, associates with heat shock protein 70 and shows similar heat induction and localisation patterns

Human cerebral malaria is caused by the protozoan parasite Plasmodium falciparum, which establishes itself within erythrocytes. The normal body temperature in the human host could constitute a possible source of heat stress to the parasite. Molecular chaperones belonging to the heat shock protein (Hsp) class are thought to be important for parasite subsistence in the host cell, as the expression of some members of this family has been reported to increase upon heat shock. In this paper we investigated the possible functions of the P. falciparum heat shock protein DnaJ homologue Pfj4, a type II Hsp40 protein. We analysed the ability of Pfj4 to functionally replace Escherichia coli Hsp40 proteins in a dnaJ cbpA mutant strain. Western analysis on cellular fractions of P. falciparum-infected erythrocytes revealed that Pfj4 expression increased upon heat shock. Localisation studies using immunofluorescence and immuno-electron microscopy suggested that Pfj4 and P. falciparum Hsp70, PfHsp70-1, were both localised to the parasites nucleus and cytoplasm. In some cases, Pfj4 was also detected in the erythrocyte cytoplasm of infected erythrocytes. Immunoprecipitation studies and size exclusion chromatography indicated that Pfj4 and PfHsp70-1 may directly or indirectly interact. Our results suggest a possible involvement of Pfj4 together with PfHsp70-1 in cytoprotection, and therefore, parasite survival inside the erythrocyte.

Heat shock protein 70 of Naegleria fowleri is important factor for proliferation and in vitro cytotoxicity

To evaluate the role of heat shock 70 protein (HSP70) in free-living amoeba, a constitutive and inducible heat shock 70 gene of pathogenic Naegleria fowleri has previously been cloned, characterized, and named as Nf-cHSP70. The Nf-cHSP70 is localized in the cytoplasm, pseudopodia, and phagocytic food-cups. To investigate the role of Nf-cHSP70 in the pathogenicity of N. fowleri, the synthesis of N. fowleri HSP70 was first inhibited with benzylidene lactam compound (KNK437), and Nf-cHSP70 gene was knock-downed with antisense oligomers, which were designed with a start region-specific antisense oligonucleotides (24 oligomers) and modified with phosphorothioate. KNK437 inhibited the induction of N. fowleri HSP70 in a dose-dependent manner. In addition, 300 muM KNK437 reduced the proliferation of N. fowleri to 79.4% of untreated control (100%). Nf-cHSP70 knock-downed N. fowleri with antisense oligomers showed 68.5% reduction of proliferation in comparison with untreated control (100%). The cytotoxicity of N. fowleri against CHO target cells was reduced to 42.1% by KNK437 and 68.6% by antisense oligomers. These results suggest that the cloned Nf-cHSP70 plays an important role in the proliferation and cytotoxicity of pathogenic N. fowleri.

The structural and functional diversity of Hsp70 proteins from Plasmodium falciparum

It is becoming increasingly apparent that heat shock proteins play an important role in the survival of Plasmodium falciparum against temperature changes associated with its passage from the cold-blooded mosquito vector to the warm-blooded human host. Interest in understanding the possible role of P. falciparum Hsp70s in the life cycle of the parasite has led to the identification of six HSP70 genes. Although most research attention has focused primarily on one of the cytosolic Hsp70s (PfHsp70-1) and its endoplasmic reticulum homolog (PfHsp70-2), further functional insights could be inferred from the structural motifs exhibited by the rest of the Hsp70 family members of P. falciparum. There is increasing evidence that suggests that PfHsp70-1 could play an important role in the life cycle of P. falciparum both as a chaperone and immunogen. In addition, P. falciparum Hsp70s and Hsp40 partners are implicated in the intracellular and extracellular trafficking of proteins. This review summarizes data emerging from studies on the chaperone role of P. falciparum Hsp70s, taking advantage of inferences gleaned from their structures and information on their cellular localization. The possible associations between P. falciparum Hsp70s with their cochaperone partners as well as other chaperones and proteins are discussed.

Molecular cloning and characterization of a cytosolic heat shock protein 70 from Naegleria fowleri

A gene encoding a cytosolic heat shock protein 70 from pathogenic Naegleria fowleri (Nf-cHSP70) was identified. The Nf-cHSP70 was 2,062 bp in length with an open reading frame of 1,980 bp encoding 659 amino acid residues. The deduced amino acid sequence of the gene shared high sequence identities with HSP70s from other parasitic organisms and mammals. The characteristic domains, including N-terminal ATPase domain, calmodulin-binding domain, and EE(D)VD motif, found in HSP70s were also well conserved in this gene. The recombinant Nf-cHSP70 protein showed strong antigenicity against the sera from mice experimentally infected with N. fowleri. Immunofluorescence assay showed that Nf-cHSP70 localized in cytosol of the parasite. The results from semi-quantitative RT-PCR and Western blot analyses demonstrated the expression levels of gene transcripts, and its products were significantly increased at high temperature (42 degrees C). The definitive biological roles of Nf-cHSP70 are not clear, but it may protect the parasite under environmental changes especially high temperature.

15-Deoxyspergualin modulates Plasmodium falciparum heat shock protein function

Heat shock proteins are essential for the survival of all cells. The C-terminal EEVD motif of Hsp70 has previously been implicated in binding 15-deoxyspergualin (DSG), an immunosuppressant with antimalarial activity whose mechanism of action is uncertain. We report the cloning, overexpression, and characterization of three members of the heat shock family, PfHsp70-1 (an Hsp70 protein with a C-terminal EEVD motif), PfHsp70-2 (an Hsp70 protein without the EEVD motif), and PfHsp70 interacting protein. The chaperone activity of PfHsp70-1, and PfHsp70-2 was enhanced by ATP and by PfHip. Interestingly, while binding of protein substrates to PfHsp70-1, PfHsp70-2 and PfHip was unaffected in the presence of DSG, the ATP enhanced chaperone activity of PfHsp70-1 but not PfHsp70-2 was stimulated further by DSG. Our finding suggests that the binding partner of DSG in the parasite cellular milieu is PfHsp70-1 and paves the way for the elucidation of the mechanism of antimalarial action of DSG.

Plasmodium falciparum heat shock protein 70 is able to suppress the thermosensitivity of an Escherichia coli DnaK mutant strain

Heat shock protein 70 (Hsp 70) and heat shock protein 40 (Hsp 40) are molecular chaperones that ensure that the proteins of the cell are properly folded and functional under both normal and stressful conditions. The malaria parasite Plasmodium falciparum is known to overproduce a heat shock protein 70 (PfHsp 70) in response to thermal stress; however, the in vivo function of this protein still needs to be explored. Using in vivo complementation assays, we found that PfHsp 70 was able to suppress the thermosensitivity of an Escherichia coli dnaK 756 strain, but not that of the corresponding deletion strain (DeltadnaK 52) or dnaK 103 strain, which produces a truncated DnaK. Constructs were generated that encoded the ATPase domain of PfHsp 70 fused to the substrate-binding domain (SBD) of E. coli DnaK (referred to as PfK), and the ATPase domain of E. coli DnaK coupled to the SBD of PfHsp 70 (KPf). PfK was unable to suppress the thermosensitivity of any of the E. coli strains. In contrast, KPf was able to suppress the thermosensitivity in the E. coli dnaK 756 strain. We also identified two key amino acid residues (V 401 and Q 402) in the linker region between the ATPase domain and SBD that are essential for the in vivo function of PfHsp 70. This is the first example of an Hsp70 from a eukaryotic parasite that can suppress thermosensitivity in a prokaryotic system. In addition, our results also suggest that interdomain communication is critical for the function of the PfHsp 70 and PfHsp 70-DnaK chimeras. We discuss the implications of these data for the mechanism of action of the Hsp70-Hsp 40 chaperone machinery.

A role for poly(dA)poly(dT) tracts in directing activity of the Plasmodium falciparum calmodulin gene promoter

Expression of the Plasmodium falciparum calmodulin gene (pfcam) is developmentally regulated throughout the blood-stage cycle. The promoter lies within approximately 1 kb of intergenic sequence that separates the pfcam open reading frame (ORF) from an upstream inverted ORF encoding a product homologous to the co-chaperone STI1. Using the oligo-capping method, which selectively reverse-transcribes cDNA from only full-length, capped transcript, we have mapped multiple transcription-initiation sites for both genes. Transcription of the pfSTI1 gene initiates over a 150 bp region centred approximately 350 bp upstream of the ORF. The pfcam transcription start sites cluster into four approximately 30 bp regions lying within 180 bp upstream of the pfcam ORF, generating transcripts with 5' untranslated regions (UTR) of 3-173 nucleotides in length. Remarkably, splicing was found to be related to UTR length, with apparent preferential splicing of longer transcripts. Activity of the pfcam promoter diminished in a linear fashion to undetectable levels upon step-wise removal of sequence between 625 and 230 bp upstream of the start ATG. Electromobility-shift assays demonstrated nuclear factor binding to eight oligonucleotide probes spanning 657 bp of the pfcam ORF proximal upstream sequence. The degree of binding correlated with the density of poly(dA)poly(dT) tracts within the probes, and in all cases could be inhibited by excess synthetic poly(dA)poly(dT), but not by poly(dAdT)poly(dAdT). The multiple transcription-initiation sites of both pfSTI1 and pfcam genes lie just downstream of 25 bp-long poly(dA)poly(dT) tracts, and the intergenic region contains over 20 poly(dA)poly(dT) tracts of 4 bp or more. Our results suggest that the basal pfcam promoter is situated between approximately -300 and -230 bp upstream of the pfcam ORF and that the P. falciparum transcription-initiation complex has a low degree of sequence-specificity for the sites of initiation but preferentially acts downstream of long poly(dA)poly(dT) tracts.

Overproduction, purification, and characterization of the Plasmodium falciparum heat shock protein 70

Plasmodium falciparum heat shock protein (PfHsp70) has been proposed to be involved in the cytoprotection of the malaria parasite through its action as a molecular chaperone. However, the biochemical and chaperone properties of PfHsp70 have not been elucidated. The heterologous overproduction of P. falciparum proteins in Escherichia coli is problematic because of its AT-rich genome and the usage of codons that are rarely used in E. coli. In this paper, we describe the successful overproduction of (His)(6)-PfHsp70 in E. coli using the pQE30 expression vector system. Initial experiments with E. coli [pQE30/PfHsp70] resulted in the overproduction of the full-length protein and truncated derivatives. The RIG plasmid, which encodes tRNAs for rare codons, was engineered into the E. coli [pQE30/PfHsp70] strain, resulting in significant reduction of the truncated (His)(6)-PfHsp70 derivatives and improved yields of the full-length protein. (His)(6)-PfHsp70 was successfully purified using nickel-chelating Sepharose affinity chromatography and its biochemical properties were determined. The V(max), K(m), and k(cat) for the basal ATPase activity of (His)(6)-PfHsp70 were found to be 14.6 nmol/min/mg, 616.5 microM, and 1.03 min(-1), respectively. Gel filtration studies indicated that (His)(6)-PfHsp70 existed largely as a monomer in solution. This is the first study to biochemically describe PfHsp70 and establishes a foundation for future studies on its chaperone properties.

Humoral immune response to the 72 kDa heat shock protein from Plasmodium falciparum in populations at hypoendemic areas of malaria in western Brazilian Amazon

The heat-shock protein Pf72/Hsp70-1 from the human malaria parasite Plasmodium falciparum has been suggested as a potential candidate antigen for a multivalent vaccine. We have investigated the prevalence and levels of IgG antibodies to the recombinant protein PfR44, derived from Pf72/Hsp70-1, in individuals from different age groups living in Candeias do Jamari, an Amazonian town characterized by unstable and hypoendemic malaria transmission. Blood were collected from a household-based random sample comprising 241 people and the sera were comparatively tested against recombinant antigen PfR44 and a detergent-soluble extract of P. falciparum (PfAg-T). The prevalence and levels of IgG antibodies to both recombinant and total P. falciparum antigens were positively correlated with cumulative exposure to malaria, as estimated by the age of the individuals and the duration of their stay in the study area. Nevertheless, correlations between antibody responses to Pf72/Hsp70-1 and the acquisition of protective anti-malarial immunity could not be derived from our data.

Enhanced expression of Plasmodium falciparum heat shock protein PFHSP70-I at higher temperatures and parasite survival

The effect of various body temperatures, encountered during malaria fever, on the synthesis of Plasmodium falciparum heat-shock protein called PFHSP70-I and parasite growth rates among five different isolates are described. The results show that after the exposure of parasites at 39 degrees C for 30 min the amount of PFHSP70-I in all five isolates increased markedly and significantly, whereas parasite growth rates and the amount of total blood stage antigens remained almost unaffected. This indicates that the PFHSP70-I gene responds to heat-shock by producing higher amounts of PFHSP70-I protein, presumably to protect the parasite from being killed during malaria fever.