Genes coding for tryptophan-rich proteins are transcribed throughout the asexual cycle of Plasmodium falciparum

The structure of a Plasmodium vivax Tryptophan Rich Antigen domain suggests a lipid binding function for a pan-Plasmodium multi-gene family

Tryptophan Rich Antigens (TRAgs) are encoded by a multi-gene family found in all Plasmodium species, but are significantly expanded in P. vivax and closely related parasites. We show that multiple P. vivax TRAgs are expressed on the merozoite surface and that one, PVP01_0000100 binds red blood cells with a strong preference for reticulocytes. Using X-ray crystallography, we solved the structure of the PVP01_0000100 C-terminal tryptophan rich domain, which defines the TRAg family, revealing a three-helical bundle that is conserved across Plasmodium and has structural homology with lipid-binding BAR domains involved in membrane remodelling. Biochemical assays confirm that the PVP01_0000100 C-terminal domain has lipid binding activity with preference for sulfatide, a glycosphingolipid present in the outer leaflet of plasma membranes. Deletion of the putative orthologue in P. knowlesi, PKNH_1300500, impacts invasion in reticulocytes, suggesting a role during this essential process. Together, this work defines an emerging molecular function for the Plasmodium TRAg family.

Humoral immune responses to a recombinant Plasmodium vivax tryptophan-rich antigen among Plasmodium vivax-infected patients and its localization in the parasite

Our recent studies have focused on the identification and characterization of the tryptophan-rich proteins of the Plasmodium vivax parasite where their role in the elicitation of humoral and cellular responses and erythrocyte-binding activity was investigated. Here, we report the humoral responses of a 32.4-kDa P. vivax tryptophan-rich antigen (PvTRAg32.4) among the sera of P. vivax-infected patients. PvTRAg32.4 also contains an unusually high percentage of tryptophan residues (10.7 %) that are positionally conserved with its orthologues in Plasmodium yoelii (PypAg1 and PypAg2) and Plasmodium falciparum (PfTryThrA and PfMATRA). Thirty-four of the 40 (85.0 %) P. vivax isolates showed seropositivity to recombinant PvTRAg32.4 by ELISA. The mean ± SD values of optical density (OD) for P. vivax subjects and naïve individuals were 1.02 ± 0.36 and 0.26 ± 0.11, respectively. In the Western blot analysis, majority of the subjects studied (n = 44) showed reactivity to the recombinant, purified PvTRAg32.4. This antigen does not show binding to the erythrocytes, but the immunofluorescence data reveals that it is expressed in the erythrocytic stages of the parasite. Sequence analysis of the clinical isolates from various parts of the country shows that PvTRAg32.4 is highly conserved. Functional in-depth characterization of more such type of novel proteins in the parasite is warranted for the development of successful malaria intervention methods.

Defining the erythrocyte binding domains of Plasmodium vivax tryptophan rich antigen 33.5

Tryptophan-rich antigens play important role in host-parasite interaction. One of the Plasmodium vivax tryptophan-rich antigens called PvTRAg33.5 had earlier been shown to be predominantly of alpha helical in nature with multidomain structure, induced immune responses in humans, binds to host erythrocytes, and its sequence is highly conserved in the parasite population. In the present study, we divided this protein into three different parts i.e. N-terminal (amino acid position 24–106), middle (amino acid position 107–192), and C-terminal region (amino acid position 185–275) and determined the erythrocyte binding activity of these fragments. This binding activity was retained by the middle and C-terminal fragments covering 107 to 275 amino acid region of the PvTRAg33.5 protein. Eight non-overlapping peptides covering this 107 to 275 amino acid region were then synthesized and tested for their erythrocyte binding activity to further define the binding domains. Only two peptides, peptide P4 (at 171–191 amino acid position) and peptide P8 (at 255–275 amino acid position), were found to contain the erythrocyte binding activity. Competition assay revealed that each peptide recognizes its own erythrocyte receptor. These two peptides were found to be located on two parallel helices at one end of the protein in the modelled structure and could be exposed on its surface to form a suitable site for protein-protein interaction. Natural antibodies present in the sera of the P. vivax exposed individuals or the polyclonal rabbit antibodies against this protein were able to inhibit the erythrocyte binding activity of PvTRAg33.5, its fragments, and these two synthetic peptides P4 and P8. Further studies on receptor-ligand interaction might lead to the development of the therapeutic reagent.

Artesunate tolerance in transgenic Plasmodium falciparum parasites overexpressing a tryptophan-rich protein

Due to their rapid, potent action on young and mature intraerythrocytic stages, artemisinin derivatives are central to drug combination therapies for Plasmodium falciparum malaria. However, the evidence for emerging parasite resistance/tolerance to artemisinins in southeast Asia is of great concern. A better understanding of artemisinin-related drug activity and resistance mechanisms is urgently needed. A recent transcriptome study of parasites exposed to artesunate led us to identify a series of genes with modified levels of expression in the presence of the drug. The gene presenting the largest mRNA level increase, Pf10_0026 (PArt), encoding a hypothetical protein of unknown function, was chosen for further study. Immunodetection with PArt-specific sera showed that artesunate induced a dose-dependent increase of the protein level. Bioinformatic analysis showed that PArt belongs to a Plasmodium-specific gene family characterized by the presence of a tryptophan-rich domain with a novel hidden Markov model (HMM) profile. Gene disruption could not be achieved, suggesting an essential function. Transgenic parasites overexpressing PArt protein were generated and exhibited tolerance to a spike exposure to high doses of artesunate, with increased survival and reduced growth retardation compared to that of wild-type-treated controls. These data indicate the involvement of PArt in parasite defense mechanisms against artesunate. This is the first report of genetically manipulated parasites displaying a stable and reproducible decreased susceptibility to artesunate, providing new possibilities to investigate the parasite response to artemisinins.

Plasmodium vivax tryptophan-rich antigen PvTRAg33.5 contains alpha helical structure and multidomain architecture

Tryptophan-rich proteins from several malarial parasites have been identified where they play an important role in host-parasite interaction. Structural characterization of these proteins is needed to develop them as therapeutic targets. Here, we describe a novel Plasmodium vivax tryptophan-rich protein named PvTRAg33.5. It is expressed by blood stage(s) of the parasite and its gene contains two exons. The exon 1 encodes for a 23 amino acids long putative signal peptide which is likely to be cleaved off whereas the exon 2 encodes for the mature protein of 252 amino acids. The mature protein contains B-cell epitopes which were recognized by the human immune system during P.vivax infection. The PvTRAg33.5 contains 24 (9.5%) tryptophan residues and six motifs whose patterns were similar among tryptophan-rich proteins. The modeled structure of the PvTRAg33.5 consists of a multidomain architecture which is stabilized by the presence of large number of tryptophan residues. The recombinant PvTRAg33.5 showed predominantly α helical structure and alpha helix to beta sheet transition at pH below 4.5. Protein acquires an irreversible non-native state at temperature more than 50°C at neutral pH. Its secondary and tertiary structures remain stable in the presence of 35% alcohol but these structures are destabilized at higher alcohol concentrations due to the disturbance of hydrophobic interactions between tryptophanyl residues. These structural changes in the protein might occur during its translocation to interact with other proteins at its final destination for biological function such as erythrocyte invasion.

Expression, purification, and characterization of the immunological response to a 40-kilodalton Plasmodium vivax tryptophan-rich antigen

We describe here an approximately 40-kDa Plasmodium vivax tryptophan-rich antigen (PvTRAg40) which contains 321 amino acids and 11.4% tryptophan residues. This protein shows 65% homology (35% identity) with the previously described PvTRAg, besides sharing 23 of 27 positionally conserved tryptophan residues and similar genomic organization. The nucleotide sequence of the entire tryptophan-rich domain of PvTRAg40 was identical among 35 P. vivax clinical isolates. The protein is expressed by ring, trophozoite, and schizont stages of the parasite. The cDNA covering exon 2 of PvTRAg40 was cloned and expressed in the pPROEXHTa vector, and recombinant protein was purified. A high humoral immune response (90.7% seropositivity; n = 43) against this recombinant protein was detected in humans during the course of natural P. vivax infection. Eighty percent of the total of 20 P. vivax-exposed individuals exhibited lymphoproliferative responses against this antigen. The T cells of these individuals produced larger amounts of interleukin-12 (IL-12), IL-4, and IL-10 than gamma interferon and tumor necrosis factor alpha cytokines in response to the recombinant protein. Production of Th2-biased cytokines, conserved T- and B-cell epitopes, and an enhanced humoral immune response indicate that PvTRAg40 could possibly induce antibody-mediated immune protection against infection.

Expression and purification of a Plasmodium vivax antigen - PvTARAg55 tryptophan- and alanine-rich antigen and its immunological responses in human subjects

Despite the immense global efforts, the malaria vaccine is not yet available and requires the identification of newer target molecules. Since tryptophan-rich proteins of P. yoelii have been proposed as vaccine candidates, we describe here the expression, purification and immunological characterization of a 55kDa Plasmodium vivax tryptophan- and alanine-rich antigen (PvTARAg55). This protein consists of 480 aa residues with a calculated molecular mass of 55.0kDa. It shows 42% aa sequence identity (64% homology) with PyPAg1 of P. yoelii and shares positional conservation of tryptophan residues. Sequence analysis of PvTARAg55 from different P. vivax isolates revealed that typtophan-rich domain which contains most of the B-cell epitopes was highly conserved in the parasite population while the alanine-rich domain showed polymorphism. Exon-2 covering major part (420 aa) of the protein including both the domains was PCR amplified, cloned, expressed in Escherichia coli, and the recombinant protein purified to its homogeneity. Majority of P. vivax-infected individuals (82.5%, n=40) produced antibodies against this antigen. Proliferative responses to the recombinant PvTARAg55 were observed in 60% (n=20) of individuals who had recently been exposed to the P. vivax infection. Measurement of Th1- (IFN-gamma, TNF-alpha, and IL-12) and Th2-type (IL-4 and IL-10) cytokine production in response to this recombinant antigen revealed a mixed type T-cell response with a Th2 response being more pronounced. These results demonstrate that PvTARAg55 elicits high humoral and cellular immune responses thus establishes its immunogenecity in humans.