The Plasmodium falciparum exported protein PF3D7_0402000 binds to erythrocyte ankyrin and band 4.1

Plasmodium vivax tryptophan-rich antigen reduces type I collagen secretion via the NF-κBp65 pathway in splenic fibroblasts

BACKGROUND

The spleen plays a critical role in the immune response against malaria parasite infection, where splenic fibroblasts (SFs) are abundantly present and contribute to immune function by secreting type I collagen (collagen I). The protein family is characterized by Plasmodium vivax tryptophan-rich antigens (PvTRAgs), comprising 40 members. PvTRAg23 has been reported to bind to human SFs (HSFs) and affect collagen I levels. Given the role of type I collagen in splenic immune function, it is important to investigate the functions of the other members within the PvTRAg protein family.

METHODS

Protein structural prediction was conducted utilizing bioinformatics analysis tools and software. A total of 23 PvTRAgs were successfully expressed and purified using an Escherichia coli prokaryotic expression system, and the purified proteins were used for co-culture with HSFs. The collagen I levels and collagen-related signaling pathway protein levels were detected by immunoblotting, and the relative expression levels of inflammatory factors were determined by quantitative real-time PCR.

RESULTS

In silico analysis showed that P. vivax has 40 genes encoding the TRAg family. The C-terminal region of all PvTRAgs is characterized by the presence of a domain rich in tryptophan residues. A total of 23 recombinant PvTRAgs were successfully expressed and purified. Only five PvTRAgs (PvTRAg5, PvTRAg16, PvTRAg23, PvTRAg30, and PvTRAg32) mediated the activation of the NF-κBp65 signaling pathway, which resulted in the production of inflammatory molecules and ultimately a significant reduction in collagen I levels in HSFs.

CONCLUSIONS

Our research contributes to the expansion of knowledge regarding the functional role of PvTRAgs, while it also enhances our understanding of the immune evasion mechanisms utilized by parasites.

Identification of Exported Plasmodium falciparum Proteins That Bind to the Erythrocyte Cytoskeleton

Plasmodium proteins are exported to the erythrocyte cytoplasm to create an environment that supports parasite replication. Although hundreds of proteins are predicted to be exported through Plasmodium export element (PEXEL)-dependent and -independent mechanisms, the functions of exported proteins are largely uncharacterized. In this study, we used a biochemical screening approach to identify putative exported P. falciparum proteins that bound to inside-out vesicles prepared from erythrocytes. Out of 69 P. falciparum PEXEL-motif proteins tested, 18 bound to inside-out vesicles (IOVs) in two or more independent assays. Using co-affinity purifications followed by mass spectrometry, pairwise co-purification experiments, and the split-luciferase assay, we identified 31 putative protein-protein interactions between erythrocyte cytoskeletal proteins and predicted exported P. falciparum proteins. We further showed that PF3D7_1401600 binds to the spectrin-binding domain of erythrocyte ankyrin via its MESA erythrocyte cytoskeleton binding (MEC) motif and to the N-terminal domains of ankyrin and 4.1R through a fragment that required an intact Plasmodium helical interspersed sub-telomeric (PHIST) domain. Introduction of PF3D7_1401600 into erythrocyte ghosts increased retention in the microsphiltration assay, consistent with previous data that reported a reduction of rigidity in red blood cells infected with PF3D7_1401600-deficient parasites.

Plasmodium vivax Protein PvTRAg23 Triggers Spleen Fibroblasts for Inflammatory Profile and Reduces Type I Collagen Secretion via NF-κBp65 Pathway

Plasmodium vivax is the most widespread human malaria parasite. The spleen is one of the most significant immune organs in the course of Plasmodium infection, and it contains splenic fibroblasts (SFs), which supports immunologic function by secreting type I collagen (collagen I). Plasmodium proteins have rarely been found to be involved in collagen alterations in the spleen during infection. Here, we selected the protein P. vivax tryptophan-rich antigen 23 (PvTRAg23), which is expressed by the spleen-dependent gene Pv-fam-a and is a member of the PvTRAgs family of export proteins, suggesting that it might have an effect on SFs. The protein specifically reduced the level of collagen I in human splenic fibroblasts (HSFs) and bound to cells with vimentin as receptors. However, such collagen changes were not mediated by binding to vimentin, but rather activating the NF-κBp65 pathway to produce inflammatory cytokines. Collagen impaired synthesis accompanied by extracellular matrix-related changes occurred in the spleen of mice infected with P. yoelii 17XNL. Overall, this study is the first one to report and verify the role of Plasmodium proteins on collagen in HSF in vitro. Results will contribute to further understanding of host spleen structural changes and immune responses after Plasmodium infection.

Glycosylphosphatidylinositol-anchored micronemal antigen (GAMA) interacts with the band 3 receptor to promote erythrocyte invasion by malaria parasites

Glycosylphosphatidylinositol-anchored micronemal antigen (GAMA) is an erythrocyte binding protein known to be involved in malarial parasite invasion. Although anti-GAMA antibodies have been shown to block GAMA attachment to the erythrocyte surface and subsequently inhibit parasite invasion, little is known about the molecular mechanisms by which GAMA promotes the invasion process. In this study, LC-MS analysis was performed on the erythrocyte membrane to identify the specific receptor that interacts with GAMA. We found that ankyrin 1 and the band 3 membrane protein showed affinity for GAMA, and characterization of their binding specificity indicated that both Plasmodium falciparum and Plasmodium vivax GAMA bound to the same extracellular loop of band 3 (loop 5). In addition, we show the interaction between GAMA and band 3 was sensitive to chymotrypsin. Furthermore, antibodies against band 3 loop 5 were able to reduce the binding activity of GAMA to erythrocytes and inhibit the invasion of P. falciparum merozoites into human erythrocytes, whereas antibodies against P. falciparum GAMA (PfGAMA)-Tr3 only slightly reduced P. falciparum invasion. The identification and characterization of the erythrocyte GAMA receptor is a novel finding that identifies an essential mechanism of parasite invasion of host erythrocytes.

Use of split-dihydrofolate reductase for the detection of protein-protein interactions and simultaneous selection of multiple plasmids in Plasmodium falciparum

Defining protein-protein interactions is fundamental to the understanding of gene function. Protein-fragment complementation assays have been used for the analysis of protein-protein interactions in various organisms. The split-dihydrofolate reductase (DHFR) protein-fragment complementation assay utilises two complementary fragments of the enzyme fused to a pair of potentially interacting proteins. If these proteins interact, the DHFR fragments associate, fold into their native structure, reconstitute their function and confer resistance to antifolate drugs. We show that murine DHFR fragments fused to interacting proteins reconstitute a functional enzyme and confer resistance to the antifolate drug WR99210 in Plasmodium falciparum. These data demonstrate that the split-DHFR method can be used to detect in vivo protein-protein interactions in the parasite. Additionally, we show that split-DHFR fragments can be used as selection markers, permitting simultaneous selection of two plasmids in the presence of a single antifolate drug. Taken together, these experiments show that split-DHFR represents a valuable tool for the characterisation of Plasmodium protein function and genetic manipulation of the parasite.

The Plasmodium falciparum MESA erythrocyte cytoskeleton-binding (MEC) motif binds to erythrocyte ankyrin

The MESA erythrocyte cytoskeleton binding (MEC) motif is a 13-amino acid sequence found in 14 exported Plasmodium falciparum proteins. First identified in the P. falciparum Mature-parasite-infected Erythrocyte Surface Antigen (MESA), the MEC motif is sufficient to target proteins to the infected red blood cell cytoskeleton. To identify host cell targets, purified MESA MEC motif was incubated with a soluble extract from uninfected erythrocytes, precipitated and subjected to mass spectrometry. The most abundant co-purifying protein was erythrocyte ankyrin (ANK1). A direct interaction between the MEC motif and ANK1 was independently verified using co-purification experiments, the split-luciferase assay, and the yeast two-hybrid assay. A systematic mutational analysis of the core MEC motif demonstrated a critical role for the conserved aspartic acid residue at the C-terminus of the MEC motif for binding to both erythrocyte inside-out vesicles and to ANK1. Using a panel of ANK1 constructs, the MEC motif binding site was localized to the ZU5^C domain, which has no known function. The MEC motif had no impact on erythrocyte deformability when introduced into uninfected erythrocyte ghosts, suggesting the MEC motif's primary function is to target exported proteins to the cytoskeleton. Finally, we show that PF3D7_0402100 (PFD0095c) binds to ANK1 and band 4.1, likely through its MEC and PHIST motifs, respectively. In conclusion, we have provided multiple lines of evidence that the MEC motif binds to erythrocyte ANK1.

Genetic conflicts with Plasmodium parasites and functional constraints shape the evolution of erythrocyte cytoskeletal proteins

Plasmodium parasites exerted a strong selective pressure on primate genomes and mutations in genes encoding erythrocyte cytoskeleton proteins (ECP) determine protective effects against Plasmodium infection/pathogenesis. We thus hypothesized that ECP-encoding genes have evolved in response to Plasmodium-driven selection. We analyzed the evolutionary history of 15 ECP-encoding genes in primates, as well as of their Plasmodium-encoded ligands (KAHRP, MESA and EMP3). Results indicated that EPB42, SLC4A1, and SPTA1 evolved under pervasive positive selection and that episodes of positive selection tended to occur more frequently in primate species that host a larger number of Plasmodium parasites. Conversely, several genes, including ANK1 and SPTB, displayed extensive signatures of purifying selection in primate phylogenies, Homininae lineages, and human populations, suggesting strong functional constraints. Analysis of Plasmodium genes indicated adaptive evolution in MESA and KAHRP; in the latter, different positively selected sites were located in the spectrin-binding domains. Because most of the positively selected sites in alpha-spectrin localized to the domains involved in the interaction with KAHRP, we suggest that the two proteins are engaged in an arms-race scenario. This observation is relevant because KAHRP is essential for the formation of “knobs”, which represent a major virulence determinant for P. falciparum.

Repetitive sequences in malaria parasite proteins

Five species of parasite cause malaria in humans with the most severe disease caused by Plasmodium falciparum. Many of the proteins encoded in the P. falciparum genome are unusually enriched in repetitive low-complexity sequences containing a limited repertoire of amino acids. These repetitive sequences expand and contract dynamically and are among the most rapidly changing sequences in the genome. The simplest repetitive sequences consist of single amino acid repeats such as poly-asparagine tracts that are found in approximately 25% of P. falciparum proteins. More complex repeats of two or more amino acids are also common in diverse parasite protein families. There is no universal explanation for the occurrence of repetitive sequences and it is possible that many confer no function to the encoded protein and no selective advantage or disadvantage to the parasite. However, there are increasing numbers of examples where repetitive sequences are important for parasite protein function. We discuss the diverse roles of low-complexity repetitive sequences throughout the parasite life cycle, from mediating protein-protein interactions to enabling the parasite to evade the host immune system.