The role of palmitoylation for protein recruitment to the inner membrane complex of the malaria parasite

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

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

Coordinated regulation of gene expression in Plasmodium female gametocytes by two transcription factors

Gametocytes play key roles in the Plasmodium lifecycle. They are essential for sexual reproduction as precursors of the gametes. They also play an essential role in parasite transmission to mosquitoes. Elucidation of the gene regulation at this stage is essential for understanding these two processes at the molecular level and for developing new strategies to break the parasite lifecycle. We identified a novel Plasmodium transcription factor (TF), designated as a partner of AP2-FG or PFG. In this article, we report that this TF regulates the gene expression in female gametocytes in concert with another female-specific TF AP2-FG. Upon the disruption of PFG, majority of female-specific genes were significantly downregulated, and female gametocyte lost the ability to produce ookinetes. ChIP-seq analysis showed that it was located in the same position as AP2-FG, indicating that these two TFs form a complex. ChIP-seq analysis of PFG in AP2-FG-disrupted parasites and ChIP-seq analysis of AP2-FG in PFG-disrupted parasites demonstrated that PFG mediates the binding of AP2-FG to a ten-base motif and that AP2-FG binds another motif, GCTCA, in the absence of PFG. In promoter assays, this five-base motif was identified as another female-specific cis-acting element. Genes under the control of the two forms of AP2-FG, with or without PFG, partly overlapped; however, each form had target preferences. These results suggested that combinations of these two forms generate various expression patterns among the extensive genes expressed in female gametocytes.

Plasmodium falciparum Development from Gametocyte to Oocyst: Insight from Functional Studies

Malaria elimination may never succeed without the implementation of transmission-blocking strategies. The transmission of Plasmodium spp. parasites from the human host to the mosquito vector depends on circulating gametocytes in the peripheral blood of the vertebrate host. Once ingested by the mosquito during blood meals, these sexual forms undergo a series of radical morphological and metabolic changes to survive and progress from the gut to the salivary glands, where they will be waiting to be injected into the vertebrate host. The design of effective transmission-blocking strategies requires a thorough understanding of all the mechanisms that drive the development of gametocytes, gametes, sexual reproduction, and subsequent differentiation within the mosquito. The drastic changes in Plasmodium falciparum shape and function throughout its life cycle rely on the tight regulation of stage-specific gene expression. This review outlines the mechanisms involved in Plasmodium falciparum sexual stage development in both the human and mosquito vector, and zygote to oocyst differentiation. Functional studies unravel mechanisms employed by P. falciparum to orchestrate the expression of stage-specific functional products required to succeed in its complex life cycle, thus providing us with potential targets for developing new therapeutics. These mechanisms are based on studies conducted with various Plasmodium species, including predominantly P. falciparum and the rodent malaria parasites P. berghei. However, the great potential of epigenetics, genomics, transcriptomics, proteomics, and functional genetic studies to improve the understanding of malaria as a disease remains partly untapped because of limitations in studies using human malaria parasites and field isolates.

Post-translational lipid modifications in Plasmodium parasites

Most eukaryotic proteins undergo post-translational modifications (PTMs) that significantly alter protein properties, regulate diverse cellular processes and increase proteome complexity. Among these PTMs, lipidation plays a unique and key role in subcellular trafficking, signalling and membrane association of proteins through altering substrate function, and hydrophobicity via the addition and removal of lipid groups. Three prevalent classes of lipid modifications in Plasmodium parasites include prenylation, myristoylation, and palmitoylation that are important for regulating parasite-specific molecular processes. The enzymes that catalyse these lipid attachments have also been explored as potential drug targets for antimalarial development. In this review, we discuss these lipidation processes in Plasmodium spp. and the methodologies that have been used to identify these modifications in the deadliest species of malaria parasite, Plasmodium falciparum. We also discuss the development status of inhibitors that block these pathways.

Inner membrane complex proteomics reveals a palmitoylation regulation critical for intraerythrocytic development of malaria parasite

Malaria is caused by infection of the erythrocytes by the parasites Plasmodium. Inside the erythrocytes, the parasites multiply via schizogony, an unconventional cell division mode. The inner membrane complex (IMC), an organelle located beneath the parasite plasma membrane, serving as the platform for protein anchorage, is essential for schizogony. So far, the complete repertoire of IMC proteins and their localization determinants remain unclear. Here we used biotin ligase (TurboID)-based proximity labeling to compile the proteome of the schizont IMC of the rodent malaria parasite Plasmodium yoelii. In total, 300 TurboID-interacting proteins were identified. 18 of 21 selected candidates were confirmed to localize in the IMC, indicating good reliability. In light of the existing palmitome of Plasmodium falciparum, 83 proteins of the P. yoelii IMC proteome are potentially palmitoylated. We further identified DHHC2 as the major resident palmitoyl-acyl-transferase of the IMC. Depletion of DHHC2 led to defective schizont segmentation and growth arrest both in vitro and in vivo. DHHC2 was found to palmitoylate two critical IMC proteins CDPK1 and GAP45 for their IMC localization. In summary, this study reports an inventory of new IMC proteins and demonstrates a central role of DHHC2 in governing the IMC localization of proteins during the schizont development.

Dynamically expressed genes provide candidate viability biomarkers in a model coccidian

Eimeria parasites cause enteric disease in livestock and the closely related Cyclosporacayetanensis causes human disease. Oocysts of these coccidian parasites undergo maturation (sporulation) before becoming infectious. Here, we assessed transcription in maturing oocysts of Eimeria acervulina, a widespread chicken parasite, predicted gene functions, and determined which of these genes also occur in C. cayetanensis. RNA-Sequencing yielded ~2 billion paired-end reads, 92% of which mapped to the E. acervulina genome. The ~6,900 annotated genes underwent temporally-coordinated patterns of gene expression. Fifty-three genes each contributed >1,000 transcripts per million (TPM) throughout the study interval, including cation-transporting ATPases, an oocyst wall protein, a palmitoyltransferase, membrane proteins, and hypothetical proteins. These genes were enriched for 285 gene ontology (GO) terms and 13 genes were ascribed to 17 KEGG pathways, defining housekeeping processes and functions important throughout sporulation. Expression differed in mature and immature oocysts for 40% (2,928) of all genes; of these, nearly two-thirds (1,843) increased their expression over time. Eight genes expressed most in immature oocysts, encoding proteins promoting oocyst maturation and development, were assigned to 37 GO terms and 5 KEGG pathways. Fifty-six genes underwent significant upregulation in mature oocysts, each contributing at least 1,000 TPM. Of these, 40 were annotated by 215 GO assignments and 9 were associated with 18 KEGG pathways, encoding products involved in respiration, carbon fixation, energy utilization, invasion, motility, and stress and detoxification responses. Sporulation orchestrates coordinated changes in the expression of many genes, most especially those governing metabolic activity. Establishing the long-term fate of these transcripts in sporulated oocysts and in senescent and deceased oocysts will further elucidate the biology of coccidian development, and may provide tools to assay infectiousness of parasite cohorts. Moreover, because many of these genes have homologues in C. cayetanensis, they may prove useful as biomarkers for risk.

Unique Endomembrane Systems and Virulence in Pathogenic Protozoa

Virulence in pathogenic protozoa is often tied to secretory processes such as the expression of adhesins on parasite surfaces or the secretion of proteases to assisted in tissue invasion and other proteins to avoid the immune system. This review is a broad overview of the endomembrane systems of pathogenic protozoa with a focus on Giardia, Trichomonas, Entamoeba, kinetoplastids, and apicomplexans. The focus is on unique features of these protozoa and how these features relate to virulence. In general, the basic elements of the endocytic and exocytic pathways are present in all protozoa. Some of these elements, especially the endosomal compartments, have been repurposed by the various species and quite often the repurposing is associated with virulence. The Apicomplexa exhibit the most unique endomembrane systems. This includes unique secretory organelles that play a central role in interactions between parasite and host and are involved in the invasion of host cells. Furthermore, as intracellular parasites, the apicomplexans extensively modify their host cells through the secretion of proteins and other material into the host cell. This includes a unique targeting motif for proteins destined for the host cell. Most notable among the apicomplexans is the malaria parasite, which extensively modifies and exports numerous proteins into the host erythrocyte. These modifications of the host erythrocyte include the formation of unique membranes and structures in the host erythrocyte cytoplasm and on the erythrocyte membrane. The transport of parasite proteins to the host erythrocyte involves several unique mechanisms and components, as well as the generation of compartments within the erythrocyte that participate in extraparasite trafficking.

Plasmodium falciparum goes bananas for sex

The sexual blood stages of the human malaria parasite Plasmodium falciparum undergo a remarkable transformation from a roughly spherical shape to an elongated crescent or "falciform" morphology from which the species gets its name. In this review, the molecular events that drive this spectacular shape change are discussed and some questions that remain regarding the mechanistic underpinnings are posed. We speculate on the role of the shape changes in promoting sequestration and release of the developing gametocyte, thereby facilitating parasite survival in the host and underpinning transmission to the mosquito vector.

Blocking Palmitoylation of Toxoplasma gondii Myosin Light Chain 1 Disrupts Glideosome Composition but Has Little Impact on Parasite Motility

Toxoplasma gondii is a widespread apicomplexan parasite that causes severe disease in immunocompromised individuals and the developing fetus. Like other apicomplexans, T. gondii uses an unusual form of substrate-dependent gliding motility to invade cells of its hosts and to disseminate throughout the body during infection. It is well established that a myosin motor consisting of a class XIVa heavy chain (TgMyoA) and two light chains (TgMLC1 and TgELC1/2) plays an important role in parasite motility. The ability of the motor to generate force at the parasite periphery is thought to be reliant upon its anchoring and immobilization within a peripheral membrane-bound compartment, the inner membrane complex (IMC). The motor does not insert into the IMC directly; rather, this interaction is believed to be mediated by the binding of TgMLC1 to the IMC-anchored protein, TgGAP45. Therefore, the binding of TgMLC1 to TgGAP45 is considered a key element in the force transduction machinery of the parasite. TgMLC1 is palmitoylated, and we show here that palmitoylation occurs on two N-terminal cysteine residues, C8 and C11. Mutations that block TgMLC1 palmitoylation completely abrogate the binding of TgMLC1 to TgGAP45. Surprisingly, the loss of TgMLC1 binding to TgGAP45 in these mutant parasites has little effect on their ability to initiate or sustain movement. These results question a key tenet of the current model of apicomplexan motility and suggest that our understanding of gliding motility in this important group of human and animal pathogens is not yet complete.

IMPORTANCE Gliding motility plays a central role in the life cycle of T. gondii and other apicomplexan parasites. The myosin motor thought to power motility is essential for virulence but distinctly different from the myosins found in humans. Consequently, an understanding of the mechanism(s) underlying parasite motility and the role played by this unusual myosin may reveal points of vulnerability that can be targeted for disease prevention or treatment. We show here that mutations that uncouple the motor from what is thought to be a key structural component of the motility machinery have little impact on parasite motility. This finding runs counter to predictions of the current, widely held “linear motor” model of motility, highlighting the need for further studies to fully understand how apicomplexan parasites generate the forces necessary to move into, out of, and between cells of the hosts they infect.

Identification of novel inner membrane complex and apical annuli proteins of the malaria parasite Plasmodium falciparum

The inner membrane complex (IMC) is a defining feature of apicomplexan parasites, which confers stability and shape to the cell, functions as a scaffolding compartment during the formation of daughter cells and plays an important role in motility and invasion during different life cycle stages of these single-celled organisms. To explore the IMC proteome of the malaria parasite Plasmodium falciparum we applied a proximity-dependent biotin identification (BioID)-based proteomics approach, using the established IMC marker protein Photosensitized INA-Labelled protein 1 (PhIL1) as bait in asexual blood-stage parasites. Subsequent mass spectrometry-based peptide identification revealed enrichment of 12 known IMC proteins and several uncharacterized candidate proteins. We validated nine of these previously uncharacterized proteins by endogenous GFP-tagging. Six of these represent new IMC proteins, while three proteins have a distinct apical localization that most likely represents structures described as apical annuli in Toxoplasma gondii. Additionally, various Kelch13 interacting candidates were identified, suggesting an association of the Kelch13 compartment and the IMC in schizont and merozoite stages. This work extends the number of validated IMC proteins in the malaria parasite and reveals for the first time the existence of apical annuli proteins in P. falciparum. Additionally, it provides evidence for a spatial association between the Kelch13 compartment and the IMC in late blood-stage parasites.

The Dynamic Roles of the Inner Membrane Complex in the Multiple Stages of the Malaria Parasite

Apicomplexan parasites, such as human malaria parasites, have complex lifecycles encompassing multiple and diverse environmental niches. Invading, replicating, and escaping from different cell types, along with exploiting each intracellular niche, necessitate large and dynamic changes in parasite morphology and cellular architecture. The inner membrane complex (IMC) is a unique structural element that is intricately involved with these distinct morphological changes. The IMC is a double membrane organelle that forms de novo and is located beneath the plasma membrane of these single-celled organisms. In Plasmodium spp. parasites it has three major purposes: it confers stability and shape to the cell, functions as an important scaffolding compartment during the formation of daughter cells, and plays a major role in motility and invasion. Recent years have revealed greater insights into the architecture, protein composition and function of the IMC. Here, we discuss the multiple roles of the IMC in each parasite lifecycle stage as well as insights into its sub-compartmentalization, biogenesis, disassembly and regulation during stage conversion of P. falciparum.

Two palmitoyl acyltransferases involved sequentially in the biogenesis of the inner membrane complex of Toxoplasma gondii

The phylum Apicomplexa includes a number of significant human pathogens like Toxoplasma gondii and Plasmodium species. These obligate intracellular parasites possess a membranous structure, the inner membrane complex (IMC), composed of flattened vesicles apposed to the plasma membrane. Numerous proteins associated with the IMC are anchored via a lipid post-translational modification termed palmitoylation. This acylation is catalysed by multi-membrane spanning protein S-acyl-transferases (PATs) containing a catalytic Asp-His-His-Cys (DHHC) motif, commonly referred to as DHHCs. Contrasting the redundancy observed in other organisms, several PATs are essential for T. gondii tachyzoite survival; 2 of them, TgDHHC2 and TgDHHC14 being IMC-resident. Disruption of either of these TgDHHCs results in a rapid collapse of the IMC in the developing daughter cells leading to dramatic morphological defects of the parasites while the impact on the other organelles is limited to their localisation but not to their biogenesis. The acyl-transferase activity of TgDHHC2 and TgDHHC14 is involved sequentially in the formation of the sub-compartments of the IMC. Investigation of proteins known to be palmitoylated and localised to these sub-compartments identified TgISP1/3 as well as TgIAP1/2 to lose their membrane association revealing them as likely substrates of TgDHHC2, while these proteins are not impacted by TgDHHC14 depletion.

A protein palmitoylation cascade regulates microtubule cytoskeleton integrity in Plasmodium

Morphogenesis of many protozoans depends on a polarized establishment of cytoskeletal structures. In malaria-causing parasites, this can be observed when a round zygote develops into an elongated motile ookinete within the mosquito stomach. This morphogenesis is mediated by the pellicle cytoskeletal structures, including the inner membrane complex (IMC) and the underlying subpellicular microtubules (SPMs). How the parasite maintains the IMC-SPM connection and establishes a dome-like structure of SPM to support cell elongation is unclear. Here, we show that palmitoylation of N-terminal cysteines of two IMC proteins (ISP1/ISP3) regulates the IMC localization of ISP1/ISP3 and zygote-to-ookinete differentiation. Palmitoylation of ISP1/ISP3 is catalyzed by an IMC-residing palmitoyl-S-acyl-transferase (PAT) DHHC2. Surprisingly, DHHC2 undergoes self-palmitoylation at C-terminal cysteines via its PAT activity, which controls DHHC2 localization in IMC after zygote formation. IMC-anchored ISP1 and ISP3 interact with microtubule component β-tubulin, serving as tethers to maintain the proper structure of SPM during zygote elongation. This study identifies the first PAT-substrate pair in malaria parasites and uncovers a protein palmitoylation cascade regulating microtubule cytoskeleton.

Palmitoylated Proteins in Plasmodium falciparum-Infected Erythrocytes: Investigation with Click Chemistry and Metabolic Labeling

The examination of the complex cell biology of the human malaria parasite Plasmodium falciparum usually relies on the time-consuming generation of transgenic parasites. Here, metabolic labeling and click chemistry are employed as a fast transfection-independent method for the microscopic examination of protein S-palmitoylation, an important post-translational modification during the asexual intraerythrocytic replication of P. falciparum. Applying various microscopy approaches such as confocal, single-molecule switching, and electron microscopy, differences in the extent of labeling within the different asexual developmental stages of P. falciparum and the host erythrocytes over time are observed.

Molecular Characterization and Immuno-Reactivity Patterns of a Novel Plasmodium falciparum Armadillo-Type Repeat Protein, PfATRP

Nearly half of the genes in the Plasmodium falciparum genome have not yet been functionally investigated. We used homology-based structural modeling to identify multiple copies of Armadillo repeats within one uncharacterized gene expressed during the intraerythrocytic stages, PF3D7_0410600, subsequently referred to as P. falciparum Armadillo-Type Repeat Protein (PfATRP). Soluble recombinant PfATRP was expressed in a bacterial expression system, purified to apparent homogeneity and the identity of the recombinant PfATRP was confirmed by mass spectrometry. Affinity-purified α-PfATRP rabbit antibodies specifically recognized the recombinant protein. Immunofluorescence assays revealed that α-PfATRP rabbit antibodies reacted with P. falciparum schizonts. Anti-PfATRP antibody exhibited peripheral staining patterns around the merozoites. Given the localization of PfATRP in merozoites, we tested for an egress phenotype during schizont arrest assays and demonstrated that native PfATRP is inaccessible on the surface of merozoites in intact schizonts. Dual immunofluorescence assays with markers for the inner membrane complex (IMC) and microtubules suggest partial colocalization in both asexual and sexual stage parasites. Using the soluble recombinant PfATRP in a screen of plasma samples revealed that malaria-infected children have naturally acquired PfATRP-specific antibodies.

Toxoplasma F-box protein 1 is required for daughter cell scaffold function during parasite replication

By binding to the adaptor protein SKP1 and serving as substrate receptors for the SKP1 Cullin, F-box E3 ubiquitin ligase complex, F-box proteins regulate critical cellular processes including cell cycle progression and membrane trafficking. While F-box proteins are conserved throughout eukaryotes and are well studied in yeast, plants, and animals, studies in parasitic protozoa are lagging. We have identified eighteen putative F-box proteins in the Toxoplasma genome of which four have predicted homologs in Plasmodium. Two of the conserved F-box proteins were demonstrated to be important for Toxoplasma fitness and here we focus on an F-box protein, named TgFBXO1, because it is the most highly expressed by replicative tachyzoites and was also identified in an interactome screen as a Toxoplasma SKP1 binding protein. TgFBXO1 interacts with Toxoplasma SKP1 confirming it as a bona fide F-box protein. In interphase parasites, TgFBXO1 is a component of the Inner Membrane Complex (IMC), which is an organelle that underlies the plasma membrane. Early during replication, TgFBXO1 localizes to the developing daughter cell scaffold, which is the site where the daughter cell IMC and microtubules form and extend from. TgFBXO1 localization to the daughter cell scaffold required centrosome duplication but before kinetochore separation was completed. Daughter cell scaffold localization required TgFBXO1 N-myristoylation and was dependent on the small molecular weight GTPase, TgRab11b. Finally, we demonstrate that TgFBXO1 is required for parasite growth due to its function as a daughter cell scaffold effector. TgFBXO1 is the first F-box protein to be studied in apicomplexan parasites and represents the first protein demonstrated to be important for daughter cell scaffold function.

Palmitoylation in apicomplexan parasites: from established regulatory roles to putative new functions

This minireview aims to provide a comprehensive synthesis on protein palmitoylation in apicomplexan parasites and higher eukaryotes where most of the data is available. Apicomplexan parasites encompass numerous obligate intracellular parasites with significant health risk to animals and humans. Protein palmitoylation is a widespread post-translational modification that plays important regulatory roles in several physiological and pathological states. Functional studies demonstrate that many processes important for parasites are regulated by protein palmitoylation. Structural analyses suggest that enzymes responsible for the palmitoylation process have a conserved architecture in eukaryotes although there are particular differences which could be related to their substrate specificities. Interestingly, with the publication of T. gondii and P. falciparum palmitoylomes new possible regulatory functions are unveiled. Here we focus our discussion on data from both palmitoylomes that suggest that palmitoylation of nuclear proteins regulate different chromatin-related processes such as nucleosome assembly and stability, transcription, translation and DNA repair.

Identification and Characterization of a Novel Palmitoyl Acyltransferase as a Druggable Rheostat of Dynamic Palmitoylome in L. donovani

Palmitoylation has been recently identified as an important post-translational rheostat for controlling protein function in eukaryotes. However, the molecular machinery underlying palmitoylation remains unclear in the neglected tropical parasite, Leishmania donovani. Herein, we have identified a catalog of 20 novel palmitoyl acyltransferases (PATs) and characterized the promastigote-specific PAT (LdPAT4) containing the canonical Asp-His-His-Cys (DHHC) domain. Immunofluorescence analysis using in-house generated LdPAT4-specific antibody demonstrated distinct expression of LdPAT4 in the flagellar pocket of promastigotes. Using metabolic labeling-coupled click chemistry method, the functionality of this recombinant enzyme could be authenticated in E. coli strain expressing LdPAT4-DHHC domain. This was evident by the cellular uptake of palmitic acid analogs, which could be successfully inhibited by 2-BMP, a PAT-specific inhibitor. Using CSS-Palm based in-silico proteomic analysis, we could predict up to 23 palmitoylated sites per protein in the promastigotes, and further identify distinctive palmitoylated protein clusters involved in microtubule assembly, flagella motility and vesicular trafficking. To highlight, proteins such as Flagellar Member proteins (FLAM1, FLAM5), Intraflagellar Transport proteins (IFT88), and flagellar motor assembly proteins including the Dynein family were found to be enriched. Furthermore, analysis of global palmitoylation in promastigotes using Acyl-biotin exchange purification identified a set of S-palmitoylated proteins overlapping with the in-silico proteomics data. The attenuation of palmitoylation using 2-BMP demonstrated several phenotypic alterations in the promastigotes including distorted morphology, reduced motility (flagellar loss or slow flagellar beating), and inefficient invasion of promastigotes to host macrophages. These analyses confirm the essential role of palmitoylation in promastigotes. In summary, the findings suggest that LdPAT4 acts as a functional acyltransferase that can regulate palmitoylation of proteins involved in parasite motility and invasion, thus, can serve as a potential target for designing chemotherapeutics in Visceral Leishmaniasis.

N-Myristoylation as a Drug Target in Malaria: Exploring the Role of N-Myristoyltransferase Substrates in the Inhibitor Mode of Action

Malaria continues to be a significant cause of death and morbidity worldwide, and there is a need for new antimalarial drugs with novel targets. We have focused as a potential target for drug development on N-myristoyl transferase (NMT), an enzyme that acylates a wide range of substrate proteins. The NMT substrates in Plasmodium falciparum include some proteins that are common to processes in eukaryotes such as secretory transport and others that are unique to apicomplexan parasites. Myristoylation facilitates a protein interaction with membranes that may be strengthened by further lipidation, and the inhibition of NMT results in incorrect protein localization and the consequent disruption of function. The diverse roles of NMT substrates mean that NMT inhibition has a pleiotropic and severe impact on parasite development, growth, and multiplication. To study the mode of action underlying NMT inhibition, it is important to consider the function of proteins upstream and downstream of NMT. In this work, we therefore present our current perspective on the different functions of known NMT substrates as well as compare the inhibition of cotranslational myristoylation to the inhibition of known targets upstream of NMT.

Cryptosporidium parvum vaccine candidates are incompletely modified with O-linked-N-acetylgalactosamine or contain N-terminal N-myristate and S-palmitate

Cryptosporidium parvum (studied here) and Cryptosporidium hominis are important causes of diarrhea in infants and immunosuppressed persons. C. parvum vaccine candidates, which are on the surface of sporozoites, include glycoproteins with Ser- and Thr-rich domains (Gp15, Gp40, and Gp900) and a low complexity, acidic protein (Cp23). Here we used mass spectrometry to determine that O-linked GalNAc is present in dense arrays on a glycopeptide with consecutive Ser derived from Gp40 and on glycopeptides with consecutive Thr derived from Gp20, a novel C. parvum glycoprotein with a formula weight of ~20 kDa. In contrast, the occupied Ser or Thr residues in glycopeptides from Gp15 and Gp900 are isolated from one another. Gly at the N-terminus of Cp23 is N-myristoylated, while Cys, the second amino acid, is S-palmitoylated. In summary, C. parvum O-GalNAc transferases, which are homologs of host enzymes, densely modify arrays of Ser or Thr, as well as isolated Ser and Thr residues on C. parvum vaccine candidates. The N-terminus of an immunodominant antigen has lipid modifications similar to those of host cells and other apicomplexan parasites. Mass spectrometric demonstration here of glycopeptides with O-glycans complements previous identification C. parvum O-GalNAc transferases, lectin binding to vaccine candidates, and human and mouse antibodies binding to glycopeptides. The significance of these post-translational modifications is discussed with regards to the function of these proteins and the design of serological tests and vaccines.

Protein S-palmitoylation in cellular differentiation

Reversible protein S-palmitoylation confers spatiotemporal control of protein function by modulating protein stability, trafficking and activity, as well as protein-protein and membrane-protein associations. Enabled by technological advances, global studies revealed S-palmitoylation to be an important and pervasive posttranslational modification in eukaryotes with the potential to coordinate diverse biological processes as cells transition from one state to another. Here, we review the strategies and tools to analyze in vivo protein palmitoylation and interrogate the functions of the enzymes that put on and take off palmitate from proteins. We also highlight palmitoyl proteins and palmitoylation-related enzymes that are associated with cellular differentiation and/or tissue development in yeasts, protozoa, mammals, plants and other model eukaryotes.

Dynamic protein S-palmitoylation mediates parasite life cycle progression and diverse mechanisms of virulence

Eukaryotic parasites possess complex life cycles and utilize an assortment of molecular mechanisms to overcome physical barriers, suppress and/or bypass the host immune response, including invading host cells where they can replicate in a protected intracellular niche. Protein S-palmitoylation is a dynamic post-translational modification in which the fatty acid palmitate is covalently linked to cysteine residues on proteins by the enzyme palmitoyl acyltransferase (PAT) and can be removed by lysosomal palmitoyl-protein thioesterase (PPT) or cytosolic acyl-protein thioesterase (APT). In addition to anchoring proteins to intracellular membranes, functions of dynamic palmitoylation include - targeting proteins to specific intracellular compartments via trafficking pathways, regulating the cycling of proteins between membranes, modulating protein function and regulating protein stability. Recent studies in the eukaryotic parasites - Plasmodium falciparum, Toxoplasma gondii, Trypanosoma brucei, Cryptococcus neoformans and Giardia lamblia - have identified large families of PATs and palmitoylated proteins. Many palmitoylated proteins are important for diverse aspects of pathogenesis, including differentiation into infective life cycle stages, biogenesis and tethering of secretory organelles, assembling the machinery powering motility and targeting virulence factors to the plasma membrane. This review aims to summarize our current knowledge of palmitoylation in eukaryotic parasites, highlighting five exemplary mechanisms of parasite virulence dependent on palmitoylation.

Palmitoylation of Plasmodium alveolins promotes cytoskeletal function

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The alveolin IMC1c is palmitoylated on a conserved cysteine motif.

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Non-palmitoylated IMC1c exhibits normal stability and trafficking.

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Palmitoylation of Plasmodium alveolins enhances tensile strength.

S-palmitoylation is a post-translational lipid modification that is widespread among Plasmodium proteins and essential for parasite development. Little is known about the contribution of palmitoylation to the function of individual parasite molecules and structures. Alveolins are major components of the subpellicular network (SPN), a cortical cytoskeleton primarily involved in providing mechanical strength to the cell. We show here that the alveolin IMC1c is palmitoylated on a conserved cysteine motif, and that non-palmitoylated IMC1c displays normal expression, stability and trafficking. However, mutant parasites exhibit reduced osmotic stress resistance and tensile strength. These findings support the hypothesis that alveolin palmitoylation enhances cytoskeletal function by strengthening the connection between the SPN and the adjoining inner membrane complex via lipid anchoring.

Progress toward Understanding Protein S-acylation: Prospective in Plants

S-acylation, also known as S-palmitoylation or palmitoylation, is a reversible post-translational lipid modification in which long chain fatty acid, usually the 16-carbon palmitate, covalently attaches to a cysteine residue(s) throughout the protein via a thioester bond. It is involved in an array of important biological processes during growth and development, reproduction and stress responses in plant. S-acylation is a ubiquitous mechanism in eukaryotes catalyzed by a family of enzymes called Protein S-Acyl Transferases (PATs). Since the discovery of the first PAT in yeast in 2002 research in S-acylation has accelerated in the mammalian system and followed by in plant. However, it is still a difficult field to study due to the large number of PATs and even larger number of putative S-acylated substrate proteins they modify in each genome. This is coupled with drawbacks in the techniques used to study S-acylation, leading to the slower progress in this field compared to protein phosphorylation, for example. In this review we will summarize the discoveries made so far based on knowledge learnt from the characterization of protein S-acyltransferases and the S-acylated proteins, the interaction mechanisms between PAT and its specific substrate protein(s) in yeast and mammals. Research in protein S-acylation and PATs in plants will also be covered although this area is currently less well studied in yeast and mammalian systems.

A Malaria Transmission-Blocking (+)-Usnic Acid Derivative Prevents Plasmodium Zygote-to-Ookinete Maturation in the Mosquito Midgut

The evolution of drug resistance is a recurrent problem that has plagued efforts to treat and control malaria. Recent emergence of artemisinin resistance in Southeast Asia underscores the need to develop novel antimalarials and identify new targetable pathways in Plasmodium parasites. Transmission-blocking approaches, which typically target gametocytes in the host bloodstream or parasite stages in the mosquito gut, are recognized collectively as a strategy that when used in combination with antimalarials that target erythrocytic stages will not only cure malaria but will also prevent subsequent transmission. We tested four derivatives of (+)-usnic acid, a metabolite isolated from lichens, for transmission-blocking activity against Plasmodium falciparum using the standard membrane feeding assay. For two of the derivatives, BT37 and BT122, we observed a consistent dose-response relationship between concentration in the blood meal and oocyst intensity in the midgut. To explore their mechanism of action, we used the murine model Plasmodium berghei and found that both derivatives prevent ookinete maturation. Using fluorescence microscopy, we demonstrated that in the presence of each compound zygote vitality was severely affected, and those that did survive failed to elongate and mature into ookinetes. The observed phenotypes were similar to those described for mutants of specific kinases (NEK2/NEK4) and of inner membrane complex 1 (IMC1) proteins, which are all vital to the zygote-to-ookinete transition. We discuss the implications of our findings and our high-throughput screening approach to identifying next generation, transmission-blocking antimalarials based on the scaffolds of these (+)-usnic acid derivatives.

Study of Plasmodium falciparum DHHC palmitoyl transferases identifies a role for PfDHHC9 in gametocytogenesis

Palmitoylation is the post-translational reversible addition of the acyl moiety, palmitate, to cysteine residues of proteins and is involved in regulating protein trafficking, localization, stability and function. The Aspartate-Histidine-Histidine-Cysteine (DHHC) protein family, named for their highly conserved DHHC signature motif, is thought to be responsible for catalysing protein palmitoylation. Palmitoylation is widespread in all eukaryotes, including the malaria parasite, Plasmodium falciparum, where over 400 palmitoylated proteins are present in the asexual intraerythrocytic schizont stage parasites, including proteins involved in key aspects of parasite maturation and development. The P. falciparum genome includes 12 proteins containing the conserved DHHC motif. In this study, we adapted a palmitoyl-transferase activity assay for use with P. falciparum proteins and demonstrated for the first time that P. falciparum DHHC proteins are responsible for the palmitoylation of P. falciparum substrates. This assay also reveals that multiple DHHCs are capable of palmitoylating the same substrate, indicating functional redundancy at least in vitro. To test whether functional redundancy also exists in vivo, we investigated the endogenous localization and essentiality of a subset of schizont-expressed PfDHHC proteins. Individual PfDHHC proteins localized to distinct organelles, including parasite-specific organelles such as the rhoptries and inner membrane complex. Knock-out studies identified individual DHHCs that may be essential for blood-stage growth and others that were functionally redundant in the blood stages but may have functions in other stages of parasite development. Supporting this hypothesis, disruption of PfDHHC9 had no effect on blood-stage growth but reduced the formation of gametocytes, suggesting that this protein could be exploited as a transmission-blocking target. The localization and stage-specific expression of the DHHC proteins may be important for regulating their substrate specificity and thus may provide a path for inhibitor development.

Maternally supplied S-acyl-transferase is required for crystalloid organelle formation and transmission of the malaria parasite

Transmission of the malaria parasite from the mammalian host to the mosquito vector requires the formation of adequately adapted parasite forms and stage-specific organelles. Here we show that formation of the crystalloid-a unique and short-lived organelle of the Plasmodium ookinete and oocyst stage required for sporogony-is dependent on the precisely timed expression of the S-acyl-transferase DHHC10. DHHC10, translationally repressed in female Plasmodium berghei gametocytes, is activated translationally during ookinete formation, where the protein is essential for the formation of the crystalloid, the correct targeting of crystalloid-resident protein LAP2, and malaria parasite transmission.

Palmitoyl transferases have critical roles in the development of mosquito and liver stages of Plasmodium

As the Plasmodium parasite transitions between mammalian and mosquito host, it has to adjust quickly to new environments. Palmitoylation, a reversible and dynamic lipid post-translational modification, plays a central role in regulating this process and has been implicated with functions for parasite morphology, motility and host cell invasion. While proteins associated with the gliding motility machinery have been described to be palmitoylated, no palmitoyl transferase responsible for regulating gliding motility has previously been identified. Here, we characterize two palmityol transferases with gene tagging and gene deletion approaches. We identify DHHC3, a palmitoyl transferase, as a mediator of ookinete development, with a crucial role for gliding motility in ookinetes and sporozoites, and we co-localize the protein with a marker for the inner membrane complex in the ookinete stage. Ookinetes and sporozoites lacking DHHC3 are impaired in gliding motility and exhibit a strong phenotype in vivo; with ookinetes being significantly less infectious to their mosquito host and sporozoites being non-infectious to mice. Importantly, genetic complementation of the DHHC3-ko parasite completely restored virulence. We generated parasites lacking both DHHC3, as well as the palmitoyl transferase DHHC9, and found an enhanced phenotype for these double knockout parasites, allowing insights into the functional overlap and compensational nature of the large family of PbDHHCs. These findings contribute to our understanding of the organization and mechanism of the gliding motility machinery, which as is becoming increasingly clear, is mediated by palmitoylation.

Plasmodium falciparum Secretome in Erythrocyte and Beyond

Plasmodium falciparum is the causative agent of deadly malaria disease. It is an intracellular eukaryote and completes its multi-stage life cycle spanning the two hosts viz, mosquito and human. In order to habituate within host environment, parasite conform several strategies to evade host immune responses such as surface antigen polymorphism or modulation of host immune system and it is mediated by secretion of proteins from parasite to the host erythrocyte and beyond, collectively known as, malaria secretome. In this review, we will discuss about the deployment of parasitic secretory protein in mechanism implicated for immune evasion, protein trafficking, providing virulence, changing permeability and cyto-adherence of infected erythrocyte. We will be covering the possibilities of developing malaria secretome as a drug/vaccine target. This gathered information will be worthwhile in depicting a well-organized picture for host-pathogen interplay during the malaria infection and may also provide some clues for the development of novel anti-malarial therapies.

The Plasmodium palmitoyl-S-acyl-transferase DHHC2 is essential for ookinete morphogenesis and malaria transmission

The post-translational addition of C-16 long chain fatty acids to protein cysteine residues is catalysed by palmitoyl-S-acyl-transferases (PAT) and affects the affinity of a modified protein for membranes and therefore its subcellular localisation. In apicomplexan parasites this reversible protein modification regulates numerous biological processes and specifically affects cell motility, and invasion of host cells by Plasmodium falciparum merozoites and Toxoplasma gondii tachyzoites. Using inhibitor studies we show here that palmitoylation is key to transformation of zygotes into ookinetes during initial mosquito infection with P. berghei. We identify DHHC2 as a unique PAT mediating ookinete formation and morphogenesis. Essential for life cycle progression in asexual blood stage parasites and thus refractory to gene deletion analyses, we used promoter swap (ps) methodology to maintain dhhc2 expression in asexual blood stages but down regulate expression in sexual stage parasites and during post-fertilization development of the zygote. The ps mutant showed normal gamete formation, fertilisation and DNA replication to tetraploid cells, but was characterised by a complete block in post-fertilisation development and ookinete formation. Our report highlights the crucial nature of the DHHC2 palmitoyl-S-acyltransferase for transmission of the malaria parasite to the mosquito vector through its essential role for ookinete morphogenesis.