Plasticity between MyoC- and MyoA-glideosomes: an example of functional compensation in Toxoplasma gondii invasion

Cytokinetic abscission in Toxoplasma gondii is governed by protein phosphatase 2A and the daughter cell scaffold complex

Cytokinetic abscission marks the final stage of cell division, during which the daughter cells physically separate through the generation of new barriers, such as the plasma membrane or cell wall. While the contractile ring plays a central role during cytokinesis in bacteria, fungi and animal cells, the process diverges in Apicomplexa. In Toxoplasma gondii, two daughter cells are formed within the mother cell by endodyogeny. The mechanism by which the progeny cells acquire their plasma membrane during the disassembly of the mother cell, allowing daughter cells to emerge, remains unknown. Here we identify and characterize five T. gondii proteins, including three protein phosphatase 2A subunits, which exhibit a distinct and dynamic localization pattern during parasite division. Individual downregulation of these proteins prevents the accumulation of plasma membrane at the division plane, preventing the completion of cellular abscission. Remarkably, the absence of cytokinetic abscission does not hinder the completion of subsequent division cycles. The resulting progeny are able to egress from the infected cells but fail to glide and invade, except in cases of conjoined twin parasites.

Characterization and functional analysis of Toxoplasma Golgi-associated proteins identified by proximity labelling

Toxoplasma gondii possesses a highly polarized secretory pathway that contains both broadly conserved eukaryotic organelles and unique apicomplexan organelles which play essential roles in the parasite's lytic cycle. As in other eukaryotes, the T. gondii Golgi apparatus sorts and modifies proteins prior to their distribution to downstream organelles. Many of the typical trafficking factors found involved in these processes are missing from apicomplexan genomes, suggesting that these parasites have evolved unique proteins to fill these roles. Here we identify a novel Golgi-localizing protein (ULP1) which contains structural homology to the eukaryotic trafficking factor p115/Uso1. We demonstrate that depletion of ULP1 leads to a dramatic reduction in parasite fitness and replicative ability. Using ULP1 as bait for TurboID proximity labelling and immunoprecipitation, we identify eleven more novel Golgi-associated proteins and demonstrate that ULP1 interacts with the T. gondii COG complex. These proteins include both conserved trafficking factors and parasite-specific proteins. Using a conditional knockdown approach, we assess the effect of each of these eleven proteins on parasite fitness. Together, this work reveals a diverse set of novel T. gondii Golgi-associated proteins that play distinct roles in the secretory pathway. As several of these proteins are absent outside of the Apicomplexa, they represent potential targets for the development of novel therapeutics against these parasites.

Importance

Apicomplexan parasites such as Toxoplasma gondii infect a large percentage of the world's population and cause substantial human disease. These widespread pathogens use specialized secretory organelles to infect their host cells, modulate host cell functions, and cause disease. While the functions of the secretory organelles are now better understood, the Golgi apparatus of the parasite remains largely unexplored, particularly regarding parasite-specific innovations that may help direct traffic intracellularly. In this work, we characterize ULP1, a protein that is unique to parasites but shares structural similarity to the eukaryotic trafficking factor p115/Uso1. We show that ULP1 plays an important role in parasite replication and demonstrate that it interacts with the conserved oligomeric Golgi (COG) complex. We then use ULP1 proximity labelling to identify eleven additional Golgi-associated proteins which we functionally analyze via conditional knockdown. This work expands our knowledge of the Toxoplasma Golgi apparatus and identifies potential targets for therapeutic intervention.

Systematic characterization of all Toxoplasma gondii TBC domain-containing proteins identifies an essential regulator of Rab2 in the secretory pathway

Toxoplasma gondii resides in its intracellular niche by employing a series of specialized secretory organelles that play roles in invasion, host cell manipulation, and parasite replication. Rab GTPases are major regulators of the parasite's secretory traffic that function as nucleotide-dependent molecular switches to control vesicle trafficking. While many of the Rab proteins have been characterized in T. gondii, precisely how these Rabs are regulated remains poorly understood. To better understand the parasite's secretory traffic, we investigated the entire family of Tre2-Bub2-Cdc16 (TBC) domain-containing proteins, which are known to be involved in vesicle fusion and secretory protein trafficking. We first determined the localization of all 18 TBC domain-containing proteins to discrete regions of the secretory pathway or other vesicles in the parasite. Second, we use an auxin-inducible degron approach to demonstrate that the protozoan-specific TgTBC9 protein, which localizes to the endoplasmic reticulum (ER), is essential for parasite survival. Knockdown of TgTBC9 results in parasite growth arrest and affects the organization of the ER and mitochondrial morphology. TgTBC9 knockdown also results in the formation of large lipid droplets (LDs) and multi-membranous structures surrounded by ER membranes, further indicating a disruption of ER functions. We show that the conserved dual-finger active site in the TBC domain of the protein is critical for its GTPase-activating protein (GAP) function and that the Plasmodium falciparum orthologue of TgTBC9 can rescue the lethal knockdown. We additionally show by immunoprecipitation and yeast 2 hybrid analyses that TgTBC9 preferentially binds Rab2, indicating that the TBC9-Rab2 pair controls ER morphology and vesicular trafficking in the parasite. Together, these studies identify the first essential TBC protein described in any protozoan and provide new insight into intracellular vesicle trafficking in T. gondii.

Myosin A and F-Actin play a critical role in mitochondrial dynamics and inheritance in Toxoplasma gondii

The single mitochondrion of the obligate intracellular parasite Toxoplasma gondii is highly dynamic. Toxoplasma's mitochondrion changes morphology as the parasite moves from the intracellular to the extracellular environment and during division. Toxoplasma's mitochondrial dynamic is dependent on an outer mitochondrion membrane-associated protein LMF1 and its interaction with IMC10, a protein localized at the inner membrane complex (IMC). In the absence of either LMF1 or IMC10, parasites have defective mitochondrial morphology and inheritance defects. As little is known about mitochondrial inheritance in Toxoplasma, we have used the LMF1/IMC10 tethering complex as an entry point to dissect the machinery behind this process. Using a yeast two-hybrid screen, we previously identified Myosin A (MyoA) as a putative interactor of LMF1. Although MyoA is known to be located at the parasite's pellicle, we now show through ultrastructure expansion microscopy (U-ExM) that this protein accumulates around the mitochondrion in the late stages of parasite division. Parasites lacking MyoA show defective mitochondrial morphology and a delay in mitochondrion delivery to the daughter parasite buds during division, indicating that this protein is involved in organellar inheritance. Disruption of the parasite's actin network also affects mitochondrion morphology. We also show that parasite-extracted mitochondrion vesicles interact with actin filaments. Interestingly, mitochondrion vesicles extracted out of parasites lacking LMF1 pulled down less actin, showing that LMF1 might be important for mitochondrion and actin interaction. Accordingly, we are showing for the first time that actin and Myosin A are important for Toxoplasma mitochondrial morphology and inheritance.

The Toxoplasma subpellicular network is highly interconnected and defines parasite shape for efficient motility and replication

Apicomplexan parasites possess several specialized structures to invade their host cells and replicate successfully. One of these is the inner membrane complex (IMC), a peripheral membrane-cytoskeletal system underneath the plasma membrane. It is composed of a series of flattened, membrane-bound vesicles and a cytoskeletal subpellicular network (SPN) comprised of intermediate filament-like proteins called alveolins. While the alveolin proteins are conserved throughout the Apicomplexa and the broader Alveolata, their precise functions and interactions remain poorly understood. Here, we describe the function of one of these alveolin proteins, TgIMC6. Disruption of IMC6 resulted in striking morphological defects that led to aberrant motility, invasion, and replication. Deletion analyses revealed that the alveolin domain alone is largely sufficient to restore localization and partially sufficient for function. As this highlights the importance of the IMC6 alveolin domain, we implemented unnatural amino acid photoreactive crosslinking to the alveolin domain and identified multiple binding interfaces between IMC6 and two other cytoskeletal proteins - IMC3 and ILP1. To our knowledge, this provides the first direct evidence of protein-protein interactions in the alveolin domain and supports the long-held hypothesis that the alveolin domain is responsible for filament formation. Collectively, our study features the conserved alveolin proteins as critical components that maintain the parasite's structural integrity and highlights the alveolin domain as a key mediator of SPN architecture.

Knocking down the expression of the molecular motors, myosin A, C and F genes in Toxoplasma gondii to decrease the parasite virulence

Toxoplasmosis is a serious parasitic infection and novel therapeutic options are highly demanded to effectively eliminate it. In current study, Toxoplasma gondii myosin A, C and F genes were knocked down using small interference RNA (siRNA) method and the parasite survival and virulence was evaluated in vitro and in vivo. The parasites were transfected with specific siRNA, virtually designed for myosin mRNAs, and co-cultured with human foreskin fibroblasts. The transfection rate and the viability of the transfected parasites were measured using flow cytometry and methyl thiazole tetrazolium (MTT) assays, respectively. Finally, the survival of BALB/c mice infected with siRNAs-transfected T. gondii was assessed. It was demonstrated that a transfection rate of 75.4% existed for siRNAs, resulting in 70% (P = 0.032), 80.6% (P = 0.017) and 85.5% (P = 0.013) gene suppression for myosin A, C and F in affected parasites, respectively, which was subsequently confirmed by Western blot analysis. Moreover, lower parasite viability was observed in those with knocked down myosin C with 80% (P = 0.0001), followed by 86.15% (P = 0.004) for myosin F and 92.3% (P = 0.083) for myosin A. Considerably higher mouse survival (about 40 h) was, also, demonstrated in mice challenged with myosin siRNA-transfected T. gondii, in comparison with control group challenged with wild-type parasites. In conclusion, myosin proteins knock down proposes a promising therapeutic strategy to combat toxoplasmosis.

Toxoplasma gondii encodes an array of TBC-domain containing proteins including an essential regulator that targets Rab2 in the secretory pathway

Toxoplasma gondii resides in its intracellular niche by employing a series of specialized secretory organelles that play roles in invasion, host-cell manipulation and parasite replication. Rab GTPases are major regulators of the parasite's secretory traffic that function as nucleotide dependent molecular switches to control vesicle trafficking. While many of the Rab proteins have been characterized in T. gondii , precisely how these Rabs are regulated remains poorly understood. To better understand the parasite's secretory traffic, we investigated the entire family of Tre2-Bub2-Cdc16 (TBC)-domain containing proteins, which are known to be involved in vesicle fusion and secretory protein trafficking. We first determined the localization of all 18 TBC-domain containing proteins to discrete regions of the secretory pathway or other vesicles in the parasite. We then use an auxin-inducible degron approach to demonstrate that the protozoan-specific TgTBC9 protein that localizes to the ER is essential for parasite survival. Knockdown of TgTBC9 results in parasite growth arrest and affects the organization of the ER and Golgi apparatus. We show that the conserved dual-finger active site in the TBC-domain of the protein is critical for its GTPase-activating protein (GAP) function and that the P. falciparum orthologue of TgTBC9 can rescue the lethal knockdown. We additionally show by immunoprecipitation and yeast two hybrid analyses that TgTBC9 directly binds Rab2, indicating that this TBC-Rab pair controls ER to Golgi traffic in the parasite. Together, these studies identify the first essential TBC protein described in any protozoan, provide new insight into intracellular vesicle trafficking in T. gondii , and reveal promising targets for the design of novel therapeutics that can specifically target apicomplexan parasites.

MyosinA is a druggable target in the widespread protozoan parasite Toxoplasma gondii

Toxoplasma gondii is a widespread apicomplexan parasite that can cause severe disease in its human hosts. The ability of T. gondii and other apicomplexan parasites to invade into, egress from, and move between cells of the hosts they infect is critical to parasite virulence and disease progression. An unusual and highly conserved parasite myosin motor (TgMyoA) plays a central role in T. gondii motility. The goal of this work was to determine whether the parasite's motility and lytic cycle can be disrupted through pharmacological inhibition of TgMyoA, as an approach to altering disease progression in vivo. To this end, we first sought to identify inhibitors of TgMyoA by screening a collection of 50,000 structurally diverse small molecules for inhibitors of the recombinant motor's actin-activated ATPase activity. The top hit to emerge from the screen, KNX-002, inhibited TgMyoA with little to no effect on any of the vertebrate myosins tested. KNX-002 was also active against parasites, inhibiting parasite motility and growth in culture in a dose-dependent manner. We used chemical mutagenesis, selection in KNX-002, and targeted sequencing to identify a mutation in TgMyoA (T130A) that renders the recombinant motor less sensitive to compound. Compared to wild-type parasites, parasites expressing the T130A mutation showed reduced sensitivity to KNX-002 in motility and growth assays, confirming TgMyoA as a biologically relevant target of KNX-002. Finally, we present evidence that KNX-002 can slow disease progression in mice infected with wild-type parasites, but not parasites expressing the resistance-conferring TgMyoA T130A mutation. Taken together, these data demonstrate the specificity of KNX-002 for TgMyoA, both in vitro and in vivo, and validate TgMyoA as a druggable target in infections with T. gondii. Since TgMyoA is essential for virulence, conserved in apicomplexan parasites, and distinctly different from the myosins found in humans, pharmacological inhibition of MyoA offers a promising new approach to treating the devastating diseases caused by T. gondii and other apicomplexan parasites.

IMC29 Plays an Important Role in Toxoplasma Endodyogeny and Reveals New Components of the Daughter-Enriched IMC Proteome

The Toxoplasma inner membrane complex (IMC) is a unique organelle that plays critical roles in parasite motility, invasion, egress, and replication. The IMC is delineated into the apical, body, and basal regions, defined by proteins that localize to these distinct subcompartments. The IMC can be further segregated by proteins that localize specifically to the maternal IMC, the daughter bud IMC, or both. While the function of the maternal IMC has been better characterized, the precise roles of most daughter IMC components remain poorly understood. Here, we demonstrate that the daughter protein IMC29 plays an important role in parasite replication. We show that Δimc29 parasites exhibit severe replication defects, resulting in substantial growth defects and loss of virulence. Deletion analyses revealed that IMC29 localization is largely dependent on the N-terminal half of the protein containing four predicted coiled-coil domains while IMC29 function requires a short C-terminal helical region. Using proximity labeling, we identify eight novel IMC proteins enriched in daughter buds, significantly expanding the daughter IMC proteome. We additionally report four novel proteins with unique localizations to the interface between two parasites or to the outer face of the IMC, exposing new subregions of the organelle. Together, this work establishes IMC29 as an important early daughter bud component of replication and uncovers an array of new IMC proteins that provides important insights into this organelle. IMPORTANCE The inner membrane complex (IMC) is a conserved structure across the Apicomplexa phylum, which includes obligate intracellular parasites that cause toxoplasmosis, malaria, and cryptosporidiosis. The IMC is critical for the parasite to maintain its intracellular lifestyle, particularly in providing a scaffold for daughter bud formation during parasite replication. While many IMC proteins in the later stages of division have been identified, components of the early stages of division remain unknown. Here, we focus on the early daughter protein IMC29, demonstrating that it is crucial for faithful parasite replication and identifying specific regions of the protein that are important for its localization and function. We additionally use proximity labeling to reveal a suite of daughter-enriched IMC proteins, which represent promising candidates to further explore this IMC subcompartment.

Importance of aspartyl protease 5 in the establishment of the intracellular niche during acute and chronic infection of Toxoplasma gondii

Virulence and persistence of the obligate intracellular parasite Toxoplasma gondii involve the secretion of effector proteins belonging to the family of dense granule proteins (GRAs) that act notably as modulators of the host defense mechanisms and participate in cyst wall formation. The subset of GRAs residing in the parasitophorous vacuole (PV) or exported into the host cell, undergo proteolytic cleavage in the Golgi upon the action of the aspartyl protease 5 (ASP5). In tachyzoites, ASP5 substrates play central roles in the morphology of the PV and the export of effectors across the translocon complex MYR1/2/3. Here, we used N-terminal amine isotopic labeling of substrates to identify novel ASP5 cleavage products by comparing the N-terminome of wild-type and Δasp5 lines in tachyzoites and bradyzoites. Validated substrates reside within the PV or PVM in an ASP5-dependent manner. Remarkably, Δasp5 bradyzoites are impaired in the formation of the cyst wall in vitro and exhibit a considerably reduced cyst burden in chronically infected animals. More specifically two-photon serial tomography of infected mouse brains revealed a comparatively reduced number and size of the cysts throughout the establishment of persistence in the absence of ASP5.

An apical membrane complex for triggering rhoptry exocytosis and invasion in Toxoplasma

Apicomplexan parasites possess secretory organelles called rhoptries that undergo regulated exocytosis upon contact with the host. This process is essential for the parasitic lifestyle of these pathogens and relies on an exocytic machinery sharing structural features and molecular components with free-living ciliates. However, how the parasites coordinate exocytosis with host interaction is unknown. Here, we performed a Tetrahymena-based transcriptomic screen to uncover novel exocytic factors in Ciliata and conserved in Apicomplexa. We identified membrane-bound proteins, named CRMPs, forming part of a large complex essential for rhoptry secretion and invasion in Toxoplasma. Using cutting-edge imaging tools, including expansion microscopy and cryo-electron tomography, we show that, unlike previously described rhoptry exocytic factors, TgCRMPs are not required for the assembly of the rhoptry secretion machinery and only transiently associate with the exocytic site-prior to the invasion. CRMPs and their partners contain putative host cell-binding domains, and CRMPa shares similarities with GPCR proteins. Collectively our data imply that the CRMP complex acts as a host-molecular sensor to ensure that rhoptry exocytosis occurs when the parasite contacts the host cell.

Identification of new components of the basal pole of Toxoplasma gondii provides novel insights into its molecular organization and functions

The Toxoplasma gondii tachyzoite is a singled-cell obligate intracellular parasite responsible for the acute phase of toxoplasmosis. This polarized cell exhibits an apical complex, a hallmark of the phylum Apicomplexa, essential for motility, invasion, and egress from the host cell. Located on the opposite end of the cell is the basal complex, an elaborated cytoskeletal structure that also plays critical roles in the lytic cycle of the parasite, being involved in motility, cell division, constriction and cytokinesis, as well as intravacuolar cell-cell communication. Nevertheless, only a few proteins of this structure have been described and functionally assessed. In this study, we used spatial proteomics to identify new basal complex components (BCC), and in situ imaging, including ultrastructure expansion microscopy, to position them. We thus confirmed the localization of nine BCCs out of the 12 selected candidates and assigned them to different sub-compartments of the basal complex, including two new domains located above the basal ring and below the posterior cup. Their functional investigation revealed that none of these BCCs are essential for parasite growth in vitro. However, one BCC is critical for constricting of the basal complex, likely through direct interaction with the class VI myosin heavy chain J (MyoJ), and for gliding motility. Four other BCCs, including a phosphatase and a guanylate-binding protein, are involved in the formation and/or maintenance of the intravacuolar parasite connection, which is required for the rosette organization and synchronicity of cell division.

Temporal and thermal profiling of the Toxoplasma proteome implicates parasite Protein Phosphatase 1 in the regulation of Ca2+-responsive pathways

Apicomplexan parasites cause persistent mortality and morbidity worldwide through diseases including malaria, toxoplasmosis, and cryptosporidiosis. Ca²⁺ signaling pathways have been repurposed in these eukaryotic pathogens to regulate parasite-specific cellular processes governing the replicative and lytic phases of the infectious cycle, as well as the transition between them. Despite the presence of conserved Ca²⁺-responsive proteins, little is known about how specific signaling elements interact to impact pathogenesis. We mapped the Ca²⁺-responsive proteome of the model apicomplexan T. gondii via time-resolved phosphoproteomics and thermal proteome profiling. The waves of phosphoregulation following PKG activation and stimulated Ca²⁺ release corroborate known physiological changes but identify specific proteins operating in these pathways. Thermal profiling of parasite extracts identified many expected Ca²⁺-responsive proteins, such as parasite Ca²⁺-dependent protein kinases. Our approach also identified numerous Ca²⁺-responsive proteins that are not predicted to bind Ca²⁺, yet are critical components of the parasite signaling network. We characterized protein phosphatase 1 (PP1) as a Ca²⁺-responsive enzyme that relocalized to the parasite apex upon Ca²⁺ store release. Conditional depletion of PP1 revealed that the phosphatase regulates Ca²⁺ uptake to promote parasite motility. PP1 may thus be partly responsible for Ca²⁺-regulated serine/threonine phosphatase activity in apicomplexan parasites.

Marine gregarine genomes reveal the breadth of apicomplexan diversity with a partially conserved glideosome machinery

Our current view of the evolutionary history, coding and adaptive capacities of Apicomplexa, protozoan parasites of a wide range of metazoan, is currently strongly biased toward species infecting humans, as data on early diverging apicomplexan lineages infecting invertebrates is extremely limited. Here, we characterized the genome of the marine eugregarine Porospora gigantea, intestinal parasite of Lobsters, remarkable for the macroscopic size of its vegetative feeding forms (trophozoites) and its gliding speed, the fastest so far recorded for Apicomplexa. Two highly syntenic genomes named A and B were assembled. Similar in size (~ 9 Mb) and coding capacity (~ 5300 genes), A and B genomes are 10.8% divergent at the nucleotide level, corresponding to 16-38 My in divergent time. Orthogroup analysis across 25 (proto)Apicomplexa species, including Gregarina niphandrodes, showed that A and B are highly divergent from all other known apicomplexan species, revealing an unexpected breadth of diversity. Phylogenetically these two species branch sisters to Cephaloidophoroidea, and thus expand the known crustacean gregarine superfamily. The genomes were mined for genes encoding proteins necessary for gliding, a key feature of apicomplexans parasites, currently studied through the molecular model called glideosome. Sequence analysis shows that actin-related proteins and regulatory factors are strongly conserved within apicomplexans. In contrast, the predicted protein sequences of core glideosome proteins and adhesion proteins are highly variable among apicomplexan lineages, especially in gregarines. These results confirm the importance of studying gregarines to widen our biological and evolutionary view of apicomplexan species diversity, and to deepen our understanding of the molecular bases of key functions such as gliding, well known to allow access to the intracellular parasitic lifestyle in Apicomplexa.

Functional Characterization of the Thrombospondin-Related Paralogous Proteins Rhoptry Discharge Factors 1 and 2 Unveils Phenotypic Plasticity in Toxoplasma gondii Rhoptry Exocytosis

To gain access to the intracellular cytoplasmic niche essential for their growth and replication, apicomplexan parasites such as Toxoplasma gondii rely on the timely secretion of two types of apical organelles named micronemes and rhoptries. Rhoptry proteins are key to host cell invasion and remodeling, however, the molecular mechanisms underlying the tight control of rhoptry discharge are poorly understood. Here, we report the identification and functional characterization of two novel T. gondii thrombospondin-related proteins implicated in rhoptry exocytosis. The two proteins, already annotated as MIC15 and MIC14, were renamed rhoptry discharge factor 1 (RDF1) and rhoptry discharge factor 2 (RDF2) and found to be exclusive of the Coccidia class of apicomplexan parasites. Furthermore, they were shown to have a paralogous relationship and share a C-terminal transmembrane domain followed by a short cytoplasmic tail. Immunofluorescence analysis of T. gondii tachyzoites revealed that RDF1 presents a diffuse punctate localization not reminiscent of any know subcellular compartment, whereas RDF2 was not detected. Using a conditional knockdown approach, we demonstrated that RDF1 loss caused a marked growth defect. The lack of the protein did not affect parasite gliding motility, host cell attachment, replication and egress, whereas invasion was dramatically reduced. Notably, while RDF1 depletion did not result in altered microneme exocytosis, rhoptry discharge was found to be heavily impaired. Interestingly, rhoptry secretion was reversed by spontaneous upregulation of the RDF2 gene in knockdown parasites grown under constant RDF1 repression. Collectively, our results identify RDF1 and RDF2 as additional key players in the pathway controlling rhoptry discharge. Furthermore, this study unveils a new example of compensatory mechanism contributing to phenotypic plasticity in T. gondii.

N-Acetylation of secreted proteins is widespread in Apicomplexa and independent of acetyl-CoA ER-transporter AT1

Acetyl-CoA participates in post-translational modification of proteins, central carbon and lipid metabolism in several cell compartments. In mammals, the acetyl-CoA transporter 1 (AT1) facilitates the flux of cytosolic acetyl-CoA into the endoplasmic reticulum (ER), enabling the acetylation of proteins of the secretory pathway, in concert with dedicated acetyltransferases including NAT8. However, the implication of the ER acetyl-CoA pool in acetylation of ER-transiting proteins in Apicomplexa is unknown. We identify homologues of AT1 and NAT8 in Toxoplasma gondii and Plasmodium berghei. Proteome-wide analyses revealed widespread N-terminal acetylation marks of secreted proteins in both parasites. Such acetylation profile of N-terminally processed proteins was never observed so far in any other organisms. AT1 deletion resulted in a considerable reduction of parasite fitness. In P. berghei, AT1 is important for growth of asexual blood stages and production of female gametocytes and male gametocytogenesis impaling its requirement for transmission. In the absence of AT1, the lysine and N-terminal acetylation sites remained globally unaltered, suggesting an uncoupling between the role of AT1 in development and active acetylation occurring along the secretory pathway.

Toxoplasma gondii’s Basal Complex: The Other Apicomplexan Business End Is Multifunctional

The Apicomplexa are famously named for their apical complex, a constellation of organelles at their apical end dedicated to invasion of their host cells. In contrast, at the other end of the cell, the basal complex (BC) has been overshadowed since it is much less prominent and specific functions were not immediately obvious. However, in the past decade a staggering array of functions have been associated with the BC and strides have been made in understanding its structure. Here, these collective insights are supplemented with new data to provide an overview of the understanding of the BC in Toxoplasma gondii. The emerging picture is that the BC is a dynamic and multifunctional complex, with a series of (putative) functions. The BC has multiple roles in cell division: it is the site where building blocks are added to the cytoskeleton scaffold; it exerts a two-step stretch and constriction mechanism as contractile ring; and it is key in organelle division. Furthermore, the BC has numerous putative roles in ‘import’, such as the recycling of mother cell remnants, the acquisition of host-derived vesicles, possibly the uptake of lipids derived from the extracellular medium, and the endocytosis of micronemal proteins. The latter process ties the BC to motility, whereas an additional role in motility is conferred by Myosin C. Furthermore, the BC acts on the assembly and/or function of the intravacuolar network, which may directly or indirectly contribute to the establishment of chronic tissue cysts. Here we provide experimental support for molecules acting in several of these processes and identify several new BC proteins critical to maintaining the cytoplasmic bridge between divided parasites. However, the dispensable nature of many BC components leaves many questions unanswered regarding its function. In conclusion, the BC in T. gondii is a dynamic and multifunctional structure at the posterior end of the parasite.

Revisiting the Role of Toxoplasma gondii ERK7 in the Maintenance and Stability of the Apical Complex

Toxoplasma gondii extracellular signal-regulated kinase 7 (ERK7) is known to contribute to the integrity of the apical complex and to participate in the final step of conoid biogenesis. In the absence of ERK7, mature parasites lose their conoid complex and are unable to glide, invade, or egress from host cells. In contrast to a previous report, we show here that the depletion of ERK7 phenocopies the depletion of the apical cap protein AC9 or AC10. The absence of ERK7 leads to the loss of the apical polar ring (APR), the disorganization of the basket of subpellicular microtubules (SPMTs), and a severe impairment in microneme secretion. Ultrastructure expansion microscopy (U-ExM), coupled to N-hydroxysuccinimide ester (NHS-ester) staining on intracellular parasites, offers an unprecedented level of resolution and highlights the disorganization of the rhoptries as well as the dilated plasma membrane at the apical pole in the absence of ERK7. Comparative proteomics analysis of wild-type and ERK7-depleted parasites confirmed the disappearance of known apical complex proteins, including markers of the apical polar ring and a new apical cap named AC11. Concomitantly, the absence of ERK7 led to an accumulation of microneme proteins, resulting from the defect in the exocytosis of the organelles. AC9-depleted parasites were included as controls and exhibited an increase in inner membrane complex proteins, with two new proteins assigned to this compartment, namely, IMC33 and IMC34. IMPORTANCE The conoid is an enigmatic, dynamic organelle positioned at the apical tip of the coccidian subgroup of the Apicomplexa, close to the apical polar ring (APR) from which the subpellicular microtubules (SPMTs) emerge and through which the secretory organelles (micronemes and rhoptries) reach the plasma membrane for exocytosis. In Toxoplasma gondii, the conoid protrudes concomitantly with microneme secretion, during egress, motility, and invasion. The conditional depletion of the apical cap structural protein AC9 or AC10 leads to a disorganization of SPMTs as well as the loss of the APR and conoid, resulting in a microneme secretion defect and a block in motility, invasion, and egress. We show here that the depletion of the kinase ERK7 phenocopies AC9 and AC10 mutants. The combination of ultrastructure expansion microscopy and NHS-ester staining revealed that ERK7-depleted parasites exhibit a dilated apical plasma membrane and an altered positioning of the rhoptries, while electron microscopy images unambiguously highlight the loss of the APR.

A Novel Toxoplasma Inner Membrane Complex Suture-Associated Protein Regulates Suture Protein Targeting and Colocalizes with Membrane Trafficking Machinery

The cytoskeleton of Toxoplasma gondii is composed of the inner membrane complex (IMC) and an array of underlying microtubules that provide support at the periphery of the parasite. Specific subregions of the IMC carry out distinct roles in replication, motility, and host cell invasion. Building on our previous in vivo biotinylation (BioID) experiments of the IMC, we identified here a novel protein that localizes to discrete puncta that are embedded in the parasite’s cytoskeleton along the IMC sutures. Gene knockout analysis showed that loss of the protein results in defects in cytoskeletal suture protein targeting, cytoskeletal integrity, parasite morphology, and host cell invasion. We then used deletion analyses to identify a domain in the N terminus of the protein that is critical for both localization and function. Finally, we used the protein as bait for in vivo biotinylation, which identified several other proteins that colocalize in similar spot-like patterns. These putative interactors include several proteins that are implicated in membrane trafficking and are also associated with the cytoskeleton. Together, these data reveal an unexpected link between the IMC sutures and membrane trafficking elements of the parasite and suggest that the suture puncta are likely a portal for trafficking cargo across the IMC.

Plasmodium falciparum PhIL1-associated complex plays an essential role in merozoite reorientation and invasion of host erythrocytes

The human malaria parasite, Plasmodium falciparum possesses unique gliding machinery referred to as the glideosome that powers its entry into the insect and vertebrate hosts. Several parasite proteins including Photosensitized INA-labelled protein 1 (PhIL1) have been shown to associate with glideosome machinery. Here we describe a novel PhIL1 associated protein complex that co-exists with the glideosome motor complex in the inner membrane complex of the merozoite. Using an experimental genetics approach, we characterized the role(s) of three proteins associated with PhIL1: a glideosome associated protein- PfGAPM2, an IMC structural protein- PfALV5, and an uncharacterized protein—referred here as PfPhIP (PhIL1 Interacting Protein). Parasites lacking PfPhIP or PfGAPM2 were unable to invade host RBCs. Additionally, the downregulation of PfPhIP resulted in significant defects in merozoite segmentation. Furthermore, the PfPhIP and PfGAPM2 depleted parasites showed abrogation of reorientation/gliding. However, initial attachment with host RBCs was not affected in these parasites. Together, the data presented here show that proteins of the PhIL1-associated complex play an important role in the orientation of P. falciparum merozoites following initial attachment, which is crucial for the formation of a tight junction and hence invasion of host erythrocytes.

Invasion of Plasmodium merozoites into RBCs is a multistep process that involves initial attachment of merozoites to the RBC surface, their reorientation, and subsequent gliding into RBCs using glideosome machinery. The glideosome machinery lies between the plasma membrane and inner membrane complex (IMC) and consists of MyoA, its interacting protein; MTIP, gliding associated proteins (GAPs), and a Photosensitized INA labeled protein (PhIL1)-associated protein complex. Here, we demonstrate that the deletion of any of two components of the PhIL1-associated complex, PfPhIP or PfGAPM2, aborts merozoite reorientation and blocks their invasion into RBCs. The study thus provides new molecular and mechanistic insights into merozoite invasion of RBCs.

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.

The multiple functions of actin in apicomplexan parasites

The cytoskeletal protein actin is highly abundant and conserved in eukaryotic cells. It occurs in two different states- the globular (G-actin) form, which can polymerise into the filamentous (F-actin) form, fulfilling various critical functions including cytokinesis, cargo trafficking and cellular motility. In higher eukaryotes, there are several actin isoforms with nearly identical amino acid sequences. Despite the high level of amino acid identity, they display regulated expression patterns and unique non-redundant roles. The number of actin isoforms together with conserved sequences may reflect the selective pressure exerted by scores of actin binding proteins (ABPs) in higher eukaryotes. In contrast, in many protozoans such as apicomplexan parasites which possess only a few ABPs, the regulatory control of actin and its multiple functions are still obscure. Here, we provide a summary of the regulation and biological functions of actin in higher eukaryotes and compare it with the current knowledge in apicomplexans. We discuss future experiments that will help us understand the multiple, critical roles of this fascinating system in apicomplexans.

The Modular Circuitry of Apicomplexan Cell Division Plasticity

The close-knit group of apicomplexan parasites displays a wide variety of cell division modes, which differ between parasites as well as between different life stages within a single parasite species. The beginning and endpoint of the asexual replication cycles is a 'zoite' harboring the defining apical organelles required for host cell invasion. However, the number of zoites produced per division round varies dramatically and can unfold in several different ways. This plasticity of the cell division cycle originates from a combination of hard-wired developmental programs modulated by environmental triggers. Although the environmental triggers and sensors differ between species and developmental stages, widely conserved secondary messengers mediate the signal transduction pathways. These environmental and genetic input integrate in division-mode specific chromosome organization and chromatin modifications that set the stage for each division mode. Cell cycle progression is conveyed by a smorgasbord of positively and negatively acting transcription factors, often acting in concert with epigenetic reader complexes, that can vary dramatically between species as well as division modes. A unique set of cell cycle regulators with spatially distinct localization patterns insert discrete check points which permit individual control and can uncouple general cell cycle progression from nuclear amplification. Clusters of expressed genes are grouped into four functional modules seen in all division modes: 1. mother cytoskeleton disassembly; 2. DNA replication and segregation (D&S); 3. karyokinesis; 4. zoite assembly. A plug-and-play strategy results in the variety of extant division modes. The timing of mother cytoskeleton disassembly is hard-wired at the species level for asexual division modes: it is either the first step, or it is the last step. In the former scenario zoite assembly occurs at the plasma membrane (external budding), and in the latter scenario zoites are assembled in the cytoplasm (internal budding). The number of times each other module is repeated can vary regardless of this first decision, and defines the modes of cell division: schizogony, binary fission, endodyogeny, endopolygeny.

Identification and Molecular Dissection of IMC32, a Conserved Toxoplasma Inner Membrane Complex Protein That Is Essential for Parasite Replication

The inner membrane complex (IMC) is a unique organelle of apicomplexan parasites that plays critical roles in parasite motility, host cell invasion, and replication. Despite the common functions of the organelle, relatively few IMC proteins are conserved across the phylum and the precise roles of many IMC components remain to be characterized. Here, we identify a novel component of the Toxoplasma gondii IMC (IMC32) that localizes to the body portion of the IMC and is recruited to developing daughter buds early during endodyogeny. IMC32 is essential for parasite survival, as its conditional depletion results in a complete collapse of the IMC that is lethal to the parasite. We demonstrate that localization of IMC32 is dependent on both an N-terminal palmitoylation site and a series of C-terminal coiled-coil domains. Using deletion analyses and functional complementation, we show that two conserved regions within the C-terminal coiled-coil domains play critical roles in protein function during replication. Together, this work reveals an essential component of parasite replication that provides a novel target for therapeutic intervention of T. gondii and related apicomplexan parasites.IMPORTANCE The IMC is an important organelle that apicomplexan parasites use to maintain their intracellular lifestyle. While many IMC proteins have been identified, only a few central players that are essential for internal budding have been described and even fewer are conserved across the phylum. Here, we identify IMC32, a novel component of the Toxoplasma gondii IMC that localizes to very early daughter buds, indicating a role in the early stages of parasite replication. We then demonstrate that IMC32 is essential for parasite survival and pinpoint conserved regions within the protein that are important for membrane association and daughter cell formation. As IMC32 is unique to these parasites and not present in their mammalian hosts, it serves as a new target for the development of drugs that exclusively affect these important intracellular pathogens.

Toxoplasma gondii GRA60 is an effector protein that modulates host cell autonomous immunity and contributes to virulence

Toxoplasma gondii infects virtually any nucleated cell and resides inside a non-phagocytic vacuole surrounded by a parasitophorous vacuolar membrane (PVM). Pivotal to the restriction of T. gondii dissemination upon infection in murine cells is the recruitment of immunity regulated GTPases (IRGs) and guanylate binding proteins (GBPs) to the PVM that leads to pathogen elimination. The virulent T. gondii type I RH strain secretes a handful of effectors including the dense granule protein GRA7, the serine-threonine kinases ROP17 and ROP18, and a pseudo-kinase ROP5, that synergistically inhibit the recruitment of IRGs to the PVM. Here, we characterise GRA60, a novel dense granule effector, which localises to the vacuolar space and PVM and contributes to virulence of RH in mice, suggesting a role in the subversion of host cell defence mechanisms. Members of the host cell IRG defence system Irgb10 and Irga6 are recruited to the PVM of RH parasites lacking GRA60 as observed previously for the avirulent RHΔrop5 mutant, with RH preventing such recruitment. Deletion of GRA60 in RHΔrop5 leads to a recruitment of IRGs comparable to the single knockouts. GRA60 therefore represents a novel parasite effector conferring resistance to IRGs in type I parasites, and found associated to ROP18, a member of the virulence complex.

Actomyosin forces and the energetics of red blood cell invasion by the malaria parasite Plasmodium falciparum

All symptoms of malaria disease are associated with the asexual blood stages of development, involving cycles of red blood cell (RBC) invasion and egress by the Plasmodium spp. merozoite. Merozoite invasion is rapid and is actively powered by a parasite actomyosin motor. The current accepted model for actomyosin force generation envisages arrays of parasite myosins, pushing against short actin filaments connected to the external milieu that drive the merozoite forwards into the RBC. In Plasmodium falciparum, the most virulent human malaria species, Myosin A (PfMyoA) is critical for parasite replication. However, the precise function of PfMyoA in invasion, its regulation, the role of other myosins and overall energetics of invasion remain unclear. Here, we developed a conditional mutagenesis strategy combined with live video microscopy to probe PfMyoA function and that of the auxiliary motor PfMyoB in invasion. By imaging conditional mutants with increasing defects in force production, based on disruption to a key PfMyoA phospho-regulation site, the absence of the PfMyoA essential light chain, or complete motor absence, we define three distinct stages of incomplete RBC invasion. These three defects reveal three energetic barriers to successful entry: RBC deformation (pre-entry), mid-invasion initiation, and completion of internalisation, each requiring an active parasite motor. In defining distinct energetic barriers to invasion, these data illuminate the mechanical challenges faced in this remarkable process of protozoan parasitism, highlighting distinct myosin functions and identifying potential targets for preventing malaria pathogenesis.

Multi-omics analysis delineates the distinct functions of sub-cellular acetyl-CoA pools in Toxoplasma gondii

BACKGROUND

Acetyl-CoA is a key molecule in all organisms, implicated in several metabolic pathways as well as in transcriptional regulation and post-translational modification. The human pathogen Toxoplasma gondii possesses at least four enzymes which generate acetyl-CoA in the nucleo-cytosol (acetyl-CoA synthetase (ACS); ATP citrate lyase (ACL)), mitochondrion (branched-chain α-keto acid dehydrogenase-complex (BCKDH)) and apicoplast (pyruvate dehydrogenase complex (PDH)). Given the diverse functions of acetyl-CoA, we know very little about the role of sub-cellular acetyl-CoA pools in parasite physiology.

RESULTS

To assess the importance and functions of sub-cellular acetyl-CoA-pools, we measured the acetylome, transcriptome, proteome and metabolome of parasites lacking ACL/ACS or BCKDH. We demonstrate that ACL/ACS constitute a synthetic lethal pair. Loss of both enzymes causes a halt in fatty acid elongation, hypo-acetylation of nucleo-cytosolic and secretory proteins and broad changes in gene expression. In contrast, loss of BCKDH results in an altered TCA cycle, hypo-acetylation of mitochondrial proteins and few specific changes in gene expression. We provide evidence that changes in the acetylome, transcriptome and proteome of cells lacking BCKDH enable the metabolic adaptations and thus the survival of these parasites.

CONCLUSIONS

Using multi-omics and molecular tools, we obtain a global and integrative picture of the role of distinct acetyl-CoA pools in T. gondii physiology. Cytosolic acetyl-CoA is essential and is required for the synthesis of parasite-specific fatty acids. In contrast, loss of mitochondrial acetyl-CoA can be compensated for through metabolic adaptations implemented at the transcriptional, translational and post-translational level.

Fussing About Fission: Defining Variety Among Mainstream and Exotic Apicomplexan Cell Division Modes

Cellular reproduction defines life, yet our textbook-level understanding of cell division is limited to a small number of model organisms centered around humans. The horizon on cell division variants is expanded here by advancing insights on the fascinating cell division modes found in the Apicomplexa, a key group of protozoan parasites. The Apicomplexa display remarkable variation in offspring number, whether karyokinesis follows each S/M-phase or not, and whether daughter cells bud in the cytoplasm or bud from the cortex. We find that the terminology used to describe the various manifestations of asexual apicomplexan cell division emphasizes either the number of offspring or site of budding, which are not directly comparable features and has led to confusion in the literature. Division modes have been primarily studied in two human pathogenic Apicomplexa, malaria-causing Plasmodium spp. and Toxoplasma gondii, a major cause of opportunistic infections. Plasmodium spp. divide asexually by schizogony, producing multiple daughters per division round through a cortical budding process, though at several life-cycle nuclear amplifications stages, are not followed by karyokinesis. T. gondii divides by endodyogeny producing two internally budding daughters per division round. Here we add to this diversity in replication mechanisms by considering the cattle parasite Babesia bigemina and the pig parasite Cystoisospora suis. B. bigemina produces two daughters per division round by a “binary fission” mechanism whereas C. suis produces daughters through both endodyogeny and multiple internal budding known as endopolygeny. In addition, we provide new data from the causative agent of equine protozoal myeloencephalitis (EPM), Sarcocystis neurona, which also undergoes endopolygeny but differs from C. suis by maintaining a single multiploid nucleus. Overall, we operationally define two principally different division modes: internal budding found in cyst-forming Coccidia (comprising endodyogeny and two forms of endopolygeny) and external budding found in the other parasites studied (comprising the two forms of schizogony, binary fission and multiple fission). Progressive insights into the principles defining the molecular and cellular requirements for internal vs. external budding, as well as variations encountered in sexual stages are discussed. The evolutionary pressures and mechanisms underlying apicomplexan cell division diversification carries relevance across Eukaryota.

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.

Essential function of the alveolin network in the subpellicular microtubules and conoid assembly in Toxoplasma gondii

The coccidian subgroup of Apicomplexa possesses an apical complex harboring a conoid, made of unique tubulin polymer fibers. This enigmatic organelle extrudes in extracellular invasive parasites and is associated to the apical polar ring (APR). The APR serves as microtubule-organizing center for the 22 subpellicular microtubules (SPMTs) that are linked to a patchwork of flattened vesicles, via an intricate network composed of alveolins. Here, we capitalize on ultrastructure expansion microscopy (U-ExM) to localize the Toxoplasma gondii Apical Cap protein 9 (AC9) and its partner AC10, identified by BioID, to the alveolin network and intercalated between the SPMTs. Parasites conditionally depleted in AC9 or AC10 replicate normally but are defective in microneme secretion and fail to invade and egress from infected cells. Electron microscopy revealed that the mature parasite mutants are conoidless, while U-ExM highlighted the disorganization of the SPMTs which likely results in the catastrophic loss of APR and conoid.

The Myosin Family of Mechanoenzymes: From Mechanisms to Therapeutic Approaches

Myosins are among the most fascinating enzymes in biology. As extremely allosteric chemomechanical molecular machines, myosins are involved in myriad pivotal cellular functions and are frequently sites of mutations leading to disease phenotypes. Human β-cardiac myosin has proved to be an excellent target for small-molecule therapeutics for heart muscle diseases, and, as we describe here, other myosin family members are likely to be potentially unique targets for treating other diseases as well. The first part of this review focuses on how myosins convert the chemical energy of ATP hydrolysis into mechanical movement, followed by a description of existing therapeutic approaches to target human β-cardiac myosin. The next section focuses on the possibility of targeting nonmuscle members of the human myosin family for several diseases. We end the review by describing the roles of myosin in parasites and the therapeutic potential of targeting them to block parasitic invasion of their hosts.

The Actomyosin Systems in Apicomplexa

The phylum of Apicomplexa groups obligate intracellular parasites that exhibit unique classes of unconventional myosin motors. These parasites also encode a limited repertoire of actins, actin-like proteins, actin-binding proteins and nucleators of filamentous actin (F-actin) that display atypical properties. In the last decade, significant progress has been made to visualize F-actin and to unravel the functional contribution of actomyosin systems in the biology of Toxoplasma and Plasmodium, the most genetically-tractable members of the phylum. In addition to assigning specific roles to each myosin, recent biochemical and structural studies have begun to uncover mechanistic insights into myosin function at the atomic level. In several instances, the myosin light chains associated with the myosin heavy chains have been identified, helping to understand the composition of the motor complexes and their mode of regulation. Moreover, the considerable advance in proteomic methodologies and especially in assignment of posttranslational modifications is offering a new dimension to our understanding of the regulation of actin dynamics and myosin function. Remarkably, the actomyosin system contributes to three major processes in Toxoplasma gondii: (i) organelle trafficking, positioning and inheritance, (ii) basal pole constriction and intravacuolar cell-cell communication and (iii) motility, invasion, and egress from infected cells. In this chapter, we summarize how the actomyosin system harnesses these key events to ensure successful completion of the parasite life cycle.

Polarization of MTIP is a signature of gliding locomotion in Plasmodium ookinetes and sporozoites

Gliding motility and cell invasion are essential for the successful transmission of Plasmodium parasites. These processes rely on an acto-myosin motor located underneath the parasite plasma membrane. The Myosin A-tail interacting protein (MTIP) connects the class XIV myosin A (MyoA) to the gliding-associated proteins and is essential for assembly of the motor at the inner membrane complex. Here, we assessed the subcellular localization of MTIP in Plasmodium berghei motile stages from wild-type parasites and mutants that lack MyoA or the small heat shock protein 20 (HSP20). We demonstrate that MTIP is recruited to the apical end of motile ookinetes independently of the presence of MyoA. We also show that infective sporozoites displayed a polarized MTIP distribution during gliding, and that this distribution was abrogated in mutant parasites with an aberrant locomotion.

Systematic analysis of Plasmodium myosins reveals differential expression, localisation, and function in invasive and proliferative parasite stages

The myosin superfamily comprises of actin-dependent eukaryotic molecular motors important in a variety of cellular functions. Although well studied in many systems, knowledge of their functions in Plasmodium, the causative agent of malaria, is restricted. Previously, six myosins were identified in this genus, including three Class XIV myosins found only in Apicomplexa and some Ciliates. The well characterized MyoA is a Class XIV myosin essential for gliding motility and invasion. Here, we characterize all other Plasmodium myosins throughout the parasite life cycle and show that they have very diverse patterns of expression and cellular location. MyoB and MyoE, the other two Class XIV myosins, are expressed in all invasive stages, with apical and basal locations, respectively. Gene deletion revealed that MyoE is involved in sporozoite traversal, MyoF and MyoK are likely essential in the asexual blood stages, and MyoJ and MyoB are not essential. Both MyoB and its essential light chain (MCL-B) are localised at the apical end of ookinetes but expressed at completely different time points. This work provides a better understanding of the role of actomyosin motors in Apicomplexan parasites, particularly in the motile and invasive stages of Plasmodium during sexual and asexual development within the mosquito.

Plasmodium myosin A drives parasite invasion by an atypical force generating mechanism

Plasmodium parasites are obligate intracellular protozoa and causative agents of malaria, responsible for half a million deaths each year. The lifecycle progression of the parasite is reliant on cell motility, a process driven by myosin A, an unconventional single-headed class XIV molecular motor. Here we demonstrate that myosin A from Plasmodium falciparum (PfMyoA) is critical for red blood cell invasion. Further, using a combination of X-ray crystallography, kinetics, and in vitro motility assays, we elucidate the non-canonical interactions that drive this motor’s function. We show that PfMyoA motor properties are tuned by heavy chain phosphorylation (Ser19), with unphosphorylated PfMyoA exhibiting enhanced ensemble force generation at the expense of speed. Regulated phosphorylation may therefore optimize PfMyoA for enhanced force generation during parasite invasion or for fast motility during dissemination. The three PfMyoA crystallographic structures presented here provide a blueprint for discovery of specific inhibitors designed to prevent parasite infection.

Here, Robert-Paganin et al. show that myosin A from Plasmodium falciparum is critical for red blood cell invasion and that non-canonical interactions and regulated phosphorylation are important for force generation during parasite invasion.

Centrin2 from the human parasite Toxoplasma gondii is required for its invasion and intracellular replication

Centrins are EF-hand containing proteins ubiquitously found in eukaryotes and are key components of centrioles/basal bodies as well as certain contractile fibers. We previously identified three centrins in the human parasite Toxoplasma gondii, all of which localized to the centrioles. However, one of them, T. gondii (Tg) Centrin2 (CEN2), is also targeted to structures at the apical and basal ends of the parasite, as well as to annuli at the base of the apical cap of the membrane cortex. The role(s) that CEN2 play in these locations were unknown. Here, we report the functional characterization of CEN2 using a conditional knockdown method that combines transcriptional and protein stability control. The knockdown resulted in an ordered loss of CEN2 from its four compartments, due to differences in incorporation kinetics and structural inheritance over successive generations. This was correlated with a major invasion deficiency at early stages of CEN2 knockdown, and replication defects at later stages. These results indicate that CEN2 is incorporated into multiple cytoskeletal structures to serve distinct functions that are required for parasite survival.

The roles of Centrin 2 and Dynein Light Chain 8a in apical secretory organelles discharge of Toxoplasma gondii

To efficiently enter host cells, apicomplexan parasites such as Toxoplasma gondii rely on an apical complex composed of tubulin-based structures as well as two sets of secretory organelles named micronemes and rhoptries. The trafficking and docking of these organelles to the apical pole of the parasite is crucial for the discharge of their contents. Here, we describe two proteins typically associated with microtubules, Centrin 2 (CEN2) and Dynein Light Chain 8a (DLC8a), that are required for efficient host cell invasion. CEN2 localizes to four different compartments, and remarkably, conditional depletion of the protein occurs in stepwise manner, sequentially depleting the protein pools from each location. This phenomenon allowed us to discern the essential function of the apical pool of CEN2 for microneme secretion, motility, invasion and egress. DLC8a localizes to the conoid, and its depletion also perturbs microneme exocytosis in addition to the apical docking of the rhoptry organelles, causing a severe defect in host cell invasion. Phenotypic characterization of CEN2 and DLC8a indicates that while both proteins participate in microneme secretion, they likely act at different steps along the cascade of events leading to organelle exocytosis.

Three F-actin assembly centers regulate organelle inheritance, cell-cell communication and motility in Toxoplasma gondii

Toxoplasma gondii possesses a limited set of actin-regulatory proteins and relies on only three formins (FRMs) to nucleate and polymerize actin. We combined filamentous actin (F-actin) chromobodies with gene disruption to assign specific populations of actin filaments to individual formins. FRM2 localizes to the apical juxtanuclear region and participates in apicoplast inheritance. Restricted to the residual body, FRM3 maintains the intravacuolar cell-cell communication. Conoidal FRM1 initiates a flux of F-actin crucial for motility, invasion and egress. This flux depends on myosins A and H and is controlled by phosphorylation via PKG (protein kinase G) and CDPK1 (calcium-dependent protein kinase 1) and by methylation via AKMT (apical lysine methyltransferase). This flux is independent of microneme secretion and persists in the absence of the glideosome-associated connector (GAC). This study offers a coherent model of the key players controlling actin polymerization, stressing the importance of well-timed post-translational modifications to power parasite motility.

Comprehensive Characterization of Toxoplasma Acyl Coenzyme A-Binding Protein TgACBP2 and Its Critical Role in Parasite Cardiolipin Metabolism

Toxoplasma gondii is one of the most successful human parasites, infecting nearly one-third of the total world population. T. gondii tachyzoites residing within parasitophorous vacuoles (PVs) can acquire fatty acids both via salvage from host cells and via de novo synthesis pathways for membrane biogenesis. However, although fatty acid fluxes are known to exist in this parasite, how fatty acids flow through Toxoplasma lipid metabolic organelles, especially mitochondria, remains unknown. In this study, we demonstrated that Toxoplasma expresses an active ankyrin repeat containing protein TgACBP2 to coordinate cardiolipin metabolism. Specifically, HMA acquisition resulting from heterologous functional expression of MAF1 rescued growth and lipid metabolism defects in ACBP2-deficient type II parasites, manifesting the complementary role of host mitochondria in parasite cardiolipin metabolism. This work highlights the importance of TgACBP2 in parasite cardiolipin metabolism and provides evidence for metabolic association of host mitochondria with T. gondii.

Acyl coenzyme A (CoA)-binding protein (ACBP) can bind acyl-CoAs with high specificity and affinity, thus playing multiple roles in cellular functions. Mitochondria of the apicomplexan parasite Toxoplasma gondii have emerged as key organelles for lipid metabolism and signaling transduction. However, the rationale for how this parasite utilizes acyl-CoA-binding protein to regulate mitochondrial lipid metabolism remains unclear. Here, we show that an ankyrin repeat-containing protein, TgACBP2, is localized to mitochondria and displays active acyl-CoA-binding activities. Dephosphorylation of TgACBP2 is associated with relocation from the plasma membrane to the mitochondria under conditions of regulation of environmental [K⁺]. Under high [K⁺] conditions, loss of ACBP2 induced mitochondrial dysfunction and apoptosis-like cell death. Disruption of ACBP2 caused growth and virulence defects in the type II strain but not in type I parasites. Interestingly, mitochondrial association factor-1 (MAF1)-mediated host mitochondrial association (HMA) restored the growth ability of ACBP2-deficient type II parasites. Lipidomics analysis indicated that ACBP2 plays key roles in the cardiolipin metabolism of type II parasites and that MAF1 expression complemented the lipid metabolism defects of ACBP2-deficient type II parasites. In addition, disruption of ACBP2 caused attenuated virulence of Prugniuad (Pru) parasites for mice. Taking the results collectively, these data indicate that ACBP2 is critical for the growth and virulence of type II parasites and for the growth of type I parasites under high [K⁺] conditions.

Structural and mechanistic insights into the function of the unconventional class XIV myosin MyoA from Toxoplasma gondii

Parasites of the phylum Apicomplexa are responsible for significant morbidity and mortality on a global scale. Central to the virulence of these pathogens are the phylum-specific, unconventional class XIV myosins that power the essential processes of parasite motility and host cell invasion. Notably, class XIV myosins differ from human myosins in key functional regions, yet they are capable of fast movement along actin filaments with kinetics rivaling previously studied myosins. Toward establishing a detailed molecular mechanism of class XIV motility, we determined the 2.6-Å resolution crystal structure of the Toxoplasma gondii MyoA (TgMyoA) motor domain. Structural analysis reveals intriguing strategies for force transduction and chemomechanical coupling that rely on a divergent SH1/SH2 region, the class-defining "HYAG"-site polymorphism, and the actin-binding surface. In vitro motility assays and hydrogen-deuterium exchange coupled with MS further reveal the mechanistic underpinnings of phosphorylation-dependent modulation of TgMyoA motility whereby localized regions of increased stability and order correlate with enhanced motility. Analysis of solvent-accessible pockets reveals striking differences between apicomplexan class XIV and human myosins. Extending these analyses to high-confidence homology models of Plasmodium and Cryptosporidium MyoA motor domains supports the intriguing potential of designing class-specific, yet broadly active, apicomplexan myosin inhibitors. The successful expression of the functional TgMyoA complex combined with our crystal structure of the motor domain provides a strong foundation in support of detailed structure-function studies and enables the development of small-molecule inhibitors targeting these devastating global pathogens.

The Actinomyosin Motor Drives Malaria Parasite Red Blood Cell Invasion but Not Egress

Apicomplexa are obligate intracellular parasites that actively invade, replicate within, and egress from host cells. The parasite actinomyosin-based molecular motor complex (often referred to as the glideosome) is considered an important mediator of parasite motility and virulence. Mature intracellular parasites often become motile just prior to egress from their host cells, and in some genera, this motility is important for successful egress as well as for subsequent invasion of new host cells. To determine whether actinomyosin-based motility is important in the red blood cell egress and invasion activities of the malaria parasite, we have used a conditional genetic approach to delete GAP45, a primary component of the glideosome, in asexual blood stages of Plasmodium falciparum Our results confirm the essential nature of GAP45 for invasion but show that P. falciparum does not require a functional motor complex to undergo egress from the red blood cell. Malarial egress therefore differs fundamentally from induced egress in the related apicomplexan Toxoplasma gondiiIMPORTANCE Clinical malaria results from cycles of replication of single-celled parasites of the genus Plasmodium in red blood cells. Intracellular parasite replication is followed by a highly regulated, protease-dependent process called egress, in which rupture of the bounding membranes allows explosive release of daughter merozoites which rapidly invade fresh red cells. A parasite actinomyosin-based molecular motor (the glideosome) has been proposed to provide the mechanical force to drive invasion. Studies of the related parasite Toxoplasma gondii have shown that induced egress requires parasite motility, mediated by a functional glideosome. However, whether the glideosome has a similar essential role in egress of malaria merozoites from red blood cells is unknown. Here, we show that although a functional glideosome is required for red blood cell invasion by Plasmodium falciparum merozoites, it is not required for egress. These findings place further emphasis on the key role of the protease cascade in malarial egress.

Post-translational modifications as key regulators of apicomplexan biology: insights from proteome-wide studies

Parasites of the Apicomplexa phylum, such as Plasmodium spp. and Toxoplasma gondii, undergo complex life cycles involving multiple stages with distinct biology and morphologies. Post-translational modifications (PTMs), such as phosphorylation, acetylation and glycosylation, regulate numerous cellular processes, playing a role in every aspect of cell biology. PTMs can occur on proteins at any time in their lifespan and through alterations of target protein activity, localization, protein-protein interactions, among other functions, dramatically increase proteome diversity and complexity. In addition, PTMs can be induced or removed on changes in cellular environment and state. Thus, PTMs are likely to be key regulators of developmental transitions, biology and pathogenesis of apicomplexan parasites. In this review we examine the roles of PTMs in both parasite-specific and conserved eukaryotic processes, and the potential crosstalk between PTMs, that together regulate the intricate lives of these protozoa.

Myosin-dependent cell-cell communication controls synchronicity of division in acute and chronic stages of Toxoplasma gondii

The obligate intracellular parasite Toxoplasma gondii possesses a repertoire of 11 myosins. Three class XIV motors participate in motility, invasion and egress, whereas the class XXII myosin F is implicated in organelle positioning and inheritance of the apicoplast. Here we provide evidence that TgUNC acts as a chaperone dedicated to the folding, assembly and function of all Toxoplasma myosins. The conditional ablation of TgUNC recapitulates the phenome of the known myosins and uncovers two functions in parasite basal complex constriction and synchronized division within the parasitophorous vacuole. We identify myosin J and centrin 2 as essential for the constriction. We demonstrate the existence of an intravacuolar cell–cell communication ensuring synchronized division, a process dependent on myosin I. This connectivity contributes to the delayed death phenotype resulting from loss of the apicoplast. Cell–cell communication is lost in activated macrophages and during bradyzoite differentiation resulting in asynchronized, slow division in the cysts.

The mechanism by which Toxoplasma gondii achieves synchronized cell division is incompletely understood. Here, the authors identify an intravacuolar cell-cell communication that ensures synchronized division and depends on myosin I.

Calmodulin-like proteins localized to the conoid regulate motility and cell invasion by Toxoplasma gondii

Toxoplasma gondii contains an expanded number of calmodulin (CaM)-like proteins whose functions are poorly understood. Using a combination of CRISPR/Cas9-mediated gene editing and a plant-like auxin-induced degron (AID) system, we examined the roles of three apically localized CaMs. CaM1 and CaM2 were individually dispensable, but loss of both resulted in a synthetic lethal phenotype. CaM3 was refractory to deletion, suggesting it is essential. Consistent with this prediction auxin-induced degradation of CaM3 blocked growth. Phenotypic analysis revealed that all three CaMs contribute to parasite motility, invasion, and egress from host cells, and that they act downstream of microneme and rhoptry secretion. Super-resolution microscopy localized all three CaMs to the conoid where they overlap with myosin H (MyoH), a motor protein that is required for invasion. Biotinylation using BirA fusions with the CaMs labeled a number of apical proteins including MyoH and its light chain MLC7, suggesting they may interact. Consistent with this hypothesis, disruption of MyoH led to degradation of CaM3, or redistribution of CaM1 and CaM2. Collectively, our findings suggest these CaMs may interact with MyoH to control motility and cell invasion.

One of the most common motifs that binds calcium to transduce intracellular signals is called an EF hand- named after the globular domain structure first characterized in ovalbumin. A conserved cluster of four EF hands, each of which that binds one calcium atom, is a conserved feature of calmodulin, centrins, and calmodulin-like proteins, including myosin light chains. Although the presence of EF hands is predictive of calcium binding, it alone does not allow classification of biological function as this set of conserved proteins have very diverse functions. Here we used modified editing procedures based on CRISPR/Cas9 combined with a plant-like degradation system to define the roles of three calmodulin-like proteins in T. gondii. These proteins all localized to a specialized apical structure called the conoid where they overlap with the motor protein called MyoH. Additionally, biochemical and genetic studies suggest they coordinately regulate cell invasion. These new genomic editing tools, combined with an efficient system for protein degradation, expand the functional tool kit for an analysis of essential genes and proteins in T. gondii.

Myosin B of Plasmodium falciparum (PfMyoB): in silico prediction of its three-dimensional structure and its possible interaction with MTIP

The mobility and invasion strategy of Plasmodium falciparum is governed by a protein complex known as the glideosome, which contains an actin-myosin motor. It has been shown that myosin A of the parasite (PfMyoA) is the myosin of the glideosome, and the interaction of PfMyoA with myosin tail domain interacting protein (MTIP) determines its correct location and its ability to function in the complex. Because PfMyoA and myosin B of P. falciparum (PfMyoB) share high sequence identity, are both small proteins without a tail domain, belong to the class XIV myosins, and are expressed in late schizonts and merozoites, we suspect that these myosins may have similar or redundant functions. Therefore, this work examined the structural similarity between PfMyoA and PfMyoB and performed a molecular docking between PfMyoB and MTIP. Three-dimensional (3D) models obtained for PfMyoA and PfMyoB achieved high scores in the structural validation programs used, and their superimposition revealed high structural similarity, supporting the hypothesis of possible similar functions for these two proteins. The 3D interaction models obtained and energy values found suggested that interaction between PfMyoB and MTIP is possible. Given the apparent abundance of PfMyoA relative to PfMyoB in the parasite, we believe that the interaction between PfMyoB and MTIP would only be detectable in specific cellular environments because under normal circumstances, it would be masked by the interaction between PfMyoA and MTIP.

Surface attachment, promoted by the actomyosin system of Toxoplasma gondii is important for efficient gliding motility and invasion

Background

Apicomplexan parasites employ a unique form of movement, termed gliding motility, in order to invade the host cell. This movement depends on the parasite’s actomyosin system, which is thought to generate the force during gliding. However, recent evidence questions the exact molecular role of this system, since mutants for core components of the gliding machinery, such as parasite actin or subunits of the MyoA-motor complex (the glideosome), remain motile and invasive, albeit at significantly reduced efficiencies. While compensatory mechanisms and unusual polymerisation kinetics of parasite actin have been evoked to explain these findings, the actomyosin system could also play a role distinct from force production during parasite movement.

Results

In this study, we compared the phenotypes of different mutants for core components of the actomyosin system in Toxoplasma gondii to decipher their exact role during gliding motility and invasion. We found that, while some phenotypes (apicoplast segregation, host cell egress, dense granule motility) appeared early after induction of the act1 knockout and went to completion, a small percentage of the parasites remained capable of motility and invasion well past the point at which actin levels were undetectable. Those act1 conditional knockout (cKO) and mlc1 cKO that continue to move in 3D do so at speeds similar to wildtype parasites. However, these mutants are virtually unable to attach to a collagen-coated substrate under flow conditions, indicating an important role for the actomyosin system of T. gondii in the formation of attachment sites.

Conclusion

We demonstrate that parasite actin is essential during the lytic cycle and cannot be compensated by other molecules. Our data suggest a conventional polymerisation mechanism in vivo that depends on a critical concentration of G-actin. Importantly, we demonstrate that the actomyosin system of the parasite functions in attachment to the surface substrate, and not necessarily as force generator.

Electronic supplementary material

The online version of this article (doi:10.1186/s12915-016-0343-5) contains supplementary material, which is available to authorized users.

An Apicomplexan Actin-Binding Protein Serves as a Connector and Lipid Sensor to Coordinate Motility and Invasion

Apicomplexa exhibit a unique form of substrate-dependent gliding motility central for host cell invasion and parasite dissemination. Gliding is powered by rearward translocation of apically secreted transmembrane adhesins via their interaction with the parasite actomyosin system. We report a conserved armadillo and pleckstrin homology (PH) domain-containing protein, termed glideosome-associated connector (GAC), that mediates apicomplexan gliding motility, invasion, and egress by connecting the micronemal adhesins with the actomyosin system. TgGAC binds to and stabilizes filamentous actin and specifically associates with the transmembrane adhesin TgMIC2. GAC localizes to the apical pole in invasive stages of Toxoplasma gondii and Plasmodium berghei, and apical positioning of TgGAC depends on an apical lysine methyltransferase, TgAKMT. GAC PH domain also binds to phosphatidic acid, a lipid mediator associated with microneme exocytosis. Collectively, these findings indicate a central role for GAC in spatially and temporally coordinating gliding motility and invasion.