Stability and function of a putative microtubule-organizing center in the human parasite Toxoplasma gondii

Architecture of the Toxoplasma gondii apical polar ring and its role in gliding motility and invasion

In Toxoplasma gondii, the conoid comprises a cone with spiraling tubulin fibers, preconoidal rings, and intraconoidal microtubules. This dynamic organelle undergoes extension and retraction through the apical polar ring (APR) during egress, gliding, and invasion. The forces involved in conoid extrusion are beginning to be understood, and its role in directing F-actin flux to the pellicular space, thereby controlling parasite motility, has been proposed. However, the contribution of the APR and its interactions with the conoid remain unclear. To gain insight into the APR architecture, ultrastructure expansion microscopy was applied to pinpoint known and newly identified APR proteins (APR2 to APR7). Our results revealed that the APR is constructed as a fixed multilayered structure. Notably, conditional depletion of APR2 resulted in significant impairments in motility and invasion. Electron microscopy and cryoelectron tomography revealed that depletion of APR2 alters APR integrity, affecting conoid extrusion and causing cytosolic leakage of F-actin. These findings implicate the APR structure in directing the apico-basal flux of F-actin to regulate parasite motility and invasion.

The initiation and early development of apical-basal polarity in Toxoplasma gondii

The body plan of the human parasite Toxoplasma gondii has a well-defined polarity. The minus ends of the 22 cortical microtubules are anchored to the apical polar ring, which is a putative microtubule-organizing center. The basal complex caps and constricts the parasite posterior end and is crucial for cytokinesis. How this apical-basal polarity is initiated is unknown. Here, we have examined the development of the apical polar ring and the basal complex using expansion microscopy. We found that substructures in the apical polar ring have different sensitivities to perturbations. In addition, apical-basal differentiation is already established upon nucleation of the cortical microtubule array: arc forms of the apical polar ring and basal complex associate with opposite ends of the microtubules. As the nascent daughter framework grows towards the centrioles, the apical and basal arcs co-develop ahead of the microtubule array. Finally, two apical polar ring components, APR2 and KinesinA, act synergistically. The removal of individual proteins has a modest impact on the lytic cycle. However, the loss of both proteins results in abnormalities in the microtubule array and in highly reduced plaquing and invasion efficiency.

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.

Toxoplasma FER1 is a versatile and dynamic mediator of differential microneme trafficking and microneme exocytosis

Toxoplasma gondii is a polarized cell concentrating several secretory organelles at the apical pole. The secretory micronemes come in two sub-populations differentiated by dependence on Rab5A/C in their biogenesis. Calcium-dependent exocytosis of micronemes occurs at the very apical tip and is critical for parasite egress from its host cell, adhesion and invasion of the next cell. Ferlins represent a protein family with roles in exocytosis containing multiple Ca2+-sensing C2 domains. We determined that T. gondii's ferlin 1 (FER1) localized dynamically to the parasite's secretory pathway. FER1 function was dissected by dominant negative overexpression strategies. We demonstrated that FER1 traffics microneme organelles along the following trajectories: (1) Along the cortex to the apical end; (2) To the apical tip for fusion with the plasma membrane; (3) Differential microneme sub-population traffic, and that FER1 could putatively be responsible for microneme protein trafficking. (4) From the trans-Golgi-endosomal network to the subpellicular cortex; (5) Retrograde transport allowing microneme recycling from mother to daughter. Finally, FER1 overexpression triggers a microneme exocytosis burst, supporting the notion that the radially organized micronemes at the apical tip comprise a readily-releasable microneme pool. In summary, FER1 is pivotal for dynamic microneme trafficking, acts differently on the two microneme subpopulations, and acts on the plasma membrane fusion step during microneme exocytosis.

Co-dependent formation of the Toxoplasma gondii sub-pellicular microtubules and inner membrane skeleton

One of the defining features of apicomplexan parasites is their cytoskeleton composed of alveolar vesicles, known as the inner membrane complex (IMC) undergirded by intermediate-like filament network and an array of subpellicular microtubules (SPMTs). In Toxoplasma gondii, this specialized cytoskeleton is involved in all aspects of the disease-causing lytic cycle, and notably acting as a scaffold for parasite offspring in the internal budding process. Despite advances in our understanding of the architecture and molecular composition, insights pertaining to the coordinated assembly of the scaffold are still largely elusive. Here, T. gondii tachyzoites were dissected by advanced, iterative expansion microscopy (pan-ExM) revealing new insights into the very early sequential formation steps of the tubulin scaffold. A comparative study of the related parasite Sarcocystis neurona revealed that different MT bundling organizations of the nascent SPMTs correlate with the number of central and basal alveolar vesicles. In absence of a so far identified MT nucleation mechanism, we genetically dissected T. gondii γ-tubulin and γ-tubulin complex protein 4 (GCP4). While γ-tubulin depletion abolished the formation of the tubulin scaffold, a set of MTs still formed that suggests SPMTs are nucleated at the outer core of the centrosome. Depletion of GCP4 interfered with the correct assembly of SPMTs into the forming daughter buds, further indicating that the parasite utilizes the γ-tubulin complex in tubulin scaffold formation .

Roles of the tubulin-based cytoskeleton in the Toxoplasma gondii apical complex

Microtubules (MTs) play a vital role as key components of the eukaryotic cytoskeleton. The phylum Apicomplexa comprises eukaryotic unicellular parasitic organisms defined by the presence of an apical complex which consists of specialized secretory organelles and tubulin-based cytoskeletal elements. One apicomplexan parasite, Toxoplasma gondii, is an omnipresent opportunistic pathogen with significant medical and veterinary implications. To ensure successful infection and widespread dissemination, T. gondii heavily relies on the tubulin structures present in the apical complex. Recent advances in high-resolution imaging, coupled with reverse genetics, have offered deeper insights into the composition, functionality, and dynamics of these tubulin-based structures. The apicomplexan tubulins differ from those of their mammalian hosts, endowing them with unique attributes and susceptibility to specific classes of inhibitory compounds.

Potent hydroxamate-derived compounds arrest endodyogeny of Toxoplasma gondii tachyzoites

Toxoplasmosis is a zoonosis that is a worldwide health problem, commonly affecting fetal development and immunodeficient patients. Treatment is carried out with a combination of pyrimethamine and sulfadiazine, which can cause cytopenia and intolerance and does not lead to a parasitological cure of the infection. Lysine deacetylases (KDACs), which remove an acetyl group from lysine residues in histone and non-histone proteins are found in the Toxoplasma gondii genome. Previous work showed the hydroxamate-type KDAC inhibitors Tubastatin A (TST) and Vorinostat (Suberoylanilide Hydroxamic Acid, SAHA) were effective against T. gondii. In the present study, the effects of three hydroxamates (KV-24, KV-30, KV-46), which were originally designed to inhibit human KDAC6, showed different effects against T. gondii. These compounds contain a heterocyclic cap group and a benzyl linker bearing the hydroxamic acid group in para-position. All compounds showed selective activity against T. gondii proliferation, inhibiting tachyzoite proliferation with IC50 values in a nanomolar range after 48h treatment. Microscopy analyses showed that after treatment, tachyzoites presented mislocalization of the apicoplast, disorganization of the inner membrane complex, and arrest in the completion of new daughter cells. The number of dividing cells with incomplete endodyogeny increased significantly after treatment, indicating the compounds can interfere in the late steps of cell division. The results obtained in this work that these new hydroxamates should be considered for future in vivo tests and the development of new compounds for treating toxoplasmosis.

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 initiation and early development of the tubulin-containing cytoskeleton in the human parasite Toxoplasma gondii

The tubulin-containing cytoskeleton of the human parasite Toxoplasma gondii includes several distinct structures: the conoid, formed of 14 ribbon-like tubulin polymers, and the array of 22 cortical microtubules (MTs) rooted in the apical polar ring. Here we analyze the structure of developing daughter parasites using both 3D-SIM and expansion microscopy. Cortical MTs and the conoid start to develop almost simultaneously, but from distinct precursors near the centrioles. Cortical MTs are initiated in a fixed sequence, starting around the periphery of a short arc that extends to become a complete circle. The conoid also develops from an open arc into a full circle, with a fixed spatial relationship to the centrioles. The patterning of the MT array starts from a "blueprint" with ∼five-fold symmetry, switching to 22-fold rotational symmetry in the final product, revealing a major structural rearrangement during daughter growth. The number of MT is essentially invariant in the wild-type array, but is perturbed by the loss of some structural components of the apical polar ring. This study provides insights into the development of tubulin-containing structures that diverge from conventional models, insights that are critical for understanding the evolutionary paths leading to construction and divergence of cytoskeletal frameworks.

Analysis of CDPK1 targets identifies a trafficking adaptor complex that regulates microneme exocytosis in Toxoplasma

Apicomplexan parasites use Ca2+-regulated exocytosis to secrete essential virulence factors from specialized organelles called micronemes. Ca2+-dependent protein kinases (CDPKs) are required for microneme exocytosis; however, the molecular events that regulate trafficking and fusion of micronemes with the plasma membrane remain unresolved. Here, we combine sub-minute resolution phosphoproteomics and bio-orthogonal labeling of kinase substrates in Toxoplasma gondii to identify 163 proteins phosphorylated in a CDPK1-dependent manner. In addition to known regulators of secretion, we identify uncharacterized targets with predicted functions across signaling, gene expression, trafficking, metabolism, and ion homeostasis. One of the CDPK1 targets is a putative HOOK activating adaptor. In other eukaryotes, HOOK homologs form the FHF complex with FTS and FHIP to activate dynein-mediated trafficking of endosomes along microtubules. We show the FHF complex is partially conserved in T. gondii, consisting of HOOK, an FTS homolog, and two parasite-specific proteins (TGGT1_306920 and TGGT1_316650). CDPK1 kinase activity and HOOK are required for the rapid apical trafficking of micronemes as parasites initiate motility. Moreover, parasites lacking HOOK or FTS display impaired microneme protein secretion, leading to a block in the invasion of host cells. Taken together, our work provides a comprehensive catalog of CDPK1 targets and reveals how vesicular trafficking has been tuned to support a parasitic lifestyle.

The cortical microtubules of Toxoplasma gondii underlie the helicity of parasite movement

Motility is essential for apicomplexan parasites to infect their hosts. In a three-dimensional (3D) environment, the apicomplexan parasite Toxoplasma gondii moves along a helical path. The cortical microtubules, which are ultra-stable and spirally arranged, have been considered to be a structure that guides the long-distance movement of the parasite. Here, we address the role of the cortical microtubules in parasite motility, invasion and egress by utilizing a previously generated mutant (dubbed 'TKO') in which these microtubules are destabilized in mature parasites. We found that the cortical microtubules in ∼80% of the non-dividing (i.e. daughter-free) TKO parasites are much shorter than normal. The extent of depolymerization was further exacerbated upon commencement of daughter formation or cold treatment, but parasite replication was not affected. In a 3D Matrigel matrix, the TKO mutant moved directionally over long distances, but along trajectories that were significantly more linear (i.e. less helical) than those of wild-type parasites. Interestingly, this change in trajectory did not impact either movement speed in the matrix or the speed and behavior of the parasite during entry into and egress from the host cell.

Analysis of CDPK1 targets identifies a trafficking adaptor complex that regulates microneme exocytosis in Toxoplasma

Apicomplexan parasites use Ca2+-regulated exocytosis to secrete essential virulence factors from specialized organelles called micronemes. Ca2+-dependent protein kinases (CDPKs) are required for microneme exocytosis; however, the molecular events that regulate trafficking and fusion of micronemes with the plasma membrane remain unresolved. Here, we combine sub-minute resolution phosphoproteomics and bio-orthogonal labeling of kinase substrates in Toxoplasma gondii to identify 163 proteins phosphorylated in a CDPK1-dependent manner. In addition to known regulators of secretion, we identify uncharacterized targets with predicted functions across signaling, gene expression, trafficking, metabolism, and ion homeostasis. One of the CDPK1 targets is a putative HOOK activating adaptor. In other eukaryotes, HOOK homologs form the FHF complex with FTS and FHIP to activate dynein-mediated trafficking of endosomes along microtubules. We show the FHF complex is partially conserved in T. gondii, consisting of HOOK, an FTS homolog, and two parasite-specific proteins (TGGT1_306920 and TGGT1_316650). CDPK1 kinase activity and HOOK are required for the rapid apical trafficking of micronemes as parasites initiate motility. Moreover, parasites lacking HOOK or FTS display impaired microneme protein secretion, leading to a block in the invasion of host cells. Taken together, our work provides a comprehensive catalog of CDPK1 targets and reveals how vesicular trafficking has been tuned to support a parasitic lifestyle.

Toxoplasma gondii HOOK-FTS-HIP Complex is Critical for Secretory Organelle Discharge during Motility, Invasion, and Egress

Members of the Apicomplexa phylum possess specialized secretory organelles that discharge, apically and in a timely regulated manner, key factors implicated in parasite motility, host cell invasion, egress and subversion of host cellular functions. The mechanisms regulating trafficking and apical docking of these secretory organelles are only partially elucidated. Here, we characterized two conserved endosomal trafficking regulators known to promote vesicle transport and/or fusion, HOOK and Fused Toes (FTS), in the context of organelle discharge in Toxoplasma gondii. TgHOOK and TgFTS form a complex with a coccidian-specific partner, named HOOK interacting partner (HIP). TgHOOK displays an apically enriched vesicular pattern and concentrates at the parasite apical tip where it colocalizes with TgFTS and TgHIP. Functional investigations revealed that TgHOOK is dispensable but fitness conferring. The protein regulates the apical positioning and secretion of micronemes and contributes to egress, motility, host cell attachment, and invasion. Conditional depletion of TgFTS or TgHIP impacted on the same processes but led to more severe phenotypes. This study provides evidence of endosomal trafficking regulators involved in the apical exocytosis of micronemes and possibly as a consequence or directly on the discharge of the rhoptries. IMPORTANCE Toxoplasma gondii affects between 30 and 80% of the human population, poses a life-threatening risk to immunocompromised individuals, and is a cause of abortion and birth defects following congenital transmission. T. gondii belongs to the phylum of Apicomplexa characterized by a set of unique apical secretory organelles called the micronemes and rhoptries. Upon host cell recognition, this obligatory intracellular parasite secretes specific effectors contained in micronemes and rhoptries to promote parasite invasion of host cells and subsequent persistence. Here, we identified novel T. gondii endosomal trafficking regulators and demonstrated that they regulate microneme organelle apical positioning and exocytosis, thereby strongly contributing to host cell invasion and parasite virulence.

Cryo-tomography reveals rigid-body motion and organization of apicomplexan invasion machinery

The apical complex is a specialized collection of cytoskeletal and secretory machinery in apicomplexan parasites, which include the pathogens that cause malaria and toxoplasmosis. Its structure and mechanism of motion are poorly understood. We used cryo-FIB-milling and cryo-electron tomography to visualize the 3D-structure of the apical complex in its protruded and retracted states. Averages of conoid-fibers revealed their polarity and unusual nine-protofilament arrangement with associated proteins connecting and likely stabilizing the fibers. Neither the structure of the conoid-fibers nor the architecture of the spiral-shaped conoid complex change during protrusion or retraction. Thus, the conoid moves as a rigid body, and is not spring-like and compressible, as previously suggested. Instead, the apical-polar-rings (APR), previously considered rigid, dilate during conoid protrusion. We identified actin-like filaments connecting the conoid and APR during protrusion, suggesting a role during conoid movements. Furthermore, our data capture the parasites in the act of secretion during conoid protrusion.

Apical anchorage and stabilization of subpellicular microtubules by apical polar ring ensures Plasmodium ookinete infection in mosquito

Morphogenesis of many protozoans depends on a polarized establishment of cortical cytoskeleton containing the subpellicular microtubules (SPMTs), which are apically nucleated and anchored by the apical polar ring (APR). In malaria parasite Plasmodium, APR emerges in the host-invading stages, including the ookinete for mosquito infection. So far, the fine structure and molecular components of APR as well as the underlying mechanism of APR-mediated apical positioning of SPMTs are largely unknown. Here, we resolve an unprecedented APR structure composed of a top ring plus approximate 60 radiating spines. We report an APR-localizing and SPMT-binding protein APR2. APR2 disruption impairs ookinete morphogenesis and gliding motility, leading to Plasmodium transmission failure in mosquitoes. The APR2-deficient ookinetes display defective apical anchorage of APR and SPMT due to the impaired integrity of APR. Using protein proximity labeling, we obtain a Plasmodium ookinete APR proteome and validate ten undescribed APR proteins. Among them, APRp2 and APRp4 directly interact with APR2 and also mediate the apical anchorage of SPMTs. This study sheds light on the molecular basis of APR in the organization of Plasmodium ookinete SPMTs.

Nanoscale imaging of the conoid and functional dissection of its dynamics in Apicomplexa

Members of the Apicomplexa phylum are unified by an apical complex tailored for motility and host cell invasion. It includes regulated secretory organelles and a conoid attached to the apical polar ring (APR) from which subpellicular microtubules emerge. In coccidia, the conoid is composed of a cone of spiraling tubulin fibers, two preconoidal rings, and two intraconoidal microtubules. The conoid extrudes through the APR in motile parasites. Recent advances in proteomics, cryo-electron tomography, super-resolution, and expansion microscopy provide a more comprehensive view of the spatial and temporal resolution of proteins belonging to the conoid subcomponents. In combination with the phenotyping of targeted mutants, the biogenesis, turnover, dynamics, and function of the conoid begin to be elucidated.

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 Taxoplasma 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.

An apical protein, Pcr2, is required for persistent movement by the human parasite Toxoplasma gondii

The phylum Apicomplexa includes thousands of species of unicellular parasites that cause a wide range of human and animal diseases such as malaria and toxoplasmosis. To infect, the parasite must first initiate active movement to disseminate through tissue and invade into a host cell, and then cease moving once inside. The parasite moves by gliding on a surface, propelled by an internal cortical actomyosin-based motility apparatus. One of the most effective invaders in Apicomplexa is Toxoplasma gondii, which can infect any nucleated cell and any warm-blooded animal. During invasion, the parasite first makes contact with the host cell "head-on" with the apical complex, which features an elaborate cytoskeletal apparatus and associated structures. Here we report the identification and characterization of a new component of the apical complex, Preconoidal region protein 2 (Pcr2). Pcr2 knockout parasites replicate normally, but they are severely diminished in their capacity for host tissue destruction due to significantly impaired invasion and egress, two vital steps in the lytic cycle. When stimulated for calcium-induced egress, Pcr2 knockout parasites become active, and secrete effectors to lyse the host cell. Calcium-induced secretion of the major adhesin, MIC2, also appears to be normal. However, the movement of the Pcr2 knockout parasite is spasmodic, which drastically compromises egress. In addition to faulty motility, the ability of the Pcr2 knockout parasite to assemble the moving junction is impaired. Both defects likely contribute to the poor efficiency of invasion. Interestingly, actomyosin activity, as indicated by the motion of mEmerald tagged actin chromobody, appears to be largely unperturbed by the loss of Pcr2, raising the possibility that Pcr2 may act downstream of or in parallel with the actomyosin machinery.

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.

Cryo-ET of Toxoplasma parasites gives subnanometer insight into tubulin-based structures

Tubulin is a conserved protein that polymerizes into different forms of filamentous structures in Toxoplasma gondii, an obligate intracellular parasite in the phylum Apicomplexa. Two key tubulin-containing cytoskeletal components are subpellicular microtubules (SPMTs) and conoid fibrils (CFs). The SPMTs help maintain shape and gliding motility, while the CFs are implicated in invasion. Here, we use cryogenic electron tomography to determine the molecular structures of the SPMTs and CFs in vitrified intact and detergent-extracted parasites. Subvolume densities from detergent-extracted parasites yielded averaged density maps at subnanometer resolutions, and these were related back to their architecture in situ. An intralumenal spiral lines the interior of the 13-protofilament SPMTs, revealing a preferred orientation of these microtubules relative to the parasite's long axis. Each CF is composed of nine tubulin protofilaments that display a comma-shaped cross-section, plus additional associated components. Conoid protrusion, a crucial step in invasion, is associated with an altered pitch of each CF. The use of basic building blocks of protofilaments and different accessory proteins in one organism illustrates the versatility of tubulin to form two distinct types of assemblies, SPMTs and CFs.

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.

Unraveling the Elusive Rhoptry Exocytic Mechanism of Apicomplexa

Apicomplexan parasites are unicellular eukaryotes that invade the cells in which they proliferate. The development of genetic tools in Toxoplasma, and then in Plasmodium, in the 1990s allowed the first description of the molecular machinery used for motility and invasion, revealing a crucial role for two different secretory organelles, micronemes and rhoptries. Rhoptry proteins are injected directly into the host cytoplasm not only to promote invasion but also to manipulate host functions. Nonetheless, the injection machinery has remained mysterious, a major conundrum in the field. Here we review recent progress in uncovering structural components and proteins implicated in rhoptry exocytosis and explain how revisiting early findings and considering the evolutionary origins of Apicomplexa contributed to some of these discoveries.

Molecular characterization of the conoid complex in Toxoplasma reveals its conservation in all apicomplexans, including Plasmodium species

The apical complex is the instrument of invasion used by apicomplexan parasites, and the conoid is a conspicuous feature of this apparatus found throughout this phylum. The conoid, however, is believed to be heavily reduced or missing from Plasmodium species and other members of the class Aconoidasida. Relatively few conoid proteins have previously been identified, making it difficult to address how conserved this feature is throughout the phylum, and whether it is genuinely missing from some major groups. Moreover, parasites such as Plasmodium species cycle through 3 invasive forms, and there is the possibility of differential presence of the conoid between these stages. We have applied spatial proteomics and high-resolution microscopy to develop a more complete molecular inventory and understanding of the organisation of conoid-associated proteins in the model apicomplexan Toxoplasma gondii. These data revealed molecular conservation of all conoid substructures throughout Apicomplexa, including Plasmodium, and even in allied Myzozoa such as Chromera and dinoflagellates. We reporter-tagged and observed the expression and location of several conoid complex proteins in the malaria model P. berghei and revealed equivalent structures in all of its zoite forms, as well as evidence of molecular differentiation between blood-stage merozoites and the ookinetes and sporozoites of the mosquito vector. Collectively, we show that the conoid is a conserved apicomplexan element at the heart of the invasion mechanisms of these highly successful and often devastating parasites.

Multivalent Interactions Drive the Toxoplasma AC9:AC10:ERK7 Complex To Concentrate ERK7 in the Apical Cap

The Toxoplasma inner membrane complex (IMC) is a specialized organelle that is crucial for the parasite to establish an intracellular lifestyle and ultimately cause disease. The IMC is composed of both membrane and cytoskeletal components, further delineated into the apical cap, body, and basal subcompartments. The apical cap cytoskeleton was recently demonstrated to govern the stability of the apical complex, which controls parasite motility, invasion, and egress. While this role was determined by individually assessing the apical cap proteins AC9, AC10, and the mitogen-activated protein kinase ERK7, how the three proteins collaborate to stabilize the apical complex is unknown. In this study, we use a combination of deletion analyses and yeast two-hybrid experiments to establish that these proteins form an essential complex in the apical cap. We show that AC10 is a foundational component of the AC9:AC10:ERK7 complex and demonstrate that the interactions among them are critical to maintaining the apical complex. Importantly, we identify multiple independent regions of pairwise interaction between each of the three proteins, suggesting that the AC9:AC10:ERK7 complex is organized by multivalent interactions. Together, these data support a model in which multiple interacting domains enable the oligomerization of the AC9:AC10:ERK7 complex and its assembly into the cytoskeletal IMC, which serves as a structural scaffold that concentrates ERK7 kinase activity in the apical cap. IMPORTANCE The phylum Apicomplexa consists of obligate, intracellular parasites, including the causative agents of toxoplasmosis, malaria, and cryptosporidiosis. Hallmarks of these parasites are the IMC and the apical complex, both of which are unique structures that are conserved throughout the phylum and required for parasite survival. The apical cap portion of the IMC has previously been shown to stabilize the apical complex. Here, we expand on those studies to determine the precise protein-protein interactions of the apical cap complex that confer this essential function. We describe the multivalent nature of these interactions and show that the resulting protein oligomers likely tether ERK7 in the apical cap. This study represents the first description of the architecture of the apical cap at a molecular level, expanding our understanding of the unique cell biology that drives Toxoplasma infections.

Coupling Polar Adhesion with Traction, Spring, and Torque Forces Allows High-Speed Helical Migration of the Protozoan Parasite Toxoplasma

Among the eukaryotic cells that navigate through fully developed metazoan tissues, protozoans from the Apicomplexa phylum have evolved motile developmental stages that move much faster than the fastest crawling cells owing to a peculiar substrate-dependent type of motility, known as gliding. Best-studied models are the Plasmodium sporozoite and the Toxoplasma tachyzoite polarized cells for which motility is vital to achieve their developmental programs in the metazoan hosts. The gliding machinery is shared between the two parasites and is largely characterized. Localized beneath the cell surface, it includes actin filaments, unconventional myosin motors housed within a multimember glideosome unit, and apically secreted transmembrane adhesins. In contrast, less is known about the force mechanisms powering cell movement. Pioneered biophysical studies on the sporozoite and phenotypic analysis of tachyzoite actin-related mutants have added complexity to the general view that force production for parasite forward movement directly results from the myosin-driven rearward motion of the actin-coupled adhesion sites. Here, we have interrogated how forces and substrate adhesion-de-adhesion cycles operate and coordinate to allow the typical left-handed helical gliding mode of the tachyzoite. By combining quantitative traction force and reflection interference microscopy with micropatterning and expansion microscopy, we unveil at the millisecond and nanometer scales the integration of a critical apical anchoring adhesion with specific traction and spring-like forces. We propose that the acto-myoA motor directs the traction force which allows transient energy storage by the microtubule cytoskeleton and therefore sets the thrust force required for T. gondii tachyzoite vital helical gliding capacity.

Rab11A regulates dense granule transport and secretion during Toxoplasma gondii invasion of host cells and parasite replication

Toxoplasma gondii possesses an armada of secreted virulent factors that enable parasite invasion and survival into host cells. These factors are contained in specific secretory organelles, the rhoptries, micronemes and dense granules that release their content upon host cell recognition. Dense granules are secreted in a constitutive manner during parasite replication and play a crucial role in modulating host metabolic and immune responses. While the molecular mechanisms triggering rhoptry and microneme release upon host cell adhesion have been well studied, constitutive secretion remains a poorly explored aspect of T. gondii vesicular trafficking. Here, we investigated the role of the small GTPase Rab11A, a known regulator of exocytosis in eukaryotic cells. Our data revealed an essential role of Rab11A in promoting the cytoskeleton driven transport of dense granules and the release of their content into the vacuolar space. Rab11A also regulates transmembrane protein trafficking and localization during parasite replication, indicating a broader role of Rab11A in cargo exocytosis at the plasma membrane. Moreover, we found that Rab11A also regulates extracellular parasite motility and adhesion to host cells. In line with these findings, MIC2 secretion was altered in Rab11A-defective parasites, which also exhibited severe morphological defects. Strikingly, by live imaging we observed a polarized accumulation of Rab11A-positive vesicles and dense granules at the apical pole of extracellular motile and invading parasites suggesting that apically polarized Rab11A-dependent delivery of cargo regulates early secretory events during parasite entry into host cells.

Toxoplasma gondii (T. gondii) is a highly prevalent parasite infecting a wide range of animals as well as humans. T. gondii secretes numerous virulent factors contained in specific organelles, termed the rhoptries, micronemes and dense granules. These factors are released upon host cell recognition and enable parasite invasion and subsequent development into an intracellular vacuole. In particular, dense granules contain critical effectors that modulate intrinsic defenses of infected host cells ensuring parasite survival and dissemination. The mechanisms regulating dense granule secretion have not been elucidated. In this study, we unraveled a novel role for the T. gondii GTPase Rab11A in promoting dense granule transport along the parasite cytoskeleton and their content release into the vacuolar space during parasite replication. We also found that T. gondii Rab11A regulates extracellular parasite motility and adhesion to host cells suggesting a broader role in distinct secretory pathways essential for parasite virulence.

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.

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.

Loss of a conserved MAPK causes catastrophic failure in assembly of a specialized cilium-like structure in Toxoplasma gondii

Primary cilia are important organizing centers that control diverse cellular processes. Apicomplexan parasites like Toxoplasma gondii have a specialized cilium-like structure called the conoid that organizes the secretory and invasion machinery critical for the parasites' lifestyle. The proteins that initiate the biogenesis of this structure are largely unknown. We identified the Toxoplasma orthologue of the conserved kinase ERK7 as essential to conoid assembly. Parasites in which ERK7 has been depleted lose their conoids late during maturation and are immotile and thus unable to invade new host cells. This is the most severe phenotype to conoid biogenesis yet reported, and is made more striking by the fact that ERK7 is not a conoid protein, as it localizes just basal to the structure. ERK7 has been recently implicated in ciliogenesis in metazoan cells, and our data suggest that this kinase has an ancient and central role in regulating ciliogenesis throughout Eukaryota.

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.

Biogenesis and secretion of micronemes in Toxoplasma gondii

One of the hallmarks of the parasitic phylum of Apicomplexa is the presence of highly specialised, apical secretory organelles, called the micronemes and rhoptries that play critical roles in ensuring survival and dissemination. Upon exocytosis, the micronemes release adhesin complexes, perforins, and proteases that are crucially implicated in egress from infected cells, gliding motility, migration across biological barriers, and host cell invasion. Recent studies on Toxoplasma gondii and Plasmodium species have shed more light on the signalling events and the machinery that trigger microneme secretion. Intracellular cyclic nucleotides, calcium level, and phosphatidic acid act as key mediators of microneme exocytosis, and several downstream effectors have been identified. Here, we review the key steps of microneme biogenesis and exocytosis, summarising the still fractal knowledge at the molecular level regarding the fusion event with the parasite plasma membrane.

Alveolar proteins stabilize cortical microtubules in Toxoplasma gondii

Single-celled protists use elaborate cytoskeletal structures, including arrays of microtubules at the cell periphery, to maintain polarity and rigidity. The obligate intracellular parasite Toxoplasma gondii has unusually stable cortical microtubules beneath the alveoli, a network of flattened membrane vesicles that subtends the plasmalemma. However, anchoring of microtubules along alveolar membranes is not understood. Here, we show that GAPM1a, an integral membrane protein of the alveoli, plays a role in maintaining microtubule stability. Degradation of GAPM1a causes cortical microtubule disorganisation and subsequent depolymerisation. These changes in the cytoskeleton lead to parasites becoming shorter and rounder, which is accompanied by a decrease in cellular volume. Extended GAPM1a depletion leads to severe defects in division, reminiscent of the effect of disrupting other alveolar proteins. We suggest that GAPM proteins link the cortical microtubules to the alveoli and are required to maintain the shape and rigidity of apicomplexan zoites.

Cortical microtubules of Toxoplasma gondii are exceptionally stable, but it isn’t known how they are anchored along membranes. Here, Harding et al. show that GAPM proteins localize to the inner membrane complex and are essential for maintaining the structural stability of parasites.

Structure of a Novel Dimeric SET Domain Methyltransferase that Regulates Cell Motility

Lysine methyltransferases (KMTs) were initially associated with transcriptional control through their methylation of histones and other nuclear proteins, but have since been found to regulate many other cellular activities. The apical complex lysine (K) methyltransferase (AKMT) of the human parasite Toxoplasma gondii was recently shown to play a critical role in regulating cellular motility. Here we report a 2.1-Å resolution crystal structure of the conserved and functional C-terminal portion (aa289-709) of T. gondii AKMT. AKMT dimerizes via a unique intermolecular interface mediated by the C-terminal tetratricopeptide repeat-like domain together with a specific zinc-binding motif that is absent from all other KMTs. Disruption of AKMT dimerization impaired both its enzyme activity and parasite egress from infected host cells in vivo. Structural comparisons reveal that AKMT is related to the KMTs in the SMYD family, with, however, a number of distinct structural features in addition to the unusual dimerization interface. These features are conserved among the apicomplexan parasites and their free-living relatives, but not found in any known KMTs in animals. AKMT therefore is the founding member of a new subclass of KMT that has important implications for the evolution of the apicomplexans.

A conserved ankyrin repeat-containing protein regulates conoid stability, motility and cell invasion in Toxoplasma gondii

Apicomplexan parasites are typified by an apical complex that contains a unique microtubule-organizing center (MTOC) that organizes the cytoskeleton. In apicomplexan parasites such as Toxoplasma gondii, the apical complex includes a spiral cap of tubulin-rich fibers called the conoid. Although described ultrastructurally, the composition and functions of the conoid are largely unknown. Here, we localize 11 previously undescribed apical proteins in T. gondii and identify an essential component named conoid protein hub 1 (CPH1), which is conserved in apicomplexan parasites. CPH1 contains ankyrin repeats that are required for structural integrity of the conoid, parasite motility, and host cell invasion. Proximity labeling and protein interaction network analysis reveal that CPH1 functions as a hub linking key motor and structural proteins that contain intrinsically disordered regions and coiled coil domains. Our findings highlight the importance of essential protein hubs in controlling biological networks of MTOCs in early-branching protozoan parasites.

Apicomplexan parasites such as Toxoplasma gondii possess a tubulin-rich structure called the conoid. Here, Long et al. identify a conoid protein that interacts with motor and structural proteins and is required for structural integrity of the conoid, parasite motility, and host cell invasion.