A Toxoplasma MORN1 null mutant undergoes repeated divisions but is defective in basal assembly, apicoplast division and cytokinesis

BCC0 collaborates with IMC32 and IMC43 to form the Toxoplasma gondii essential daughter bud assembly complex

Toxoplasma gondii divides by endodyogeny, in which two daughter buds are formed within the cytoplasm of the maternal cell using the inner membrane complex (IMC) as a scaffold. During endodyogeny, components of the IMC are synthesized and added sequentially to the nascent daughter buds in a tightly regulated manner. We previously showed that the early recruiting proteins IMC32 and IMC43 form an essential daughter bud assembly complex which lays the foundation of the daughter cell scaffold in T. gondii. In this study, we identify the essential, early recruiting IMC protein BCC0 as a third member of this complex by using IMC32 as bait in both proximity labeling and yeast two-hybrid screens. We demonstrate that BCC0's localization to daughter buds depends on the presence of both IMC32 and IMC43. Deletion analyses and functional complementation studies reveal that residues 701-877 of BCC0 are essential for both its localization and function and that residues 1-899 are sufficient for function despite minor mislocalization. Pairwise yeast two-hybrid assays additionally demonstrate that BCC0's essential domain binds to the coiled-coil region of IMC32 and that BCC0 and IMC43 do not directly interact. This data supports a model for complex assembly in which an IMC32-BCC0 subcomplex initially recruits to nascent buds via palmitoylation of IMC32 and is locked into the scaffold once bud elongation begins by IMC32 binding to IMC43. Together, this study dissects the organization and function of a complex of three early recruiting daughter proteins which are essential for the proper assembly of the IMC during endodyogeny.

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.

Elucidating the spatio-temporal dynamics of the Plasmodium falciparum basal complex

Asexual replication of Plasmodium falciparum occurs via schizogony, wherein 16-36 daughter cells are produced within the parasite during one semi-synchronized cytokinetic event. Schizogony requires a divergent contractile ring structure known as the basal complex. Our lab has previously identified PfMyoJ (PF3D7_1229800) and PfSLACR (PF3D7_0214700) as basal complex proteins recruited midway through segmentation. Using ultrastructure expansion microscopy, we localized both proteins to a novel basal complex subcompartment. While both colocalize with the basal complex protein PfCINCH upon recruitment, they form a separate, more basal subcompartment termed the posterior cup during contraction. We also show that PfSLACR is recruited to the basal complex prior to PfMyoJ, and that both proteins are removed unevenly as segmentation concludes. Using live-cell microscopy, we show that actin dynamics are dispensable for basal complex formation, expansion, and contraction. We then show that EF-hand containing P. falciparum Centrin 2 partially localizes to this posterior cup of the basal complex and that it is essential for growth and replication, with variable defects in basal complex contraction and synchrony. Finally, we demonstrate that free intracellular calcium is necessary but not sufficient for basal complex contraction in P. falciparum. Thus, we demonstrate dynamic spatial compartmentalization of the Plasmodium falciparum basal complex, identify an additional basal complex protein, and begin to elucidate the unique mechanism of contraction utilized by P. falciparum, opening the door for further exploration of Apicomplexan cellular division.

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 .

The dynamin-related protein Dyn2 is essential for both apicoplast and mitochondrial fission in Plasmodium falciparum

Dynamins, or dynamin-related proteins (DRPs), are large mechano-sensitive GTPases mediating membrane dynamics or organellar fission/fusion events. Plasmodium falciparum encodes three dynamin-like proteins whose functions are poorly understood. Here, we demonstrate that PfDyn2 mediates both apicoplast and mitochondrial fission. Using super-resolution and ultrastructure expansion microscopy, we show that PfDyn2 is expressed in the schizont stage and localizes to both the apicoplast and mitochondria. Super-resolution long-term live cell microscopy shows that PfDyn2-deficient parasites cannot complete cytokinesis because the apicoplast and mitochondria do not undergo fission. Further, the basal complex or cytokinetic ring in Plasmodium cannot fully contract upon PfDyn2 depletion, a phenotype secondary to physical blockage of undivided organelles in the middle of the ring. Our data suggest that organellar fission defects result in aberrant schizogony, generating unsuccessful merozoites. The unique biology of PfDyn2, mediating both apicoplast and mitochondrial fission, has not been observed in other organisms possessing two endosymbiotic organelles.

Highlights

PfDyn2 is essential for schizont-stage development.PfDyn2 mediates both apicoplast and mitochondrial fission.Deficiency of PfDyn2 leads to organellar fission failures and blockage of basal complex contraction.Addition of apicoplast-derived metabolite IPP does not rescue the growth defects.

The SUN-like protein TgSLP1 is essential for nuclear division in the apicomplexan parasite Toxoplasma gondii

Connections between the nucleus and the cytoskeleton are important for positioning and division of the nucleus. In most eukaryotes, the linker of nucleoskeleton and cytoskeleton (LINC) complex spans the outer and inner nuclear membranes and connects the nucleus to the cytoskeleton. In opisthokonts, it is composed of Klarsicht, ANC-1 and Syne homology (KASH) domain proteins and Sad1 and UNC-84 (SUN) domain proteins. Given that the nucleus is positioned at the posterior pole of Toxoplasma gondii, we speculated that apicomplexan parasites must have a similar mechanism that integrates the nucleus and the cytoskeleton. Here, we identified three UNC family proteins in the genome of the apicomplexan parasite T. gondii. Whereas the UNC-50 protein TgUNC1 localised to the Golgi and appeared to be not essential for the parasite, the SUN domain protein TgSLP2 showed a diffuse pattern throughout the parasite. The second SUN domain protein, TgSLP1, was expressed in a cell cycle-dependent manner and was localised close to the mitotic spindle and, more detailed, at the kinetochore. We demonstrate that conditional knockout of TgSLP1 leads to failure of nuclear division and loss of centrocone integrity.

A PPP-type pseudophosphatase is required for the maintenance of basal complex integrity in Plasmodium falciparum

During its asexual blood stage, P. falciparum replicates via schizogony, wherein dozens of daughter cells are formed within a single parent. The basal complex, a contractile ring that separates daughter cells, is critical for schizogony. In this study, we identify a Plasmodium basal complex protein essential for basal complex maintenance. Using multiple microscopy techniques, we demonstrate that PfPPP8 is required for uniform basal complex expansion and maintenance of its integrity. We characterize PfPPP8 as the founding member of a novel family of pseudophosphatases with homologs in other Apicomplexan parasites. By co-immunoprecipitation, we identify two additional new basal complex proteins. We characterize the unique temporal localizations of these new basal complex proteins (late-arriving) and of PfPPP8 (early-departing). In this work, we identify a novel basal complex protein, determine its specific role in segmentation, identify a new pseudophosphatase family, and establish that the P. falciparum basal complex is a dynamic structure.

TgMORN2, a MORN Family Protein Involved in the Regulation of Endoplasmic Reticulum Stress in Toxoplasma gondii

MORN proteins play a key role in the cytoskeletal structure of eukaryotes and are essential for the close arrangement of the endoplasmic reticulum and plasma membrane. A gene with nine MORN motifs (TGGT1_292120, named TgMORN2) was identified in the Toxoplasma gondii genome; it was presumed to belong to the MORN protein family and to have the function of forming the cytoskeleton, which affects the survival of T. gondii. However, the genetic deletion of MORN2 did not noticeably affect parasite growth and virulence. Using adjacent protein labeling techniques, we identified a network of TgMORN2 interactions, which mainly included endoplasmic reticulum stress (ER stress)-related proteins. In exploring these data, we found that the pathogenicity of the KO-TgMORN2 strain was significantly reduced in the case of tunicamycin-induced ER stress. Reticulon TgRTN (TGGT1_226430) and tubulin β-Tubulin were identified as interaction proteins of TgMORN2. Collectively, TgMORN2 plays a role in ER stress, which lays a foundation for further research on the function of the MORN protein in T. gondii.

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.

Identification of basal complex protein that is essential for maturation of transmission-stage malaria parasites

Malaria remains a global driver of morbidity and mortality. To generate new antimalarials, one must elucidate the fundamental cell biology of Plasmodium falciparum, the parasite responsible for the deadliest cases of malaria. A membranous and proteinaceous scaffold called the inner membrane complex (IMC) supports the parasite during morphological changes, including segmentation of daughter cells during asexual replication and formation of transmission-stage gametocytes. The basal complex lines the edge of the IMC during segmentation and likely facilitates IMC expansion. It is unknown, however, what drives IMC expansion during gametocytogenesis. We describe the discovery of a basal complex protein, PfBLEB, which we find to be essential for gametocytogenesis. Parasites lacking PfBLEB harbor defects in IMC expansion and are unable to form mature gametocytes. This article demonstrates a role for a basal complex protein outside of asexual division, and, importantly, highlights a potential molecular target for the ablation of malaria transmission.

Proteomic characterization of the Toxoplasma gondii cytokinesis machinery portrays an expanded hierarchy of its assembly and function

The basal complex (BC) is essential for T. gondii cell division but mechanistic details are lacking. Here we report a reciprocal proximity based biotinylation approach to map the BC’s proteome. We interrogate the resulting map for spatiotemporal dynamics and function by disrupting the expression of components. This highlights four architecturally distinct BC subcomplexes, the compositions of which change dynamically in correlation with changes in BC function. We identify BCC0 as a protein undergirding BC formation in five foci that precede the same symmetry seen in the apical annuli and IMC sutures. Notably, daughter budding from BCC0 progresses bidirectionally: the apical cap in apical and the rest of the IMC in basal direction. Furthermore, the essential role of the BC in cell division is contained in BCC4 and MORN1 that form a ‘rubber band’ to sequester the basal end of the assembling daughter cytoskeleton. Finally, we assign BCC1 to the non-essential, final BC constriction step.

The basal complex is orchestrating Toxoplasma gondii cell division steps. Here, the authors use proximity biotinylation to map the proteome of this contractile ring, identify components acting on its formation, stability and constriction, and reveal bidirectional daughter budding.

Membrane Occupation and Recognition Nexus (MORN) motif controls protein localization and function

Membrane Occupation and Recognition Nexus (MORN) motif was first defined in 2000, when it was identified in the junctophilin protein family. Dozens of studies have been published ever since, mainly focusing on the function of a given MORN motif-containing protein in parasites, plants or animal cells. Proteins with MORN motifs are not only expressed in most animal and plant cell types but also significantly differ in their intracellular localization, suggesting that the MORN motifs may fulfil multiple physiological functions. Recent studies have found that MORN motif-containing proteins junctophilin 1/2 and MORN3 play a role in cardiac hypertrophy, skeletal muscle fiber stability and cancer. Hence, MORN motif-containing proteins may be exploited to develop improved treatments for various pathological conditions, such as cardiovascular diseases. Here, we review current research on MORN motif-containing proteins in different organisms and provide both ideas and approaches for follow-up exploration of their functions and applications.

A comprehensive ultrastructural analysis of the Toxoplasma gondii cytoskeleton

The invasive nature of Toxoplasma gondii is closely related to the properties of its cytoskeleton, which is constituted by a group of diverse structural and dynamic components that play key roles during the infection. Even if there have been numerous reports about the composition and function of the Toxoplasma cytoskeleton, the ultrastructural organization of some of these components has not yet been fully characterized. This study used a detergent extraction process and several electron microscopy contrast methods that allowed the successful isolation of the cytoskeleton of Toxoplasma tachyzoites. This process allowed for the conservation of the structures known to date and several new structures that had not been characterized at the ultrastructural level. For the first time, characterization was achieved for a group of nanofibers that allow the association between the polar apical ring and the conoid as well as the ultrastructural characterization of the apical cap of the parasite. The ultrastructure and precise location of the peripheral rings were also found, and the annular components of the basal complex were characterized. Finally, through immunoelectron microscopy, the exact spatial location of the subpellicular network inside the internal membrane system that forms the pellicle was found. The findings regarding these new structures contribute to the knowledge concerning the biology of the Toxoplasma gondii cytoskeleton. They also provide new opportunities in the search for therapeutic strategies aimed at these components with the purpose of inhibiting invasion and thus parasitism.

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.

The Basal Complex Protein PfMORN1 Is Not Required for Asexual Replication of Plasmodium falciparum

Plasmodium falciparum, the Apicomplexan parasite that causes the most severe form of human malaria, divides via schizogony during the asexual blood stage of its life cycle. In this method of cell division, multiple daughter cells are generated from a single schizont by segmentation. During segmentation, the basal complex forms at the basal end of the nascent daughter parasites and likely facilitates cell shape and cytokinesis. The requirement and function for each of the individual protein components within the basal complex remain largely unknown in P. falciparum. In this work, we demonstrate that the P. falciparum membrane occupation and recognition nexus repeat-containing protein 1 (PfMORN1) is not required for asexual replication. Following inducible knockout of PfMORN1, we find no detectable defect in asexual parasite morphology or replicative fitness.

IMPORTANCEPlasmodium falciparum parasites cause the most severe form of human malaria. During the clinically relevant blood stage of its life cycle, the parasites divide via schizogony. In this divergent method of cell division, the components for multiple daughter cells are generated within a common cytoplasm. At the end of schizogony, segmentation partitions the organelles into invasive daughter parasites. The basal complex is a ring-shaped molecular machine that is critical for segmentation. The requirement for individual proteins within the basal complex is incompletely understood. We demonstrate that the PfMORN1 protein is dispensable for blood stage replication of P. falciparum. This result highlights important differences between Plasmodium parasites and Toxoplasma gondii, where the ortholog T. gondii MORN1 (TgMORN1) is required for asexual replication.

The Ringleaders: Understanding the Apicomplexan Basal Complex Through Comparison to Established Contractile Ring Systems

The actomyosin contractile ring is a key feature of eukaryotic cytokinesis, conserved across many eukaryotic kingdoms. Recent research into the cell biology of the divergent eukaryotic clade Apicomplexa has revealed a contractile ring structure required for asexual division in the medically relevant genera Toxoplasma and Plasmodium; however, the structure of the contractile ring, known as the basal complex in these parasites, remains poorly characterized and in the absence of a myosin II homolog, it is unclear how the force required of a cytokinetic contractile ring is generated. Here, we review the literature on the basal complex in Apicomplexans, summarizing what is known about its formation and function, and attempt to provide possible answers to this question and suggest new avenues of study by comparing the Apicomplexan basal complex to well-studied, established cytokinetic contractile rings and their mechanisms in organisms such as S. cerevisiae and D. melanogaster. We also compare the basal complex to structures formed during mitochondrial and plastid division and cytokinetic mechanisms of organisms beyond the Opisthokonts, considering Apicomplexan diversity and divergence.

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 characterization of a new 34 kDa MORN motif-containing sporozoite surface-exposed protein, Cp-P34, unique to Cryptosporidium

Despite the public health impact of childhood diarrhea caused by Cryptosporidium, effective drugs and vaccines against this parasite are unavailable. Efforts to identify vaccine targets have focused on critical externally exposed virulence factors expressed in the parasite s invasive stages. However, no single surface antigen has yet been found that can elicit a significant protective immune response and it is likely that pooling multiple immune targets will be necessary. Discovery of surface proteins on Cryptosporidium sporozoites is therefore vital to this effort to develop a multi-antigenic vaccine. In this study we applied a novel single-domain camelid antibody (VHH) selection method to identify immunogenic proteins expressed on the surface of Cryptosporidium parvum sporozoites. By this approach, VHHs were identified that recognize two sporozoite surface-exposed antigens, the previously identified gp900 and an unrecognized immunogenic protein, Cp-P34. This Cp-P34 antigen, which contains multiple Membrane Occupation and Recognition Nexus (MORN) repeats, is found in excysted sporozoites as well as in the parasite s intracellular stages. Cp-P34 appears to accumulate inside the parasite and transiently appears on the surface of sporozoites to be shed in trails. Identical or nearly identical orthologs of Cp-P34 are found in the Cryptosporidium hominis and Cryptosporidium tyzzeri genomes. Except for the conserved MORN motifs, the Cp-P34 gene shares no significant homology with genes of other protozoans and thus appears to be unique to Cryptosporidium spp. Cp-P34 elicits immune responses in naturally exposed alpacas and warrants further investigation as a potential vaccine candidate.

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

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

TgAP2IX-5 is a key transcriptional regulator of the asexual cell cycle division in Toxoplasma gondii

Apicomplexan parasites have evolved efficient and distinctive strategies for intracellular replication where the timing of emergence of the daughter cells (budding) is a decisive element. However, the molecular mechanisms that provide the proper timing of parasite budding remain unknown. Using Toxoplasma gondii as a model Apicomplexan, we identified a master regulator that controls the timing of the budding process. We show that an ApiAP2 transcription factor, TgAP2IX-5, controls cell cycle events downstream of centrosome duplication. TgAP2IX-5 binds to the promoter of hundreds of genes and controls the activation of the budding-specific cell cycle expression program. TgAP2IX-5 regulates the expression of specific transcription factors that are necessary for the completion of the budding cycle. Moreover, TgAP2IX-5 acts as a limiting factor that ensures that asexual proliferation continues by promoting the inhibition of the differentiation pathway. Therefore, TgAP2IX-5 is a master regulator that controls both cell cycle and developmental pathways.

The control of the proper timing of emergence of apicomplexan parasite daughter cells during replication is crucial for their proliferation. Here, Khelifa et al. identify a key transcriptional regulator in the model Apicomplexa Toxoplasma gondii, which regulates the expression of transcription factors necessary for completion of the budding cycle.

Biogenesis and maintenance of the apicoplast in model apicomplexan parasites

The apicoplast is a non-photosynthetic relict plastid of Apicomplexa that evolved from a secondary symbiotic system. During its evolution, most of the genes derived from its alga ancestor were lost. Only genes involved in several valuable metabolic pathways, such as the synthesis of isoprenoid precursors, heme, and fatty acids, have been transferred to the host genome and retained to help these parasites adapt to a complex life cycle and various living environments. The biological function of an apicoplast is essential for most apicomplexan parasites. Considering their potential as drug targets, the metabolic functions of this symbiotic organelle have been intensively investigated through computational and biological means. Moreover, we know that not only organellar metabolic functions are linked with other organelles, but also their biogenesis processes have developed and evolved to tailor their biological functions and proper inheritance. Several distinct features have been found in the biogenesis process of apicoplasts. For example, the apicoplast borrows a dynamin-related protein (DrpA) from its host to implement organelle division. The autophagy system has also been repurposed for linking the apicoplast and centrosome during replication and the division process. However, many vital questions remain to be answered about how these parasites maintain and properly inherit this symbiotic organelle. Here we review our current knowledge about its biogenesis process and discuss several critical questions remaining to be answered in this field.

Structures of three MORN repeat proteins and a re-evaluation of the proposed lipid-binding properties of MORN repeats

MORN (Membrane Occupation and Recognition Nexus) repeat proteins have a wide taxonomic distribution, being found in both prokaryotes and eukaryotes. Despite this ubiquity, they remain poorly characterised at both a structural and a functional level compared to other common repeats. In functional terms, they are often assumed to be lipid-binding modules that mediate membrane targeting. We addressed this putative activity by focusing on a protein composed solely of MORN repeats-Trypanosoma brucei MORN1. Surprisingly, no evidence for binding to membranes or lipid vesicles by TbMORN1 could be obtained either in vivo or in vitro. Conversely, TbMORN1 did interact with individual phospholipids. High- and low-resolution structures of the MORN1 protein from Trypanosoma brucei and homologous proteins from the parasites Toxoplasma gondii and Plasmodium falciparum were obtained using a combination of macromolecular crystallography, small-angle X-ray scattering, and electron microscopy. This enabled a first structure-based definition of the MORN repeat itself. Furthermore, all three structures dimerised via their C-termini in an antiparallel configuration. The dimers could form extended or V-shaped quaternary structures depending on the presence of specific interface residues. This work provides a new perspective on MORN repeats, showing that they are protein-protein interaction modules capable of mediating both dimerisation and oligomerisation.

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.

Who Needs a Contractile Actomyosin Ring? The Plethora of Alternative Ways to Divide a Protozoan Parasite

Cytokinesis, or the division of the cytoplasm, following the end of mitosis or meiosis, is accomplished in animal cells, fungi, and amoebae, by the constriction of an actomyosin contractile ring, comprising filamentous actin, myosin II, and associated proteins. However, despite this being the best-studied mode of cytokinesis, it is restricted to the Opisthokonta and Amoebozoa, since members of other evolutionary supergroups lack myosin II and must, therefore, employ different mechanisms. In particular, parasitic protozoa, many of which cause significant morbidity and mortality in humans and animals as well as considerable economic losses, employ a wide diversity of mechanisms to divide, few, if any, of which involve myosin II. In some cases, cell division is not only myosin II-independent, but actin-independent too. Mechanisms employed range from primitive mechanical cell rupture (cytofission), to motility- and/or microtubule remodeling-dependent mechanisms, to budding involving the constriction of divergent contractile rings, to hijacking host cell division machinery, with some species able to utilize multiple mechanisms. Here, I review current knowledge of cytokinesis mechanisms and their molecular control in mammalian-infective parasitic protozoa from the Excavata, Alveolata, and Amoebozoa supergroups, highlighting their often-underappreciated diversity and complexity. Billions of people and animals across the world are at risk from these pathogens, for which vaccines and/or optimal treatments are often not available. Exploiting the divergent cell division machinery in these parasites may provide new avenues for the treatment of protozoal disease.

TgZFP2 is a novel zinc finger protein involved in coordinating mitosis and budding in Toxoplasma

Zinc finger proteins (ZFPs) are one of the most abundant groups of proteins with a wide range of molecular functions. We have characterised a Toxoplasma protein that we named TgZFP2, as it bears a zinc finger domain conserved in eukaryotes. However, this protein has little homology outside this region and contains no other conserved domain that could hint for a particular function. We thus investigated TgZFP2 function by generating a conditional mutant. We showed that depletion of TgZFP2 leads to a drastic arrest in the parasite cell cycle, and complementation assays demonstrated the zinc finger domain is essential for TgZFP2 function. More precisely, whereas replication of the nuclear material is initially essentially unaltered, daughter cell budding is seriously impaired: to a large extent newly formed buds fail to incorporate nuclear material. TgZFP2 is found at the basal complex in extracellular parasites and after invasion, but as the parasites progress into cell division, it relocalises to cytoplasmic punctate structures and, strikingly, accumulates in the pericentrosomal area at the onset of daughter cell elongation. Centrosomes have emerged as major coordinators of the budding and nuclear cycles in Toxoplasma, and our study identifies a novel and important component of this machinery.

The apical annuli of Toxoplasma gondii are composed of coiled-coil and signalling proteins embedded in the inner membrane complex sutures

The apical annuli are among the most intriguing and understudied structures in the cytoskeleton of the apicomplexan parasite Toxoplasma gondii. We mapped the proteome of the annuli in Toxoplasma by reciprocal proximity biotinylation (BioID), and validated five apical annuli proteins (AAP1-5), Centrin2, and an apical annuli methyltransferase. Moreover, inner membrane complex (IMC) suture proteins connecting the alveolar vesicles were also detected and support annuli residence within the sutures. Super-resolution microscopy identified a concentric organisation comprising four rings with diameters ranging from 200 to 400 nm. The high prevalence of domain signatures shared with centrosomal proteins in the AAPs together with Centrin2 suggests that the annuli are related and/or derived from the centrosomes. Phylogenetic analysis revealed that the AAPs are conserved narrowly in coccidian, apicomplexan parasites that multiply by an internal budding mechanism. This suggests a role in replication, for example, to provide pores in the mother IMC permitting exchange of building blocks and waste products. However, presence of multiple signalling domains and proteins are suggestive of additional functions. Knockout of AAP4, the most conserved compound forming the largest ring-like structure, modestly decreased parasite fitness in vitro but had no significant impact on acute virulence in vivo. In conclusion, the apical annuli are composed of coiled-coil and signalling proteins assembled in a pore-like structure crossing the IMC barrier maintained during internal budding.

An essential contractile ring protein controls cell division in Plasmodium falciparum

During the blood stage of human malaria, Plasmodium falciparum parasites divide by schizogony-a process wherein components for several daughter cells are produced within a common cytoplasm and then segmentation, a synchronized cytokinesis, produces individual invasive daughters. The basal complex is hypothesized to be required for segmentation, acting as a contractile ring to establish daughter cell boundaries. Here we identify an essential component of the basal complex which we name PfCINCH. Using three-dimensional reconstructions of parasites at electron microscopy resolution, we show that while parasite organelles form and divide normally, PfCINCH-deficient parasites develop inviable conjoined daughters that contain components for multiple cells. Through biochemical evaluation of the PfCINCH-containing complex, we discover multiple previously undescribed basal complex proteins. Therefore, this work provides genetic evidence that the basal complex is required for precise segmentation and lays the groundwork for a mechanistic understanding of how the parasite contractile ring drives cell division.

A plasma membrane localized protein phosphatase in Toxoplasma gondii, PPM5C, regulates attachment to host cells

The propagation of Toxoplasma gondii is accomplished by repeated lytic cycles of parasite attachment to a host cell, invasion, replication within a parasitophorous vacuole, and egress from the cell. This lytic cycle is delicately regulated by calcium-dependent reversible phosphorylation of the molecular machinery that drives invasion and egress. While much progress has been made elucidating the protein kinases and substrates central to parasite propagation, little is known about the relevant protein phosphatases. In this study, we focused on the five protein phosphatases that are predicted to be membrane-associated either integrally or peripherally. We have determined that of these only PPM5C, a PP2C family member, localizes to the plasma membrane of Toxoplasma. Disruption of PPM5C results in a slow propagation phenotype in tissue culture. Interestingly, parasites lacking PPM5C divide and undergo egress at a normal rate, but have a deficiency in attaching to host cells. Both membrane localization and phosphatase activity are required for PPM5C’s role in attachment. Phosphoproteomic analysis show relatively few phosphorylation sites being affected by PPM5C deletion in extracellular parasites of which several are found on proteins involved in signaling cascades. This implies that PPM5C is part of a wider regulatory network important for attachment to host cells.

ARC3 Activation by PARC6 Promotes FtsZ-Ring Remodeling at the Chloroplast Division Site

Chloroplast division is initiated by assembly of the stromal Z ring, composed of cytoskeletal Filamenting temperature-sensitive Z (FtsZ) proteins. Midplastid Z-ring positioning is governed by the chloroplast Min (Minicell) system, which inhibits Z-ring assembly everywhere except the division site. The central Min-system player is the FtsZ-assembly inhibitor ACCUMULATION AND REPLICATION OF CHLOROPLASTS3 (ARC3). Here, we report Arabidopsis (Arabidopsis thaliana) chloroplasts contain two pools of ARC3: one distributed throughout the stroma, which presumably fully inhibits Z-ring assembly at nondivision sites, and the other localized to a midplastid ring-like structure. We show that ARC3 is recruited to the middle of the plastid by the inner envelope membrane protein PARALOG OF ARC6 (PARC6). ARC3 bears a C-terminal Membrane Occupation and Recognition Nexus (MORN) domain; previous yeast two-hybrid experiments with full-length and MORN-truncated ARC3 showed the MORN domain mediates ARC3-PARC6 interaction but prevents ARC3-FtsZ interaction. Using yeast three-hybrid experiments, we demonstrate that the MORN-dependent ARC3-PARC6 interaction enables full-length ARC3 to bind FtsZ. The resulting PARC6/ARC3/FtsZ complex enhances the dynamics of Z rings reconstituted in a heterologous system. Our findings lead to a model whereby activation of midplastid-localized ARC3 by PARC6 facilitates Z-ring remodeling during chloroplast division by promoting Z-ring dynamics and reveal a novel function for MORN domains in regulating protein-protein interactions.

A unique dynamin-related protein is essential for mitochondrial fission in Toxoplasma gondii

The single mitochondrion of apicomplexan protozoa is thought to be critical for all stages of the life cycle, and is a validated drug target against these important human and veterinary parasites. In contrast to other eukaryotes, replication of the mitochondrion is tightly linked to the cell cycle. A key step in mitochondrial segregation is the fission event, which in many eukaryotes occurs by the action of dynamins constricting the outer membrane of the mitochondria from the cytosolic face. To date, none of the components of the apicomplexan fission machinery have been identified and validated. We identify here a highly divergent, dynamin-related protein (TgDrpC), conserved in apicomplexans as essential for mitochondrial biogenesis and potentially for fission in Toxoplasma gondii. We show that TgDrpC is found adjacent to the mitochondrion, and is localised both at its periphery and at its basal part, where fission is expected to occur. We demonstrate that depletion or dominant negative expression of TgDrpC results in interconnected mitochondria and ultimately in drastic changes in mitochondrial morphology, as well as in parasite death. Intriguingly, we find that the canonical adaptor TgFis1 is not required for mitochondrial fission. The identification of an Apicomplexa-specific enzyme required for mitochondrial biogenesis and essential for parasite growth highlights parasite adaptation. This work paves the way for future drug development targeting TgDrpC, and for the analysis of additional partners involved in this crucial step of apicomplexan multiplication.

Red Blood Cell Invasion by the Malaria Parasite Is Coordinated by the PfAP2-I Transcription Factor

Obligate intracellular parasites must efficiently invade host cells in order to mature and be transmitted. For the malaria parasite Plasmodium falciparum, invasion of host red blood cells (RBCs) is essential. Here we describe a parasite-specific transcription factor PfAP2-I, belonging to the Apicomplexan AP2 (ApiAP2) family, that is responsible for regulating the expression of genes involved in RBC invasion. Our genome-wide analysis by ChIP-seq shows that PfAP2-I interacts with a specific DNA motif in the promoters of target genes. Although PfAP2-I contains three AP2 DNA-binding domains, only one is required for binding of the target genes during blood stage development. Furthermore, we find that PfAP2-I associates with several chromatin-associated proteins, including the Plasmodium bromodomain protein PfBDP1 and that complex formation is associated with transcriptional regulation. As a key regulator of red blood cell invasion, PfAP2-I represents a potential new antimalarial therapeutic target.

Dual role of the Toxoplasma gondii clathrin adaptor AP1 in the sorting of rhoptry and microneme proteins and in parasite division

Toxoplasma gondii possesses a highly polarized secretory system, which efficiently assembles de novo micronemes and rhoptries during parasite replication. These apical secretory organelles release their contents into host cells promoting parasite invasion and survival. Using a CreLox-based inducible knock-out strategy and the ddFKBP over-expression system, we unraveled novel functions of the clathrin adaptor complex TgAP1. First, our data indicate that AP1 in T. gondii likely functions as a conserved heterotetrameric complex composed of the four subunits γ, β, μ1, σ1 and interacts with known regulators of clathrin-mediated vesicular budding such as the unique ENTH-domain containing protein, which we named Epsin-like protein (TgEpsL). Disruption of the μ1 subunit resulted in the mis-sorting of microneme proteins at the level of the Trans-Golgi-Network (TGN). Furthermore, we demonstrated that TgAP1 regulates rhoptry biogenesis by activating rhoptry protein exit from the TGN, but also participates in the post-Golgi maturation process of preROP compartments into apically anchored club-shaped mature organelles. For this latter activity, our data indicate a specific functional relationship between TgAP1 and the Rab5A-positive endosome-like compartment. In addition, we unraveled an original role for TgAP1 in the regulation of parasite division. APμ1-depleted parasites undergo normal daughter cell budding and basal complex assembly but fail to segregate at the end of cytokinesis.

The phylum Apicomplexa comprises a large group of obligate intracellular parasites of wide human and agricultural significance. Most notable are Plasmodium, the causative agent of malaria, and Toxoplasma gondii, one of the most common human parasites, responsible for disease of the developing fetus and immune-compromised individuals. Apicomplexa are characterized by the presence of an apical complex consisting of secretory organelles named micronemes (MIC) and rhoptries (ROP). MIC and ROP proteins, released upon host cell recognition, are essential for host cell invasion and parasite survival. After invasion, these organelles are neo-synthesized at each parasite replication cycle. In our study, we demonstrate a crucial role for the T. gondii clathrin adaptor complex AP1 in the vesicular transport of neo-synthesized MIC and ROP proteins, thereby regulating mature apical organelle formation. In addition, we unravel an original role for TgAP1 in the late stages of the parasite division process during daughter cell segregation. Therefore, our study provides new insights into key regulatory mechanisms of the vesicular trafficking system essential for host invasion and intracellular survival of Toxoplasma gondii.

Expression of a MORN repeat protein from Euplotes octocarinatus requires a +1 programmed ribosomal frameshifting

Analysis of transcriptome revealed that a membrane occupation and recognition nexus (MORN) repeat protein-encoding gene of Euplotes octocarinatus (Eo-morn-9-31) was a candidate for programmed +1 ribosomal frameshifting (+1 PRF). In this study, a dual-luciferase assay was performed to detect its expression. The result showed that the MORN repeat protein (Eo-MORN-9-31) could be produced by the +1 PRF event during the process of translation in yeast and the frameshifting efficiency was about 4-5%. We further confirmed its reality by western blot and mass spectrometry. This study provided experimental evidence indicating that the expression of the Eo-MORN-9-31 of E. octocarinatus required the +1 PRF.

MORN5 Expression during Craniofacial Development and Its Interaction with the BMP and TGFβ Pathways

MORN5 (MORN repeat containing 5) is encoded by a locus positioned on chromosome 17 in the chicken genome. The MORN motif is found in multiple copies in several proteins including junctophilins or phosphatidylinositol phosphate kinase family and the MORN proteins themselves are found across the animal and plant kingdoms. MORN5 protein has a characteristic punctate pattern in the cytoplasm in immunofluorescence imaging. Previously, MORN5 was found among differentially expressed genes in a microarray profiling experiment of the chicken embryo head. Here, we provided in situ hybridization to analyse, in detail, the MORN5 expression in chick craniofacial structures. The expression of MORN5 was first observed at stage HH17-18 (E2.5). MORN5 expression gradually appeared on either side of the primitive oral cavity, within the maxillary region. At stage HH20 (E3), prominent expression was localized in the mandibular prominences lateral to the midline. From stage HH20 up to HH29 (E6), there was strong expression in restricted regions of the maxillary and mandibular prominences. The frontonasal mass (in the midline of the face) expressed MORN5, starting at HH27 (E5). The expression was concentrated in the corners or globular processes, which will ultimately fuse with the cranial edges of the maxillary prominences. MORN5 expression was maintained in the fusion zone up to stage HH29. In sections MORN5 expression was localized preferentially in the mesenchyme. Previously, we examined signals that regulate MORN5 expression in the face based on a previous microarray study. Here, we validated the array results with in situ hybridization and QPCR. MORN5 was downregulated 24 h after Noggin and/or RA treatment. We also determined that BMP pathway genes are downstream of MORN5 following siRNA knockdown. Based on these results, we conclude that MORN5 is both regulated by and required for BMP signaling. The restricted expression of MORN5 in the lip fusion zone shown here supports the human genetic data in which MORN5 variants were associated with increased risk of non-syndromic cleft lip with or without cleft palate.

An evolutionarily conserved SSNA1/DIP13 homologue is a component of both basal and apical complexes of Toxoplasma gondii

Microtubule-based cytoskeletal structures have fundamental roles in several essential eukaryotic processes, including transport of intracellular constituents as well as ciliary and flagellar mobility. Temporal and spatial organisation of microtubules is determined by microtubule organising centers and a number of appendages and accessory proteins. Members of the SSNA1/DIP13 family are coiled coil proteins that are known to localise to microtubular structures like centrosomes and flagella, but are otherwise poorly characterised. We have identified a homologue of SSNA1/DIP13 in the parasitic protist Toxoplasma gondii and found it localises to parasite-specific cytoskeletal structures: the conoid in the apical complex of mature and dividing cells, and the basal complex in elongating daughter cells during cell division. This protein is dispensable for parasite growth in vitro. However, quite remarkably, this coiled coil protein is able to self-associate into higher order structures both in vitro and in vivo, and its overexpression is impairing parasite division.

Characterization of membrane occupation and recognition nexus repeat containing 3, meiosis expressed gene 1 binding partner, in mouse male germ cells

Mammalian spermatogenesis is a well-organized process of cell development and differentiation. Meiosis expressed gene 1 (MEIG1) plays an essential role in the regulation of spermiogenesis. To explore potential mechanisms of MEIG1's action, a yeast two-hybrid screen was conducted, and several potential binding partners were identified; one of them was membrane occupation and recognition nexus repeat containing 3 (MORN3). MORN3 mRNA is only abundant in mouse testis. In the testis, Morn3 mRNA is highly expressed in the spermiogenesis stage. Specific anti-MORN3 polyclonal antibody was generated against N-terminus of the full-length MORN3 protein, and MORN3 expression and localization was examined in vitro and in vivo. In transfected Chinese hamster ovary cells, the antibody specifically crossed-reacted the full-length MORN3 protein, and immunofluorescence staining revealed that MORN3 was localized throughout the cytoplasm. Among multiple mouse tissues, about 25 kDa protein, was identified only in the testis. The protein was highly expressed after day 20 of birth. Immunofluorescence staining on mixed testicular cells isolated from adult wild-type mice demonstrated that MORN3 was expressed in the acrosome in germ cells throughout spermiogenesis. The protein was also present in the manchette of elongating spermatids. The total MORN3 expression and acrosome localization were not changed in the Meig 1-deficient mice. However, its expression in manchette was dramatically reduced in the mutant mice. Our studies suggest that MORN3 is another regulator for spermatogenesis, probably together with MEIG1.

An essential malaria protein defines the architecture of blood-stage and transmission-stage parasites

Blood-stage replication of the human malaria parasite Plasmodium falciparum occurs via schizogony, wherein daughter parasites are formed by a specialized cytokinesis known as segmentation. Here we identify a parasite protein, which we name P. falciparum Merozoite Organizing Protein (PfMOP), as essential for cytokinesis of blood-stage parasites. We show that, following PfMOP knockdown, parasites undergo incomplete segmentation resulting in a residual agglomerate of partially divided cells. While organelles develop normally, the structural scaffold of daughter parasites, the inner membrane complex (IMC), fails to form in this agglomerate causing flawed segmentation. In PfMOP-deficient gametocytes, the IMC formation defect causes maturation arrest with aberrant morphology and death. Our results provide insight into the mechanisms of replication and maturation of malaria parasites.

Blood-stage malaria parasites replicate through a specialised type of cell division known as schizogony. Here, Absalon et al. identify a parasite protein that is essential during schizogony for cytokinesis and formation of the inner membrane complex, the structural scaffold of daughter parasites.

A MORN1-associated HAD phosphatase in the basal complex is essential for Toxoplasma gondii daughter budding

Apicomplexan parasites replicate by several budding mechanisms with two well-characterized examples being Toxoplasma endodyogeny and Plasmodium schizogony. Completion of budding requires the tapering of the nascent daughter buds toward the basal end, driven by contraction of the basal complex. This contraction is not executed by any of the known cell division associated contractile mechanisms and in order to reveal new components of the unusual basal complex we performed a yeast two-hybrid screen with its major scaffolding protein, TgMORN1. Here we report on a conserved protein with a haloacid dehalogenase (HAD) phosphatase domain, hereafter named HAD2a, identified by yeast two-hybrid. HAD2a has demonstrated enzyme-activity in vitro, localizes to the nascent daughter buds, and co-localizes with MORN1 to the basal complex during its contraction. Conditional knockout of HAD2a in Toxoplasma interferes with basal complex assembly, which leads to incomplete cytokinesis and conjoined daughters that ultimately results in disrupted proliferation. In Plasmodium, we further confirmed localization of the HAD2a ortholog to the basal complex toward the end of schizogony. In conclusion, our work highlights an essential role for this HAD phosphatase across apicomplexan budding and suggests a regulatory mechanism of differential phosphorylation on the structure and/or contractile function of the basal complex.

The FIKK kinase of Toxoplasma gondii is not essential for the parasite's lytic cycle

FIKK kinases are a novel family of kinases unique to the Apicomplexa. While most apicomplexans encode a single FIKK kinase, Plasmodium falciparum expresses 21 and piroplasms do not encode a FIKK kinase. FIKK kinases share a conserved C-terminal catalytic domain, but the N-terminal region is highly variable and contains no known functional domains. To date, FIKK kinases have been primarily studied in P. falciparum and Plasmodium berghei. Those that have been studied are exported from the parasite and associate with diverse locations in the infected erythrocyte cytosol or membrane. Deletion of individual P. falciparum FIKK kinases indicates that they may play a role in modification of the infected erythrocyte. The current study characterises the single FIKK gene in Toxoplasma gondii to evaluate the importance of the FIKK kinase in an apicomplexan that has a single FIKK kinase. The TgFIKK gene encoded a protein of approximately 280kDa. Endogenous tagging of the FIKK protein with Yellow Fluorescent Protein showed that the FIKK protein exclusively localised to the posterior end of tachyzoites. A Yellow Fluorescent Protein-tagged FIKK and a Ty-tagged FIKK both co-localised with T. gondii membrane occupation and recognition nexus protein to the basal complex and were localised apical to inner membrane complex protein-5 and Centrin2. Deletion of TgFIKK, surprisingly, had no detectable effect on the parasite's lytic cycle in vitro in human fibroblast cells or in acute virulence in vivo. Thus, our results clearly show that while the FIKK kinase is expressed in tachyzoites, it is not essential for the lytic cycle of T. gondii.

Toxoplasma gondii Arginine Methyltransferase 1 (PRMT1) Is Necessary for Centrosome Dynamics during Tachyzoite Cell Division

The arginine methyltransferase family (PRMT) has been implicated in a variety of cellular processes, including signal transduction, epigenetic regulation, and DNA repair pathways. PRMT1 is thought to be responsible for the majority of PRMT activity in Toxoplasma gondii, but its exact function is unknown. To further define the biological function of the PRMT family, we generated T. gondii mutants lacking PRMT1 (Δprmt1) by deletion of the PRMT1 gene. Δprmt1 parasites exhibit morphological defects during cell division and grow slowly, and this phenotype reverses in the Δprmt::PRMT1mRFP complemented strain. Tagged PRMT1 localizes primarily in the cytoplasm with enrichment at the pericentriolar material, and the strain lacking PRMT1 is unable to segregate progeny accurately. Unlike wild-type and complemented parasites, Δprmt1 parasites have abnormal daughter buds, perturbed centrosome stoichiometry, and loss of synchronous replication. Whole-genome expression profiling demonstrated differences in expression of cell-cycle-regulated genes in the Δprmt1 strain relative to the complemented Δprmt1::PRMT1mRFP and parental wild-type strains, but these changes do not correlate with a specific block in cell cycle. Although PRMT1’s primary biological function was previously proposed to be methylation of histones, our studies suggest that PRMT1 plays an important role within the centrosome to ensure the proper replication of the parasite.

Apicomplexan parasites include several important pathogens, including Toxoplasma gondii, a major cause of opportunistic infections and congenital birth defects. These parasites divide using a unique form of cell division called endodyogeny that is different from those of most eukaryotes. PRMT1 is a conserved arginine methyltransferase that was thought to regulate gene expression of T. gondii by modifying histone methylation. Using genetic techniques, we show that disruption of PRMT1 affects the parasite’s ability to perform accurate cell division. Our studies reveal an unexpected role for arginine methylation in centrosome biology and regulation of parasite replication.

Pellicle formation in the malaria parasite

The intraerythrocytic developmental cycle of Plasmodium falciparum is completed with the release of up to 32 invasive daughter cells, the merozoites, into the blood stream. Before release, the final step of merozoite development is the assembly of the cortical pellicle, a multi-layered membrane structure. This unique apicomplexan feature includes the inner membrane complex (IMC) and the parasite's plasma membrane. A dynamic ring structure, referred to as the basal complex, is part of the IMC and helps to divide organelles and abscises in the maturing daughter cells. Here, we analyze the dynamics of the basal complex of P. falciparum. We report on a novel transmembrane protein of the basal complex termed BTP1, which is specific to the genus Plasmodium. It colocalizes with the known basal complex marker protein MORN1 and shows distinct dynamics as well as localization when compared to other IMC proteins during schizogony. Using a parasite plasma membrane marker cell line, we correlate dynamics of the basal complex with the acquisition of the maternal membrane. We show that plasma membrane invagination and IMC propagation are interlinked during the final steps of cell division.

Form, Fabric, and Function of a Flagellum-Associated Cytoskeletal Structure

Trypanosoma brucei is a uniflagellated protist and the causative agent of African trypanosomiasis, a neglected tropical disease. The single flagellum of T. brucei is essential to a number of cellular processes such as motility, and has been a longstanding focus of scientific enquiry. A number of cytoskeletal structures are associated with the flagellum in T. brucei, and one such structure—a multiprotein complex containing the repeat motif protein TbMORN1—is the focus of this review. The TbMORN1-containing complex, which was discovered less than ten years ago, is essential for the viability of the mammalian-infective form of T. brucei. The complex has an unusual asymmetric morphology, and is coiled around the flagellum to form a hook shape. Proteomic analysis using the proximity-dependent biotin identification (BioID) technique has elucidated a number of its components. Recent work has uncovered a role for TbMORN1 in facilitating protein entry into the cell, thus providing a link between the cytoskeleton and the endomembrane system. This review summarises the extant data on the complex, highlights the outstanding questions for future enquiry, and provides speculation as to its possible role in a size-exclusion mechanism for regulating protein entry. The review additionally clarifies the nomenclature associated with this topic, and proposes the adoption of the term “hook complex” to replace the former name “bilobe” to describe the complex.

Lytic Cycle of Toxoplasma gondii: 15 Years Later

Toxoplasmosis is the clinical and pathological consequence of acute infection with the obligate intracellular apicomplexan parasite Toxoplasma gondii. Symptoms result from tissue destruction that accompanies lytic parasite growth. This review updates current understanding of the host cell invasion, parasite replication, and eventual egress that constitute the lytic cycle, as well as the ways T. gondii manipulates host cells to ensure its survival. Since the publication of a previous iteration of this review 15 years ago, important advances have been made in our molecular understanding of parasite growth and mechanisms of host cell egress, and knowledge of the parasite's manipulation of the host has rapidly progressed. Here we cover molecular advances and current conceptual frameworks that include each of these topics, with an eye to what may be known 15 years from now.

Ciprofloxacin Derivatives Affect Parasite Cell Division and Increase the Survival of Mice Infected with Toxoplasma gondii

Toxoplasmosis, caused by the protozoan Toxoplasma gondii, is a worldwide disease whose clinical manifestations include encephalitis and congenital malformations in newborns. Previously, we described the synthesis of new ethyl-ester derivatives of the antibiotic ciprofloxacin with ~40-fold increased activity against T. gondii in vitro, compared with the original compound. Cipro derivatives are expected to target the parasite's DNA gyrase complex in the apicoplast. The activity of these compounds in vivo, as well as their mode of action, remained thus far uncharacterized. Here, we examined the activity of the Cipro derivatives in vivo, in a model of acute murine toxoplasmosis. In addition, we investigated the cellular effects T. gondii tachyzoites in vitro, by immunofluorescence and transmission electron microscopy (TEM). When compared with Cipro treatment, 7-day treatments with Cipro derivatives increased mouse survival significantly, with 13-25% of mice surviving for up to 60 days post-infection (vs. complete lethality 10 days post-infection, with Cipro treatment). Light microscopy examination early (6 and 24h) post-infection revealed that 6-h treatments with Cipro derivatives inhibited the initial event of parasite cell division inside host cells, in an irreversible manner. By TEM and immunofluorescence, the main cellular effects observed after treatment with Cipro derivatives and Cipro were cell scission inhibition--with the appearance of 'tethered' parasites--malformation of the inner membrane complex, and apicoplast enlargement and missegregation. Interestingly, tethered daughter cells resulting from Cipro derivatives, and also Cipro, treatment did not show MORN1 cap or centrocone localization. The biological activity of Cipro derivatives against C. parvum, an apicomplexan species that lacks the apicoplast, is, approximately, 50 fold lower than that in T. gondii tachyzoites, supporting that these compounds targets the apicoplast. Our results show that Cipro derivatives improved the survival of mice acutely infected with T. gondii and inhibited parasite replication early in the first cycle of infection in vitro, highlighting their therapeutic potential for the treatment of toxoplasmosis.

Tetrahymena Expresses More than a Hundred Proteins with Lipid-binding MORN Motifs that can Differ in their Subcellular Localisations

Proteins with membrane occupation and recognition nexus (MORN) motifs are associated with cell fission in apicomplexan parasites, chloroplast division in Arabidopsis and the motility of sperm cells. We found that ciliates are among those that encode the largest variety of MORN proteins. Tetrahymena thermophila expresses 129 MORN protein-encoding genes, some of which are specifically up-regulated during conjugation. A lipid-binding assay underpins the assumption that the predominant function of MORN motifs themselves is to confer the ability of lipid binding. The localisation of four MORN candidate proteins with similar characteristics highlights the functional diversity of this group especially in ciliates.

A single mutation in the gatekeeper residue in TgMAPKL-1 restores the inhibitory effect of a bumped kinase inhibitor on the cell cycle

Toxoplasma gondii is the causative pathogen for Toxoplasmosis. Bumped kinase inhibitor 1NM-PP1 inhibits the growth of T. gondii by targeting TgCDPK1. However, we recently reported that resistance to 1NM-PP1 can be acquired via a mutation in T. gondii mitogen-activated protein kinase like 1 (TgMAPKL-1). Further characterization of how this TgMAPKL-1 mutation restores the inhibitory effect of 1NM-PP1 would shed further light on the function of TgMAPKL-1 in the parasite life cycle. Therefore, we made parasite clones with TgMAPKL-1 mutated at the gatekeeper residue Ser 191, which is critical for 1NM-PP1 susceptibility. Host cell lysis of RH/ku80(-)/HA-TgMAPKL-1(S191A) was completely inhibited at 250 nM 1NM-PP1, whereas that of RH/ku80(-)/HA-TgMAPKL-1(S191Y) was not. By comparing 1NM-PP1-sensitive (RH/ku80(-)/HA-TgMAPKL-1(S191A)) and -resistant (RH/ku80(-)/HA-TgMAPKL-1(S191Y)) clones, we observed that inhibition of TgMAPKL-1 blocked cell cycle progression after DNA duplication. Morphological analysis revealed that TgMAPKL-1 inhibition caused enlarged parasite cells with many daughter cell scaffolds and imcomplete cytokinesis. We conclude that the mutation in TgMAPKL-1 restored the cell cycle-arresting effect of 1NM-PP1 on T. gondii endodyogeny. Given that endodyogeny is the primary mechanism of cell division for both the tachyzoite and bradyzoite stages of this parasite, TgMAPKL-1 may be a promising target for drug development. Exploration of the signals that regulate TgMAPKL-1 will provide further insights into the unique mode of T. gondii cell division.

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

The glideosome is an actomyosin-based machinery that powers motility in Apicomplexa and participates in host cell invasion and egress from infected cells. The central component of the glideosome, myosin A (MyoA), is a motor recruited at the pellicle by the acylated gliding-associated protein GAP45. In Toxoplasma gondii, GAP45 also contributes to the cohesion of the pellicle, composed of the inner membrane complex (IMC) and the plasma membrane, during motor traction. GAP70 was previously identified as a paralog of GAP45 that is tailored to recruit MyoA at the apical cap in the coccidian subgroup of the Apicomplexa. A third member of this family, GAP80, is demonstrated here to assemble a new glideosome, which recruits the class XIV myosin C (MyoC) at the basal polar ring. MyoC shares the same myosin light chains as MyoA and also interacts with the integral IMC proteins GAP50 and GAP40. Moreover, a central component of this complex, the IMC-associated protein 1 (IAP1), acts as the key determinant for the restricted localization of MyoC to the posterior pole. Deletion of specific components of the MyoC-glideosome underscores the installation of compensatory mechanisms with components of the MyoA-glideosome. Conversely, removal of MyoA leads to the relocalization of MyoC along the pellicle and at the apical cap that accounts for residual invasion. The two glideosomes exhibit a considerable level of plasticity to ensure parasite survival.

Toxoplasma gondii can infect most warm-blooded animals, and is an important opportunistic pathogen for humans. This obligate intracellular parasite is able to invade virtually all nucleated cells, and as with most parasites of the Apicomplexa phylum, relies on a substrate-dependent gliding motility to actively penetrate into host cells and egress from infected cells. The conserved molecular machine (named glideosome) powering motility is located at the periphery of the parasite and involves the molecular motor, myosin A (MyoA). The glideosome exists in three flavors, exhibiting the same overall organization and sharing some common components while being spatially restricted to the central IMC, the apical cap and the basal pole of the parasite, respectively. The central and apical glideosomes are associated with MyoA (MyoA-glideosome) whereas the basal complex recruits myosin C (MyoC). Deleting components of the MyoC-glideosome uncovers the existence of complementary and compensatory mechanisms that ensure successful establishment of infection. This study highlights a higher degree of complexity and plasticity of the gliding machinery.