Polo-like kinase is necessary for flagellum inheritance in Trypanosoma brucei

Characterisation of TbSmee1 suggests endocytosis allows surface-bound cargo to enter the trypanosome flagellar pocket

All endocytosis and exocytosis in the African trypanosome Trypanosoma brucei occurs at a single subdomain of the plasma membrane. This subdomain, the flagellar pocket, is a small vase-shaped invagination containing the root of the single flagellum of the cell. Several cytoskeleton-associated multiprotein complexes are coiled around the neck of the flagellar pocket on its cytoplasmic face. One of these, the hook complex, was proposed to affect macromolecule entry into the flagellar pocket lumen. In previous work, knockdown of T. brucei (Tb)MORN1, a hook complex component, resulted in larger cargo being unable to enter the flagellar pocket. In this study, the hook complex component TbSmee1 was characterised in bloodstream form T. brucei and found to be essential for cell viability. TbSmee1 knockdown resulted in flagellar pocket enlargement and impaired access to the flagellar pocket membrane by surface-bound cargo, similar to depletion of TbMORN1. Unexpectedly, inhibition of endocytosis by knockdown of clathrin phenocopied TbSmee1 knockdown, suggesting that endocytic activity itself is a prerequisite for the entry of surface-bound cargo into the flagellar pocket.

FAZ assembly in bloodstream form Trypanosoma brucei requires kinesin KIN-E

Trypanosoma brucei, the causative agent of African sleeping sickness, uses its flagellum for movement, cell division, and signaling. The flagellum is anchored to the cell body membrane via the flagellum attachment zone (FAZ), a complex of proteins, filaments, and microtubules that spans two membranes with elements on both flagellum and cell body sides. How FAZ components are carried into place to form this complex is poorly understood. Here, we show that the trypanosome-specific kinesin KIN-E is required for building the FAZ in bloodstream-form parasites. KIN-E is localized along the flagellum with a concentration at its distal tip. Depletion of KIN-E by RNAi rapidly inhibits flagellum attachment and leads to cell death. A detailed analysis reveals that KIN-E depletion phenotypes include failure in cytokinesis completion, kinetoplast DNA missegregation, and transport vesicle accumulation. Together with previously published results in procyclic form parasites, these data suggest KIN-E plays a critical role in FAZ assembly in T. brucei.

Morphogenesis in Trypanosoma cruzi epimastigotes proceeds via a highly asymmetric cell division

Trypanosoma cruzi is a protist parasite that is the causative agent of Chagas' disease, a neglected tropical disease endemic to the Americas. T. cruzi cells are highly polarized and undergo morphological changes as they cycle within their insect and mammalian hosts. Work on related trypanosomatids has described cell division mechanisms in several life-cycle stages and identified a set of essential morphogenic proteins that serve as markers for key events during trypanosomatid division. Here, we use Cas9-based tagging of morphogenic genes, live-cell imaging, and expansion microscopy to study the cell division mechanism of the insect-resident epimastigote form of T. cruzi, which represents an understudied trypanosomatid morphotype. We find that T. cruzi epimastigote cell division is highly asymmetric, producing one daughter cell that is significantly smaller than the other. Daughter cell division rates differ by 4.9 h, which may be a consequence of this size disparity. Many of the morphogenic proteins identified in T. brucei have altered localization patterns in T. cruzi epimastigoes, which may reflect fundamental differences in the cell division mechanism of this life cycle stage, which widens and shortens the cell body to accommodate the duplicated organelles and cleavage furrow rather than elongating the cell body along the long axis of the cell, as is the case in life-cycle stages that have been studied in T. brucei. This work provides a foundation for further investigations of T. cruzi cell division and shows that subtle differences in trypansomatid cell morphology can alter how these parasites divide.

Polo-like kinase and Aurora B kinase phosphorylate and cooperate with the CIF1-CIF2 complex to promote cytokinesis initiation in Trypanosoma brucei

Cytokinesis in eukaryotes is regulated by a Polo-like kinase-mediated and Aurora B kinase-mediated signalling pathway that promotes the assembly of the actomyosin contractile ring, a cytokinesis machinery conserved across evolution from yeast to humans. Trypanosoma brucei, an early divergent parasitic protozoan, employs an actomyosin-independent mechanism for its unusual cytokinesis that is controlled by a regulatory pathway comprising the Polo-like kinase TbPLK, the Aurora B kinase TbAUK1 and multiple trypanosomatid-specific regulators. However, whether any of these trypanosomatid-specific regulators function as substrates of TbPLK and/or TbAUK1 and how they cooperate with TbPLK and TbAUK1 to promote cytokinesis remain unknown. Here, we demonstrate that TbPLK and TbAUK1 phosphorylate the cytokinesis regulators CIF1 and CIF2 on multiple sites within their intrinsically disordered regions. We further show that TbPLK localization depends on its interaction with CIF1 from S/G2 phases, that TbPLK maintains CIF1 and CIF2 localization from G2 phase until early mitosis, and that TbAUK1 maintains CIF1 and CIF2 localization from late mitosis. Finally, we demonstrate that the cytokinesis regulators CIF4 and FPRC are not substrates of TbPLK and TbAUK1, and that they function upstream of TbPLK and TbAUK1 in the cytokinesis regulatory pathway. Together, these results provide insights into the functional interplay and the order of actions between the two protein kinases and the trypanosomatid-specific cytokinesis regulators in T. brucei.

TFK1, a basal body transition fibre protein that is essential for cytokinesis in Trypanosoma brucei

In Trypanosoma brucei, transition fibres (TF) form a nine-bladed pattern-like structure connecting the base of the flagellum to the flagellar pocket membrane. Despite the characterization of two TF proteins, CEP164C and TbRP2, little is known about the organization of these fibres. Here, we report the identification and characterization of the first kinetoplastid-specific TF protein named TFK1 (Tb927.6.1180). Bioinformatics and functional domain analysis identified three TFK1 distinct domains: an N-terminal domain of an unpredicted function, a coiled-coil domain involved in TFK1-TFK1 interaction and a C-terminal intrinsically disordered region potentially involved in protein interaction. Cellular immuno-localization showed that TFK1 is a newly identified basal body maturation marker. Further, using ultrastructure expansion and immuno-electron microscopies we localized CEP164C and TbRP2 at the TF and TFK1 on the distal appendage matrix of the TF. Importantly, RNAi knockdown of TFK1 in bloodstream form cells induced misplacement of basal bodies, a defect in the furrow or fold generation and eventually cell death. We hypothesize that TFK1 is a basal body positioning specific actor and a key regulator of cytokinesis in the bloodstream form Trypanosoma brucei.

Structural Domains of CIF3 Required for Interaction with Cytokinesis Regulatory Proteins and for Cytokinesis Initiation in Trypanosoma brucei

Cytokinesis in Trypanosoma brucei occurs unidirectionally from the anterior toward the posterior through mechanisms distinct from those of its human host and is controlled by a signaling pathway comprising evolutionarily conserved and trypanosome-specific regulatory proteins. The mechanistic roles and the functional interplay of these cytokinesis regulators remain poorly understood. Here, we investigate the requirement of the structural motifs in the trypanosome-specific cytokinesis regulator CIF3 for the initiation of cytokinesis, the interaction with other cytokinesis regulators, and the recruitment of CIF3-interacting proteins to the cytokinesis initiation site. We demonstrate that the internal and C-terminal coiled-coil motifs, but not the N-terminal coiled-coil motif, of CIF3 play essential roles in cytokinesis and interact with distinct cytokinesis regulators. CIF3 interacts with TbPLK, CIF1, CIF4, and FPRC through the N-terminal and C-terminal coiled-coil motifs and with KAT80 through all three coiled-coil motifs. The C-terminal coiled-coil motif of CIF3 is required for the localization of CIF3 and all of its interacting proteins, and additionally, the internal coiled-coil motif of CIF3 is required for KAT80 localization. Conversely, all the CIF3-interacting proteins are required to maintain CIF3 at the cytokinesis initiation site at different cell cycle stages. These results demonstrate that CIF3 cooperates with multiple interacting partner proteins to promote cytokinesis in T. brucei.

IMPORTANCE Cytokinesis is the final stage of cell division and is regulated by a signaling pathway conserved from yeast to humans. Cytokinesis in Trypanosoma brucei, an early-branching protozoan parasite causing human sleeping sickness, is regulated by mechanisms that are distinct from those of its human host, employing a number of trypanosome-specific regulatory proteins to cooperate with evolutionarily conserved regulators. The functional interplay of these cytokinesis regulators is still poorly understood. In this work, we investigated the structural requirement of the trypanosome-specific cytokinesis regulator CIF3 for the initiation of cytokinesis, the interaction with other cytokinesis regulatory proteins, and the recruitment of CIF3-interacting proteins. We demonstrated that different structural motifs of CIF3 played distinct roles in cytokinesis, interacted with distinct cytokinesis regulatory proteins, and were required for the recruitment of distinct cytokinesis regulatory proteins. These findings provided novel insights into the cooperative roles of cytokinesis regulators in promoting cytokinesis in T. brucei.

The kinetoplastid-specific phosphatase KPP1 attenuates PLK activity to facilitate flagellum inheritance in Trypanosoma brucei

Trypanosoma brucei, an important human parasite, has a flagellum that controls cell motility, morphogenesis, proliferation, and cell-cell communication. Inheritance of the newly assembled flagellum during the cell cycle requires the Polo-like kinase homolog TbPLK and the kinetoplastid-specific protein phosphatase KPP1, although whether TbPLK acts on KPP1 or vice versa has been unclear. Here, we showed that dephosphorylation of TbPLK on Thr¹²⁵ by KPP1 maintained low TbPLK activity in the flagellum-associated hook complex structure, thereby ensuring proper flagellum positioning and attachment. This dephosphorylation event required the recognition of phosphorylated Thr¹⁹⁸ in the activation loop of TbPLK by the N-terminal Plus3 domain of KPP1 and the dephosphorylation of phosphorylated Thr¹²⁵ in TbPLK by the C-terminal catalytic domain of KPP1. Dephosphorylation of TbPLK by KPP1 prevented hyperphosphorylation of the hook complex protein TbCentrin2, thereby allowing timely dephosphorylation of phosphorylated TbCentrin2 for hook complex duplication and flagellum positioning and attachment. Thus, KPP1 attenuates TbPLK activity by dephosphorylating TbPLK to facilitate flagellum inheritance.

Polo-like kinase in trypanosomes: an odd member out of the Polo family

Polo-like kinases (Plks) are evolutionarily conserved serine/threonine protein kinases playing crucial roles during multiple stages of mitosis and cytokinesis in yeast and animals. Plks are characterized by a unique Polo-box domain, which plays regulatory roles in controlling Plk activation, interacting with substrates and targeting Plk to specific subcellular locations. Plk activity and protein abundance are subject to temporal and spatial control through transcription, phosphorylation and proteolysis. In the early branching protists, Plk orthologues are present in some taxa, such as kinetoplastids and Giardia, but are lost in apicomplexans, such as Plasmodium. Works from characterizing a Plk orthologue in Trypanosoma brucei, a kinetoplastid protozoan, discover its essential roles in regulating the inheritance of flagellum-associated cytoskeleton and the initiation of cytokinesis, but not any stage of mitosis. These studies reveal evolutionarily conserved and species-specific features in the control of Plk activation, substrate recognition and protein abundance, and suggest the divergence of Plk function and regulation for specialized needs in this flagellated unicellular eukaryote.

Trypanosomes have divergent kinesin-2 proteins that function differentially in flagellum biosynthesis and cell viability

Trypanosoma brucei, the causative agent of African sleeping sickness, has a flagellum that is crucial for motility, pathogenicity, and viability. In most eukaryotes, the intraflagellar transport (IFT) machinery drives flagellum biogenesis, and anterograde IFT requires kinesin-2 motor proteins. In this study, we investigated the function of the two T. brucei kinesin-2 proteins, TbKin2a and TbKin2b, in bloodstream form trypanosomes. We found that, compared to kinesin-2 proteins across other phyla, TbKin2a and TbKin2b show greater variation in neck, stalk and tail domain sequences. Both kinesins contributed additively to flagellar lengthening. Silencing TbKin2a inhibited cell proliferation, cytokinesis and motility, whereas silencing TbKin2b did not. TbKin2a was localized on the flagellum and colocalized with IFT components near the basal body, consistent with it performing a role in IFT. TbKin2a was also detected on the flagellar attachment zone, a specialized structure that connects the flagellum to the cell body. Our results indicate that kinesin-2 proteins in trypanosomes play conserved roles in flagellar biosynthesis and exhibit a specialized localization, emphasizing the evolutionary flexibility of motor protein function in an organism with a large complement of kinesins.

Basal Body Protein TbSAF1 Is Required for Microtubule Quartet Anchorage to the Basal Bodies in Trypanosoma brucei

Trypanosoma brucei contains a large array of single-copied organelles and structures. Through extensive interorganelle connections, these structures replicate and divide following a strict temporal and spatial order. A microtubule quartet (MtQ) originates from the basal bodies and extends toward the anterior end of the cell, stringing several cytoskeletal structures together along its path. In this study, we examined the interaction network of TbSpef1, the only protein specifically located to the MtQ. We identified an interaction between TbSpef1 and a basal body protein TbSAF1, which is required for MtQ anchorage to the basal bodies. This study thus provides the first molecular description of MtQ association with the basal bodies, since the discovery of this association ∼30 years ago. The results also reveal a general mechanism of the evolutionarily conserved Spef1/CLAMP, which achieves specific cellular functions via their conserved microtubule functions and their diverse molecular interaction networks.

Sperm flagellar protein 1 (Spef1, also known as CLAMP) is a microtubule-associated protein involved in various microtubule-related functions from ciliary motility to polarized cell movement and planar cell polarity. In Trypanosoma brucei, the causative agent of trypanosomiasis, a single Spef1 ortholog (TbSpef1) is associated with a microtubule quartet (MtQ), which is in close association with several single-copied organelles and is required for their coordinated biogenesis during the cell cycle. Here, we investigated the interaction network of TbSpef1 using BioID, a proximity-dependent protein-protein interaction screening method. Characterization of selected candidates provided a molecular description of TbSpef1-MtQ interactions with nearby cytoskeletal structures. Of particular interest, we identified a new basal body protein TbSAF1, which is required for TbSpef1-MtQ anchorage to the basal bodies. The results demonstrate that MtQ-basal body anchorage is critical for the spatial organization of cytoskeletal organelles, as well as the morphology of the membrane-bound flagellar pocket where endocytosis takes place in this parasite.

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.

Coordination of the Cell Cycle in Trypanosomes

Trypanosomes have complex life cycles within which there are both proliferative and differentiation cell divisions. The coordination of the cell cycle to achieve these different divisions is critical for the parasite to infect both host and vector. From studying the regulation of the proliferative cell cycle of the Trypanosoma brucei procyclic life cycle stage, three subcycles emerge that control the duplication and segregation of (a) the nucleus, (b) the kinetoplast, and (c) a set of cytoskeletal structures. We discuss how the clear dependency relationships within these subcycles, and the potential for cross talk between them, are likely required for overall cell cycle coordination. Finally, we look at the implications this interdependence has for proliferative and differentiation divisions through the T. brucei life cycle and in related parasitic trypanosomatid species.

BOH1 cooperates with Polo-like kinase to regulate flagellum inheritance and cytokinesis initiation in Trypanosoma brucei

Trypanosoma brucei possesses a motile flagellum that determines cell morphology and the cell division plane. Inheritance of the newly assembled flagellum during the cell cycle is controlled by the Polo-like kinase homolog TbPLK, which also regulates cytokinesis initiation. How TbPLK is targeted to its subcellular locations remains poorly understood. Here we report the trypanosome-specific protein BOH1 that cooperates with TbPLK to regulate flagellum inheritance and cytokinesis initiation in the procyclic form of T. brucei BOH1 localizes to an unusual sub-domain in the flagellum-associated hook complex, bridging the hook complex, the centrin arm and the flagellum attachment zone. Depletion of BOH1 disrupts hook-complex morphology, inhibits centrin-arm elongation and abolishes flagellum attachment zone assembly, leading to flagellum mis-positioning and detachment. Further, BOH1 deficiency impairs the localization of TbPLK and the cytokinesis regulator CIF1 to the cytokinesis initiation site, providing a molecular mechanism for its role in cytokinesis initiation. These findings reveal the roles of BOH1 in maintaining hook-complex morphology and regulating flagellum inheritance, and establish BOH1 as an upstream regulator of the TbPLK-mediated cytokinesis regulatory pathway.

Identification of TOEFAZ1-interacting proteins reveals key regulators of Trypanosoma brucei cytokinesis

The protist parasite Trypanosoma brucei is an obligate extracellular pathogen that retains its highly polarized morphology during cell division and has evolved a novel cytokinetic process independent of non-muscle myosin II. The polo-like kinase homolog TbPLK is essential for transmission of cell polarity during division and for cytokinesis. We previously identified a putative TbPLK substrate named Tip of the Extending FAZ 1 (TOEFAZ1) as an essential kinetoplastid-specific component of the T. brucei cytokinetic machinery. We performed a proximity-dependent biotinylation identification (BioID) screen using TOEFAZ1 as a means to identify additional proteins that are involved in cytokinesis. Using quantitative proteomic methods, we identified nearly 500 TOEFAZ1-proximal proteins and characterized 59 in further detail. Among the candidates, we identified an essential putative phosphatase that regulates the expression level and localization of both TOEFAZ1 and TbPLK, a previously uncharacterized protein that is necessary for the assembly of a new cell posterior, and a microtubule plus-end directed orphan kinesin that is required for completing cleavage furrow ingression. The identification of these proteins provides new insight into T. brucei cytokinesis and establishes TOEFAZ1 as a key component of this essential and uniquely configured process in kinetoplastids.

Flagellum inheritance in Trypanosoma brucei requires a kinetoplastid-specific protein phosphatase

Trypanosoma brucei causes sleeping sickness in humans and nagana in cattle in sub-Saharan Africa and alternates between its mammalian hosts and its insect vector, the tsetse fly. T. brucei uses a flagellum for motility, cell division, and cell-cell communication. Proper positioning and attachment of the newly assembled flagellum rely on the faithful duplication and segregation of flagellum-associated cytoskeletal structures. These processes are regulated by the polo-like kinase homolog TbPLK, whose activity and abundance are under stringent control to ensure spatiotemporally regulated phosphorylation of its substrates. However, it remains unclear whether a protein phosphatase that counteracts TbPLK activity is also involved in this regulation. Here, we report that a putative kinetoplastid-specific protein phosphatase, named KPP1, has essential roles in regulating flagellum positioning and attachment in T. brucei KPP1 localized to multiple flagellum-associated cytoskeletal structures and co-localized with TbPLK in several cytoskeletal structures at different cell-cycle stages. KPP1 depletion abolished basal body segregation, inhibited the duplication of the centrin arm and the hook complex of the bilobe structure, and disrupted the elongation of the flagellum attachment zone, leading to flagellum misplacement and detachment and cytokinesis arrest. Importantly, KPP1-depleted cells lacked dephosphorylation of TbCentrin2, a TbPLK substrate, at late cell-cycle stages. Together, these results suggest that KPP1-mediated protein dephosphorylation regulates the duplication and segregation of flagellum-associated cytoskeletal structures, thereby promoting flagellum positioning and attachment. These findings highlight the requirement of reversible protein phosphorylation, mediated by TbPLK and KPP1, in regulating flagellum inheritance in T. brucei.

TbSmee1 regulates hook complex morphology and the rate of flagellar pocket uptake in Trypanosoma brucei

Trypanosoma brucei uses multiple mechanisms to evade detection by its insect and mammalian hosts. The flagellar pocket (FP) is the exclusive site of uptake from the environment in trypanosomes and shields receptors from exposure to the host. The FP neck is tightly associated with the flagellum via a series of cytoskeletal structures that include the hook complex (HC) and the centrin arm. These structures are implicated in facilitating macromolecule entry into the FP and nucleating the flagellum attachment zone (FAZ), which adheres the flagellum to the cell surface. TbSmee1 (Tb927.10.8820) is a component of the HC and a putative substrate of polo-like kinase (TbPLK), which is essential for centrin arm and FAZ duplication. We show that depletion of TbSmee1 in the insect-resident (procyclic) form of the parasite causes a 40% growth decrease and the appearance of multinucleated cells that result from defective cytokinesis. Cells lacking TbSmee1 contain HCs with aberrant morphology and show delayed uptake of both fluid-phase and membrane markers. TbPLK localization to the tip of the new FAZ is also blocked. These results argue that TbSmee1 is necessary for maintaining HC morphology, which is important for the parasite's ability to take up molecules from its environment.

Interaction between the flagellar pocket collar and the hook complex via a novel microtubule-binding protein in Trypanosoma brucei

Trypanosoma brucei belongs to a group of unicellular, flagellated parasites that are responsible for human African trypanosomiasis. An essential aspect of parasite pathogenicity is cytoskeleton remodelling, which occurs during the life cycle of the parasite and is accompanied by major changes in morphology and organelle positioning. The flagellum originates from the basal bodies and exits the cell body through the flagellar pocket (FP) but remains attached to the cell body via the flagellum attachment zone (FAZ). The FP is an invagination of the pellicular membrane and is the sole site for endo- and exocytosis. The FAZ is a large complex of cytoskeletal proteins, plus an intracellular set of four specialised microtubules (MtQ) that elongate from the basal bodies to the anterior end of the cell. At the distal end of the FP, an essential, intracellular, cytoskeletal structure called the flagellar pocket collar (FPC) circumvents the flagellum. Overlapping the FPC is the hook complex (HC) (a sub-structure of the previously named bilobe) that is also essential and is thought to be involved in protein FP entry. BILBO1 is the only functionally characterised FPC protein and is necessary for FPC and FP biogenesis. Here, we used a combination of in vitro and in vivo approaches to identify and characterize a new BILBO1 partner protein-FPC4. We demonstrate that FPC4 localises to the FPC, the HC, and possibly to a proximal portion of the MtQ. We found that the C-terminal domain of FPC4 interacts with the BILBO1 N-terminal domain, and we identified the key amino acids required for this interaction. Interestingly, the FPC4 N-terminal domain was found to bind microtubules. Over-expression studies highlight the role of FPC4 in its association with the FPC, HC and FPC segregation. Our data suggest a tripartite association between the FPC, the HC and the MtQ.

A functional analysis of TOEFAZ1 uncovers protein domains essential for cytokinesis in Trypanosoma brucei

The parasite Trypanosoma brucei is highly polarized, including a flagellum that is attached along the cell surface by the flagellum attachment zone (FAZ). During cell division, the new FAZ positions the cleavage furrow, which ingresses from the anterior tip of the cell towards the posterior. We recently identified TOEFAZ1 (for 'Tip of the Extending FAZ protein 1') as an essential protein in trypanosome cytokinesis. Here, we analyzed the localization and function of TOEFAZ1 domains by performing overexpression and RNAi complementation experiments. TOEFAZ1 comprises three domains with separable functions: an N-terminal α-helical domain that may be involved in FAZ recruitment, a central intrinsically disordered domain that keeps the morphogenic kinase TbPLK at the new FAZ tip, and a C-terminal zinc finger domain necessary for TOEFAZ1 oligomerization. Both the N-terminal and C-terminal domains are essential for TOEFAZ1 function, but TbPLK retention at the FAZ is not necessary for cytokinesis. The feasibility of alternative cytokinetic pathways that do not employ TOEFAZ1 are also assessed. Our results show that TOEFAZ1 is a multimeric scaffold for recruiting proteins that control the timing and location of cleavage furrow ingression.

Protein diversity in discrete structures at the distal tip of the trypanosome flagellum

The distal end of the eukaryotic flagellum/cilium is important for axonemal growth and signaling and has distinct biomechanical properties. Specific flagellum tip structures exist, yet their composition, dynamics, and functions are largely unknown. We used biochemical approaches to identify seven constituents of the flagella connector at the tip of an assembling trypanosome flagellum and three constituents of the axonemal capping structure at the tips of both assembling and mature flagella. Both tip structures contain evolutionarily conserved as well as kinetoplastid-specific proteins, and component assembly into the structures occurs very early during flagellum extension. Localization and functional studies reveal that the flagella connector membrane junction is attached to the tips of extending microtubules of the assembling flagellum by a kinesin-15 family member. On the opposite side, a kinetoplastid-specific kinesin facilitates attachment of the junction to the microtubules in the mature flagellum. Functional studies also suggest roles of several other components and the definition of subdomains in the tip structures.

AEE788 Inhibits Basal Body Assembly and Blocks DNA Replication in the African Trypanosome

Trypanosoma brucei causes human African trypanosomiasis (HAT). The pyrrolopyrimidine AEE788 (a hit for anti-HAT drug discovery) associates with three trypanosome protein kinases. Herein we delineate the effects of AEE788 on T. brucei using chemical biology strategies. AEE788 treatment inhibits DNA replication in the kinetoplast (mitochondrial nucleoid) and nucleus. In addition, AEE788 blocks duplication of the basal body and the bilobe without affecting mitosis. Thus, AEE788 prevents entry into the S-phase of the cell division cycle. To study the kinetics of early events in trypanosome division, we employed an "AEE788 block and release" protocol to stage entry into the S-phase. A time-course of DNA synthesis (nuclear and kinetoplast DNA), duplication of organelles (basal body, bilobe, kinetoplast, nucleus), and cytokinesis was obtained. Unexpected findings include the following: 1) basal body and bilobe duplication are concurrent; 2) maturation of probasal bodies, marked by TbRP2 recruitment, is coupled with nascent basal body assembly, monitored by localization of TbSAS6 at newly forming basal bodies; and 3) kinetoplast division is observed in G2 after completion of nuclear DNA synthesis. Prolonged exposure of trypanosomes to AEE788 inhibited transferrin endocytosis, altered cell morphology, and decreased cell viability. To discover putative effectors for the pleiotropic effects of AEE788, proteome-wide changes in protein phosphorylation induced by the drug were determined. Putative effectors include an SR protein kinase, bilobe proteins, TbSAS4, TbRP2, and BILBO-1. Loss of function of one or more of these effectors can, from published literature, explain the polypharmacology of AEE788 on trypanosome biology.

CRL4WDR1 Controls Polo-like Kinase Protein Abundance to Promote Bilobe Duplication, Basal Body Segregation and Flagellum Attachment in Trypanosoma brucei

The Polo-like kinase homolog in Trypanosoma brucei, TbPLK, plays essential roles in basal body segregation, flagellum attachment and cytokinesis. The level of TbPLK protein is tightly controlled, but the underlying mechanism remains elusive. Here, we report a Cullin-RING ubiquitin ligase composed of Cullin4, the DNA damage-binding protein 1 homolog TbDDB1 and a WD40-repeat protein WDR1 that controls TbPLK abundance in the basal body and the bilobe. WDR1, through its C-terminal domain, interacts with the PEST motif in TbPLK and, through its N-terminal WD40 motif, binds to TbDDB1. Depletion of WDR1 inhibits bilobe duplication and basal body segregation, disrupts the assembly of the new flagellum attachment zone filament and detaches the new flagellum. Consistent with its role in TbPLK degradation, depletion of WDR1 causes excessive accumulation of TbPLK in the basal body and the bilobe, leading to continuous phosphorylation of TbCentrin2 in the bilobe at late cell cycle stages. Together, these results identify a novel WD40-repeat protein as a TbPLK receptor in the Cullin4-DDB1 ubiquitin ligase complex for degrading TbPLK in the basal body and the bilobe after the G1/S cell cycle transition, thereby promoting bilobe duplication, basal body separation and flagellum-cell body adhesion.

The Trypanosome Flagellar Pocket Collar and Its Ring Forming Protein-TbBILBO1

Sub-species of Trypanosoma brucei are the causal agents of human African sleeping sickness and Nagana in domesticated livestock. These pathogens have developed an organelle-like compartment called the flagellar pocket (FP). The FP carries out endo- and exocytosis and is the only structure this parasite has evolved to do so. The FP is essential for parasite viability, making it an interesting structure to evaluate as a drug target, especially since it has an indispensible cytoskeleton component called the flagellar pocket collar (FPC). The FPC is located at the neck of the FP where the flagellum exits the cell. The FPC has a complex architecture and division cycle, but little is known concerning its organization. Recent work has focused on understanding how the FP and the FPC are formed and as a result of these studies an important calcium-binding, polymer-forming protein named TbBILBO1 was identified. Cellular biology analysis of TbBILBO1 has demonstrated its uniqueness as a FPC component and until recently, it was unknown what structural role it played in forming the FPC. This review summarizes the recent data on the polymer forming properties of TbBILBO1 and how these are correlated to the FP cytoskeleton.

Two distinct cytokinesis pathways drive trypanosome cell division initiation from opposite cell ends

Cytokinesis in Trypanosoma brucei, an early branching protozoan, occurs along its longitudinal axis uni-directionally from the anterior tip of the new flagellum attachment zone filament toward the cell's posterior end. However, the underlying mechanisms remain elusive. Here we report that cytokinesis in T. brucei is regulated by a concerted action of Polo-like kinase, Aurora B kinase, and a trypanosome-specific protein CIF1. Phosphorylation of CIF1 by Polo-like kinase targets it to the anterior tip of the new flagellum attachment zone filament, where it subsequently recruits Aurora B kinase to initiate cytokinesis. Consistent with its role, CIF1 depletion inhibits cytokinesis initiation from the anterior end of the cell, but, surprisingly, triggers cytokinesis initiation from the posterior end of the cell, suggesting the activation of an alternative cytokinesis from the opposite cell end. Our results reveal the mechanistic roles of CIF1 and Polo-like kinase in cytokinesis initiation and elucidate the mechanism underlying the recruitment of Aurora B kinase to the cytokinesis initiation site at late anaphase. These findings also delineate a signaling cascade controlling cytokinesis initiation from the anterior end of the cell and uncover a backup cytokinesis that is initiated from the posterior end of the cell when the typical anterior-to-posterior cytokinesis is compromised.

The Flagellum Attachment Zone: 'The Cellular Ruler' of Trypanosome Morphology

A defining feature of Trypanosoma brucei cell shape is the lateral attachment of the flagellum to the cell body, mediated by the flagellum attachment zone (FAZ). The FAZ is a complex cytoskeletal structure that connects the flagellum skeleton through two membranes to the cytoskeleton. The FAZ acts as a ‘cellular ruler’ of morphology by regulating cell length and organelle position and is therefore critical for both cell division and life cycle differentiations. Here we provide an overview of the advances in our understanding of the composition, assembly, and function of the FAZ.

The flagellum attachment zone (FAZ) is a large cytoskeletal structure that connects the flagellum skeleton to the cell body cytoskeleton through the membrane of both the flagellum and the cell body. The structure can be divided into eight zones.

The FAZ is a key morphogenetic structure regulating both cell length and organelle positioning.

Recent studies have identified numerous FAZ proteins. The function of a subset of these proteins has been studied by RNAi, revealing a range of different phenotypes from flagellum detachment to organelle positioning effects.

The assembly of the FAZ occurs at its proximal end – the opposite polarity to that of the flagellar axoneme and paraflagellar rod.

Knockdown of Inner Arm Protein IC138 in Trypanosoma brucei Causes Defective Motility and Flagellar Detachment

Motility in the protozoan parasite Trypanosoma brucei is conferred by a single flagellum, attached alongside the cell, which moves the cell forward using a beat that is generated from tip-to-base. We are interested in characterizing components that regulate flagellar beating, in this study we extend the characterization of TbIC138, the ortholog of a dynein intermediate chain that regulates axonemal inner arm dynein f/I1. TbIC138 was tagged In situ-and shown to fractionate with the inner arm components of the flagellum. RNAi knockdown of TbIC138 resulted in significantly reduced protein levels, mild growth defect and significant motility defects. These cells tended to cluster, exhibited slow and abnormal motility and some cells had partially or fully detached flagella. Slight but significant increases were observed in the incidence of mis-localized or missing kinetoplasts. To document development of the TbIC138 knockdown phenotype over time, we performed a detailed analysis of flagellar detachment and motility changes over 108 hours following induction of RNAi. Abnormal motility, such as slow twitching or irregular beating, was observed early, and became progressively more severe such that by 72 hours-post-induction, approximately 80% of the cells were immotile. Progressively more cells exhibited flagellar detachment over time, but this phenotype was not as prevalent as immotility, affecting less than 60% of the population. Detached flagella had abnormal beating, but abnormal beating was also observed in cells with no flagellar detachment, suggesting that TbIC138 has a direct, or primary, effect on the flagellar beat, whereas detachment is a secondary phenotype of TbIC138 knockdown. Our results are consistent with the role of TbIC138 as a regulator of motility, and has a phenotype amenable to more extensive structure-function analyses to further elucidate its role in the control of flagellar beat in T. brucei.

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.

A Novel Basal Body Protein That Is a Polo-like Kinase Substrate Is Required for Basal Body Segregation and Flagellum Adhesion in Trypanosoma brucei

The Polo-like kinase (PLK) in Trypanosoma brucei plays multiple roles in basal body segregation, flagellum attachment, and cytokinesis. However, the mechanistic role of TbPLK remains elusive, mainly because most of its substrates are not known. Here, we report a new substrate of TbPLK, SPBB1, and its essential roles in T. brucei. SPBB1 was identified through yeast two-hybrid screening with the kinase-dead TbPLK as the bait. It interacts with TbPLK in vitro and in vivo, and is phosphorylated by TbPLK in vitro. SPBB1 localizes to both the mature basal body and the probasal body throughout the cell cycle, and co-localizes with TbPLK at the basal body during early cell cycle stages. RNAi against SPBB1 in procyclic trypanosomes inhibited basal body segregation, disrupted the new flagellum attachment zone filament, detached the new flagellum, and caused defective cytokinesis. Moreover, RNAi of SPBB1 confined TbPLK at the basal body and the bilobe structure, resulting in constitutive phosphorylation of TbCentrin2 at the bilobe. Altogether, these results identified a basal body protein as a TbPLK substrate and its essential role in promoting basal body segregation and flagellum attachment zone filament assembly for flagellum adhesion and cytokinesis initiation.

Proteomic identification of novel cytoskeletal proteins associated with TbPLK, an essential regulator of cell morphogenesis in Trypanosoma brucei

Trypanosoma brucei is the causative agent of African sleeping sickness, a devastating disease endemic to sub-Saharan Africa with few effective treatment options. The parasite is highly polarized, including a single flagellum that is nucleated at the posterior of the cell and adhered along the cell surface. These features are essential and must be transmitted to the daughter cells during division. Recently we identified the T. brucei homologue of polo-like kinase (TbPLK) as an essential morphogenic regulator. In the present work, we conduct proteomic screens to identify potential TbPLK binding partners and substrates to better understand the molecular mechanisms of kinase function. These screens identify a cohort of proteins, most of which are completely uncharacterized, which localize to key cytoskeletal organelles involved in establishing cell morphology, including the flagella connector, flagellum attachment zone, and bilobe structure. Depletion of these proteins causes substantial changes in cell division, including mispositioning of the kinetoplast, loss of flagellar connection, and prevention of cytokinesis. The proteins identified in these screens provide the foundation for establishing the molecular networks through which TbPLK directs cell morphogenesis in T. brucei.

The Centriole Cartwheel Protein SAS-6 in Trypanosoma brucei Is Required for Probasal Body Biogenesis and Flagellum Assembly

The centriole in eukaryotes functions as the cell's microtubule-organizing center (MTOC) to nucleate spindle assembly, and its biogenesis requires an evolutionarily conserved protein, SAS-6, which assembles the centriole cartwheel. Trypanosoma brucei, an early branching protozoan, possesses the basal body as its MTOC to nucleate flagellum biogenesis. However, little is known about the components of the basal body and their roles in basal body biogenesis and flagellum assembly. Here, we report that the T. brucei SAS-6 homolog, TbSAS-6, is localized to the mature basal body and the probasal body throughout the cell cycle. RNA interference (RNAi) of TbSAS-6 inhibited probasal body biogenesis, compromised flagellum assembly, and caused cytokinesis arrest. Surprisingly, overexpression of TbSAS-6 in T. brucei also impaired probasal body duplication and flagellum assembly, contrary to SAS-6 overexpression in humans, which produces supernumerary centrioles. Furthermore, we showed that depletion of T. brucei Polo-like kinase, TbPLK, or inhibition of TbPLK activity did not abolish TbSAS-6 localization to the basal body, in contrast to the essential role of Polo-like kinase in recruiting SAS-6 to centrioles in animals. Altogether, these results identified the essential role of TbSAS-6 in probasal body biogenesis and flagellum assembly and suggest the presence of a TbPLK-independent pathway governing basal body duplication in T. brucei.

Motility and more: the flagellum of Trypanosoma brucei

Trypanosoma brucei is a pathogenic unicellular eukaryote that infects humans and other mammals in sub-Saharan Africa. A central feature of trypanosome biology is the single flagellum of the parasite, which is an essential and multifunctional organelle that facilitates cell propulsion, controls cell morphogenesis and directs cytokinesis. Moreover, the flagellar membrane is a specialized subdomain of the cell surface that mediates attachment to host tissues and harbours multiple virulence factors. In this Review, we discuss the structure, assembly and function of the trypanosome flagellum, including canonical roles in cell motility as well as novel and emerging roles in cell morphogenesis and host-parasite interactions.

Centrin3 in trypanosomes maintains the stability of a flagellar inner-arm dynein for cell motility

Centrin is a conserved component of centrioles in animals and basal bodies in flagellated organisms. It also associates with axonemal inner-arm dyneins and regulates cell motility, but the underlying mechanism remains elusive. In Trypanosoma brucei, three of the five centrins associate with the flagellar basal body, but no centrin has been found to regulate flagellar motility. Here we show that TbCentrin3 is a flagellar protein and knockdown of TbCentrin3 compromises cell motility. Tandem affinity purification followed by mass spectrometry identifies an inner-arm dynein, TbIAD5-1, as the TbCentrin3 partner, and knockdown of TbIAD5-1 causes similar cell motility defect. Further, we demonstrate the interdependence of TbCentrin3 and TbIAD5-1 for maintaining a stable complex in the flagellar axoneme. Together, these results identify the essential role of TbCentrin3 in cell motility by maintaining the stability of an inner-arm dynein in the flagellum, which may be shared by all the centrin-containing flagellated and ciliated organisms.

New insights into the molecular mechanisms of mitosis and cytokinesis in trypanosomes

Trypanosoma brucei, a unicellular eukaryote and the causative agent of human sleeping sickness, possesses multiple single-copy organelles that all need to be duplicated and segregated during cell division. Trypanosomes undergo a closed mitosis in which the mitotic spindle is anchored on the nuclear envelope and connects the kinetochores made of novel protein components. Cytokinesis in trypanosomes is initiated from the anterior tip of the new flagellum attachment zone, and proceeds along the longitudinal axis without the involvement of the actomyosin contractile ring, the well-recognized cytokinesis machinery conserved from yeast to humans. Trypanosome appears to employ both evolutionarily conserved and trypanosome-specific proteins to regulate its cell cycle, and has evolved certain cell cycle regulatory pathways that are either distinct between its life cycle stages or different from its human host. Understanding the mechanisms of mitosis and cytokinesis in trypanosomes not only would shed novel light on the evolution of cell cycle control, but also could provide new drug targets for chemotherapy.

Polo-like kinase phosphorylation of bilobe-resident TbCentrin2 facilitates flagellar inheritance in Trypanosoma brucei

In the protist parasite Trypanosoma brucei, the single Polo-like kinase (TbPLK) controls the inheritance of a suite of organelles that help position the parasite's single flagellum. These include the basal bodies, the bilobe, and the flagellar attachment zone (FAZ). TbCentrin2 was previously shown to be a target for TbPLK in vitro, and this is extended in this study to in vivo studies, highlighting a crucial role for serine 54 in the N-terminal domain. Duplication of the bilobe correlates with the presence of TbPLK and phospho-TbCentrin2, identified using phosphospecific antiserum. Mutation of S54 leads to slow growth (S54A) or no growth (S54D), the latter suggesting that dephosphorylation is needed to complete bilobe duplication and subsequent downstream events necessary for flagellum inheritance.

An analogue-sensitive approach identifies basal body rotation and flagellum attachment zone elongation as key functions of PLK in Trypanosoma brucei

The Polo-like kinase homologue in Trypanosoma brucei (TbPLK) regulates the assembly of a series of organelles necessary for positioning the parasite's flagellum. An analogue-sensitive strategy is used to acutely and specifically inhibit the kinase with a small molecule, making it possible to identify novel TbPLK functions.

Polo-like kinases are important regulators of cell division, playing diverse roles in mitosis and cytoskeletal inheritance. In the parasite Trypanosoma brucei, the single PLK homologue TbPLK is necessary for the assembly of a series of essential organelles that position and adhere the flagellum to the cell surface. Previous work relied on RNA interference or inhibitors of undefined specificity to inhibit TbPLK, both of which have significant experimental limitations. Here we use an analogue-sensitive approach to selectively and acutely inhibit TbPLK. T. brucei cells expressing only analogue-sensitive TbPLK (TbPLK^as) grow normally, but upon treatment with inhibitor develop defects in flagellar attachment and cytokinesis. TbPLK cannot migrate effectively when inhibited and remains trapped in the posterior of the cell throughout the cell cycle. Using synchronized cells, we show that active TbPLK is a direct requirement for the assembly and extension of the flagellum attachment zone, which adheres the flagellum to the cell surface, and for the rotation of the duplicated basal bodies, which positions the new flagellum so that it can extend without impinging on the old flagellum. This approach should be applicable to the many kinases found in the T. brucei genome that lack an ascribed function.

Biochemical characterization of the bi-lobe reveals a continuous structural network linking the bi-lobe to other single-copied organelles in Trypanosoma brucei

Trypanosoma brucei, a unicellular parasite, contains several single-copied organelles that duplicate and segregate in a highly coordinated fashion during the cell cycle. In the procyclic stage, a bi-lobed structure is found adjacent to the single ER exit site and Golgi apparatus, forming both stable and dynamic association with other cytoskeletal components including the basal bodies that seed the flagellum and the flagellar pocket collar that is critical for flagellar pocket biogenesis. To further understand the bi-lobe and its association with adjacent organelles, we performed proteomic analyses on the immunoisolated bi-lobe complex. Candidate proteins were localized to the flagellar pocket, the basal bodies, a tripartite attachment complex linking the basal bodies to the kinetoplast, and a segment of microtubule quartet linking the flagellar pocket collar and bi-lobe to the basal bodies. These results supported an extensive connection among the single-copied organelles in T. brucei, a strategy employed by the parasite for orderly organelle assembly and inheritance during the cell cycle.