iqing Hu

Assembly and maintenance of the flagellum attachment zone filament in Trypanosoma brucei

ABSTRACT Adhesion of motile flagella to the cell body in Trypanosoma brucei requires a filamentous cytoskeletal structure termed the flagellum attachment zone (FAZ). Despite its essentiality, the complete molecular composition of the FAZ filament and its roles in FAZ filament assembly remain poorly understood. By localization-based screening, we here identified a new FAZ protein, which we called FAZ2. Knockdown of FAZ2 disrupted the FAZ filament, destabilized multiple FAZ filament proteins and caused a cytokinesis defect. We also showed that FAZ2 depletion destabilized another new FAZ filament protein and several flagellum and cytoskeleton proteins. Furthermore, we identified CC2D and KMP11 as FAZ2 partners through affinity purification, and showed that they are each required for maintaining a stable complex. Finally, we demonstrated that FAZ filament proteins are incorporated into the FAZ filament from the proximal region, in contrast to the flagellum components, which are incorporated from the distal tip. In summary, we identified three new FAZ filament proteins and a FAZ filament protein complex, and our results suggest that assembly of the FAZ filament occurs at the proximal region and is essential to maintain the stability of FAZ filament proteins. Highlighted Article: Three new T. brucei flagellum attachment zone filament proteins are identified, and it is shown that assembly of the filament occurs at the proximal base and is essential for filament protein stability.

An EF-hand-containing Protein in Trypanosoma brucei Regulates Cytokinesis Initiation by Maintaining the Stability of the Cytokinesis Initiation Factor CIF1.

Trypanosoma brucei undergoes cytokinesis uni-directionally from the anterior tip of the new flagellum attachment zone (FAZ) toward the posterior end of the cell. We recently delineated a novel signaling pathway composed of polo-like kinase, cytokinesis initiation factor 1 (CIF1), and aurora B kinase that acts in concert at the new FAZ tip to regulate cytokinesis initiation. To identify new cytokinesis regulators, we carried out proximity-dependent biotin identification and identified many CIF1 binding partners and near neighbors. Here we report a novel CIF1-binding protein, named CIF2, and its mechanistic role in cytokinesis initiation. CIF2 interacts with CIF1 in vivo and co-localizes with CIF1 at the new FAZ tip during early cell cycle stages. RNAi of CIF2 inhibited the normal, anterior-to-posterior cytokinesis but activated an alternative, posterior-to-anterior cytokinesis. CIF2 depletion destabilized CIF1 and disrupted the localization of polo-like kinase and aurora B kinase to the new FAZ tip, thus revealing the mechanistic role of CIF2 in cytokinesis initiation. Surprisingly, overexpression of CIF2 also inhibited the normal, anterior-to-posterior cytokinesis and triggered the alternative, posterior-to-anterior cytokinesis, suggesting a tight control of CIF2 protein abundance. These results identified a new regulator in the cytokinesis regulatory pathway and reiterated that a backup cytokinesis pathway is activated by inhibiting the normal cytokinesis pathway.

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.

SAS-4 Protein in Trypanosoma brucei Controls Life Cycle Transitions by Modulating the Length of the Flagellum Attachment Zone Filament.

The evolutionarily conserved centriole/basal body protein SAS-4 regulates centriole duplication in metazoa and basal body duplication in flagellated and ciliated organisms. Here, we report that the SAS-4 homolog in the flagellated protozoan Trypanosoma brucei, TbSAS-4, plays an unusual role in controlling life cycle transitions by regulating the length of the flagellum attachment zone (FAZ) filament, a specialized cytoskeletal structure required for flagellum adhesion and cell morphogenesis. TbSAS-4 is concentrated at the distal tip of the FAZ filament, and depletion of TbSAS-4 in the trypomastigote form disrupts the elongation of the new FAZ filament, generating cells with a shorter FAZ associated with a longer unattached flagellum and repositioned kinetoplast and basal body, reminiscent of epimastigote-like morphology. Further, we show that TbSAS-4 associates with six additional FAZ tip proteins, and depletion of TbSAS-4 disrupts the enrichment of these FAZ tip proteins at the new FAZ tip, suggesting a role of TbSAS-4 in maintaining the integrity of this FAZ tip protein complex. Together, these results uncover a novel function of TbSAS-4 in regulating the length of the FAZ filament to control basal body positioning and life cycle transitions in T. brucei.

A kinetoplastid‐specific kinesin is required for cytokinesis and for maintenance of cell morphology in Trypanosoma brucei

Kinesins are motor‐based transport proteins that play diverse roles in various cellular processes. The trypanosome genome lacks the homologues of many conserved mitotic kinesins, but encodes a number of trypanosome‐specific kinesins with unknown function. Here, we report the biochemical and functional characterization of TbKIN‐C, a trypanosome‐specific kinesin, which was initially identified through an RNAi screen for cytokinesis genes in T. brucei. TbKIN‐C possesses in vitro ATPase activity and associates with cytoskeletal tubulin microtubules in vivo. It is distributed throughout the cytoskeleton with a focal enrichment at the posterior end of the cell during early cell cycle stages. RNAi of TbKIN‐C resulted in distorted cell shape with an elongated posterior filled with tyrosinated tubulin microtubules. Silencing of TbKIN‐C impaired the segregation of organelles and cytoskeletal structures and led to detachment of the new flagellum and a small portion of the cytoplasm. We also show that RNAi of TbKIN‐C compromised cytokinesis and abolished the trans‐localization of TbCPC1, a subunit of the chromosomal passenger complex, from the central spindle to the initiation site of cytokinesis. Our results suggest an essential role of TbKIN‐C in maintaining cell morphology, likely through regulating microtubule dynamics at the posterior end of the cell.

An orphan kinesin in trypanosomes cooperates with a kinetoplastid-specific kinesin to maintain cell morphology by regulating subpellicular microtubules

Summary Microtubules are a vital part of the cytoskeleton of eukaryotic cells and are involved in various cellular processes. The cytoskeleton of Trypanosoma brucei is characterized by an array of subpellicular microtubules and is essential for maintenance of cell shape and polarity, but little is known about the regulation of the assembly and organization of the subpellicular microtubule corset. Here, we report that the orphan kinesin TbKIN-D regulates the organization of subpellicular microtubules and is required for maintaining cell morphology. TbKIN-D possesses in vitro ATPase activity, associates with cytoskeletal microtubules and is distributed throughout the cytoskeleton at all cell cycle stages. RNAi of TbKIN-D disrupts the organization of the subpellicular microtubule corset and distorts cell morphology, resulting in round cells with an elongated posterior filled with newly assembled microtubules. Depletion of TbKIN-D also abolishes the segregation of organelles and cytoskeletal structures, suggesting that cellular morphogenesis is essential for proper organelle segregation. Moreover, TbKIN-D deficiency impairs the attachment of the new flagellum without compromising the formation of the flagellum attachment zone. Finally, we identified TbKIN-C, a kinetoplastid-specific kinesin known to regulate subpellicular microtubules and cell morphogenesis in T. brucei, as a partner of TbKIN-D. Further, we demonstrate that interaction between TbKIN-C and TbKIN-D requires the coiled-coil motifs in the C-termini of both proteins. Altogether, our results suggest that TbKIN-D cooperates with TbKIN-C to maintain cell morphology by regulating the organization of the subpellicular microtubule corset.

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

ABSTRACT The centriole in eukaryotes functions as the cells 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.

The cooperative roles of PHO80‐like cyclins in regulating the G1/S transition and posterior cytoskeletal morphogenesis in Trypanosoma brucei

Cyclins and cyclin‐dependent kinases (CDKs) represent the fundamental, crucial regulators of the cell division cycle in eukaryotes. Trypanosoma brucei expresses a large number of cyclins and Cdc2‐related kinases (CRKs). However, how these cyclins and CRKs cooperate to regulate cell cycle progression remains poorly understood. Here, we carry out directional yeast two‐hybrid assays to identify the interactions between the 10 cyclins and the 11 CRKs and detect a total of 26 cyclin–CRK pairs, among which 20 pairs are new. Our current efforts are focused on four PHO80‐like cyclins, CYC2, CYC4, CYC5 and CYC7, and their physical and functional interactions with CRK1. Silencing of the four cyclins and CRK1 leads to the increase of G1 cells and defective DNA replication, suggesting their important roles in promoting the G1/S transition. Additionally, CYC2‐, CYC7‐ and CRK1‐deficient cells possess an elongated posterior that is filled with newly assembled microtubules. Further, we show that the four cyclins display distinct subcellular localizations and half‐lives, suggesting that they likely undergo distinct regulation. Altogether, our results demonstrate the involvement of four CRK1‐associated cyclins, CYC2, CYC4, CYC5 and CYC7, in promoting the G1/S transition and the requirement of CYC2 and CYC7 in maintaining posterior cytoskeletal morphogenesis during the G1/S transition.

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.

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.