Swimming with protists: perception, motility and flagellum assembly

Structural basis of cofactor-mediated stabilization and substrate recognition of the α-tubulin acetyltransferase αTAT1

The functions of microtubules are controlled in part by tubulin post-translational modification including acetylation of Lys⁴⁰ in α-tubulin. αTAT1 (α-tubulin acetyltransferase 1), an enzyme evolutionarily conserved among eukaryotes, has recently been identified as the major α-tubulin Lys⁴⁰ acetyltransferase, in which AcCoA (acetyl-CoA) serves as an acetyl group donor. The regulation and substrate recognition of this enzyme, however, have not been fully understood. In the present study, we show that AcCoA and CoA each form a stable complex with human αTAT1 to maintain the protein integrity both in vivo and in vitro. The invariant residues Arg¹³² and Ser¹⁶⁰ in αTAT1 participate in the stable interaction not only with AcCoA but also with CoA, which is supported by analysis of the present crystal structures of the αTAT1 catalytic domain in complex with CoA. Alanine substitution for Arg¹³² or Ser¹⁶⁰ leads to a drastic misfolding of the isolated αTAT1 catalytic domain in the absence of CoA and AcCoA but not in the presence of excess amounts of either cofactor. A mutant αTAT1 carrying the R132A or S160A substitution is degraded much faster than the wild-type protein when expressed in mammalian Madin-Darby canine kidney cells. Furthermore, alanine-scanning experiments using Lys⁴⁰-containing peptides reveal that α-tubulin Ser³⁸ is crucial for substrate recognition of αTAT1, whereas Asp³⁹, Ile⁴², the glycine stretch (amino acid residues 43-45) and Asp⁴⁶ are also involved. The requirement for substrate selection is totally different from that in various histone acetyltransferases, which appears to be consistent with the inability of αTAT1 to acetylate histones.

Optical trapping reveals propulsion forces, power generation and motility efficiency of the unicellular parasites Trypanosoma brucei brucei

Unicellular parasites have developed sophisticated swimming mechanisms to survive in a wide range of environments. Cell motility of African trypanosomes, parasites responsible for fatal illness in humans and animals, is crucial both in the insect vector and the mammalian host. Using millisecond-scale imaging in a microfluidics platform along with a custom made optical trap, we are able to confine single cells to study trypanosome motility. From the trapping characteristics of the cells, we determine the propulsion force generated by cells with a single flagellum as well as of dividing trypanosomes with two fully developed flagella. Estimates of the dissipative energy and the power generation of single cells obtained from the motility patterns of the trypanosomes within the optical trap indicate that specific motility characteristics, in addition to locomotion, may be required for antibody clearance. Introducing a steerable second optical trap we could further measure the force, which is generated at the flagellar tip. Differences in the cellular structure of the trypanosomes are correlated with the trapping and motility characteristics and in consequence with their propulsion force, dissipative energy and power generation.

Proteomics analysis of heterogeneous flagella in brown algae (stramenopiles)

Flagella are conserved organelles among eukaryotes and they are composed of many proteins, which are necessary for flagellar assembly, maintenance and function. Stramenopiles, which include brown algae, diatoms and oomycetes, possess two laterally inserted flagella. The anterior flagellum (AF) extends forward and bears tripartite mastigonemes, whilst the smooth posterior flagellum (PF) often has a paraflagellar body structure. These heterogeneous flagella have served as crucial structures in algal studies especially from a viewpoint of phylogeny. However, the protein compositions of the flagella are still largely unknown. Here we report a LC-MS/MS based proteomics analysis of brown algal flagella. In total, 495 flagellar proteins were identified. Functional annotation of the proteome data revealed that brown algal flagellar proteins were associated with cell motility, signal transduction and various metabolic activities. We separately isolated AF and PF and analyzed their protein compositions. This analysis led to the identification of several AF- and PF-specific proteins. Among the PF-specific proteins, we found a candidate novel blue light receptor protein involved in phototaxis, and named it HELMCHROME because of the steering function of PF. Immunological analysis revealed that this protein was localized along the whole length of the PF and concentrated in the paraflagellar body.

Evidence for loss of a partial flagellar glycolytic pathway during trypanosomatid evolution

Classically viewed as a cytosolic pathway, glycolysis is increasingly recognized as a metabolic pathway exhibiting surprisingly wide-ranging variations in compartmentalization within eukaryotic cells. Trypanosomatid parasites provide an extreme view of glycolytic enzyme compartmentalization as several glycolytic enzymes are found exclusively in peroxisomes. Here, we characterize Trypanosoma brucei flagellar proteins resembling glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and phosphoglycerate kinase (PGK): we show the latter associates with the axoneme and the former is a novel paraflagellar rod component. The paraflagellar rod is an essential extra-axonemal structure in trypanosomes and related protists, providing a platform into which metabolic activities can be built. Yet, bioinformatics interrogation and structural modelling indicate neither the trypanosome PGK-like nor the GAPDH-like protein is catalytically active. Orthologs are present in a free-living ancestor of the trypanosomatids, Bodo saltans: the PGK-like protein from B. saltans also lacks key catalytic residues, but its GAPDH-like protein is predicted to be catalytically competent. We discuss the likelihood that the trypanosome GAPDH-like and PGK-like proteins constitute molecular evidence for evolutionary loss of a flagellar glycolytic pathway, either as a consequence of niche adaptation or the re-localization of glycolytic enzymes to peroxisomes and the extensive changes to glycolytic flux regulation that accompanied this re-localization. Evidence indicating loss of localized ATP provision via glycolytic enzymes therefore provides a novel contribution to an emerging theme of hidden diversity with respect to compartmentalization of the ubiquitous glycolytic pathway in eukaryotes. A possibility that trypanosome GAPDH-like protein additionally represents a degenerate example of a moonlighting protein is also discussed.

Modes of flagellar assembly in Chlamydomonas reinhardtii and Trypanosoma brucei

Defects in flagella growth are related to a number of human diseases. Central to flagellar growth is the organization of microtubules that polymerize from basal bodies to form the axoneme, which consists of hundreds of proteins. Flagella exist in all eukaryotic phyla, but neither the mechanism by which flagella grow nor the conservation of this process in evolution are known. Here, we study how protein complexes assemble onto the growing axoneme tip using (cryo) electron tomography. In Chlamydomonas reinhardtii microtubules and associated proteins are added simultaneously. However, in Trypanosoma brucei, disorganized arrays of microtubules are arranged into the axoneme structure by the later addition of preformed protein complexes. Post assembly, the T. brucei transition zone alters structure and its association with the central pair loosens. We conclude that there are multiple ways to form a flagellum and that species-specific structural knowledge is critical before evaluating flagellar defects.

DOI:http://dx.doi.org/10.7554/eLife.01479.001

Some cells have a whip-like appendage called a flagellum. This is most often used to propel the cell, notably in sperm cells, but it can also be involved in sensing cues in the surrounding environment. Flagella are found in all three domains of life—the eukaryotes (which include the animals), bacteria and ancient, single-celled organisms called Archaea—and they perform similar functions in each domain. However, they also differ significantly in their protein composition, overall structure, and mechanism of propulsion.

The core of the flagellum in eukaryotes is made up of 20 hollow filaments called ‘microtubules’ arranged so that nine pairs of microtubules form a ring around two central microtubules. The core also contains many other proteins, but it is not clear how all these components come together to make a working flagellum. Moreover, it is not known if the flagella of different groups of eukaryotes are all assembled in the same way.

Now, Höög et al. have discovered that although the core structure of the eukaryote flagellum is highly conserved, it can be assembled in markedly different ways. Some species of eukaryote—such as Chlamydomonas reinhardtii, a single-celled green alga, and Trypanosoma brucei, the protist parasite that causes African sleeping sickness—must grow new flagella when their cells divide, so that each new cell can swim. Using a form of electron microscopy called electron tomography, Höög et al. could see the detailed structure of the growing flagella in three dimensions. At first the cores of the flagella in these two distantly related species grow in the same way. However as the flagella get longer their cores grow in completely different ways. The microtubule filaments in longer flagella grow in a synchronized manner in the alga, but in a disorganized way in the protist.

The results of Höög et al. illustrate that it is not advisable to draw generalised conclusions based on studies of a few model species. However, since defects in flagella are known to cause several diseases in humans, this knowledge might inform future studies aimed at developing treatments for infertility, respiratory problems, and certain kinds of cancer.

DOI:http://dx.doi.org/10.7554/eLife.01479.002

Mouse infection and pathogenesis by Trypanosoma brucei motility mutants

The flagellum of Trypanosoma brucei is an essential and multifunctional organelle that drives parasite motility and is receiving increased attention as a potential drug target. In the mammalian host, parasite motility is suspected to contribute to infection and disease pathogenesis. However, it has not been possible to test this hypothesis owing to lack of motility mutants that are viable in the bloodstream life cycle stage that infects the mammalian host. We recently identified a bloodstream-form motility mutant in 427-derived T. brucei in which point mutations in the LC1 dynein subunit disrupt propulsive motility but do not affect viability. These mutants have an actively beating flagellum, but cannot translocate. Here we demonstrate that the LC1 point mutant fails to show enhanced cell motility upon increasing viscosity of the surrounding medium, which is a hallmark of wild type T. brucei, thus indicating that motility of the mutant is fundamentally altered compared with wild type cells. We next used the LC1 point mutant to assess the influence of trypanosome motility on infection in mice. Wesurprisingly found that disrupting parasite motility has no discernible effect on T. brucei bloodstream infection. Infection time-course, maximum parasitaemia, number of waves of parasitaemia, clinical features and disease outcome are indistinguishable between motility mutant and control parasites. Our studies provide an important step toward understanding the contribution of parasite motility to infection and a foundation for future investigations of T. brucei interaction with the mammalian host.

Amoeboid swimming: a generic self-propulsion of cells in fluids by means of membrane deformations

Microorganisms, such as bacteria, algae, or spermatozoa, are able to propel themselves forward thanks to flagella or cilia activity. By contrast, other organisms employ pronounced changes of the membrane shape to achieve propulsion, a prototypical example being the Eutreptiella gymnastica. Cells of the immune system as well as dictyostelium amoebas, traditionally believed to crawl on a substratum, can also swim in a similar way. We develop a model for these organisms: the swimmer is mimicked by a closed incompressible membrane with force density distribution (with zero total force and torque). It is shown that fast propulsion can be achieved with adequate shape adaptations. This swimming is found to consist of an entangled pusher-puller state. The autopropulsion distance over one cycle is a universal linear function of a simple geometrical dimensionless quantity A/V(2/3) (V and A are the cell volume and its membrane area). This study captures the peculiar motion of Eutreptiella gymnastica with simple force distribution.

The conserved ciliary protein Bug22 controls planar beating of Chlamydomonas flagella

Eukaryotic flagella and cilia can exhibit planar and non-planar beating, and the mechanism controlling these beating patterns is not well understood. Chlamydomonas reinhardtii flagella beat in approximately the same plane with either an asymmetric ciliary-type or symmetric flagellar-type waveform. Each B-tubule of the number 1, 5 and 6 doublets of the flagellar axoneme possesses a beak-like structure. The number 5 and 6 beak structures are implicated in conversion of ciliary motion into flagellar motion. Here, we show that in a null mutant of Bug22, the asymmetric ciliary waveform is converted into a three-dimensional (non-planar) symmetric flagellar waveform. Bug22 is localized to approximately the proximal half to two-thirds of the flagellum, similar to localization of beak-like structures. However, as shown by immunogold labeling, Bug22 associates with axonemal microtubules without apparent preference for any particular doublets. Interestingly, bug22 mutants lack all beak-like structures. We propose that one function of Bug22 is to regulate the anchoring of the beak-like structures to the doublet microtubules and confine flagellar beating to a plane.

The ciliary cytoskeleton

Cilia and flagella are surface-exposed, finger-like organelles whose core consists of a microtubule (MT)-based axoneme that grows from a modified centriole, the basal body. Cilia are found on the surface of many eukaryotic cells and play important roles in cell motility and in coordinating a variety of signaling pathways during growth, development, and tissue homeostasis. Defective cilia have been linked to a number of developmental disorders and diseases, collectively called ciliopathies. Cilia are dynamic organelles that assemble and disassemble in tight coordination with the cell cycle. In most cells, cilia are assembled during growth arrest in a multistep process involving interaction of vesicles with appendages present on the distal end of mature centrioles, and addition of tubulin and other building blocks to the distal tip of the basal body and growing axoneme; these building blocks are sorted through a region at the cilium base known as the ciliary necklace, and then transported via intraflagellar transport (IFT) along the axoneme toward the tip for assembly. After assembly, the cilium frequently continues to turn over and incorporate tubulin at its distal end in an IFT-dependent manner. Prior to cell division, the cilia are usually resorbed to liberate centrosomes for mitotic spindle pole formation. Here, we present an overview of the main cytoskeletal structures associated with cilia and centrioles with emphasis on the MT-associated appendages, fibers, and filaments at the cilium base and tip. The composition and possible functions of these structures are discussed in relation to cilia assembly, disassembly, and length regulation.

Evidence of intraflagellar transport and apical complex formation in a free-living relative of the apicomplexa

Since its first description, Chromera velia has attracted keen interest as the closest free-living relative of parasitic Apicomplexa. The life cycle of this unicellular alga is complex and involves a motile biflagellate form. Flagella are thought to be formed in the cytoplasm, a rare phenomenon shared with Plasmodium in which the canonical mode of flagellar assembly, intraflagellar transport, is dispensed with. Here we demonstrate the expression of intraflagellar transport components in C. velia, answering the question of whether this organism has the potential to assemble flagella via the canonical route. We have developed and characterized a culturing protocol that favors the generation of flagellate forms. From this, we have determined a marked shift in the mode of daughter cell production from two to four daughter cells per division as a function of time after passage. We conduct an ultrastructural examination of the C. velia flagellate form by using serial TEM and show that flagellar biogenesis in C. velia occurs prior to cytokinesis. We demonstrate a close association of the flagellar apparatus with a complex system of apical structures, including a micropore, a conoid, and a complex endomembrane system reminiscent of the apical complex of parasitic apicomplexans. Recent work has begun to elucidate the possible flagellar origins of the apical complex, and we show that in C. velia these structures are contemporaneous within a single cell and share multiple connections. We propose that C. velia therefore represents a vital piece in the puzzle of the origins of the apical complex.

CMF22 is a broadly conserved axonemal protein and is required for propulsive motility in Trypanosoma brucei

The eukaryotic flagellum (or cilium) is a broadly conserved organelle that provides motility for many pathogenic protozoa and is critical for normal development and physiology in humans. Therefore, defining core components of motile axonemes enhances understanding of eukaryotic biology and provides insight into mechanisms of inherited and infectious diseases in humans. In this study, we show that component of motile flagella 22 (CMF22) is tightly associated with the flagellar axoneme and is likely to have been present in the last eukaryotic common ancestor. The CMF22 amino acid sequence contains predicted IQ and ATPase associated with a variety of cellular activities (AAA) motifs that are conserved among CMF22 orthologues in diverse organisms, hinting at the importance of these domains in CMF22 function. Knockdown by RNA interference (RNAi) and rescue with an RNAi-immune mRNA demonstrated that CMF22 is required for propulsive cell motility in Trypanosoma brucei. Loss of propulsive motility in CMF22-knockdown cells was due to altered flagellar beating patterns, rather than flagellar paralysis, indicating that CMF22 is essential for motility regulation and likely functions as a fundamental regulatory component of motile axonemes. CMF22 association with the axoneme is weakened in mutants that disrupt the nexin-dynein regulatory complex, suggesting potential interaction with this complex. Our results provide insight into the core machinery required for motility of eukaryotic flagella.

KHARON1 mediates flagellar targeting of a glucose transporter in Leishmania mexicana and is critical for viability of infectious intracellular amastigotes

The LmxGT1 glucose transporter is selectively targeted to the flagellum of the kinetoplastid parasite Leishmania mexicana, but the mechanism for targeting this and other flagella-specific membrane proteins among the Kinetoplastida is unknown. To address the mechanism of flagellar targeting, we employed in vivo cross-linking, tandem affinity purification, and mass spectrometry to identify a novel protein, KHARON1 (KH1), which is important for the flagellar trafficking of LmxGT1. Kh1 null mutant parasites are strongly impaired in flagellar targeting of LmxGT1, and trafficking of the permease was arrested in the flagellar pocket. Immunolocalization revealed that KH1 is located at the base of the flagellum, within the flagellar pocket, where it associates with the proximal segment of the flagellar axoneme. We propose that KH1 mediates transit of LmxGT1 from the flagellar pocket into the flagellar membrane via interaction with the proximal portion of the flagellar axoneme. KH1 represents the first component involved in flagellar trafficking of integral membrane proteins among parasitic protozoa. Of considerable interest, Kh1 null mutants are strongly compromised for growth as amastigotes within host macrophages. Thus, KH1 is also important for the disease causing stage of the parasite life cycle.

Modeling and analysis of propulsion in the multiflagellated micoorganism Giardia lamblia

The goal of this work was to analyze the propulsion of multiflagellated microorganisms, and to draw insight to the underlying physics and biology of the movement. Giardia lamblia was chosen as the model organism due to its unique ability to mechanically attach to various surfaces, its rapid movement, and its fine control over steering and navigation. In this work, a mechanics model was utilized to study the mechanics and propulsive contribution of the ventral and anterior flagella in Giardia. It was discovered that energy is supplied mainly at the proximal portion of these flagella, supporting the hypothesis that a decreasing adenosine triphosphate (ATP) gradient along the length of the flagella would not affect the motion observed. Similarly, the elasticity of the flagella allows the energy input at the proximal portion to be transferred to the distal portion, where the majority of thrust is generated. Specifically, we found that the ventral flagella are the driving force for planar propulsion and turning, while the anterior flagella are used for steering and control.

Calmodulin is required for paraflagellar rod assembly and flagellum-cell body attachment in trypanosomes

In the flagellum of the African sleeping sickness parasite Trypanosoma brucei calmodulin (CaM) is found within the paraflagellar rod (PFR), an elaborate extra-axonemal structure, and the axoneme. In dissecting mechanisms of motility regulation we analysed CaM function using RNAi. Unexpectedly CaM depletion resulted in total and catastrophic failure in PFR assembly; even connections linking axoneme to PFR failed to form following CaM depletion. This provides an intriguing parallel with the role in the green alga Chlamydomonas of a CaM-related protein in docking outer-dynein arms to axoneme outer-doublet microtubules. Absence of CaM had no discernible effect on axoneme assembly, but the failure in PFR assembly was further compounded by loss of the normal linkage between PFR and axoneme to the flagellum attachment zone of the cell body. Thus, flagellum detachment was a secondary, time-dependent consequence of CaM RNAi, and coincided with the loss of normal trypomastigote morphology, thereby linking the presence of PFR architecture with maintenance of cell form, as well as cell motility. Finally, wider comparison between the flagellum detachment phenotypes of RNAi mutants for CaM and the FLA1 glycoprotein potentially provides new perspective into the function of the latter into establishing and maintaining flagellum-cell body attachment.

Predicting the proteins of Angomonas deanei, Strigomonas culicis and their respective endosymbionts reveals new aspects of the trypanosomatidae family

Endosymbiont-bearing trypanosomatids have been considered excellent models for the study of cell evolution because the host protozoan co-evolves with an intracellular bacterium in a mutualistic relationship. Such protozoa inhabit a single invertebrate host during their entire life cycle and exhibit special characteristics that group them in a particular phylogenetic cluster of the Trypanosomatidae family, thus classified as monoxenics. In an effort to better understand such symbiotic association, we used DNA pyrosequencing and a reference-guided assembly to generate reads that predicted 16,960 and 12,162 open reading frames (ORFs) in two symbiont-bearing trypanosomatids, Angomonas deanei (previously named as Crithidia deanei) and Strigomonas culicis (first known as Blastocrithidia culicis), respectively. Identification of each ORF was based primarily on TriTrypDB using tblastn, and each ORF was confirmed by employing getorf from EMBOSS and Newbler 2.6 when necessary. The monoxenic organisms revealed conserved housekeeping functions when compared to other trypanosomatids, especially compared with Leishmania major. However, major differences were found in ORFs corresponding to the cytoskeleton, the kinetoplast, and the paraflagellar structure. The monoxenic organisms also contain a large number of genes for cytosolic calpain-like and surface gp63 metalloproteases and a reduced number of compartmentalized cysteine proteases in comparison to other TriTryp organisms, reflecting adaptations to the presence of the symbiont. The assembled bacterial endosymbiont sequences exhibit a high A+T content with a total of 787 and 769 ORFs for the Angomonas deanei and Strigomonas culicis endosymbionts, respectively, and indicate that these organisms hold a common ancestor related to the Alcaligenaceae family. Importantly, both symbionts contain enzymes that complement essential host cell biosynthetic pathways, such as those for amino acid, lipid and purine/pyrimidine metabolism. These findings increase our understanding of the intricate symbiotic relationship between the bacterium and the trypanosomatid host and provide clues to better understand eukaryotic cell evolution.

Light control of the flow of phototactic microswimmer suspensions

Some microalgae are sensitive to light intensity gradients. This property is known as phototaxis: The algae swim toward a light source (positive phototaxis). We use this property to control the motion of microalgae within a Poiseuille flow using light. The combination of flow vorticity and phototaxis results in a concentration of algae around the center of the flow. Intermittent light exposure allows analysis of the dynamics of this phenomenon and its reversibility. With this phenomenon, we hope to pave the way toward new algae concentration techniques (a bottleneck challenge in biofuel algal production) and toward the improvement of pollutant biodetector technology.

Global quantitative SILAC phosphoproteomics reveals differential phosphorylation is widespread between the procyclic and bloodstream form lifecycle stages of Trypanosoma brucei

We report a global quantitative phosphoproteomic study of bloodstream and procyclic form Trypanosoma brucei using SILAC labeling of each lifecycle stage. Phosphopeptide enrichment by SCX and TiO₂ led to the identification of a total of 10096 phosphorylation sites on 2551 protein groups and quantified the ratios of 8275 phosphorylation sites between the two lifecycle stages. More than 9300 of these sites (92%) have not previously been reported. Model-based gene enrichment analysis identified over representation of Gene Ontology terms relating to the flagella, protein kinase activity, and the regulation of gene expression. The quantitative data reveal that differential protein phosphorylation is widespread between bloodstream and procyclic form trypanosomes, with significant intraprotein differential phosphorylation. Despite a lack of dedicated tyrosine kinases, 234 phosphotyrosine residues were identified, and these were 3–4 fold over-represented among site changing >10-fold between the two lifecycle stages. A significant proportion of the T. brucei kinome was phosphorylated, with evidence that MAPK pathways are functional in both lifecycle stages. Regulation of gene expression in T. brucei is exclusively post-transcriptional, and the extensive phosphorylation of RNA binding proteins observed may be relevant to the control of mRNA stability in this organism.

Crystal structure of the small GTPase Arl6/BBS3 from Trypanosoma brucei

Arl6/BBS3 is a small GTPase, mutations in which are implicated in the human ciliopathy Bardet-Biedl Syndrome (BBS). Arl6 is proposed to facilitate the recruitment of a large protein complex known as the BBSome to the base of the primary cilium, mediating specific trafficking of molecules to this important sensory organelle. Orthologues of Arl6 and the BBSome core subunits have been identified in the genomes of trypanosomes. Flagellum function and motility are crucial to the survival of Trypanosoma brucei, the causative agent of human African sleeping sickness, in the human bloodstream stage of its lifecycle and so the function of the BBSome proteins in trypanosomes warrants further study. RNAi knockdown of T. brucei Arl6 (TbArl6) has recently been shown to result in shortening of the trypanosome flagellum. Here we present the crystal structure of TbArl6 with the bound non-hydrolysable GTP analog GppNp at 2.0 Å resolution and highlight important differences between the trypanosomal and human proteins. Analysis of the TbArl6 active site confirms that it lacks the key glutamine that activates the nucleophile during GTP hydrolysis in other small GTPases. Furthermore, the trypanosomal proteins are significantly shorter at their N-termini suggesting a different method of membrane insertion compared to humans. Finally, analysis of sequence conservation suggests two surface patches that may be important for protein-protein interactions. Our structural analysis thus provides the basis for future biochemical characterisation of this important family of small GTPases.

Trypanosome motion represents an adaptation to the crowded environment of the vertebrate bloodstream

Blood is a remarkable habitat: it is highly viscous, contains a dense packaging of cells and perpetually flows at velocities varying over three orders of magnitude. Only few pathogens endure the harsh physical conditions within the vertebrate bloodstream and prosper despite being constantly attacked by host antibodies. African trypanosomes are strictly extracellular blood parasites, which evade the immune response through a system of antigenic variation and incessant motility. How the flagellates actually swim in blood remains to be elucidated. Here, we show that the mode and dynamics of trypanosome locomotion are a trait of life within a crowded environment. Using high-speed fluorescence microscopy and ordered micro-pillar arrays we show that the parasites mode of motility is adapted to the density of cells in blood. Trypanosomes are pulled forward by the planar beat of the single flagellum. Hydrodynamic flow across the asymmetrically shaped cell body translates into its rotational movement. Importantly, the presence of particles with the shape, size and spacing of blood cells is required and sufficient for trypanosomes to reach maximum forward velocity. If the density of obstacles, however, is further increased to resemble collagen networks or tissue spaces, the parasites reverse their flagellar beat and consequently swim backwards, in this way avoiding getting trapped. In the absence of obstacles, this flagellar beat reversal occurs randomly resulting in irregular waveforms and apparent cell tumbling. Thus, the swimming behavior of trypanosomes is a surprising example of micro-adaptation to life at low Reynolds numbers. For a precise physical interpretation, we compare our high-resolution microscopic data to results from a simulation technique that combines the method of multi-particle collision dynamics with a triangulated surface model. The simulation produces a rotating cell body and a helical swimming path, providing a functioning simulation method for a microorganism with a complex swimming strategy.

The destructive citrus pathogen, 'Candidatus Liberibacter asiaticus' encodes a functional flagellin characteristic of a pathogen-associated molecular pattern

Huanglongbing (HLB) is presently the most devastating citrus disease worldwide. As an intracellular plant pathogen and insect symbiont, the HLB bacterium, 'Candidatus Liberibacter asiaticus' (Las), retains the entire flagellum-encoding gene cluster in its significantly reduced genome. Las encodes a flagellin and hook-associated protein (Fla) of 452 amino acids that contains a conserved 22 amino acid domain (flg22) at positions 29 to 50 in the N-terminus. The phenotypic alteration in motility of a Sinorhizobium meliloti mutant lacking the fla genes was partially restored by constitutive expression of Fla(Las). Agrobacterium-mediated transient expression in planta revealed that Fla(Las) induced cell death and callose deposition in Nicotiana benthamiana, and that the transcription of BAK1 and SGT1, which are associated with plant innate immunity, was upregulated. Amino acid substitution experiments revealed that residues 38 (serine) and 39 (aspartate) of Fla(Las) were essential for callose induction. The synthetic flg22(Las) peptide could not induce plant cell death but retained the ability to induce callose deposition at a concentration of 20 µM or above. This demonstrated that the pathogen-associated molecular pattern (PAMP) activity of flg22 in Las was weaker than those in other well-studied plant pathogenic bacteria. These results indicate that Fla(Las) acts as a PAMP and may play an important role in triggering host plant resistance to the HLB bacteria.

The evolution of land plant cilia

Eukaryotic cilia/flagella are ancient organelles with motility and sensory functions. Cilia display significant ultrastructural conservation where present across the eukaryotic phylogeny; however, diversity in ciliary biology exists and the ability to produce cilia has been lost independently on a number of occasions. Land plants provide an excellent system for the investigation of cilia evolution and loss across a broad phylogeny, because early divergent land plant lineages produce cilia, whereas most seed plants do not. This review highlights the differences in cilia form and function across land plants and discusses how recent advances in genomics are providing novel insights into the evolutionary trajectory of ciliary proteins. We propose a renewed effort to adopt ciliated land plants as models to investigate the mechanisms underpinning complex ciliary processes, such as number control, the coordination of basal body placement and the regulation of beat patterns.

Morphology of the trypanosome bilobe, a novel cytoskeletal structure

The trypanosome bilobe is a cytoskeletal structure of unclear function. To date, four proteins have been shown to localize stably to it: TbMORN1, TbLRRP1, TbCentrin2, and TbCentrin4. In this study, a combination of immunofluorescence microscopy and electron microscopy was used to explore the morphology of the bilobe and its relationship to other nearby cytoskeletal structures in the African trypanosome procyclic trypomastigote. The use of detergent/salt-extracted flagellum preparations was found to be an effective way of discerning features of the cytoskeletal ultrastructure that are normally obscured. TbMORN1 and TbCentrin4 together define a hairpin structure comprising an arm of TbCentrin4 and a fishhook of TbMORN1. The two arms flank a specialized microtubule quartet and the flagellum attachment zone filament, with TbMORN1 running alongside the former and TbCentrin4 alongside the latter. The hooked part of TbMORN1 sits atop the flagellar pocket collar marked by TbBILBO1. The TbMORN1 bilobe occasionally exhibits tendrillar extensions that seem to be connected to the basal and probasal bodies. The TbMORN1 molecules present on these tendrils undergo higher rates of turnover than those for molecules on the main bilobe structure. These observations have been integrated with previous detailed descriptions of the cytoskeletal elements in trypanosome cells.

Both sequence and context are important for flagellar targeting of a glucose transporter

Many of the cilia- and flagella-specific integral membrane proteins identified to date function to sense the extracellular milieu, and there is considerable interest in defining pathways for targeting such proteins to these sensory organelles. The flagellar glucose transporter of Leishmania mexicana, LmxGT1, is targeted selectively to the flagellar membrane, whereas two other isoforms, LmxGT2 and LmxGT3, are targeted to the pellicular plasma membrane of the cell body. To define the flagellar targeting signal, deletions and point mutations were generated in the N-terminal hydrophilic domain of LmxGT1, which mediates flagellar localization. Three amino acids, N95-P96-M97, serve critical roles in flagellar targeting, resulting in strong mistargeting phenotypes when mutagenized. However, to facilitate flagellar targeting of other non-flagellar membrane proteins, it was necessary to attach a larger region surrounding the NPM motif containing amino acids 81-113. Molecular modeling suggests that this region might present the critical NPM residues at the surface of the N-terminal domain. It is likely that the NPM motif is recognized by currently unknown protein-binding partners that mediate flagellar targeting of membrane-associated proteins.

Integrated control of axonemal dynein AAA(+) motors

Axonemal dyneins are AAA(+) enzymes that convert ATP hydrolysis to mechanical work. This leads to the sliding of doublet microtubules with respect to each other and ultimately the generation of ciliary/flagellar beating. However, in order for useful work to be generated, the action of individual dynein motors must be precisely controlled. In addition, cells modulate the motility of these organelles through a variety of second messenger systems and these signals too must be integrated by the dynein motors to yield an appropriate output. This review describes the current status of efforts to understand dynein control mechanisms and their connectivity focusing mainly on studies of the outer dynein arm from axonemes of the unicellular biflagellate green alga Chlamydomonas.

[The rebirth of the ultrastructure of cilia and flagella]

The sensory and motility functions of eukaryotic cilia and flagella are essential for cell survival in protozoans and for cell differentiation and homoeostasis in metazoans. Ciliary biology has benefited early on from the input of electron microscopy. Over the last decade, the visualization of cellular structures has greatly progressed, thus it becomes timely to review the ultrastructure of cilia and flagella. Briefly touching upon the typical features of a 9+2 axoneme, we dwell extensively on the transition zone, the singlet zone, the ciliary necklace, cap and crown. The relation of the singlet zone to sensory and/or motile function, the link of the ciliary cap to microtubule dynamics and to ciliary beat, the involvement of the ciliary crown in ovocyte and mucosal propulsion, and the role of the transition zone/the ciliary necklace in axonemal stabilization, autotomy and as a diffusion barrier will all be discussed.

Three-dimensional structure of the Trypanosome flagellum suggests that the paraflagellar rod functions as a biomechanical spring

Flagellum motility is critical for normal human development and for transmission of pathogenic protozoa that cause tremendous human suffering worldwide. Biophysical principles underlying motility of eukaryotic flagella are conserved from protists to vertebrates. However, individual cells exhibit diverse waveforms that depend on cell-specific elaborations on basic flagellum architecture. Trypanosoma brucei is a uniflagellated protozoan parasite that causes African sleeping sickness. The T. brucei flagellum is comprised of a 9+2 axoneme and an extra-axonemal paraflagellar rod (PFR), but the three-dimensional (3D) arrangement of the underlying structural units is poorly defined. Here, we use dual-axis electron tomography to determine the 3D architecture of the T. brucei flagellum. We define the T. brucei axonemal repeating unit. We observe direct connections between the PFR and axonemal dyneins, suggesting a mechanism by which mechanochemical signals may be transmitted from the PFR to axonemal dyneins. We find that the PFR itself is comprised of overlapping laths organized into distinct zones that are connected through twisting elements at the zonal interfaces. The overall structure has an underlying 57nm repeating unit. Biomechanical properties inferred from PFR structure lead us to propose that the PFR functions as a biomechanical spring that may store and transmit energy derived from axonemal beating. These findings provide insight into the structural foundations that underlie the distinctive flagellar waveform that is a hallmark of T. brucei cell motility.

Cryoelectron tomography of radial spokes in cilia and flagella

Cryo-EM tomography of wild-type and mutant cilia and flagella from Tetrahymena and Chlamydomonas reveals new information on the substructure of radial spokes.

Radial spokes (RSs) are ubiquitous components in the 9 + 2 axoneme thought to be mechanochemical transducers involved in local control of dynein-driven microtubule sliding. They are composed of >23 polypeptides, whose interactions and placement must be deciphered to understand RS function. In this paper, we show the detailed three-dimensional (3D) structure of RS in situ in Chlamydomonas reinhardtii flagella and Tetrahymena thermophila cilia that we obtained using cryoelectron tomography (cryo-ET). We clarify similarities and differences between the three spoke species, RS1, RS2, and RS3, in T. thermophila and in C. reinhardtii and show that part of RS3 is conserved in C. reinhardtii, which only has two species of complete RSs. By analyzing C. reinhardtii mutants, we identified the specific location of subsets of RS proteins (RSPs). Our 3D reconstructions show a twofold symmetry, suggesting that fully assembled RSs are produced by dimerization. Based on our cryo-ET data, we propose models of subdomain organization within the RS as well as interactions between RSPs and with other axonemal components.

Ultrastructure of cilia and flagella - back to the future!

Eukaryotic cilia and flagella perform motility and sensory functions which are essential for cell survival in protozoans, and to organism development and homoeostasis in metazoans. Their ultrastructure has been studied from the early beginnings of electron microscopy, and these studies continue to contribute to much of our understanding about ciliary biology. In the light of the progress made in the visualization of cellular structures over the last decade, we revisit the ultrastructure of cilia and flagella. We briefly describe the typical features of a 9+2 axoneme before focusing extensively on the transition zone, the ciliary necklace, the singlet zone, the ciliary cap and the ciliary crown. We discuss how the singlet zone is linked to sensory and/or motile function, the contribution of the ciliary crown to ovocyte and mucosal propulsion, and the relationship between the ciliary cap and microtubule growth and shortening, and its relation to ciliary beat. We further examine the involvement of the transition zone/the ciliary necklace in axonemal stabilization, autotomy and as a diffusion barrier.

Giardia flagellar motility is not directly required to maintain attachment to surfaces

Giardia trophozoites attach to the intestinal microvilli (or inert surfaces) using an undefined “suction-based” mechanism, and remain attached during cell division to avoid peristalsis. Flagellar motility is a key factor in Giardia's pathogenesis and colonization of the host small intestine. Specifically, the beating of the ventral flagella, one of four pairs of motile flagella, has been proposed to generate a hydrodynamic force that results in suction-based attachment via the adjacent ventral disc. We aimed to test this prevailing “hydrodynamic model” of attachment mediated by flagellar motility. We defined four distinct stages of attachment by assessing surface contacts of the trophozoite with the substrate during attachment using TIRF microscopy (TIRFM). The lateral crest of the ventral disc forms a continuous perimeter seal with the substrate, a cytological indication that trophozoites are fully attached. Using trophozoites with two types of molecularly engineered defects in flagellar beating, we determined that neither ventral flagellar beating, nor any flagellar beating, is necessary for the maintenance of attachment. Following a morpholino-based knockdown of PF16, a central pair protein, both the beating and morphology of flagella were defective, but trophozoites could still initiate proper surface contacts as seen using TIRFM and could maintain attachment in several biophysical assays. Trophozoites with impaired motility were able to attach as well as motile cells. We also generated a strain with defects in the ventral flagellar waveform by overexpressing a dominant negative form of alpha2-annexin::GFP (D122A, D275A). This dominant negative alpha2-annexin strain could initiate attachment and had only a slight decrease in the ability to withstand normal and shear forces. The time needed for attachment did increase in trophozoites with overall defective flagellar beating, however. Thus while not directly required for attachment, flagellar motility is important for positioning and orienting trophozoites prior to attachment. Drugs affecting flagellar motility may result in lower levels of attachment by indirectly limiting the number of parasites that can position the ventral disc properly against a surface and against peristaltic flow.

Giardia is a widespread, single-celled, intestinal parasite that infects millions of people and animals each year. Colonization of the small intestine is a critical part of Giardia's life cycle in any host. This colonization is initiated when cells attach to the intestinal wall via a specialized suction cup-like structure, the ventral disc. In the host, Giardia moves by beating four pairs of flagella; movement of the ventral pair has been implicated in attachment. This study shows that the beating of the flagella is not important for attachment, but rather for positioning Giardia close to the intestinal wall prior to attachment, and thus disproves the commonly held model of giardial attachment. This work implies that drugs targeting Giardia motility could prevent or slow attachment, leading to lower rates of infection.

Structure of Trypanosoma brucei flagellum accounts for its bihelical motion

Trypanosoma brucei is a parasitic protozoan that causes African sleeping sickness. It contains a flagellum required for locomotion and viability. In addition to a microtubular axoneme, the flagellum contains a crystalline paraflagellar rod (PFR) and connecting proteins. We show here, by cryoelectron tomography, the structure of the flagellum in three bending states. The PFR lattice in straight flagella repeats every 56 nm along the length of the axoneme, matching the spacing of the connecting proteins. During flagellar bending, the PFR crystallographic unit cell lengths remain constant while the interaxial angles vary, similar to a jackscrew. The axoneme drives the expansion and compression of the PFR lattice. We propose that the PFR modifies the in-plane axoneme motion to produce the characteristic trypanosome bihelical motility as captured by high-speed light microscope videography.

Impact of microscopic motility on the swimming behavior of parasites: straighter trypanosomes are more directional

Microorganisms, particularly parasites, have developed sophisticated swimming mechanisms to cope with a varied range of environments. African Trypanosomes, causative agents of fatal illness in humans and animals, use an insect vector (the Tsetse fly) to infect mammals, involving many developmental changes in which cell motility is of prime importance. Our studies reveal that differences in cell body shape are correlated with a diverse range of cell behaviors contributing to the directional motion of the cell. Straighter cells swim more directionally while cells that exhibit little net displacement appear to be more bent. Initiation of cell division, beginning with the emergence of a second flagellum at the base, correlates to directional persistence. Cell trajectory and rapid body fluctuation correlation analysis uncovers two characteristic relaxation times: a short relaxation time due to strong body distortions in the range of 20 to 80 ms and a longer time associated with the persistence in average swimming direction in the order of 15 seconds. Different motility modes, possibly resulting from varying body stiffness, could be of consequence for host invasion during distinct infective stages.

Transitions between three swimming gaits in Paramecium escape

Paramecium and other protists are able to swim at velocities reaching several times their body size per second by beating their cilia in an organized fashion. The cilia beat in an asymmetric stroke, which breaks the time reversal symmetry of small scale flows. Here we show that Paramecium uses three different swimming gaits to escape from an aggression, applied in the form of a focused laser heating. For a weak aggression, normal swimming is sufficient and produces a steady swimming velocity. As the heating amplitude is increased, a higher acceleration and faster swimming are achieved through synchronized beating of the cilia, which begin by producing oscillating swimming velocities and later give way to the usual gait. Finally, escape from a life-threatening aggression is achieved by a "jumping" gait, which does not rely on the cilia but is achieved through the explosive release of a group of trichocysts in the direction of the hot spot. Measurements through high-speed video explain the role of trichocysts in defending against aggressions while showing unexpected transitions in the swimming of microorganisms. These measurements also demonstrate that Paramecium optimizes its escape pattern by taking advantage of its inertia.

Arginine kinase isoforms in the closest protozoan relative of metazoans

The genome of the choanoflagellate Monosiga brevicollis contains at least three genes for the phosphoryl transfer enzyme, arginine kinase (AK; EC 2.7.3.3). Bioinformatic analyses of the deduced amino acid sequences of the proteins coded for by two of these genes showed that one of these AKs is cytoplasmic (denoted AK1) while the other appears to have an N-terminal mitochondrial targeting peptide (denoted AK2). Cloning and expression of the cDNA for AK1 yielded considerable soluble AK activity. Three AK2 constructs were expressed - one corresponding to the full length protein and two corresponding to truncated versions in which the signal peptide had been deleted. Expression of the former construct yielded minimal soluble activity. In contrast, significant AK activity was found in both truncated constructs confirming the importance of removal of the targeting peptide for proper folding and catalytic activity. Both AK1 and AK2 are functional oligomers unlike typical AKs which are monomeric. A phylogenetic analysis showed that these choanoflagellate AKs group more closely with a supercluster consisting of cytoplasmic and mitochondrial CKs and invertebrate AKs that evolved secondarily from a CK-like ancestor. Reaction-diffusion constraints in choanoflagellates are likely mitigated by the presence of AK isoforms which facilitate energy transport in these highly polarized cells.

Structure-function analysis of dynein light chain 1 identifies viable motility mutants in bloodstream-form Trypanosoma brucei

The flagellum of Trypanosoma brucei is an essential and multifunctional organelle that is receiving increasing attention as a potential drug target and as a system for studying flagellum biology. RNA interference (RNAi) knockdown is widely used to test the requirement for a protein in flagellar motility and has suggested that normal flagellar motility is essential for viability in bloodstream-form trypanosomes. However, RNAi knockdown alone provides limited functional information because the consequence is often loss of a multiprotein complex. We therefore developed an inducible system that allows functional analysis of point mutations in flagellar proteins in T. brucei. Using this system, we identified point mutations in the outer dynein light chain 1 (LC1) that allow stable assembly of outer dynein motors but do not support propulsive motility. In procyclic-form trypanosomes, the phenotype of LC1 mutants with point mutations differs from the motility and structural defects of LC1 knockdowns, which lack the outer-arm dynein motor. Thus, our results distinguish LC1-specific functions from broader functions of outer-arm dynein. In bloodstream-form trypanosomes, LC1 knockdown blocks cell division and is lethal. In contrast, LC1 point mutations cause severe motility defects without affecting viability, indicating that the lethal phenotype of LC1 RNAi knockdown is not due to defective motility. Our results demonstrate for the first time that normal motility is not essential in bloodstream-form T. brucei and that the presumed connection between motility and viability is more complex than might be interpreted from knockdown studies alone. These findings open new avenues for dissecting mechanisms of flagellar protein function and provide an important step in efforts to exploit the potential of the flagellum as a therapeutic target in African sleeping sickness.

Role of adenylate kinase type 7 expression on cilia motility: possible link in primary ciliary dyskinesia

BACKGROUND

Adenylate kinase 7 (AK7) mediates the reaction 2ADP <--> ATP + AMP, providing energy for the beating of cilia. A study recently showed that AK7 expression may be correlated with the primary ciliary dyskinesia (PCD) phenotype in mice. In this study, we characterized AK7 expression in vitro in an air-liquid interface (ALI) model and in middle nasal turbinate biopsy specimens from a cohort of patients with PCD to elucidate whether AK7 expression is correlated with ciliary malfunction.

METHODS

AK7 expression was measured by real-time reverse-transcription polymerase chain reaction and Western blotting. In vitro differentiated nasal human epithelial cell siRNA experiments were performed to investigate the effect of AK7 expression on ciliary beat frequency (CBF). Ciliary motility and ultrastructure were evaluated in a cohort of 29 patients with PCD (PCD, n = 17; Kartagener's syndrome, n = 12) and 26 healthy control donors.

RESULTS

AK7 expression was mainly located on the apical surface of differentiated nasal ALI cells, and targeted suppression of the AK7 gene decreased CBF by 41%. AK7 expression was diminished significantly in patients with PCD (0.54 +/- 0.1-fold; p < 0.05) compared with healthy controls (1.1 +/- 0.08-fold). Furthermore, AK7 expression was correlated with CBF in patients with PCD (r = 0.5; p = 0.009).

CONCLUSION

AK7 expression was correlated with CBF in vitro and in nasal biopsy specimens from patients with PCD, which may have contributed to the ciliary malfunction observed in our patients with PCD.

CMF70 is a subunit of the dynein regulatory complex

Flagellar motility drives propulsion of several important pathogens and is essential for human development and physiology. Motility of the eukaryotic flagellum requires coordinate regulation of thousands of dynein motors arrayed along the axoneme, but the proteins underlying dynein regulation are largely unknown. The dynein regulatory complex, DRC, is recognized as a focal point of axonemal dynein regulation, but only a single DRC subunit, trypanin/PF2, is currently known. The component of motile flagella 70 protein, CMF70, is broadly and uniquely conserved among organisms with motile flagella, suggesting a role in axonemal motility. Here we demonstrate that CMF70 is part of the DRC from Trypanosoma brucei. CMF70 is located along the flagellum, co-sediments with trypanin in sucrose gradients and co-immunoprecipitates with trypanin. RNAi knockdown of CMF70 causes motility defects in a wild-type background and suppresses flagellar paralysis in cells with central pair defects, thus meeting the functional definition of a DRC subunit. Trypanin and CMF70 are mutually conserved in at least five of six extant eukaryotic clades, indicating that the DRC was probably present in the last common eukaryotic ancestor. We have identified only the second known subunit of this ubiquitous dynein regulatory system, highlighting the utility of combined genomic and functional analyses for identifying novel subunits of axonemal sub-complexes.

Functional characterisation and drug target validation of a mitotic kinesin-13 in Trypanosoma brucei

Mitotic kinesins are essential for faithful chromosome segregation and cell proliferation. Therefore, in humans, kinesin motor proteins have been identified as anti-cancer drug targets and small molecule inhibitors are now tested in clinical studies. Phylogenetic analyses have assigned five of the approximately fifty kinesin motor proteins coded by Trypanosoma brucei genome to the Kinesin-13 family. Kinesins of this family have unusual biochemical properties because they do not transport cargo along microtubules but are able to depolymerise microtubules at their ends, therefore contributing to the regulation of microtubule length. In other eukaryotic genomes sequenced to date, only between one and three Kinesin-13s are present. We have used immunolocalisation, RNAi-mediated protein depletion, biochemical in vitro assays and a mouse model of infection to study the single mitotic Kinesin-13 in T. brucei. Subcellular localisation of all five T. brucei Kinesin-13s revealed distinct distributions, indicating that the expansion of this kinesin family in kinetoplastids is accompanied by functional diversification. Only a single kinesin (TbKif13-1) has a nuclear localisation. Using active, recombinant TbKif13-1 in in vitro assays we experimentally confirm the depolymerising properties of this kinesin. We analyse the biological function of TbKif13-1 by RNAi-mediated protein depletion and show its central role in regulating spindle assembly during mitosis. Absence of the protein leads to abnormally long and bent mitotic spindles, causing chromosome mis-segregation and cell death. RNAi-depletion in a mouse model of infection completely prevents infection with the parasite. Given its essential role in mitosis, proliferation and survival of the parasite and the availability of a simple in vitro activity assay, TbKif13-1 has been identified as an excellent potential drug target.

Parasites in motion: flagellum-driven cell motility in African trypanosomes

Motility of the sleeping sickness parasite, Trypanosoma brucei, impacts disease transmission and pathogenesis. Trypanosome motility is driven by a flagellum that harbors a canonical 9+2 axoneme, together with trypanosome-specific elaborations. Trypanosome flagellum biology and motility have been the object of intense research over the last two years. These studies have led to the discovery of a novel form of motility, termed social motility, and provided revision of long-standing models for cell propulsion. Recent work has also uncovered novel structural features and motor proteins associated with the flagellar apparatus and has identified candidate signaling molecules that are predicted to regulate flagellar motility. Together with earlier inventories of flagellar proteins from proteomic and genomic studies, the stage is now set to move forward with functional studies to elucidate molecular mechanisms and investigate parasite motility in the context of host-parasite interactions.

Flagellum assembly and function during the Leishmania life cycle

During a complex digenetic life cycle flagellated Leishmania parasites alternate between promastigote and amastigote forms which differ significantly in cellular morphology and flagellum length. Recent studies have provided important new insights into mechanisms by which Leishmania regulate expression of genes required for flagellum assembly, and mechanisms used to modify flagellum length. While the critical role of the promastigote flagellum in parasite biology has long been appreciated, the importance of the amastigote flagellum has often been disregarded. However, recent work suggests that the 'rudimentary' amastigote flagellum may serve indispensable roles in cellular organisation, and/or sensory perception, which are critical for intracellular survival of Leishmania within host macrophages.

Life with eight flagella: flagellar assembly and division in Giardia

Flagellar movement in Giardia, a common intestinal parasitic protist, is crucial to its survival in the host. Each axoneme is unique in possessing a long, cytoplasmic portion as well as a membrane-bound portion. Intraflagellar transport (IFT) is required for the assembly of membrane-bound regions, yet the cytoplasmic regions may be assembled by IFT-independent mechanisms. Steady-state axoneme length is maintained by IFT and by intrinsic and active microtubule dynamics. Following mitosis and before their segregation, giardial flagella undergo a multigenerational division cycle in which the parental eight flagella migrate and reposition to different cellular locations; eight new flagella are assembled de novo. Each daughter cell thus inherits four mature and four newly synthesized flagella.

Phytophthora nicotianae transformants lacking dynein light chain 1 produce non-flagellate zoospores

Biflagellate zoospores of the highly destructive plant pathogens in the genus Phytophthora are responsible for the initiation of infection of host plants. Zoospore motility is a critical component of the infection process because it allows zoospores to actively target suitable infection sites on potential hosts. Flagellar assembly and function in eukaryotes depends on a number of dynein-based molecular motors that facilitate retrograde intraflagellar transport and sliding of adjacent microtubule doublets in the flagellar axonemes. Dynein light chain 1 (DLC1) is one of a number of proteins in the dynein outer arm multiprotein complex. It is a 22 kDa leucine-rich repeat protein that binds to the catalytic motor domain of the dynein gamma heavy chain. We report the cloning and characterization of DLC1 homologues in Phytophthora cinnamomi and Phytophthora nicotianae (PcDLC1 and PnDLC1). PcDLC1 and PnDLC1 are single copy genes that are more highly expressed in sporulating hyphae than in vegetative hyphae, zoospores or germinated cysts. Polyclonal antibodies raised against PnDLC1 locallized PnDLC1 along the length of the flagella of P. nicotianae zoospores. RNAi-mediated silencing of PnDLC1 expression yielded transformants that released non-flagellate, non-motile zoospores from their sporangia. Our observations indicate that zoospore motility is not required for zoospore release from P. nicotianae sporangia or for breakage of the evanescent vesicle into which zoospores are initially discharged.

Convergent evolution of RFX transcription factors and ciliary genes predated the origin of metazoans

BACKGROUND

Intraflagellar transport (IFT) genes, which are critical for the development and function of cilia and flagella in metazoans, are tightly regulated by the Regulatory Factor X (RFX) transcription factors (TFs). However, how and when their evolutionary relationship was established remains unknown.

RESULTS

We have identified evidence suggesting that RFX TFs and IFT genes evolved independently and their evolution converged before the first appearance of metazoans. Both ciliary genes and RFX TFs exist in all metazoans as well as some unicellular eukaryotes. However, while RFX TFs and IFT genes are found simultaneously in all sequenced metazoan genomes, RFX TFs do not co-exist with IFT genes in most pre-metazoans and thus do not regulate them in these organisms. For example, neither the budding yeast nor the fission yeast possesses cilia although both have well-defined RFX TFs. Conversely, most unicellular eukaryotes, including the green alga Chlamydomonas reinhardtii, have typical cilia and well conserved IFT genes but lack RFX TFs. Outside of metazoans, RFX TFs and IFT genes co-exist only in choanoflagellates including M. brevicollis, and only one fungus Allomyces macrogynus of the 51 sequenced fungus genomes. M. brevicollis has two putative RFX genes and a full complement of ciliary genes.

CONCLUSIONS

The evolution of RFX TFs and IFT genes were independent in pre-metazoans. We propose that their convergence in evolution, or the acquired transcriptional regulation of IFT genes by RFX TFs, played a pivotal role in the establishment of metazoan.

Rewiring and regulation of cross-compartmentalized metabolism in protists

Plastid acquisition, endosymbiotic associations, lateral gene transfer, organelle degeneracy or even organelle loss influence metabolic capabilities in many different protists. Thus, metabolic diversity is sculpted through the gain of new metabolic functions and moderation or loss of pathways that are often essential in the majority of eukaryotes. What is perhaps less apparent to the casual observer is that the sub-compartmentalization of ubiquitous pathways has been repeatedly remodelled during eukaryotic evolution, and the textbook pictures of intermediary metabolism established for animals, yeast and plants are not conserved in many protists. Moreover, metabolic remodelling can strongly influence the regulatory mechanisms that control carbon flux through the major metabolic pathways. Here, we provide an overview of how core metabolism has been reorganized in various unicellular eukaryotes, focusing in particular on one near universal catabolic pathway (glycolysis) and one ancient anabolic pathway (isoprenoid biosynthesis). For the example of isoprenoid biosynthesis, the compartmentalization of this process in protists often appears to have been influenced by plastid acquisition and loss, whereas for glycolysis several unexpected modes of compartmentalization have emerged. Significantly, the example of trypanosomatid glycolysis illustrates nicely how mathematical modelling and systems biology can be used to uncover or understand novel modes of pathway regulation.

Social motility in african trypanosomes

African trypanosomes are devastating human and animal pathogens that cause significant human mortality and limit economic development in sub-Saharan Africa. Studies of trypanosome biology generally consider these protozoan parasites as individual cells in suspension cultures or in animal models of infection. Here we report that the procyclic form of the African trypanosome Trypanosoma brucei engages in social behavior when cultivated on semisolid agarose surfaces. This behavior is characterized by trypanosomes assembling into multicellular communities that engage in polarized migrations across the agarose surface and cooperate to divert their movements in response to external signals. These cooperative movements are flagellum-mediated, since they do not occur in trypanin knockdown parasites that lack normal flagellum motility. We term this behavior social motility based on features shared with social motility and other types of surface-induced social behavior in bacteria. Social motility represents a novel and unexpected aspect of trypanosome biology and offers new paradigms for considering host-parasite interactions.

African trypanosomes, e.g. Trypanosoma brucei, and related kinetoplastid parasites cause morbidity and mortality in several million people worldwide. Trypanosomes are protists and are thus generally considered to behave as single-celled microorganisms. In other microorganisms, social interactions among individuals lead to development of multicellular communities with specialized and advantageous capabilities versus single cells. The concept of bacteria acting as groups of cells communicating and cooperating with one another has had a major impact on our understanding of bacterial physiology and pathogenesis, but this paradigm has not been applied to parasitic protozoa. Here we report that T. brucei is capable of social behavior when exposed to semisolid surfaces. This behavior, termed social motility, is characterized by the assembly of parasites into multicellular communities with emergent properties that are not evident in single cells. Parasites within communities exhibit polarized movements and cooperate to coordinate their movements in response to an external stimulus. Social motility offers many potential advantages, such as facilitating colonization and navigation through host tissues. The identification of social behavior in T. brucei reveals a novel and unexpected aspect of parasite biology and provides new concepts for considering host-parasite interactions.

Membrane protein SMP-1 is required for normal flagellum function in Leishmania

Eukaryotic flagella and cilia are surrounded by a membrane that is continuous with, but distinct from, the rest of the plasma membrane. In Leishmania parasites, the inner leaflet of the flagellar membrane is coated with the acylated membrane protein, SMP-1. Here, we provide evidence that SMP-1 stabilizes the flagellar membrane and is required for flagella elongation and function. The expression and flagella targeting of SMP-1 is tightly associated with flagella elongation during amastigote to promastigote differentiation. Deletion of the genes encoding SMP-1 and the flagellar pocket protein SMP-2, led to the production of short flagella and defects in motility. Alterations in the physical properties of the smp-1/smp-2(-/-) flagellar membrane were suggested by: (1) the accumulation of membrane vesicles in the flagellar matrix, and (2) further retraction of flagella following partial inhibition of sterol and sphingolipid biosynthesis. The flagella phenotype of the smp-1/smp-2(-/-) null mutant was reversed by re-expression of SMP-1, but not SMP-2. SMP-1 contains a jelly-roll beta-sheet structure that is probably conserved in all SMP proteins, and forms stable homo-oligomers in vivo. We propose that the SMP-1 coat generates and/or stabilizes sterol- and sphingolipid-rich domains in the flagellar membrane.

Functional genomics of intraflagellar transport-associated proteins in C. elegans

The nematode Caenorhabditis elegans presents numerous advantages for the identification and molecular analysis of intraflagellar transport (IFT)-associated proteins, which play a critical role in the formation of cilia. Many proteins were first described as participating in IFT in this organism, including IFTA-1 (IFT121), DYF-1 (fleer/IFT70), DYF-2 (IFT144), DYF-3 (Qilin), DYF-11 (MIP-T3/IFT54), DYF-13, XBX-1 (dynein light intermediate chain), XBX-2 (dynein light chain), CHE-13 (IFT57/HIPPI), orthologs of Bardet-Biedl syndrome proteins, and potential regulatory protein, IFTA-2 (RABL5/IFT22). Transgenic animals bearing green fluorescent protein (GFP)-tagged proteins can be generated with ease, and in vivo imaging of IFT in both wild-type and cilia mutant strains can be performed quickly. The analyses permit detailed information on the localization and dynamic properties (velocities along the ciliary axoneme) of the relevant proteins, providing insights into their potential functions in processes such as anterograde and retrograde transport and cilium formation, as well as association with distinct modules of the IFT machinery (e.g., IFT subcomplexes A or B). Behavioral studies of the corresponding IFT-associated gene mutants further enable an understanding of the ciliary role of the proteins-e.g., in chemosensation, lipid homeostasis, lifespan control, and signaling-in a multicellular animal. In this chapter, we discuss how C. elegans can be used for the identification and characterization of IFT-associated proteins, focusing on methods for the generation of GFP-tagged IFT reporter strains, time-lapse microscopy, and IFT rate measurements.