The extended tubulin superfamily

The flagellum of trypanosomes

Eukaryotic cilia and flagella are cytoskeletal organelles that are remarkably conserved from protists to mammals. Their basic unit is the axoneme, a well-defined cylindrical structure composed of microtubules and up to 250 associated proteins. These complex organelles are assembled by a dynamic process called intraflagellar transport. Flagella and cilia perform diverse motility and sensitivity functions in many different organisms. Trypanosomes are flagellated protozoa, responsible for various tropical diseases such as sleeping sickness and Chagas disease. In this review, we first describe general knowledge on the flagellum: its occurrence in the living world, its molecular composition, and its mode of assembly, with special emphasis on the exciting developments that followed the discovery of intraflagellar transport. We then present recent progress regarding the characteristics of the trypanosome flagellum, highlighting the original contributions brought by this organism. The most striking phenomenon is the involvement of the flagellum in several aspects of the trypanosome cell cycle, including cell morphogenesis, basal body migration, and cytokinesis.

A novel form of actin in Leishmania: molecular characterisation, subcellular localisation and association with subpellicular microtubules

To study the occurrence and subcellular distribution of actin in trypanosomatid parasites, we have cloned and overexpressed Leishmania donovani actin gene in bacteria, purified the protein, and employed the affinity purified rabbit polyclonal anti-recombinant actin antibodies as a probe to study the organisation and subcellular distribution of actin in Leishmania cells. The Leishmania actin did not cross react with antimammalian actin antibodies but was readily recognized by the anti-Leishmania actin antibodies in both the promastigote and amastigote forms of the parasite. About 10(6) copies per cell of this protein (M(r) 42.05 kDa) were present in the Leishmania promastigote. Unlike other eukaryotic actins, the oligomeric forms of Leishmania actin were not stained by phalloidin nor were dissociated by actin filament-disrupting agents, like Latrunculin B and Cytochalasin D. Analysis of the primary structure of this protein revealed that these unusual characteristics may be related to the presence of highly diverged amino acids in the DNase I-binding loop (amino acids 40-50) and the hydrophobic plug (amino acids 262-272) regions of Leishmania actin. The subcellular distribution of actin was studied in the Leishmania promastigotes by employing immunoelectron and immunofluorescence microscopies. This protein was present not only in the flagella, flagellar pocket, nucleus and the kinetoplast but it was also localized on the nuclear, vacuolar and cytoplasmic face of the plasma membranes. Further, the plasma membrane-associated actin was colocalised with subpellicular microtubules, while most of the actin present in the kinetoplast colocalised with the k-DNA network. These results clearly indicate that Leishmania contains a novel form of actin which may structurally and functionally differ from other eukaryotic actins. The functional significance of these observations is discussed.

Toward an HDAC6 inhibitor: synthesis and conformational analysis of cyclic hexapeptide hydroxamic acid designed from α-tubulin sequence

A cyclic hexapeptide hydroxamic acid inhibitor for HDAC6 has been designed and synthesized on the basis of the facts that alpha-tubulin is the substrate of HDAC6 and of the excellent inhibitory activity of cyclic tetrapeptide hydroxamic acids (CHAPs) for HDACs. Unexpectedly, cyclic hexapeptide hydroxamic acid showed very low HDAC inhibitory activity. To explain the low activity, we have carried out conformation analysis and compared it to the crystal structure of alpha-tubulin. The conformation around the acetylated lysine of the cyclic hexapeptide substrate or the aminosuberate hydroxamic acid [Asu(NHOH)] of cyclic hexapeptide inhibitor is different from that around alpha-tubulin's lysine-40. The difference in the conformation seems to cause some steric hindrance at the capping site resulting in poor binding capacity.

?-Tubulin and acetylated ?-tubulin during ovarian follicle differentiation in the lizardPodarcis sicula Raf

During most of the previtellogenic oocyte growth, the follicular epithelium of the lizard Podarcis sicula shows a polymorphic structure, due to the presence of different follicle cells. These include small cells which divide and move from the periphery of the follicle to the oocyte surface, intermediate cells which represent an initial step in the process of cell enlargement, and large pyriform cells engaged in the transport of different materials to the oocyte through intercellular bridges. We have studied, by immunolocalization and immunoblotting, the localization of alpha-tubulin and its acetylated form in different follicle cells and in the oocyte during the main steps of ovarian follicle differentiation. Our results indicate that alpha-tubulin is present in all follicle cells at different stages of ovarian follicle differentiation, while its acetylated form is detectable exclusively in the small proliferating and migrating follicle cells. In pyriform cells, alpha-tubulin is localized around the nucleus, extends to the cell apex, and crosses the zona pellucida into the oocyte cortex. The presence of acetylated tubulin in the small follicle cells may be related to the proliferation and/or migration of these cells. The absence of acetylated tubulin form in the cytoplasm of intermediate and pyriform cells can be related to the colocalization of alpha-tubulin with the keratin cytoskeleton in these cells, as detected by confocal microscopy. We have also identified the colocalization of alpha-tubulin with keratin in the cortical region of the oocyte, in particular when the cortex is engaged in the uptake of the yolk proteins.

Meiosis-specific failure of cell cycle progression in fission yeast by mutation of a conserved beta-tubulin residue

The microtubule cytoskeleton is involved in regulation of cell morphology, differentiation, and cell cycle progression. Precisely controlled dynamic properties are required for these microtubule functions. To better understand how tubulin's dynamics are embedded in its primary sequence, we investigated in vivo the consequences of altering a single, highly conserved residue in beta-tubulin that lies at the interface between two structural domains. The residue differs between the cold-adapted Antarctic fish and temperate animals in a manner that suggests a role in microtubule stability. Fungi, like the Antarctic fish, have a phenylalanine in this position, whereas essentially all other animals have tyrosine. We mutated the corresponding residue in fission yeast to tyrosine. Temperature effects were subtle, but time-lapse microscopy of microtubule dynamics revealed reduced depolymerization rates and increased stability. Mitotic exit signaled by breakdown of the mitotic spindle was delayed. In meiosis, microtubules displayed prolonged contact to the cell cortex during horsetail movement, followed by completion of meiosis I but frequent asymmetric failure of meiosis II spindle formation. Our results indicate that depolymerization dynamics modulated through interdomain motion may be important for regulating a subset of plus-end microtubule complexes in Schizosaccharomyces pombe.

Class III beta-tubulin isotype: a key cytoskeletal protein at the crossroads of developmental neurobiology and tumor neuropathology

The expression of the cytoskeletal protein class III beta-tubulin isotype is reviewed in the context of human central nervous system development and neoplasia. Compared to systemic organs and tissues, class III beta-tubulin is abundant in the brain, where it is prominently expressed during fetal and postnatal development. As exemplified in cerebellar neurogenesis, the distribution of class III beta-tubulin is neuron associated, exhibiting different temporospatial gradients in the neuronal progeny of the external granule layer versus the neuroepithelial germinal matrix of the velum medullare. However, transient expression of this protein is also present in the telencephalic subventricular zones comprising putative neuronal and/or glial precursor cells. This temporospatially restricted, potentially non-neuronal expression of class III beta-tubulin may have implications in the accurate identification of presumptive neurons derived from transplanted embryonic stem cells. In the adult central nervous system, the distribution of class III beta-tubulin is almost exclusively neuron specific. Altered patterns of expression are noted in brain tumors. In "embryonal"-type neuronal/neuroblastic tumors of the central nervous system, such as the medulloblastomas, class III beta-tubulin expression is associated with neuronal differentiation and decreased cell proliferation. In contrast, the expression of class III beta-tubulin in gliomas is associated with an ascending grade of histologic malignancy and with correspondingly high proliferative indices. Thus, class III beta-tubulin expression in neuronal or neuroblastic tumors is differentiation dependent, whereas in glial tumors, it is aberrant and/or represents "dedifferentiation" associated with the acquisition of glial progenitor-like phenotype(s). From a diagnostic perspective, the detection of class III beta-tubulin immunostaining in neoplastic cells should not be construed as categorical evidence of divergent neuronal differentiation in tumors, which are otherwise phenotypically glial. Because class III beta-tubulin is present in neoplastic but not in normal differentiated glial cells, the elucidation of molecular mechanisms responsible for the altered expression of this isotype may provide critical insights into the dynamics of the microtubule cytoskeleton in the growth and progression of gliomas.

Altered expression of transcripts for ?-tubulin and an unidentified gene in the spinal cord of phenyl saligenin phosphate treated hens (Gallus gallus)

Phenyl saligenin phosphate (PSP) induces a central-peripheral distal axonopathy in domestic fowl that develops 7-21 days after a single exposure. Neurotoxic esterase (NTE) is the initial molecular target for this neurotoxicity. PSP has to covalently bind to NTE and chemically "age" for induction of axonopathy. It was hypothesized that exposure to PSP results in early changes in spinal cord gene expression that do not occur with phenylmethylsulfonyl fluoride, a non-neuropathic compound that also inhibits NTE, or DMSO controls. Targeted display was used to screen approximately 15,000 gel bands. Three candidate genes were identified, but only the transcript designated P1 showed decreased expression following PSP exposure (2 mg/kg i.m.) in subsequent Northern blot and in situ hybridization experiments in samples taken <48 h after exposure. Additional experiments revealed that a approximately 2.5 kb alpha-tubulin transcript had decreased expression at 12-48 h after PSP exposure, with maximum change at 48 h (33%, p = 0.0479). A approximately 4.5 kb alpha-tubulin transcript had increased expression at 12 h (38%, p = 0.0125) and decreased expression at 48 h (28%, p = 0.0576). In situ hybridization on spinal cord revealed neuronal expression of P1 and alpha-tubulin transcripts. Decreased expression of transcripts for P1 and alpha-tubulin was present at 12 and 48 h, respectively. This decrease occurred in all neurons, not just those whose axons degenerate. Results suggest that (1) in PSP-induced OPIDN (organophosphorus-induced delayed neurotoxicity) some gene transcript expression changes are associated with initiation of axonopathy, and (2) PSP modulates spinal cord gene expression in neuronal types that do not undergo axonal degeneration.

Molecular characterization and expression of a divergent α-tubulin in planarian Schmidtea polychroa

We report the cloning and sequencing of a cDNA from planarian Schmidtea polychroa (Platyhelminthes, Turbellaria, Tricladida) encoding for an unusual tubulin isoform (SpTub-1) which is specifically expressed in testis. Sequence comparison of SpTub-1 with other known tubulins reveals that it has the highest homology with alpha-tubulins, even though the analysis of the molecular features shows that this isoform is significantly divergent. Hybridization of SpTub-1 to restriction-digested genomic DNA to Southern blotting produced a multiple banding pattern indicating that in planarian, a tubulin multigene family exists. Using in situ hybridization, we showed that the transcript is specifically detectable in planarian testis, suggesting that it may play a role in spermatogenesis.

Microtubule alterations and mutations induced by desoxyepothilone B: implications for drug-target interactions

Epothilones, like paclitaxel, bind to beta-tubulin and stabilize microtubules. We selected a series of four leukemia sublines that display increasing levels of resistance to the epothilone analog desoxyepothilone B (dEpoB). The dEpoB cells selected in 30-140 nM were approximately 15-fold cross-resistant to paclitaxel, while 300 nM selected cells were 467-fold resistant to this agent. The dEpoB-selected cells are hypersensitive to microtubule destabilizing agents, and express increased levels of class III beta-tubulin and MAP4. A novel class I beta-tubulin mutation, A231T, that affects microtubule stability but does not alter paclitaxel binding, was identified. The 300 nM selected cells acquired a second mutation, Q292E, situated near the M loop of class I beta-tubulin. These cells fail to undergo drug-induced tubulin polymerization due to dramatically reduced drug binding. The dEpoB-resistant leukemia cells provide novel insights into microtubule dynamics and, in particular, drug-target interactions.

Class III beta-tubulin in human development and cancer

The differential cellular expression of class III beta-tubulin isotype (betaIII) is reviewed in the context of human embryological development and neoplasia. As compared to somatic organs and tissues, betaIII is abundant in the central and peripheral nervous systems (CNS and PNS) where it is prominently expressed during fetal and postnatal development. As exemplified in cerebellar and sympathoadrenal neurogenesis, the distribution of betaIII is neuron-associated, exhibiting distinct temporospatial gradients according to the regional neuroepithelia of origin. However, transient expression of this protein is also present in the subventricular zones of the CNS comprising putative neuronal- and/or glial precursor cells, as well as in Kulchitsky neuroendocrine cells of the fetal respiratory epithelium. This temporally restricted, potentially non-neuronal expression may have implications in the identification of presumptive neurons derived from embryonic stem cells. In adult tissues, the distribution of betaIII is almost exclusively neuron-specific. Altered patterns of expression are noted in cancer. In "embryonal"- and "adult-type" neuronal tumors of the CNS and PNS, betaIII is associated with neuronal differentiation and decreased cell proliferation. In contrast, the presence of betaIII in gliomas and lung cancer is associated with an ascending histological grade of malignancy. Thus, betaIII expression in neuronal tumors is differentiation-dependent, while in non-neuronal tumors it is aberrant and/or represents "dedifferentiation" associated with the acquisition of progenitor-like phenotypic properties. Increased expression in various epithelial cancer cell lines is associated with chemoresistance to taxanes. Because betaIII is present in subpopulations of neoplastic, but not in normal differentiated glial or somatic epithelial cells, the elucidation of mechanisms responsible for the altered expression of this isotype may provide insights into the role of the microtubule cytoskeleton in tumorigenesis and tumor progression.

Molecular dissection of a yeast septin: distinct domains are required for septin interaction, localization, and function

The septins are a family of cytoskeletal proteins present in animal and fungal cells. They were first identified for their essential role in cytokinesis, but more recently, they have been found to play an important role in many cellular processes, including bud site selection, chitin deposition, cell compartmentalization, and exocytosis. Septin proteins self-associate into filamentous structures that, in yeast cells, form a cortical ring at the mother bud neck. Members of the septin family share common structural domains: a GTPase domain in the central region of the protein, a stretch of basic residues at the amino terminus, and a predicted coiled-coil domain at the carboxy terminus. We have studied the role of each domain in the Saccharomyces cerevisiae septin Cdc11 and found that the three domains are responsible for distinct and sometimes overlapping functions. All three domains are important for proper localization and function in cytokinesis and morphogenesis. The basic region was found to bind the phosphoinositides phosphatidylinositol 4-phosphate and phosphatidylinositol 5-phosphate. The coiled-coil domain is important for interaction with Cdc3 and Bem4. The GTPase domain is involved in Cdc11-septin interaction and targeting to the mother bud neck. Surprisingly, GTP binding appears to be dispensable for Cdc11 function, localization, and lipid binding. Thus, we find that septins are multifunctional proteins with specific domains involved in distinct molecular interactions required for assembly, localization, and function within the cell.

Proteome mapping of the protozoan parasite Leishmania and application to the study of drug targets and resistance mechanisms

Leishmania is a protozoan parasite responsible for significant morbidity and mortality worldwide. Few parasites have been subjected to proteomic analysis to date, but a genome sequencing project for Leishmania major is currently underway, making these studies possible. Here we present a high resolution proteome for L. major comprising almost 3700 spots, making it the most complete two-dimensional gel representation of a parasite proteome generated to date. We have identified a number of landmark proteins by mass spectrometry and show that several of these are valid for the related species Leishmania donovani infantum. We have also observed several forms and fragments of alpha- and beta-tubulins and show that the number and amount of these fragments increase with the age of the parasite culture. Trypanothione reductase (TRYR), which replaces glutathione reductase in trypanosomatid parasites, is an essential protein specific to these parasites and as such is under considerable scrutiny as a drug target. Two-dimensional gel analysis of a L. major strain overexpressing TRYR revealed increased amounts of five spots, all at the predicted molecular weight for TRYR and differing by 0.08 pH units in pI. Mass spectrometry identified four of these as TRYR, leading to the novel suggestion that it could be post-translationally modified. Finally quantitative comparative analysis of a methotrexate-resistant mutant of L. major generated in vitro found that a known primary resistance mediator, the pteridine reductase PTR1, was overexpressed. This constitutes the first proteomic analysis of drug resistance in a parasite and also the clearest identification of a primary drug resistance mechanism using this approach. Together these results provide a framework for further proteomic studies of Leishmania species and demonstrate that these tools are valuable for the essential study of potential drug targets and drug resistance mechanisms.

Basal body/centriole assembly and continuity

The long-standing interest in centrioles and basal bodies stems from the evolutionary conservation of their structural design and from their dual mode of assembly (templated versus de novo), revealed by electron microscopic studies nearly four decades ago and unique for a subcellular organelle. Molecular dissection of the assembly pathway during the past few years has recently progressed, essentially through direct and reverse genetic approaches. These studies revealed essential roles for centrins and the gamma-, delta-, epsilon - and eta-tubulins in assembly or as specific signals for centriole duplication. Identification of further components of basal bodies and centrioles might help to unravel the two assembly pathways and their regulation.

Tubulin superfamily: giving birth to triplets

Two new studies show that e tubulin is required for centriole/basal body duplication in both Chlamydomonas and Paramecium, adding to the list of new tubulin family members specifically involved in forming the centriole triplet microtubules. The function of these triplets, and the precise role of e tubulin in triplet formation, remains unclear.

The cytoskeleton of Giardia lamblia

Giardia lamblia is a ubiquitous intestinal pathogen of mammals. Evolutionary studies have also defined it as a member of one of the earliest diverging eukaryotic lineages that we are able to cultivate and study in the laboratory. Despite early recognition of its striking structure resembling a half pear endowed with eight flagella and a unique ventral disk, a molecular understanding of the cytoskeleton of Giardia has been slow to emerge. Perhaps most importantly, although the association of Giardia with diarrhoeal disease has been known for several hundred years, little is known of the mechanism by which Giardia exacts such a toll on its host. What is clear, however, is that the flagella and disk are essential for parasite motility and attachment to host intestinal epithelial cells. Because peristaltic flow expels intestinal contents, attachment is necessary for parasites to remain in the small intestine and cause diarrhoea, underscoring the essential role of the cytoskeleton in virulence. This review presents current day knowledge of the cytoskeleton, focusing on its role in motility and attachment. As the advent of new molecular technologies in Giardia sets the stage for a renewed focus on the cytoskeleton and its role in Giardia virulence, we discuss future research directions in cytoskeletal function and regulation.

Assembly of centrosomal proteins and microtubule organization depends on PCM-1

The protein PCM-1 localizes to cytoplasmic granules known as "centriolar satellites" that are partly enriched around the centrosome. We inhibited PCM-1 function using a variety of approaches: microinjection of antibodies into cultured cells, overexpression of a PCM-1 deletion mutant, and specific depletion of PCM-1 by siRNA. All approaches led to reduced targeting of centrin, pericentrin, and ninein to the centrosome. Similar effects were seen upon inhibition of dynactin by dynamitin, and after prolonged treatment of cells with the microtubule inhibitor nocodazole. Inhibition or depletion of PCM-1 function further disrupted the radial organization of microtubules without affecting microtubule nucleation. Loss of microtubule organization was also observed after centrin or ninein depletion. Our data suggest that PCM-1-containing centriolar satellites are involved in the microtubule- and dynactin-dependent recruitment of proteins to the centrosome, of which centrin and ninein are required for interphase microtubule organization.

Functional role of epsilon-tubulin in the assembly of the centriolar microtubule scaffold

Centrioles and basal bodies fascinate by their spectacular architecture, featuring an arrangement of nine microtubule triplets into an axial symmetry, whose biogenesis relies on yet elusive mechanisms. However, the recent discovery of new tubulins, such as delta-, epsilon-, or eta-tubulin, could constitute a breakthrough for deciphering the assembly steps of this unconventional microtubule scaffold. Here, we report the functional analysis in vivo of epsilon-tubulin, based on gene silencing in Paramecium, which demonstrates that this protein, which localizes at the basal bodies, is essential for the assembly and anchorage of the centriolar microtubules.

A beta-tubulin-like cDNA expressed specifically in elongating cotton fibers induces longitudinal growth of fission yeast

Using cDNA Representational Difference Analysis (RDA) techniques, we isolated a cDNA that was expressed specifically in cotton fibers but not in the ovules of a fuzzless-lintless mutant (fl). We designated it as Gh-BTubL for it shares high sequence identity with known plant and yeast beta-tubulins. RT-PCR and robotic cDNA dot blot analyses indicated that the expression of Gh-BTubL was correlated with the elongation pattern of cotton fibers. In situ hybridization results verified that there was no Gh-BTubL mRNA in fl ovules while it was easily detected in the elongating wild type cotton fiber cells. Overexpression of Gh-BTubL in fission yeast induced longitudinal growth of the host cells by 1.74-fold, with no apparent effect on other aspects of the host cells. We suggest that Gh-BTubL plays an important role in cotton fiber elongation and we believe that elucidation of the control mechanisms for expression of tubulin-like proteins may help improve fiber quality and productivity.

Immunization with a tubulin-rich preparation from Trypanosoma brucei confers broad protection against African trypanosomosis

Tubulin from Trypanosoma brucei was purified to near homogeneity using a protocol which involved treatment with urea with subsequent renaturation and was then used to immunize mice. Renatured tubulin further purified by SDS-PAGE (denatured), synthetic tubulin peptides (STP), and rat brain tubulin (RbTub) were also used. Immunized mice were challenged with T. brucei, Trypanosoma congolense or Trypanosoma rhodesiense. Renatured T. brucei tubulin (nTbTub) induced protection in all mice tested, of which 60-80% (n = 81) was complete and the remainder partial. Denatured T. brucei tubulin (dTbTub), STP, or RbTub induced lower antibody levels than nTbTub and did not offer protection. However, in culture, the antibodies against dTbTub or STP killed trypanosomes although at lower dilutions than nTbTub, but those against RbTub did not. In Western blots anti-trypanosome antibodies recognized the tubulin of all the trypanosome species investigated but not vertebrate tubulin, whereas the anti-RbTUB antibodies recognized both trypanosome and vertebrate tubulin. Of the five mice given passive immunity by the transfer of anti-nTbTub serum, four were completely protected and one partially protected. These data suggest that tubulin is the relevant immunogen in the preparation used and could therefore be a promising target for the development of a parasite-specific, broad spectrum vaccine.

Two pathways of homologous recombination in Trypanosoma brucei

African trypanosomes are unicellular parasites that use DNA recombination to evade the mammalian immune response. They do this in a process called antigenic variation, in which the parasites periodically switch the expression of VSG genes that encode distinct Variant Surface Glycoprotein coats. Recombination is used to move new VSG genes into specialised bloodstream VSG transcription sites. Genetic and molecular evidence has suggested that antigenic variation uses homologous recombination, but the detailed reaction pathways are not understood. In this study, we examine the recombination pathways used by trypanosomes to integrate transformed DNA into their genome, and show that they possess at least two pathways of homologous recombination. The primary mechanism is dependent upon RAD51, but a subsidiary pathway exists that is RAD51-independent. Both pathways contribute to antigenic variation. We show that the RAD51-independent pathway is capable of recombining DNA substrates with very short lengths of sequence homology and in some cases aberrant recombination reactions can be detected using such microhomologies.

A fourth component of the fission yeast gamma-tubulin complex, Alp16, is required for cytoplasmic microtubule integrity and becomes indispensable when gamma-tubulin function is compromised

gamma-Tubulin functions as a multiprotein complex, called the gamma-tubulin complex (gamma-TuC), and composes the microtubule organizing center (MTOC). Fission yeast Alp4 and Alp6 are homologues of two conserved gamma-TuC proteins, hGCP2 and hGCP3, respectively. We isolated a novel gene, alp16(+), as a multicopy suppressor of temperature-sensitive alp6-719 mutants. alp16(+) encodes a 759-amino-acid protein with two conserved regions found in all other members of gamma-TuC components. In addition, Alp16 contains an additional motif, which shows homology to hGCP6/Xgrip210. Gene disruption shows that alp16(+) is not essential for cell viability. However, alp16 deletion displays abnormally long cytoplasmic microtubules, which curve around the cell tip. Furthermore, alp16-deleted mutants are hypersensitive to microtubule-depolymerizing drugs and synthetically lethal with either temperature-sensitive alp4-225, alp4-1891, or alp6-719 mutants. Overproduction of Alp16 is lethal, with defective phenotypes very similar to loss of Alp4 or Alp6. Alp16 localizes to the spindle pole body throughout the cell cycle and to the equatorial MTOC at postanaphase. Alp16 coimmunoprecipitates with gamma-tubulin and cosediments with the gamma-TuC in a large complex (>20 S). Alp16 is, however, not required for the formation of this large complex. We discuss evolutional conservation and divergence of structure and function of the gamma-TuC between yeast and higher eukaryotes.