The highly divergent alpha- and beta-tubulins from Dictyostelium discoideum are encoded by single genes

Morphological and Phylogenetic Analyses Reveal Dictyostelids (Cellular Slime Molds) Colonizing the Ascocarp of Morchella

Morchella spp. (true morels) are precious edible mushrooms consumed around the world, with a delicious taste, rich nutritional value, and unique healthcare effects. Various fungi and bacteria have been reported to colonize the ascocarps of Morchella, damaging their fruiting bodies and leading to serious economic losses in cultivation. The species identification of these colonizing organisms is crucial for understanding their colonization mechanisms on morels. Slime molds, which have characteristics of both "fungi" and "animals", can occasionally colonize crops and edible fungi. However, there have been no reports of dictyostelid cellular slime molds (dictyostelids) colonizing plants and fungi to date. In this study, we discovered that dictyostelids colonized the surface of one wild ascoma of Morchella in the forest of Chongqing, China, with the tissues being black and rotten. Macro- and micro-morphological observations, along with molecular phylogenetic analyses, identified the specimens investigated in this study as Dictyostelium implicatum and Morchella sp. Mel-21. The results provide new knowledge of dictyostelid colonization on organisms and contribute to the diversity of species colonizing true morels. Moreover, this is also the first report of dictyostelids distributed in Chongqing, China. This study enhances our insights into the life history and potential ecological significance of dictyostelids and updates their distribution area in China. Further research will be conducted to uncover the mechanisms behind the colonization observed in this study.

Toxin-Producing Endosymbionts Shield Pathogenic Fungus against Micropredators

The fungus Rhizopus microsporus harbors a bacterial endosymbiont (Mycetohabitans rhizoxinica) for the production of the antimitotic toxin rhizoxin. Although rhizoxin is the causative agent of rice seedling blight, the toxinogenic bacterial-fungal alliance is, not restricted to the plant disease. It has been detected in numerous environmental isolates from geographically distinct sites covering all five continents, thus raising questions regarding the ecological role of rhizoxin beyond rice seedling blight. Here, we show that rhizoxin serves the fungal host in fending off protozoan and metazoan predators. Fluorescence microscopy and coculture experiments with the fungivorous amoeba Protostelium aurantium revealed that ingestion of R. microsporus spores is toxic to P. aurantium. This amoebicidal effect is caused by the dominant bacterial rhizoxin congener rhizoxin S2, which is also lethal toward the model nematode Caenorhabditis elegans. By combining stereomicroscopy, automated image analysis, and quantification of nematode movement, we show that the fungivorous nematode Aphelenchus avenae actively feeds on R. microsporus that is lacking endosymbionts, whereas worms coincubated with symbiotic R. microsporus are significantly less lively. This study uncovers an unexpected ecological role of rhizoxin as shield against micropredators. This finding suggests that predators may function as an evolutionary driving force to maintain toxin-producing endosymbionts in nonpathogenic fungi.

Cloning and sequencing of beta-tubulin and internal transcribed spacer-2 (ITS-2) of Eimeria tenella isolate from India

Beta-tubulin is an important multifunctional protein of eukaryotes abundant in the cytoskeleton and responsible for the formation of tubulin, structures responsible for cell morphology and which aid in motility and intracellular transportation. It has been used as a genotypic marker for studying the evolutionary history and phylogenetic relationships between eukaryotic organisms. Internal transcribed spacers of the ribosomal RNA genes have been widely used for typing inter-species and intra-species variation. An Indian isolate of Eimeria tenella was genotyped following the cloning and sequencing of beta-tubulin and internal transcribed spacer-2 (ITS-2) and compared with other reference isolates of E. tenella. The β-tubulin has 99 % intra-species similarity at the gene level and the functional homology of the protein is very high with more than 95 % amino-acid similarity across the Apicomplexa. The ITS-2 sequence had a similar pattern of nucleotide base arrangement with 99 % homology to Houghton and Nippon strains of E. tenella.

The Rac GEF ZizB regulates development, cell motility and cytokinesis in Dictyostelium

Dock (dedicator of cytokinesis) proteins represent a family of guanine nucleotide exchange factors (GEFs) that include the well-studied Dock180 family and the poorly characterised zizimin family. Our current understanding of Dock180 function is that it regulates Rho small GTPases and thus has a role in a number of cell processes, including cell migration, development and division. Here, we use a tractable model for cell motility research, Dictyostelium discoideum, to help elucidate the role of the related zizimin proteins. We show that gene ablation of zizA causes no change in development, whereas ablation of zizB gives rise to an aberrant developmental morphology and a reduction in cell directionality and velocity, and altered cell shape. Fluorescently labelled ZizA protein associates with the microtubule-organising centre (MTOC), whereas ZizB is enriched in the cortex. Overexpression of ZizB also causes an increase in the number of filopodia and a partial inhibition of cytokinesis. Analysis of ZizB protein binding partners shows that it interacts with Rac1a and a range of actin-associated proteins. In conclusion, our work provides insight into the molecular and cellular functions of zizimin GEF proteins, which are shown to have a role in cell movement, filopodia formation and cytokinesis.

Population structure of the pumpkin fruit flyBactrocera depressa(Tephritidae) in Korea and Japan: Pliocene allopatry or recent invasion?

Because of their widespread agricultural impact and rapid range expansions, true fruit flies (Tephritidae) are the subject of quarantine and control efforts worldwide. Among these flies, the pumpkin fruit fly Bactrocera depressa, which infests squash and other cucurbitaceous plants in Korea, Japan and Taiwan, was recently isolated from produce shipments entering Japan and identified as a regulatory target. This species was described in 1933 from collections in Japan and discovered in 1974 in Korea, suggesting that it may have recently invaded mainland Asia. We analysed the genetic structure of Asian populations of B. depressa using sequence variation for mitochondrial gene cytochrome-oxidase I and three nuclear loci: elongation factor 1alpha, tubulinbeta1 and tubulinbeta3, using frequency-based approaches, nested clade analysis and assignment tests. Contrary to the hypothesis of recent invasion, high levels of genetic subdivision were found among five Korean and three Japanese populations. Nested clade analysis suggested a variety of processes operating over different time scales, including ancient isolation between Korea and Japan and more recent range expansions within each country. Contrary to a priori expectations, the results also suggested the recent introduction of a mitochondrial haplotype into Yokohama, Japan that is related closely to a widespread haplotype found throughout Korea. Assignment tests also supported these conclusions. The combination of a genealogical approach and probabilistic assignments of individuals to populations of origin was able to provide statistical support for the identification of cryptic introductions within an otherwise widespread indigenous species.

Tubulins in Trichomonas vaginalis: molecular characterization of alpha-tubulin genes, posttranslational modifications, and homology modeling of the tubulin dimer

We have isolated and analysed an alpha-tubulin-encoding gene (atub1) in an early-diverging eukaryote, Trichomonas vaginalis. The complete atub1 open reading frame included 1.356 bp encoding a polypeptide of 452 amino-acyl residues. A second alpha-tubulin gene (atub2) was amplified by PCR using primers derived from consensus alpha-tubulin amino acid sequences. Both T. vaginalis alpha-tubulin sequences showed high identity to those described in other parabasalids (94.4%-97.3%), and exhibited a high degree of similarity to sequences from Metazoa (such as pig brain) and diplomonads (such as Giardia). Despite large evolutionary distances previously observed between trichomonads and mammals, the three-dimensional model of the T. vaginalis tubulin dimer was very similar to that of pig brain. Possible correlations between alpha-tubulin sequences and posttranslational modifications (PTMs) were examined. Our observations corroborated previous data obtained in T. vaginalis using specific anti-PTMs antibodies. As described in the related species Tritrichomonas mobilensis, microtubules are likely acetylated, non-tyrosinated, glutamylated, and non-glycylated in T. vaginalis. Evolutionary considerations concerning the time of appearance of these tubulin PTMs are also discussed since trichomonads are potentially one of the earliest diverging eukaryotic lineages.

Molecular motors and membrane traffic in Dictyostelium

Phagocytosis and membrane traffic in general are largely dependent on the cytoskeleton and their associated molecular motors. The myosin family of motors, especially the unconventional myosins, interact with the actin cortex to facilitate the internalization of external materials during the early steps of phagocytosis. Members of the kinesin and dynein motor families, which mediate transport along microtubules (MTs), facilitate the intracellular processing of the internalized materials and the movement of membrane. Recent studies indicate that some unconventional myosins are also involved in membrane transport, and that the MT- and actin-dependent transport systems might interact with each other. Studies in Dictyostelium have led to the discovery of many motors involved in critical steps of phagocytosis and membrane transport. With the ease of genetic and biochemical approaches, the established functional analysis to test phagocytosis and vesicle transport, and the effort of the Dictyostelium cDNA and Genome Projects, Dictyostelium will continue to be a superb model system to study phagocytosis in particular and cytoskeleton and motors in general.

Microtubule organization and the effects of GFP-tubulin expression in dictyostelium discoideum

We have labeled microtubules in living Dictyostelium amoebae by incorporation of a GFP-alpha-tubulin fusion protein. The GFP-alpha-tubulin incorporates into microtubules and, as reported by others [Neujahr et al., 1998], the labeled microtubules are highly motile. Electron microscopy (EM) analysis of the distribution and organization of microtubules in the amoebae shows that some cytoplasmic microtubules form close associations. These associations could allow motor proteins attached to one microtubule to walk along an adjacent microtubule and thus generate some of the observed motility. Protein blot analysis indicates that the GFP-alpha-tubulin incorporates into microtubules at a lower efficiency than does the endogenous alpha-tubulin. EM and immunofluorescence (IF) analyses suggest that the GFP-alpha-tubulin interferes with microtubule nucleation. We have also observed an increased sensitivity of the GFP-alpha-tubulin expressing cells to blue light, as compared to wild-type cells. These results suggest that although GFP-alpha-tubulin can be used as a marker for microtubules in living cells, the use of this marker is not recommended for certain types of studies such as assembly dynamics.

Phosphofructokinase from Dictyostelium discoideum is a potent inhibitor of tubulin polymerization

We identified the nonallosteric phosphofructokinase from the slime mold Dictyostelium discoideum as a potent protein factor that inhibits the rate of polymerization of tubulin at a molar ratio of 1 molecule to about 300 tubulin dimers for half-maximal action (IC50 = 32 nM). This effect was (i) assessed by turbidity measurements, pelleting of microtubules, and electron microscopy, (ii) observed when tubulin assembly was induced by taxol as well as by GTP in the presence of microtubule-associated proteins or glutamate, and (iii) specific as it was not produced by the phosphofructokinase from rabbit muscle. Also in contrast to the latter, neither tubulin nor microtubules modified the catalytic activity of the slime mold isozyme. Immunoelectron microscopy provided further evidence that D. discoideumphosphofructokinase physically interacts with tubulin, leading to the formation of aggregates. The process seems to be reversible since microtubules eventually formed in the presence of the inhibitor with concomitant reduction of tubulin aggregates. Limited proteolysis by subtilisin showed that the hypervariable C-termini of tubulin is not involved in the interaction with the enzyme. The possible physiological relevance of this novel function of D. discoideum phosphofructokinase different from its glycolytic action is discussed.

Microtubule-mediated centrosome motility and the positioning of cleavage furrows in multinucleate myosin II-null cells

To study centrosome motility and the interaction of microtubules with the cell cortex in mitotic, post-mitotic and interphase cells, (alpha)-tubulin was tagged in Dictyostelium discoideum with green fluorescent protein. Multinucleate cells formed by myosin II-null mutants proved to be especially suited for the analysis of the control of cleavage furrow formation by the microtubule system. After docking of the mitotic apparatus onto the cell cortex during anaphase, the cell surface is activated to form ruffles on top of the asters of microtubules that emanate from the centrosomes. Cleavage furrows are initiated at spaces between the asters independently of the positions of spindles. Once initiated, the furrows expand as deep folds without a continued connection to the microtubule system. Occurrence of unilateral furrows indicates that a closed contractile ring is dispensable for cytokinesis in Dictyostelium. The progression of cytokinesis in the multinucleate cells underlines the importance of proteins other than myosin II in specifying a cleavage furrow. The analysis of centrosome motility suggests a major role for a minus-end directed motor protein, probably cytoplasmic dynein, in applying traction forces on guiding microtubules that connect the centrosome with the cell cortex.

Multiple forms of tubulin: different gene products and covalent modifications

Tubulin, the subunit protein of microtubules, is an alpha/beta heterodimer. In many organisms, both alpha and beta exist in numerous isotypic forms encoded by different genes. In addition, both alpha and beta undergo a variety of posttranslational covalent modifications, including acetylation, phosphorylation, detyrosylation, polyglutamylation, and polyglycylation. In this review the distribution and possible functional significance of the various forms of tubulin are discussed. In analyzing the differences among tubulin isotypes encoded by different genes, some appear to have no functional significance, some increase the overall adaptability of the organism to environmental challenges, and some appear to perform specific functions including formation of particular organelles and interactions with specific proteins. Purified isotypes also display different properties in vitro. Although the significance of all the covalent modification of tubulin is not fully understood, some of them may influence the stability of modified microtubules in vivo as well as interactions with certain proteins and may help to determine the functional role of microtubules in the cell. The review also discusses isotypes of gamma-tubulin and puts various forms of tubulin in an evolutionary context.

Multiple lobster tubulin isoforms are encoded by a simple gene family

Microtubule proteins isolated from pleopod tegumental gland (PTG) tissue of the American lobster, Homarus americanus, reveal a complex tubulin (Tub) profile. To determine whether Tub heterogeneity in PTG is due to expression of a large tub gene family or the result of post-translational modification, a PTG cDNA library was constructed. Clones coding for both alpha- and beta-Tub were isolated, sequenced and found to contain open reading frames (ORFs) of 451 amino acids (aa). Alignments reveal phylogenetic clustering with other arthropods and identify unique changes in primary structure which may have functional significance. These clones, when used to probe restriction enzyme-digested lobster genomic DNA in transfer-hybridization experiments, revealed a simple banding pattern indicating a lobster tub gene family of limited complexity. Lobsters appear to make use of a small tub gene family and fulfill the varied functional requirements imposed upon cellular microtubules through post-translational modifications of relatively few gene products.

The Dictyostelium cytoskeleton

New avenues of cytoskeleton research in Dictyostelium discoideum have opened up with the cloning of the alpha- and beta-tubulin genes and the characterization of kinesins and cytoplasmic dynein. Much research, however, continues to focus on the actin cytoskeleton and its dynamics during chemotaxis, morphogenesis, and other motile processes. New actin-associated proteins are being identified and characterized by biochemical means and through isolation of mutants lacking individual components. This work is shedding light on the roles of specific actin assemblies in various biological processes.

The role of Ca2+ during spore germination in Dictyostelium: autoactivation is mediated by the mobilization of Ca2+ while amoebal emergence requires entry of external Ca2+

One of the developmental pathways used by the social amoeba Dictyostelium discoideum produces dormant spores. As with any temporary resistant stage, these spores must be able to germinate rapidly in response to positive environmental stimuli. One such stimulus is the autoactivator, an endogenous, diffusible molecule that is secreted by spores. Previous work has shown that three phases of germination, autoactivation, spore swelling and amoebal emergence, require the activity of the Ca(2+)-dependent, regulatory protein calmodulin, implicating Ca2+ as an essential cation during germination. In this study we used a pharmacological approach coupled with the direct measurement of Ca2+ levels in germinating spore populations by atomic adsorption to examine Ca(2+)-dependent signal transduction during spore activation and germination in D. discoideum. Inhibitors of both phospholipase C and internal Ca2+ release inhibited autoactivation while exogenously added Ins(1,4,5)P3, acted synergistically with the autoactivator. The antagonists specifically affected spore activation as mediated by the autoactivator, since neither had any effect on heat-activated spores. In contrast, La3+, an inhibitor of Ca2+ uptake, had little or no effect on either autoactivation or the swelling of autoactivated spores. However, an inhibition of Ca2+ influx by La3+ inhibited both the swelling of heat-activated spores and amoebal emergence following each period of autoactivation or heat activation. Ca2+ levels change in the spore population during germination. During activation and swelling, Ca2+ efflux occurs from the spores. Both of the activating stimuli used here, the autoactivator and heat, caused this Ca2+ efflux. The efflux is reversed during emergence when there is a net Ca2+ uptake by the spores and cells from the medium. Together these data provide the first evidence that autoactivation is mediated by Ca(2+)-dependent signal transduction, leading to Ca2+ efflux, and that the late event of germination, amoebal emergence, requires Ca2+ uptake to proceed. The data also suggest that the responses of the spore to the each of autoactivator and heat, i.e. Ca2+ movements and germination, are mediated by different mechanisms.

The alpha- and beta-tubulin genes of Euplotes octocarinatus

The alpha- and the beta-tubulin genes of the hypotrichous ciliate Euplotes octocarinatus were isolated from a size-selected macronuclear DNA library. The alpha-tubulin gene is located on a 1,587 bp macronuclear DNA molecule and the beta-tubulin gene on a 1,524 bp macronuclear DNA molecule. Sequencing revealed that all the cysteine residues of the two genes are encoded by the common cysteine codons UGU and UGC and none by an UGA codon. This is in contrast to the genes of E. octocarinatus sequenced so far, where some of the cysteines are encoded by the opal codon UGA. The tubulin genes end like other Euplotes genes with a TAA. They do not contain introns. The last codon for an amino acid in the alpha-tubulin gene is a GAA which codes for glutamic acid. This is in contrast to what has been reported for most alpha-tubulin genes, but it supports for other hypotrichous ciliates. No evidence for the existence of more than one type of alpha- and one type of beta-tubulin genes could be obtained.