Developmental distribution of RNA and protein products of the Drosophila alpha-tubulin gene family

A force-sensitive mutation reveals a non-canonical role for dynein in anaphase progression

The diverse roles of the dynein motor in shaping microtubule networks and cargo transport complicate in vivo analysis of its functions significantly. To address this issue, we have generated a series of missense mutations in Drosophila Dynein heavy chain. We show that mutations associated with human neurological disease cause a range of defects, including impaired cargo trafficking in neurons. We also describe a novel microtubule-binding domain mutation that specifically blocks the metaphase-anaphase transition during mitosis in the embryo. This effect is independent from dynein's canonical role in silencing the spindle assembly checkpoint. Optical trapping of purified dynein complexes reveals that this mutation only compromises motor performance under load, a finding rationalized by the results of all-atom molecular dynamics simulations. We propose that dynein has a novel function in anaphase progression that depends on it operating in a specific load regime. More broadly, our work illustrates how in vivo functions of motors can be dissected by manipulating their mechanical properties.

A force-sensitive mutation reveals a spindle assembly checkpoint-independent role for dynein in anaphase progression

The cytoplasmic dynein-1 (dynein) motor organizes cells by shaping microtubule networks and moving a large variety of cargoes along them. However, dynein's diverse roles complicate in vivo studies of its functions significantly. To address this issue, we have used gene editing to generate a series of missense mutations in Drosophila Dynein heavy chain (Dhc). We find that mutations associated with human neurological disease cause a range of defects in larval and adult flies, including impaired cargo trafficking in neurons. We also describe a novel mutation in the microtubule-binding domain (MTBD) of Dhc that, remarkably, causes metaphase arrest of mitotic spindles in the embryo but does not impair other dynein-dependent processes. We demonstrate that the mitotic arrest is independent of dynein's well-established roles in silencing the spindle assembly checkpoint. In vitro reconstitution and optical trapping assays reveal that the mutation only impairs the performance of dynein under load. In silico all-atom molecular dynamics simulations show that this effect correlates with increased flexibility of the MTBD, as well as an altered orientation of the stalk domain, with respect to the microtubule. Collectively, our data point to a novel role of dynein in anaphase progression that depends on the motor operating in a specific load regime. More broadly, our work illustrates how cytoskeletal transport processes can be dissected in vivo by manipulating mechanical properties of motors.

The polarity protein Dlg5 regulates collective cell migration during Drosophila oogenesis

Collective migration plays critical roles in animal development, physiological events, and cancer metastasis. However, the molecular mechanisms of collective cell migration are not well understood. Drosophila border cells represent an excellent in vivo genetic model to study collective cell migration and identify novel regulatory genes for cell migration. Using the Mosaic Analysis with a Repressible Cell Marker (MARCM) system, we screened 240 P-element insertion lines to identify essential genes for border cell migration. Two genes were uncovered, including dlg5 (discs large 5) and CG31689. Further analysis showed that Dlg5 regulates the apical-basal polarity and cluster integrity in border cell clusters. Dlg5 is enriched in lateral surfaces between border cells and central polar cells but also shows punctate localization between border cells. We found that the distribution of Dlg5 in border cell clusters is regulated by Armadillo. Structure-function analysis revealed that the N-terminal Coiled-coil domain and the C-terminal PDZ3-PDZ4-SH3-GUK domains but not the PDZ1-PDZ2 domains of Dlg5 are required for BC migration. The Coiled-coil domain and the PDZ4-SH3-GUK domains are critical for Dlg5’s cell surface localization in border cell clusters.

A modular toolset of phiC31-based fluorescent protein tagging vectors for Drosophila

The Drosophila transgenic technology and fluorescent protein fusions are powerful tools to analyze protein expression patterns, subcellular localization and protein dynamics. Recently, the Drosophila transgenic technology has been improved by the highly efficient phiC31 site-specific integration system. Many new and improved fluorescent proteins with desirable advantages have been developed. However, the phiC31 system and the newly developed fluorescent proteins have not been systematically applied in Drosophila transgenic vectors. Here, we have constructed a modular toolset of C-terminal fluorescent protein fusion vectors based on phiC31 site-specific integration system for the generation of transgenic Drosophila lines. These cloning vectors contain a variety of fluorescent tags, including blue, cyan, green or red fluorescent proteins, photoactivatable or photoswitchable fluorescent proteins, fluorescent timers, photosensitizers and bimolecular fluorescence complementation tags. These vectors provide a range of transcriptional regulation options including UAST, UASP, UASC, LexAop, QUAS, Ubi, αTub67C and αTub84B promoters, and two screening marker options including white and vermilion gene. The vectors have been tested in vivo and can produce fluorescent chimeric proteins that are functional.

Analysis of Drosophila melanogaster testis transcriptome

Background

The formation of matured and individual sperm involves a series of molecular and spectacular morphological changes of the developing cysts in Drosophila melanogaster testis. Recent advances in RNA Sequencing (RNA-Seq) technology help us to understand the complexity of eukaryotic transcriptomes by dissecting different tissues and developmental stages of organisms. To gain a better understanding of cellular differentiation of spermatogenesis, we applied RNA-Seq to analyse the testis-specific transcriptome, including coding and non-coding genes.

Results

We isolated three different parts of the wild-type testis by dissecting and cutting the different regions: 1.) the apical region, which contains stem cells and developing spermatocytes 2.) the middle region, with enrichment of meiotic cysts 3.) the basal region, which contains elongated post-meiotic cysts with spermatids. Total RNA was isolated from each region and analysed by next-generation sequencing. We collected data from the annotated 17412 Drosophila genes and identified 5381 genes with significant transcript accumulation differences between the regions, representing the main stages of spermatogenesis. We demonstrated for the first time the presence and region specific distribution of 2061 lncRNAs in testis, with 203 significant differences. Using the available modENCODE RNA-Seq data, we determined the tissue specificity indices of Drosophila genes. Combining the indices with our results, we identified genes with region-specific enrichment in testis.

Conclusion

By multiple analyses of our results and integrating existing knowledge about Drosophila melanogaster spermatogenesis to our dataset, we were able to describe transcript composition of different regions of Drosophila testis, including several stage-specific transcripts. We present searchable visualizations that can facilitate the identification of new components that play role in the organisation and composition of different stages of spermatogenesis, including the less known, but complex regulation of post-meiotic stages.

Electronic supplementary material

The online version of this article (10.1186/s12864-018-5085-z) contains supplementary material, which is available to authorized users.

Effects of mutating α-tubulin lysine 40 on sensory dendrite development

Microtubules are essential for neuronal structure and function. Axonal and dendritic microtubules are enriched in post-translational modifications that impact microtubule dynamics, transport and microtubule-associated proteins. Acetylation of α-tubulin lysine 40 (K40) is a prominent and conserved modification of neuronal microtubules. However, the cellular role of microtubule acetylation remains controversial. To resolve how microtubule acetylation might affect neuronal morphogenesis, we mutated endogenous α-tubulin in vivo using a new Drosophila strain that facilitates the rapid knock-in of designer αTub84B alleles (the predominant α-tubulin-encoding gene in flies). Leveraging our new strain, we found that microtubule acetylation, as well as polyglutamylation and (de)tyrosination, is not essential for survival. However, we found that dendrite branch refinement in sensory neurons relies on α-tubulin K40. Mutagenesis of K40 reveals moderate yet significant changes in dendritic lysosome transport, microtubule polymerization and Futsch protein distribution in dendrites but not in axons. Our studies point to an unappreciated role for α-tubulin K40 and acetylation in dendrite morphogenesis. While our results are consistent with the idea that acetylation tunes microtubule function within neurons, they also suggest there may be an acetylation-independent requirement for α-tubulin K40.

This article has an associated First Person interview with the first author of the paper.

Highlighted Article: Neurons are enriched in post-translationally modified microtubules. Targeted mutagenesis of endogenous α-tubulin in flies reveals that dendrite branch refinement is altered by acetylation-blocking mutations.

Increased acetylation of microtubules rescues human tau-induced microtubule defects and neuromuscular junction abnormalities in Drosophila

Tau normally associates with and stabilizes microtubules (MTs), but is hyperphosphorylated and aggregated into neurofibrillary tangles in Alzheimer's disease and related neurodegenerative diseases, which are collectively known as tauopathies. MTs are regulated by different forms of post-translational modification, including acetylation; acetylated MTs represent a more stable microtubule population. In our previous study, we showed that inhibition of histone deacetylase 6 (HDAC6), which deacetylates tubulin at lysine 40, rescues defects in MTs and in neuromuscular junction growth caused by tau overexpression. However, HDAC6 also acts on other proteins that are involved in distinct biological processes unrelated to tubulins. In order to examine directly the role of increased tubulin acetylation against tau toxicity, we generated a site-directed α-tubulin^K40Q mutation by CRISPR/Cas9 technology to mimic the acetylated MTs and found that acetylation-mimicking α-tubulin rescued tau-induced MT defects and neuromuscular junction developmental abnormalities. We also showed that late administration of ACY-1215 and tubastatin A, two potent and selective inhibitors of HDAC6, rescued the tau-induced MT defects after the abnormalities had already become apparent. Overall, our results indicate that increasing MT acetylation by either genetic manipulations or drugs might be used as potential strategies for intervention in tauopathies.

Highlighted Article: Increased acetylation of microtubules by genetic and pharmacological approaches rescues human tau overexpression-induced toxicity in Drosophila muscles and neurons.

An Amino-Terminal Polo Kinase Interaction Motif Acts in the Regulation of Centrosome Formation and Reveals a Novel Function for centrosomin (cnn) in Drosophila

The formation of the pericentriolar matrix (PCM) and a fully functional centrosome in syncytial Drosophila melanogaster embryos requires the rapid transport of Cnn during initiation of the centrosome replication cycle. We show a Cnn and Polo kinase interaction is apparently required during embryogenesis and involves the exon 1A-initiating coding exon, suggesting a subset of Cnn splice variants is regulated by Polo kinase. During PCM formation exon 1A Cnn-Long Form proteins likely bind Polo kinase before phosphorylation by Polo for Cnn transport to the centrosome. Loss of either of these interactions in a portion of the total Cnn protein pool is sufficient to remove native Cnn from the pool, thereby altering the normal localization dynamics of Cnn to the PCM. Additionally, Cnn-Short Form proteins are required for polar body formation, a process known to require Polo kinase after the completion of meiosis. Exon 1A Cnn-LF and Cnn-SF proteins, in conjunction with Polo kinase, are required at the completion of meiosis and for the formation of functional centrosomes during early embryogenesis.

αTubulin 67C and Ncd are essential for establishing a cortical microtubular network and formation of the Bicoid mRNA gradient in Drosophila

The Bicoid (Bcd) protein gradient in Drosophila serves as a paradigm for gradient formation in textbooks. To explain the generation of the gradient, the ARTS model, which is based on the observation of a bcd mRNA gradient, proposes that the bcd mRNA, localized at the anterior pole at fertilization, migrates along microtubules (MTs) at the cortex to the posterior to form a bcd mRNA gradient which is translated to form a protein gradient. To fulfil the criteria of the ARTS model, an early cortical MT network is thus a prerequisite. We report hitherto undiscovered MT activities in the early embryo important for bcd mRNA transport: (i) an early and omnidirectional MT network exclusively at the anterior cortex of early nuclear cycle embryos showing activity during metaphase and anaphase only, (ii) long MTs up to 50 µm extending into the yolk at blastoderm stage to enable basal-apical transport. The cortical MT network is not anchored to the actin cytoskeleton. The posterior transport of the mRNA via the cortical MT network critically depends on maternally-expressed αTubulin67C and the minus-end motor Ncd. In either mutant, cortical transport of the bcd mRNA does not take place and the mRNA migrates along another yet undisclosed interior MT network, instead. Our data strongly corroborate the ARTS model and explain the occurrence of the bcd mRNA gradient.

Probing the boundaries of orthology: the unanticipated rapid evolution of Drosophila centrosomin

The rapid evolution of essential developmental genes and their protein products is both intriguing and problematic. The rapid evolution of gene products with simple protein folds and a lack of well-characterized functional domains typically result in a low discovery rate of orthologous genes. Additionally, in the absence of orthologs it is difficult to study the processes and mechanisms underlying rapid evolution. In this study, we have investigated the rapid evolution of centrosomin (cnn), an essential gene encoding centrosomal protein isoforms required during syncytial development in Drosophila melanogaster. Until recently the rapid divergence of cnn made identification of orthologs difficult and questionable because Cnn violates many of the assumptions underlying models for protein evolution. To overcome these limitations, we have identified a group of insect orthologs and present conserved features likely to be required for the functions attributed to cnn in D. melanogaster. We also show that the rapid divergence of Cnn isoforms is apparently due to frequent coding sequence indels and an accelerated rate of intronic additions and eliminations. These changes appear to be buffered by multi-exon and multi-reading frame maximum potential ORFs, simple protein folds, and the splicing machinery. These buffering features also occur in other genes in Drosophila and may help prevent potentially deleterious mutations due to indels in genes with large coding exons and exon-dense regions separated by small introns. This work promises to be useful for future investigations of cnn and potentially other rapidly evolving genes and proteins.

Multiple functionally divergent and conserved copies of alpha tubulin in bdelloid rotifers

BACKGROUND

Bdelloid rotifers are microscopic animals that have apparently survived without sex for millions of years and are able to survive desiccation at all life stages through a process called anhydrobiosis. Both of these characteristics are believed to have played a role in shaping several unusual features of bdelloid genomes discovered in recent years. Studies into the impact of asexuality and anhydrobiosis on bdelloid genomes have focused on understanding gene copy number. Here we investigate copy number and sequence divergence in alpha tubulin. Alpha tubulin is conserved and normally present in low copy numbers in animals, but multiplication of alpha tubulin copies has occurred in animals adapted to extreme environments, such as cold-adapted Antarctic fish. Using cloning and sequencing we compared alpha tubulin copy variation in four species of bdelloid rotifers and four species of monogonont rotifers, which are facultatively sexual and cannot survive desiccation as adults. Results were verified using transcriptome data from one bdelloid species, Adineta ricciae.

RESULTS

In common with the typical pattern for animals, monogonont rotifers contain either one or two copies of alpha tubulin, but bdelloid species contain between 11 and 13 different copies, distributed across five classes. Approximately half of the copies form a highly conserved group that vary by only 1.1% amino acid pairwise divergence with each other and with the monogonont copies. The other copies have divergent amino acid sequences that evolved significantly faster between classes than within them, relative to synonymous changes, and vary in predicted biochemical properties. Copies of each class were expressed under the laboratory conditions used to construct the transcriptome.

CONCLUSIONS

Our findings are consistent with recent evidence that bdelloids are degenerate tetraploids and that functional divergence of ancestral copies of genes has occurred, but show how further duplication events in the ancestor of bdelloids led to proliferation in both conserved and functionally divergent copies of this gene.

Drosophila spermiogenesis: Big things come from little packages

Drosophila melanogaster spermatids undergo dramatic morphological changes as they differentiate from small round cells approximately 12 μm in diameter into highly polarized, 1.8 mm long, motile sperm capable of participating in fertilization. During spermiogenesis, syncytial cysts of 64 haploid spermatids undergo synchronous differentiation. Numerous changes occur at a subcellular level, including remodeling of existing organelles (mitochondria, nuclei), formation of new organelles (flagellar axonemes, acrosomes), polarization of elongating cysts and plasma membrane addition. At the end of spermatid morphogenesis, organelles, mitochondrial DNA and cytoplasmic components not needed in mature sperm are stripped away in a caspase-dependent process called individualization that results in formation of individual sperm. Here, we review the stages of Drosophila spermiogenesis and examine our current understanding of the cellular and molecular mechanisms involved in shaping male germ cell-specific organelles and forming mature, fertile sperm.

The Anopheles gambiae alpha-tubulin-1b promoter directs neuronal, testes and developing imaginal tissue specific expression and is a sensitive enhancer detector

A knowledge gap in mosquito functional genetic analysis is the dearth of characterized regulatory regions that can target tissue specific transgene expression. To broaden the tools available, a promoter region of the Anopheles gambiaeα-tubulin1b gene has been assayed following fusion to the green fluorescent protein (GFP) reporter gene and stable transformation of An. gambiae. In eight transgenic lines, the Angtub α1b regulatory region directed a core profile of tissue specific expression in the head, chordotonal organs, ventral nerve cord and testes. This profile overlaps those seen for α2-tubulin expression in Drosophila melanogaster and Bombyx mori. In addition, widespread position dependant expression was observed in other specific tissues that were unique to each line. For example, in different lines, expression was observed in larval and adult muscles, fatbody, cuticle and midgut secretory cells. The majority of genomic transgene insertions were mapped to within 10 kb of a gene, suggesting that the Angtub α1b basal promoter is particularly sensitive to enhancers and may be suitable to form the basis of a sensitive enhancer trapping construct, in combination with a binary expression system such as Gal4-UAS.

Grainyhead and Zelda compete for binding to the promoters of the earliest-expressed Drosophila genes

Maternally contributed mRNAs and proteins control the initial stages of development following fertilization. During this time, most of the zygotic genome remains transcriptionally silent. The initiation of widespread zygotic transcription is coordinated with the degradation of maternally provided mRNAs at the maternal-to-zygotic transition (MZT). While most of the genome is silenced prior to the MZT, a small subset of zygotic genes essential for the future development of the organism is transcribed. Previous work in our laboratory and others identified the TAGteam element, a set of related heptameric DNA-sequences in the promoters of many early-expressed Drosophila genes required to drive their unusually early transcription. To understand how this unique subset of genes is regulated, we identified a TAGteam-binding factor Grainyhead (Grh). We demonstrated that Grh and the previously characterized transcriptional activator Zelda (Zld) bind to different TAGteam sequences with varying affinities, and that Grh competes with Zld for TAGteam occupancy. Moreover, overexpression of Grh in the early embryo causes defects in cell division, phenocopying Zld depletion. Our findings indicate that during early embryonic development the precise timing of gene expression is regulated by both the sequence of the TAGteam elements in the promoter and the relative levels of the transcription factors Grh and Zld.

The SILAC fly allows for accurate protein quantification in vivo

Stable isotope labeling by amino acids in cell culture (SILAC) is widely used to quantify protein abundance in tissue culture cells. Until now, the only multicellular organism completely labeled at the amino acid level was the laboratory mouse. The fruit fly Drosophila melanogaster is one of the most widely used small animal models in biology. Here, we show that feeding flies with SILAC-labeled yeast leads to almost complete labeling in the first filial generation. We used these "SILAC flies" to investigate sexual dimorphism of protein abundance in D. melanogaster. Quantitative proteome comparison of adult male and female flies revealed distinct biological processes specific for each sex. Using a tudor mutant that is defective for germ cell generation allowed us to differentiate between sex-specific protein expression in the germ line and somatic tissue. We identified many proteins with known sex-specific expression bias. In addition, several new proteins with a potential role in sexual dimorphism were identified. Collectively, our data show that the SILAC fly can be used to accurately quantify protein abundance in vivo. The approach is simple, fast, and cost-effective, making SILAC flies an attractive model system for the emerging field of in vivo quantitative proteomics.

Tubulin evolution in insects: gene duplication and subfunctionalization provide specialized isoforms in a functionally constrained gene family

Background

The completion of 19 insect genome sequencing projects spanning six insect orders provides the opportunity to investigate the evolution of important gene families, here tubulins. Tubulins are a family of eukaryotic structural genes that form microtubules, fundamental components of the cytoskeleton that mediate cell division, shape, motility, and intracellular trafficking. Previous in vivo studies in Drosophila find a stringent relationship between tubulin structure and function; small, biochemically similar changes in the major alpha 1 or testis-specific beta 2 tubulin protein render each unable to generate a motile spermtail axoneme. This has evolutionary implications, not a single non-synonymous substitution is found in beta 2 among 17 species of Drosophila and Hirtodrosophila flies spanning 60 Myr of evolution. This raises an important question, How do tubulins evolve while maintaining their function? To answer, we use molecular evolutionary analyses to characterize the evolution of insect tubulins.

Results

Sixty-six alpha tubulins and eighty-six beta tubulin gene copies were retrieved and subjected to molecular evolutionary analyses. Four ancient clades of alpha and beta tubulins are found in insects, a major isoform clade (alpha 1, beta 1) and three minor, tissue-specific clades (alpha 2-4, beta 2-4). Based on a Homarus americanus (lobster) outgroup, these were generated through gene duplication events on major beta and alpha tubulin ancestors, followed by subfunctionalization in expression domain. Strong purifying selection acts on all tubulins, yet maximum pairwise amino acid distances between tubulin paralogs are large (0.464 substitutions/site beta tubulins, 0.707 alpha tubulins). Conversely orthologs, with the exception of reproductive tissue isoforms, show little sequence variation except in the last 15 carboxy terminus tail (CTT) residues, which serve as sites for post-translational modifications (PTMs) and interactions with microtubule-associated proteins. CTT residues overwhelming comprise the co-evolving residues between Drosophila alpha 2 and beta 3 tubulin proteins, indicating CTT specializations can be mediated at the level of the tubulin dimer. Gene duplications post-dating separation of the insect orders are unevenly distributed, most often appearing in major alpha 1 and minor beta 2 clades. More than 40 introns are found in tubulins. Their distribution among tubulins reveals that insertion and deletion events are common, surprising given their potential for disrupting tubulin coding sequence. Compensatory evolution is found in Drosophila beta 2 tubulin cis-regulation, and reveals selective pressures acting to maintain testis expression without the use of previously identified testis cis-regulatory elements.

Conclusion

Tubulins have stringent structure/function relationships, indicated by strong purifying selection, the loss of many gene duplication products, alpha-beta co-evolution in the tubulin dimer, and compensatory evolution in beta 2 tubulin cis-regulation. They evolve through gene duplication, subfunctionalization in expression domain and divergence of duplication products, largely in CTT residues that mediate interactions with other proteins. This has resulted in the tissue-specific minor insect isoforms, and in particular the highly diverse α3, α4, and β2 reproductive tissue-specific tubulin isoforms, illustrating that even a highly conserved protein family can participate in the adaptive process and respond to sexual selection.

Glu415 in the alpha-tubulins plays a key role in stabilizing the microtubule-ADP-kinesin complexes

Kavar(21g), a dominant female-sterile mutation of Drosophila, identifies the alphaTubulin67C gene that encodes alpha4-tubulin, the maternally provided alpha-tubulin isoform. Although alpha4-tubulin is synthesized during oogenesis, its function is required only in the early cleavage embryos. However, once present in the developing oocyte, much of the alpha4-tubulin and the Kavar(21g)-encoded E426K-alpha4-tubulin molecules become incorporated into the microtubules. We analyzed ooplasmic streaming and lipid-droplet transport, with confocal reflection microscopy, in the developing egg primordia in the presence and absence of alpha4-tubulin and E426K-alpha4-tubulin and learnt that the E426K-alpha4-tubulin molecules eliminate ooplasmic streaming and alter lipid-droplet transport. Apparently, Glu426 is involved in stabilization of the microtubule-kinesin complexes when the kinesins are in the most labile, ADP-bound state. Replacement of Glu426 by Lys results in frequent detachments of the kinesins from the microtubules leading to reduced transport efficiency and death of the embryos derived from the Kavar(21g)-carrying females. Glu426 is a component of the twelfth alpha-helix, which is the landing and binding platform for the mechanoenzymes. Since the twelfth alpha-helix is highly conserved in the alpha-tubulin family, Glu415, which corresponds to Glu426 in the constitutively expressed alpha-tubulins, seems be a key component of microtubule-kinesin interaction and thus the microtubule-based transport.

Cooperativity between the beta-tubulin carboxy tail and the body of the molecule is required for microtubule function

Using Drosophila spermatogenesis as a model, we show that function of the beta-tubulin C-terminal tail (CTT) is not independent of the body of the molecule. For optimal microtubule function, the beta-tubulin CTT and body must match. beta2 is the only beta-tubulin used in meiosis and spermatid differentiation. beta1-tubulin is used in basal bodies, but beta1 cannot replace beta2. However, when beta1 is co-expressed with beta2, both beta-tubulins are equally incorporated into all microtubules, and males exhibit near wild type fertility. In contrast, co-expression of beta2beta1C and beta1beta2C, two reciprocal chimeric molecules with bodies and tails swapped, results in defects in meiosis, cytoskeletal microtubules, and axonemes; males produce few functional sperm and few or no progeny. In these experiments, all the same beta-tubulin parts are present, but unlike the co-assembled native beta-tubulins, the "trans" configuration of the co-assembled chimeras is poorly functional. Our data thus reveal essential intra-molecular interactions between the CTT and other parts of the beta-tubulin molecule, even though the CTT is a flexible surface feature of tubulin heterodimers and microtubules. In addition, we show that Drosophila sperm tail length depends on the total tubulin pool available for axoneme assembly and spermatid elongation. D. melanogaster and other Drosophila species have extraordinarily long sperm tails, the length of which is remarkably constant in wild type flies. We show that in males of experimental genotypes that express wild type tubulins but have half the amount of the normal tubulin pool size, sperm tails are substantially shorter than wild type.

Centrosomin: a complex mix of long and short isoforms is required for centrosome function during early development in Drosophila melanogaster

Centrosomin (Cnn) is a required core component in mitotic centrosomes during syncytial development and the presence of Cnn at centrosomes has become synonymous with fully functional centrosomes in Drosophila melanogaster. Previous studies of Cnn have attributed this embryonic function to a single isoform or splice variant. In this study, we present new evidence that significantly increases the complexity of cnn. Rather than a single isoform, Cnn function can be attributed to two unique classes of proteins that comprise a total of at least 10 encoded protein isoforms. We present the initial characterization of a new class of Cnn short isoforms required for centrosome function during gametogenesis and embryogenesis. We also introduce new evidence for a complex mix of Cnn isoforms present during early embryogenesis. Finally, we reexamine cnn mutations, in light of the short isoforms, and find previously overlooked differences attributable to allele-specific mutant phenotypes. This study addresses several questions surrounding Cnn function at the centrosome during embryogenesis and shows that cnn function cannot be ascribed to a single protein.

The cloning and expression of alpha-tubulin in Monochamus alternatus

The Japanese pine sawyer Monochamus alternatus is one of the major forest pests. It damages pine directly and transfers the nematode Bursaphelenchus xylophilus to pine wood; resulting in serious economic losses around the world every year. Alpha-tubulin is one of most important proteins in most species. We cloned a ubiquitously expressed M. alternatus alpha-tubulin gene and analysed its nucleotides and protein structure; its sequence characters are consistent with what have been reported in other insects. The alignment of proteins showed that there is high homology of alpha-tubulin between M. alternatus and other species. Western blot and immunocytochemistry analyses suggested a common epitope of alpha-tubulin between M. alternatus and Strongylcentrotus purpuratus. We also expressed the protein in Escherichia coli for further functional studies.

Axoneme-dependent tubulin modifications in singlet microtubules of the Drosophila sperm tail

Drosophila melanogaster sperm tubulins are posttranslationally glutamylated and glycylated. We show here that axonemes are the substrate for these tubulin C-terminal modifications. Axoneme architecture is required, but full length, motile axonemes are not necessary. Tubulin glutamylation occurs during or shortly after assembly into the axoneme; only glutamylated tubulins are glycylated. Tubulins in other testis microtubules are not modified. Only a small subset of total Drosophila sperm axoneme tubulins have these modifications. Biochemical fractionation of Drosophila sperm showed that central pair and accessory microtubules have the majority of poly-modified tubulins, whereas doublet microtubules have only small amounts of mono- and oligo-modified tubulins. Glutamylation patterns for different beta-tubulins experimentally assembled into axonemes were consistent with utilization of modification sites corresponding to those identified in other organisms, but surrounding sequence context was also important. We compared tubulin modifications in the 9 + 9 + 2 insect sperm tail axonemes of Drosophila with the canonical 9 + 2 axonemes of sperm of the sea urchin Lytichinus pictus and the 9 + 0 motile sperm axonemes of the eel Anguilla japonica. In contrast to Drosophila sperm, L. pictus sperm have equivalent levels of modified tubulins in both doublet and central pair microtubule fractions, whereas the doublets of A. japonica sperm exhibit little glutamylation but extensive glycylation. Tubulin C-terminal modifications are a prevalent feature of motile axonemes, but there is no conserved pattern for placement or amount of these

Regulation and function of maternal mRNA destabilization during early Drosophila development

Early embryonic development in all animals depends on maternally provided gene products. Posttranscriptional and posttranslational processes control spatial and temporal readout of the maternal information. This review focuses on the control of maternal transcript stability in the early Drosophila embryo and how transcript destabilization is necessary for normal development. The molecular pathways that regulate transcript stability are often intimately linked with other posttranscriptional mechanisms such as mRNA localization and translational regulation. These additional mechanisms are explored here with an emphasis on their relationship to transcript decay.

α4-Tubulin is involved in rapid formation of long microtubules to push apart the daughter centrosomes during earlyxDrosophilaembryogenesis

Although α4-tubulin comprises only about one-fifth of the α-tubulin pool in every Drosophila egg, in the absence of α4-tubulin - in eggs of the kavar0/- hemizygous females - only a tassel of short microtubules forms with two barely separated daughter centrosomes. We report that α4-tubulin is enriched in the long microtubules that embrace the nuclear envelope and suggest that they push apart daughter centrosomes along the nuclear perimeter during the initial cleavage divisions. In vitro tubulin polymerization showed that α4-tubulin is required for rapid tubulin polymerization. Since tubulin polymerization is slow inside eggs of the kavar0/- females, only short microtubules can form within the 4 to 5 minutes allowed for the process. A tassel of short microtubules with two barely separated centrosomes forms in every egg of the Kavar18c/+ females, in which the cytoplasm contains both wild-type and Kavar18c-encoded α4-tubulin with an E82K amino acid substitution (E82K-α4-tubulin). E82K-α4-tubulin is incorporated into the microtubules and renders them unstable. When injected into wild-type early cleavage embryos E82K-α4-tubulin slows down the formation of long microtubules and the separation of the daughter centrosomes. Surprisingly, when injected into late cleavage embryos E82K-α4-tubulin is non-toxic. Similarly, in the neuroblasts, ectopically expressed E82K-α4-tubulin becomes incorporated into the microtubules that grow sufficiently long and function normally.

centrosomin's beautiful sister (cbs) encodes a GRIP-domain protein that marks Golgi inheritance and functions in the centrosome cycle in Drosophila

The mechanism of inheritance of the Golgi complex is an important problem in cell biology. In this study, we examine the localization and function of a Golgi protein encoded by centrosomin's beautiful sister (cbs) during cleavage in Drosophila melanogaster. Cbs contains a GRIP domain that is 57% identical to vertebrate Golgin-97. Cbs undergoes a dramatic relocalization during mitosis from the cytoplasm to an association with chromosomes from late prometaphase to early telophase, by a transport mechanism that requires the GRIP domain and Arl1, the product of the Arf72A locus. Additionally, Cbs remains independent of the endoplasmic reticulum throughout cleavage. The use of RNAi, Arf72A mutant analysis and ectopic expression of the GRIP domain, shows that cycling of Cbs during mitosis is required for the centrosome cycle. The effects on the centrosome cycle depend on Cbs concentration and Cbs transport from the cytoplasm to DNA. When Cbs levels are reduced centrosomes fail to mature, and when Cbs transport is impeded by ectopic expression of the GRIP domain, centrosomes undergo hypertrophy. We propose that, Cbs is a trans-Golgi protein that links Golgi inheritance to the cell cycle and the Drosophila Golgi is more vertebrate-like than previously recognized.

Functional constraint underlies 60 million year stasis of Dipteran testis-specific beta-tubulin

How do proteins evolve while maintaining their function? Previous studies find a highly stringent structure/function relationship between the Drosophila melanogaster testis-specific tubulin beta2 and the spermtail axoneme, such that small changes in the beta2 protein render it unable to generate a motile axoneme. This raises the question, how does beta2 evolve while maintaining its function? To address this question we cloned full- and partial-length beta2 sequences from 17 species of Drosophila and Hirtodrosophila flies spanning 60 Myr of evolution. Not a single amino acid difference is coded among them-beta2 maintains its function by not evolving. We also performed gene genealogical analyses to determine ortholog/paralog relationships among insect tubulins. We find that the Lepidopteran and Dipteran testis-specific beta-tubulins are likely orthologs, and surprisingly, despite functioning in the same structure, the Lepidopteran orthologs are evolving rapidly. We argue that differences in tubulin isoform use in the testes cause the Dipteran axoneme to be less evolvable than the Lepidopteran axoneme, which has facilitated the evolution of a unique amino acid synergism in Drosophila and Hirtodrosophilabeta2 that is resistant to change, contributing to its evolutionary stasis.

Bio-Object, a stochastic simulator for post-transcriptional regulation

MOTIVATION

Recently, biologists learnt that the transport and degradation of transcribed mRNA and protein present critically important steps for the regulation of gene expression through extensive studies of RNA interference, none-sense mediated decay and ubiquitination. However, adequate consideration of these factors has not been done in the past in in silico analysis compared with transcriptional regulations.

RESULTS

We have developed a bio-system simulator 'Bio-Object' and assessed the contribution of numerous factors including movements, stability and interactions of both mRNAs and proteins in the virtual cell space to the Drosophila circadian rhythm. The oscillations of period (per), timeless (tim) and Drosophila Clock (dClk) mRNAs and proteins predicted by the simulations agreed with the observed data in Drosophila and were lost with the knock-out of either the per or the dClk gene as observed experimentally. Bio-Object predicts that (1) the stability of dClk mRNA, (2) the stability of dCLK and (3) the affinity of the PER-TIM complex are determinants of the circadian duration.

AVAILABILITY

The source code is available for download from http://www.tmd.ac.jp/mri/mri-end/bio-object/download/

The Kavar D dominant female-sterile mutations of Drosophila reveal a role for the maternally provided α-tubulin4 isoform in cleavage spindle maintenance and elongation

The dominant-negative female-sterile Kavar(D) mutations and their revertant kavar(r) alleles identify the alphaTubulin67C gene of Drosophila melanogaster, which codes for the maternally provided alpha-tubulin(4) isoform. The mutations result in the formation of monopolar, collapsed spindles (each with two nearby centrosomes, a tassel of microtubules and overcondensed chromosomes), thus revealing a novel function for alpha-tubulin(4) in spindle maintenance and elongation. Molecular features of the two Kavar(D) alleles and a kavar(null) allele are described and models for their actions are discussed.

The proximal region of the β-tubulin C-terminal tail is sufficient for axoneme assembly

We have used Drosophila testis-specific beta2-tubulin to determine sequence requirements for different microtubules. The beta2-tubulin C-terminal tail has unique sperm-specific functions [Dev Biol 158:267-286 (2003)] and is also important for forming stable heterodimers with alpha-tubulin, a general function common to all microtubules [Mol Biol Cell 12(7):2185-2194 (2001)]. beta-tubulins utilized in motile 9 + 2 axonemes contain a C-terminal sequence "axoneme motif" [Science 275 (1997) 70-73]. C-terminal truncated beta2-tubulin cannot form the sperm tail axoneme. Here we show that a partially truncated beta2-tubulin (beta2Delta7) containing only the proximal portion of the C-terminal tail, including the axoneme motif, can support production of functional motile sperm. We conclude that these proximal eight amino acids specify the binding site for protein(s) essential to support assembly of the motile axoneme. Males that express beta2Delta7, although they are fertile, produce fewer sperm than wild type males. Beta2Delta7 causes a slightly increased error rate in spermatogenesis attributable to loss of stabilizing properties intrinsic to the full-length C-terminal tail. Therefore, beta2Delta7 males would be at a selective disadvantage and it is likely that the full-length C-terminus would be essential in the wild and in evolution.

Analysis of alpha- and beta-tubulin genes of Bombyx mori using an EST database

Tubulin is one of the most widespread classes of multiprotein families and is well known to construct microtubules with two different subunits, alpha- and beta-tubulin. In the course of genome analysis of Bombyx mori, we have constructed an EST database by large-scale sequencing of clones that were randomly selected from cDNA libraries of various tissues and organs belonging to different developmental stages. Using this EST database, we have identified four types of beta-tubulin gene and three types of alpha-tubulin gene. Based on the analysis of deduced amino acid sequences, we have determined the phylogenetic relationships of tubulins between Bombyx and Drosophila melanogaster as well as two other moth species, suggesting that each tubulin is classified into at least three distinct subfamilies: a ubiquitously expressed one, a developmentally regulated one and a testis specific one.

RNAi is activated during Drosophila oocyte maturation in a manner dependent on aubergine and spindle-E

Gene silencing by double-stranded RNA is a widespread phenomenon called RNAi, involving homology-dependent degradation of mRNAs. Here we show that RNAi is established in the Drosophila female germ line. mRNA transcripts are translationally quiescent at the arrested oocyte stage and are insensitive to RNAi. Upon oocyte maturation, transcripts that are translated become sensitive to degradation while untranslated transcripts remain resistant. Mutations in aubergine and spindle-E, members of the PIWI/PAZ and DE-H helicase gene families, respectively, block RNAi activation during egg maturation and perturb translation control during oogenesis, supporting a connection between gene silencing and translation in the oocyte.

The best of all worlds or the best possible world? Developmental constraint in the evolution of beta-tubulin and the sperm tail axoneme

Through evolutionary history, some features of the phenotype show little variation. Stabilizing selection could produce this result, but the possibility also exists that a feature is conserved because it is developmentally constrained--only one or a few developmental mechanisms can produce that feature. We present experimental data documenting developmental constraint in the assembly of the motile sperm tail axoneme. The 9+2 microtubule architecture of the eukaryotic axoneme has been deeply conserved. We argue that the quality of motility supported by axonemes with this morphology explains their long conservation, rather than a developmental necessity for the 9+2 architecture. However, our functional tests in Drosophila spermatogenesis reveal considerable constraint in the coevolution of testis-specific beta-tubulin and the sperm tail axoneme. The evolution of testis beta-tubulins used in insect sperm tail axonemes is highly punctuated, indicating some pressure acting on their evolution. We provide a mechanistic explanation for their punctuated evolution by testing structure-function relationships between testis beta-tubulin and the motile axoneme in D. melanogaster. We discovered that a highly conserved sequence feature of beta-tubulins used in motile axonemes is needed to specify central pair formation. Second, our data suggest that cooperativity in the function of internal beta-tubulin amino acids is needed to support the long axonemes characteristic of Drosophila sperm tails. Thus, central pair formation constrains the evolution of the axoneme motif, and intramolecular cooperativity makes the evolution of the internal residues path dependent, which slows their evolution. Our results explain why a highly specialized beta-tubulin is needed to construct the Drosophila sperm tail axoneme. We conclude that these constraints have fixed testis-specific beta-tubulin identity in Drosophila.

Cloning and characterization of a divergent alpha-tubulin that is expressed specifically in dividing amebae of Naegleria gruberi

A novel alpha-tubulin gene (alpha6) was cloned from a genomic library of Naegleria gruberi strain NB-1 and characterized. The open reading frame of alpha6 contained 1359 nucleotides encoding a protein of 452 amino acids (aa) with a calculated molecular weight of 50.5 kDa. The nucleotide sequence of the open reading frame of alpha6 showed considerable divergence (68.4% identity) when compared with previously cloned N. gruberi alpha-tubulin genes, which share about 97% identity in DNA sequences. The deduced aa sequence of alpha6-tubulin was 61.9% identical to that of alpha13-tubulin, which was cloned from the same strain, and showed similar identities to those of alpha-tubulins from other species (54 approximately 62%). These data showed that alpha6-tubulin is one of the most divergent alpha-tubulins so far known. Alpha6-tubulin was found to be expressed in actively growing cells and repressed quickly when these cells were induced to differentiate. Immunostaining with an antibody against alpha6-tubulin showed that alpha6-tubulin is present in the nuclei and mitotic spindle-fibers but absent in flagellar axonemes or cytoskeletal microtubules. These data finally established the presence of an alpha-tubulin that is specifically utilized for spindle-fiber microtubules and distinct from the flagellar axonemal alpha-tubulins in N. gruberi, hence confirmed the multi-tubulin hypothesis in this organism.

Separable and redundant regulatory determinants in Cactus mediate its dorsal group dependent degradation

Dorsal-ventral polarity within the Drosophila syncytial blastoderm embryo is determined by the maternally encoded dorsal group signal transduction pathway that regulates nuclear localization of the transcription factor Dorsal. Nuclear uptake of Dorsal, a Rel/NFκB homolog, is controlled by the interaction with its cognate IκB inhibitor protein Cactus, which is degraded on the ventral side of the embryo in response to dorsal group signaling. Previous studies have suggested that an N-terminally located kinase target motif similar to that found in IκB proteins is involved in the spatially controlled degradation of Cactus. We report studies of the in vivo function and distribution of fusion proteins comprising segments of Cactus attached to Escherichia coli β-galactosidase (lacZ). Full-length Cactus-lacZ expressed in vivo normalizes the ventralized phenotype of embryos that lack Cactus and faithfully reconstitutes dorsal group-regulated degradation, while fusion protein constructs that lack the first 125 amino acids of Cactus escape dorsal group-dependent degradation. Furthermore, Cactus-lacZ constructs that lack only the putative IκB-dependent kinase target-like motif can nevertheless undergo spatially regulated dorsal group-dependent degradation and we have identified the regulatory determinant responsible for dorsal group-dependent degradation of Cactus in the absence of this motif. Taken together, our studies indicate the presence of two distinct redundantly acting determinants in the N terminus of Cactus that direct dorsal group-dependent degradation. Strikingly, the regulatory domain of human IκBα can also direct polarized degradation of Cactus-lacZ fusion protein.

Tubulin sorting during dimerization in vivo

We demonstrate sorting of beta-tubulins during dimerization in the Drosophila male germ line. Different beta-tubulin isoforms exhibit distinct affinities for alpha-tubulin during dimerization. Our data suggest that differences in dimerization properties are important in determining isoform-specific microtubule functions. The differential use of beta-tubulin during dimerization reveals structural parameters of the tubulin heterodimer not discernible in the resolved three-dimensional structure. We show that the variable beta-tubulin carboxyl terminus, a surface feature in the heterodimer and in microtubules, and which is disordered in the crystallographic structure, is of key importance in forming a stable alpha-beta heterodimer. If the availability of alpha-tubulin is limiting, alpha-beta dimers preferentially incorporate intact beta-tubulins rather than a beta-tubulin missing the carboxyl terminus (beta 2 Delta C). When alpha-tubulin is not limiting, beta 2 Delta C forms stable alpha-beta heterodimers. Once dimers are formed, no further sorting occurs during microtubule assembly: alpha-beta 2 Delta C dimers are incorporated into axonemes in proportion to their contribution to the total dimer pool. Co-incorporation of beta 2 Delta C and wild-type beta 2-tubulin results in nonmotile axonemes because of a disruption of the periodicity of nontubulin axonemal elements. Our data show that the beta-tubulin carboxyl terminus has two distinct roles: 1) forming the alpha-beta heterodimer, important for all microtubules and 2) providing contacts for nontubulin components required for specific microtubule structures, such as axonemes.

Axoneme-specific beta-tubulin specialization: a conserved C-terminal motif specifies the central pair

Axonemes are ancient organelles that mediate motility of cilia and flagella in animals, plants, and protists. The long evolutionary conservation of axoneme architecture, a cylinder of nine doublet microtubules surrounding a central pair of singlet microtubules, suggests all motile axonemes may share common assembly mechanisms. Consistent with this, alpha- and beta-tubulins utilized in motile axonemes fall among the most conserved tubulin sequences [1, 2], and the beta-tubulins contain a sequence motif at the same position in the carboxyl terminus [3]. Axoneme doublet microtubules are initiated from the corresponding triplet microtubules of the basal body [4], but the large macromolecular "central apparatus" that includes the central pair microtubules and associated structures [5] is a specialization unique to motile axonemes. In Drosophila spermatogenesis, basal bodies and axonemes utilize the same alpha-tubulin but different beta-tubulins [6--13]. beta 1 is utilized for the centriole/basal body, and beta 2 is utilized for the motile sperm tail axoneme. beta 2 contains the motile axoneme-specific sequence motif, but beta 1 does not [3]. Here, we show that the "axoneme motif" specifies the central pair. beta 1 can provide partial function for axoneme assembly but cannot make the central microtubules [14]. Introducing the axoneme motif into the beta 1 carboxyl terminus, a two amino acid change, conferred upon beta 1 the ability to assemble 9 + 2 axonemes. This finding explains the conservation of the axoneme-specific sequence motif through 1.5 billion years of evolution.

Conserved axoneme symmetry altered by a component beta-tubulin

Ninefold microtubule symmetry of the eukaryotic basal body and motile axoneme has been long established [1-3]. In Drosophila, these organelles contain distinct but similar beta-tubulin isoforms [4-10]: basal bodies contain only beta1-tubulin, and only beta2-tubulin is used for assembly of sperm axonemes. A single alpha-tubulin functions throughout spermatogenesis [11,12]. Thus, differences in organelle assembly reside in beta-tubulin. We tested the ability of beta1 to function in axonemes and found that beta1 alone could not generate axonemes. Small sequence differences between the two isoforms therefore mediate large differences in assembly capacity, even though these two related organelles have a common evolutionarily ancient architecture. In males with equal beta1 and beta2, beta1 was co-incorporated at equimolar ratio into functional sperm axonemes. When beta1 exceeded beta2, however, axonemes with 10 doublets were produced, an alteration unprecedented in natural phylogeny. Addition of the tenth doublet occurred by a novel mechanism, bypassing the basal body. It has been assumed that the instructions for axoneme morphogenesis reside primarily in the basal body, which normally serves as the axonemal template. Our data reveal that beta-tubulin requirements for basal bodies and axonemes are distinct, and that key information for axoneme architecture resides in the axonemal beta-tubulin.

Localization of gurken RNA in Drosophila oogenesis requires elements in the 5' and 3' regions of the transcript

During Drosophila oogenesis, signaling between the germline and the soma leads to the establishment of polarity in the egg and embryo. This process involves the interaction of gurken (grk), a TGFalpha-like protein, with torpedo (top), the Drosophila EGF receptor (Egfr). In early stage egg chambers, grk RNA is present predominantly along the posterior cortex of the oocyte, and in mid stage egg chambers, the grk transcript becomes tightly localized to the future dorsal anterior corner of the oocyte. This localization of grk RNA restricts the distribution of Gurken protein and is critical in defining both the anterior-posterior and dorsal-ventral axes of the egg. We have determined the genomic sequence of the grk gene. By testing the requirement of various fragments of grk RNA in the localization process, we find localization signals present in both the 5' and 3' regions of the gene. Sequences in the 5' noncoding region allow for accumulation of the transcript within the oocyte in early stage egg chambers, while signals in the coding region and the 3'UTR are necessary for localization in mid to late stage egg chambers. Active translation is not required for localization of the grk RNA. The mechanism of gurken RNA localization, therefore, differs from that of other localized RNAs studied to date.

Quantitative analysis of gene function in the Drosophila embryo

The specific functions of gene products frequently depend on the developmental context in which they are expressed. Thus, studies on gene function will benefit from systems that allow for manipulation of gene expression within model systems where the developmental context is well defined. Here we describe a system that allows for genetically controlled overexpression of any gene of interest under normal physiological conditions in the early Drosophila embryo. This regulated expression is achieved through the use of Drosophila lines that express a maternal mRNA for the yeast transcription factor GAL4. Embryos derived from females that express GAL4 maternally activate GAL4-dependent UAS transgenes at uniform levels throughout the embryo during the blastoderm stage of embryogenesis. The expression levels can be quantitatively manipulated through the use of lines that have different levels of maternal GAL4 activity. Specific phenotypes are produced by expression of a number of different developmental regulators with this system, including genes that normally do not function during Drosophila embryogenesis. Analysis of the response to overexpression of runt provides evidence that this pair-rule segmentation gene has a direct role in repressing transcription of the segment-polarity gene engrailed. The maternal GAL4 system will have applications both for the measurement of gene activity in reverse genetic experiments as well as for the identification of genetic factors that have quantitative effects on gene function in vivo.

The Drosophila Polycomb Group proteins ESC and E(Z) bind directly to each other and co-localize at multiple chromosomal sites

The Polycomb Group gene esc encodes an evolutionarily conserved protein required for transcriptional silencing of the homeotic genes. Unlike other Polycomb Group genes, esc is expressed and apparently required only during early embryogenesis, suggesting it is required for the initial establishment of silencing but not for its subsequent maintenance. We present evidence that the ESC protein interacts directly with E(Z), another Polycomb Group protein required for silencing of the homeotic genes. We show that the most highly conserved region of ESC, containing seven WD motifs that are predicted to fold into a beta-propeller structure, mediate its binding to a conserved N-terminal region of E(Z). Mutations in the WD region that perturb ESC silencing function in vivo also perturb binding to E(Z) in vitro. The entire WD region forms a trypsin-resistant structure, like known beta -propeller domains, and mutations that would affect the predicted ESC beta-propeller perturb its trypsin-resistance, while a putative structure-conserving mutation does not. We show by co-immunoprecipitation that ESC and E(Z) are directly associated in vivo and that they also co-localize at many chromosomal binding sites. Since E(Z) is required for binding of other Polycomb Group proteins to chromosomes, these results suggest that formation of an E(Z):ESC complex at Polycomb Response Elements may be an essential prerequisite for the establishment of silencing.

The organization of the alpha-tubulin gene family in the Drosophila montium subgroup of the melanogaster species group

The alpha 1-, alpha 2-, alpha 3-, and alpha 4-tubulin genes have been mapped by in situ hybridization to the polytene chromosomes of five species representative of the Drosophila montium subgroup geographical distribution. A lambda phage clone containing alpha 1-tubulin specific sequences was isolated from a genomic DNA library of Drosophila auraria and its restriction endonuclease pattern is presented. Both well-characterized heterologous and homologous probes were used to assess orthogonality of gene members between species groups. The in situ hybridization pattern observed in all species studied is consistent with that of Drosophila melanogaster, since alpha 1-, alpha 2-, and alpha 3-tubulin genes are located on the same polytene arm, and the alpha 4-tubulin gene is found on a different arm. Cross-hybridization was observed among alpha 1-, alpha 2-, and alpha 3-tubulin specific sequences in all species studied, using either heterologous or homologous probes. However, unlike D. melanogaster, in all montium species studied, both alpha 1- and alpha 3-tubulin specific probes hybridize to the same polytene band, indicating a clustered organization of the above genes. The chromosomal organization of this gene family would suggest that taxa within the montium subgroup are closer to their common ancestors than are the taxa in the melanogaster species group. A mode of evolution for this gene family in Drosophila is proposed.

Src64 is required for ovarian ring canal morphogenesis during Drosophila oogenesis

The Src family of protein tyrosine kinases have been implicated as important regulators of cellular proliferation, differentiation and function. In order to understand further the role of Src family kinases, we have generated loss-of-function mutations in Src64, one of two Src family kinases known in Drosophila melanogaster. Animals with reduced Src64 function develop normally and are fully viable. However, Src64 female flies have reduced fertility, which is associated with the incomplete transfer of cytoplasm from nurse cells to the developing oocyte. Analysis of Src64 egg chambers showed defects in the ring canals that interconnect the oocyte and its 15 associated nurse cells. Src64 ring canals fail to accumulate the high levels of tyrosine phosphorylation that are normally present. Despite the reduced tyrosine phosphorylation, known ring canal components such as filamentous actin, a ring canal-specific product of the hu-li tai shao gene, and the kelch protein localize properly. However, Src64 ring canals are reduced in size and frequently degenerate. These results indicate that Src64 is required for the proper growth and stability of the ovarian ring canals.

The Tomaj mutant alleles of alpha Tubulin67C reveal a requirement for the encoded maternal specific tubulin isoform in the sperm aster, the cleavage spindle apparatus and neurogenesis during embryonic development in Drosophila

The three dominant TomajD and their eleven revertant (TomajR) alleles have been localized to the alpha Tubulin67C gene of Drosophila melanogaster. Although the meiotic divisions are normally completed in eggs laid by TomajD/+, TomajD/-, TomajR/- females, embryogenesis arrests prior to the gonomeric division. The arrest is caused by: (1) the failure of prominent sperm aster formation; and (2) a consequent lack of female pronuclear migration towards the male pronucleus. Concomitant with the sperm aster defect, the four female meiotic products fuse (tetra-fusion), similar to what is seen in eggs of wild-type virgin females. In eggs of females heterozygous for weaker TomajR alleles, embryogenesis comes to a cessation before or shortly after cortical migration of cleavage nuclei. The apparent source of embryonic defect is the cleavage spindle apparatus. One of the three TomajD alleles is cold-sensitive and its cold-sensitive period coincides with the completion of female meiosis and pronuclear migration. Disorganized central and peripheral nervous systems are also characteristic of embryos derived from the temperature-sensitive TomajD/+ females. The Tomaj mutant phenotypes indicate an involvement of the normal alpha Tubulin67C gene product in: (1) the formation of the sperm aster; (2) cleavage spindle apparatus formation/function; and (3) the differentiation of the embryonic nervous system. The TomajD alleles encode a normal-sized alpha Tubulin67C isotype. Sequence analyses of the TomajD alleles revealed the replacement in different positions of a single negatively charged or neutral amino acid with a positively charged one. These residues presumably identify important functional sites.

Identification and developmental expression of a Bombyx mori alpha-tubulin gene

A cDNA clone isolated from the wing discs at the metamorphosis of Bombyx mori during the period of morphogenesis has been characterized. The amino acid sequence predicted for the putative protein is highly homologous to the Drosophila alpha1-tubulin. This is the first alpha-tubulin gene isolated in Bombyx mori and other isotype sequences are present in the Bombyx genome. The transcript is detected in the wing discs at every postembryonic stage examined, and is also expressed in other tissues, but at different levels. Although the mRNA level is maximum when the 20-hydroxyecdysone titre is high, its accumulation is independent of the hormone level both in vivo and in vitro. Significance of the accumulation of the mRNA of an ubiquitous alpha-tubulin in developing wing discs is discussed by comparison with our knowledge of the alpha-tubulin family in Drosophila and in other organisms.

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.

The mago nashi gene is required for the polarisation of the oocyte and the formation of perpendicular axes in Drosophila

BACKGROUND

Drosophila axis formation requires a series of inductive interactions between the oocyte and the somatic follicle cells. Early in oogenesis, Gurken protein, a member of the transforming growth factor alpha family, is produced by the oocyte to induce the adiacent follicle cells to adopt a posterior cell fate. These cells subsequently send an unidentified signal back to the oocyte to induce the formation of a polarised microtubule array that defines the anterior-posterior axis. The polarised microtubules also direct the movement of the nucleus and gurken mRNA from the posterior to the anterior of the oocyte, where Gurken signals a second time to induce the dorsal follicle cells, thereby polarising the dorsal-ventral axis.

RESULTS

In addition to its previously described role in the localisation of oskar mRNA, the mago nashi gene is required in the germ line for the transduction of the polarising signal from the posterior follicle cells. Using a new in vivo marker for microtubules, we show that mago nashi mutant oocytes develop a symmetric microtubule cytoskeleton that leads to the transient localisation of bicoid mRNA to both poles. Furthermore, the oocyte nucleus often fails to migrate to the anterior, causing the second Gurken signal to be sent in the same direction as the first. This results in a novel phenotype in which the anterior of the egg is ventralised and the posterior dorsalised, demonstrating that the migration of the oocyte nucleus determines the relative orientation of the two principal axes of Drosophila. The mago nashi gene is highly conserved from plants to animals, and encodes a protein that is predominantly localised to nuclei.

CONCLUSIONS

The mago nashi gene plays two essential roles in Drosophila axis formation: it is required downstream of the signal from the posterior follicle cells for the polarisation of the oocyte microtubule cytoskeleton, and has a second, independent role in the localisation of oskar mRNA to the posterior of the oocyte.

Structurally similar Drosophila alpha-tubulins are functionally distinct in vivo

We used transgenic analysis in Drosophila to compare the ability of two structurally similar alpha-tubulin isoforms to support microtubule assembly in vivo. Our data revealed that even closely related alpha-tubulin isoforms have different functional capacities. Thus, in multicellular organisms, even small changes in tubulin structure may have important consequences for regulation of the microtubule cytoskeleton. In spermatogenesis, all microtubule functions in the postmitotic male germ cells are carried out by a single tubulin heterodimer composed of the major Drosophila alpha-84B tubulin isoform and the testis-specific beta 2-tubulin isoform. We tested the ability of the developmentally regulated alpha 85E-tubulin isoform to replace alpha 84B in spermatogenesis. Even though it is 98% similar in sequence, alpha 85E is not functionally equivalent to alpha 84B. alpha 85E can support some functional microtubules in the male germ cells, but alpha 85E causes dominant male sterility if it makes up more than one-half of the total alpha-tubulin pool in the spermatids. alpha 85E does not disrupt meiotic spindle or cytoplasmic microtubules but causes defects in morphogenesis of the two classes of singlet microtubules in the sperm tail axoneme, the central pair and the accessory microtubules. Axonemal defects caused by alpha 85E are precisely reciprocal to dominant defects in doublet microtubules we observed in a previous study of ectopic germ-line expression of the developmentally regulated beta 3-tubulin isoform. These data demonstrate that the doublet and singlet axoneme microtubules have different requirements for alpha- and beta-tubulin structure. In their normal sites of expression, alpha 85E and beta 3 are coexpressed during differentiation of several somatic cell types, suggesting that alpha 85E and beta 3 might form a specialized heterodimer. Our tests of different alpha-beta pairs in spermatogenesis did not support this model. We conclude that if alpha 85E and beta 3 have specialized properties required for their normal functions, they act independently to modulate the properties of microtubules into which they are incorporated.

Enhancement of the ncdD microtubule motor mutant by mutants of alpha Tub67C

Ncd is a kinesin-related microtubule motor protein required for chromosome segregation in Drosophila oocytes and early embryos. In tests for interactions with other proteins, we find that mutants of alpha Tub67C, which affect an oocyte- and early embryo-specific alpha-tubulin, enhance meiotic nondisjunction and zygotic loss of ncdD, a partial loss-of-function mutant of ncd. The enhancement is dominant and allele-specific with respect to alpha Tub67C, and depends on the recessive effects of ncdD. Cytologically, embryos of alpha Tub67C/+ show delayed meiotic divisions and defective female pronucleus formation, while meiotic spindle assembly is abnormal in embryos of ncdD/ncdD. Doubly mutant alpha Tub67C ncdD/ncdD embryos are rescued for female pronucleus formation, but show delayed meiotic progression and defective pronuclear conjugation or fusion. Delayed completion of meiosis, together with failure of pronuclear fusion, prevents normal interactions of maternal with paternal chromosomes, enhancing the ncdD mutant phenotype. The genetics and cytology of doubly mutant embryos and the molecular defect of NcdD provide evidence for interaction of Ncd with alpha Tub67C in vivo. These results imply that a specific alpha-tubulin isoform is required for normal cellular function of a kinesin motor protein.