Structural analysis of mutations in the Drosophila beta 2-tubulin isoform reveals regions in the beta-tubulin molecular required for general and for tissue-specific microtubule functions

β-tubulin functions in spermatogenesis in Lygus hesperus Knight

Lygus hesperus Knight is an important insect pest of crops across western North America, with field management heavily reliant on the use of chemical insecticides. Because of the evolution of resistance to these insecticides, effective and environmentally benign pest management strategies are needed. Traditional sterile insect technique (SIT) has been successfully employed to manage or eradicate some insect pests but involves introducing irradiated insects with random mutations into field populations. New genetically-driven SIT techniques are a safer alternative, causing fixed mutations that manipulate individual genes in target pests to produce sterile individuals for release. Here, we identified seven β-tubulin coding genes from L. hesperus and show that Lhβtub2 is critical in male sperm production and fertility. Lhβtub2 is expressed primarily in the male testes and targeting of this gene by RNA interference or gene editing leads to male sterility.

Using the Culex pipiens sperm proteome to identify elements essential for mosquito reproduction

Mature sperm from Culex pipiens were isolated and analyzed by mass spectrometry to generate a mature sperm proteome dataset. In this study, we highlight subsets of proteins related to flagellar structure and sperm motility and compare the identified protein components to previous studies examining essential functions of sperm. The proteome includes 1700 unique protein IDs, including a number of uncharacterized proteins. Here we discuss those proteins that may contribute to the unusual structure of the Culex sperm flagellum, as well as potential regulators of calcium mobilization and phosphorylation pathways that regulate motility. This database will prove useful for understanding the mechanisms that activate and maintain sperm motility as well as identify potential molecular targets for mosquito population control.

Tubulin isotypes - functional insights from model organisms

The microtubule cytoskeleton is assembled from the α- and β-tubulin subunits of the canonical tubulin heterodimer, which polymerizes into microtubules, and a small number of other family members, such as γ-tubulin, with specialized functions. Overall, microtubule function involves the collective action of multiple α- and β-tubulin isotypes. However, despite 40 years of awareness that most eukaryotes harbor multiple tubulin isotypes, their role in the microtubule cytoskeleton has remained relatively unclear. Various model organisms offer specific advantages for gaining insight into the role of tubulin isotypes. Whereas simple unicellular organisms such as yeast provide experimental tractability that can facilitate deeper access to mechanistic details, more complex organisms, such as the fruit fly, nematode and mouse, can be used to discern potential specialized functions of tissue- and structure-specific isotypes. Here, we review the role of α- and β-tubulin isotypes in microtubule function and in associated tubulinopathies with an emphasis on the advances gained using model organisms. Overall, we argue that studying tubulin isotypes in a range of organisms can reveal the fundamental mechanisms by which they mediate microtubule function. It will also provide valuable perspectives on how these mechanisms underlie the functional and biological diversity of the cytoskeleton.

Characterization of the Drosophila suzukii β2-tubulin gene and the utilization of its promoter to monitor sex separation and insemination

The Drosophila melanogaster β2-tubulin gene (Dm-β2t) controls the function of microtubules in the testis and sperm, and has been evaluated for use in biocontrol strategies based on the sterile insect technique, including sexing and the induction of male sterility. The spotted-wing Drosophila (Drosophila suzukii) is native to eastern Asia but has spread globally as an invasive pest of fruit crops, so biocontrol strategies are urgently required for this species. We therefore isolated the β2tubulin ortholog Ds-β2t from the USA laboratory strain of D. suzukii and confirmed the presence of functional motifs by aligning orthologs from multiple insects. The developmental expression profile of Ds-β2t was determined by RT-PCR using gene-specific primers and was similar to that of Dm-β2t. We then isolated the Ds-β2t promoter and used it to generate transgenic strains expressing a testis-specific fluorescent protein starting from the thirdinstar larvae. Efficient sexing was achieved based on fluorescence detection, and the transgenic males showed a similar survival rate to wild-type males. Fluorescence imaging and PCR were also used to confirm the insemination of wild-type females by transgenic males. We therefore confirm that D. suzukii strains expressing fluorescent markers under the control of the Ds-β2t promoter can be used for sexing and the confirmation of mating, and we discuss the wider potential of the Ds-β2t promoter in the context of genetic control strategies for D. suzukii.

Distinct effects of tubulin isotype mutations on neurite growth in Caenorhabditis elegans

Different tubulin isotypes perform different functions in the regulation of microtubule (MT) structure and neurite growth, and missense mutations of tubulin genes have three types of distinct effects on MT stability and neurite growth. One α-tubulin isotype appears to induce relative instability due to the lack of potential posttranslational modification sites.

Tubulins, the building block of microtubules (MTs), play a critical role in both supporting and regulating neurite growth. Eukaryotic genomes contain multiple tubulin isotypes, and their missense mutations cause a range of neurodevelopmental defects. Using the Caenorhabditis elegans touch receptor neurons, we analyzed the effects of 67 tubulin missense mutations on neurite growth. Three types of mutations emerged: 1) loss-of-function mutations, which cause mild defects in neurite growth; 2) antimorphic mutations, which map to the GTP binding site and intradimer and interdimer interfaces, significantly reduce MT stability, and cause severe neurite growth defects; and 3) neomorphic mutations, which map to the exterior surface, increase MT stability, and cause ectopic neurite growth. Structure-function analysis reveals a causal relationship between tubulin structure and MT stability. This stability affects neuronal morphogenesis. As part of this analysis, we engineered several disease-associated human tubulin mutations into C. elegans genes and examined their impact on neuronal development at the cellular level. We also discovered an α-tubulin (TBA-7) that appears to destabilize MTs. Loss of TBA-7 led to the formation of hyperstable MTs and the generation of ectopic neurites; the lack of potential sites for polyamination and polyglutamination on TBA-7 may be responsible for this destabilization.

Structural differences between yeast and mammalian microtubules revealed by cryo-EM

Yeast MTs do not appear to undergo the lattice compaction seen in mammalian MTs upon GTP hydrolysis. Binding of the +TIP Bim1, both between and within αβ-tubulin dimers, causes compaction of yeast MTs and their rapid disassembly.

Microtubules are polymers of αβ-tubulin heterodimers essential for all eukaryotes. Despite sequence conservation, there are significant structural differences between microtubules assembled in vitro from mammalian or budding yeast tubulin. Yeast MTs were not observed to undergo compaction at the interdimer interface as seen for mammalian microtubules upon GTP hydrolysis. Lack of compaction might reflect slower GTP hydrolysis or a different degree of allosteric coupling in the lattice. The microtubule plus end–tracking protein Bim1 binds yeast microtubules both between αβ-tubulin heterodimers, as seen for other organisms, and within tubulin dimers, but binds mammalian tubulin only at interdimer contacts. At the concentrations used in cryo-electron microscopy, Bim1 causes the compaction of yeast microtubules and induces their rapid disassembly. Our studies demonstrate structural differences between yeast and mammalian microtubules that likely underlie their differing polymerization dynamics. These differences may reflect adaptations to the demands of different cell size or range of physiological growth temperatures.

Disruption of Ttll5/stamp gene (tubulin tyrosine ligase-like protein 5/SRC-1 and TIF2-associated modulatory protein gene) in male mice causes sperm malformation and infertility

TTLL5/STAMP (tubulin tyrosine ligase-like family member 5) has multiple activities in cells. TTLL5 is one of 13 TTLLs, has polyglutamylation activity, augments the activity of p160 coactivators (SRC-1 and TIF2) in glucocorticoid receptor-regulated gene induction and repression, and displays steroid-independent growth activity with several cell types. To examine TTLL5/STAMP functions in whole animals, mice were prepared with an internal deletion that eliminated several activities of the Stamp gene. This mutation causes both reduced levels of STAMP mRNA and C-terminal truncation of STAMP protein. Homozygous targeted mutant (Stamp(tm/tm)) mice appear normal except for marked decreases in male fertility associated with defects in progressive sperm motility. Abnormal axonemal structures with loss of tubulin doublets occur in most Stamp(tm/tm) sperm tails in conjunction with substantial reduction in α-tubulin polyglutamylation, which closely correlates with the reduction in mutant STAMP mRNA. The axonemes in other structures appear unaffected. There is no obvious change in the organs for sperm development of WT versus Stamp(tm/tm) males despite the levels of WT STAMP mRNA in testes being 20-fold higher than in any other organ examined. This defect in male fertility is unrelated to other Ttll genes or 24 genes previously identified as important for sperm function. Thus, STAMP appears to participate in a unique, tissue-selective TTLL-mediated pathway for α-tubulin polyglutamylation that is required for sperm maturation and motility and may be relevant for male fertility.

Identification of a tubulin-α gene specifically expressed in testis and adductor muscle during stable reference gene selection in the hermaphrodite gonad of the lion's paw scallop Nodipecten subnodosus

For non-model species, as many used for aquaculture, with minimal or no genomic information, relative quantification of gene expression studies requires preliminary research including the isolation of potential reference genes and the identification of those stably expressed under the biological conditions of interest. Here we report on the isolation of five partial gene sequences from gonad tissue cDNA in the functional hermaphrodite scallop Nodipecten subnodosus to be evaluated as reference genes: 18S-rRNA, riboprotein l8 (rp-l8), actin-β (act-β), elongation factor 1α (ef-1α) and alpha-tubulin-α (tub-α). We found that 18S-rRNA was stably expressed independently of the priming method used to reverse transcribe RNA to cDNA, oligo-dT or random hexamer. Stability analysis for the five putative reference genes with geNorm and NormFinder indicated that 18S together with rp-l8 were the most stable genes for normalization of gene expression during gonad development in both, male and female sexual regions of the hermaphrodite N. subnodosus. The least stable gene was tub-α, showing a biased expression profile between sexual regions of the gonad, therefore this gene was analyzed thereafter as a target gene together with vitellogenin (vit) and a DEAD-box RNA helicase (dbx) gene. Relative expression, estimated by normalization with the combination of 18S and rp-l8 as reference genes, indicated that as gonad development advanced two of the target genes were up-regulated, tub-α in the male region and vit in the female region. Whereas an increased expression was expected during development for vit for its known role in vitellogenesis, the increased expression of tub-α in the male sexual region was unexpected, and pointed toward this gene being a testis-specific α-tubulin isotype. Further analyses of gene expression among tissues indicated that tub-α is specifically and highly expressed in the male gonad, although expression in adductor muscle was also observed at significantly lower levels. The existence of testis specific α- and β-tubulins has been previously reported in other taxa, relating their function to sperm axoneme formation. Tissue-specific tubulin genes, particularly their promoters, have recently found an application as native promoters for transgene tissue-specific expression in research and reproductive control of insect plagues. The third target gene, a putative member of the DEAD-box RNA helicase family (dbx), showed no changes in expression during gonad development or between sexual regions, therefore it was chosen to discuss the different statistical inferences resulting from the arbitrary use of 'randomly chosen' reference genes when normalizing gene expression.

Proteome mapping of the Drosophila melanogaster male reproductive system

The fruit fly Drosophila melanogaster is an excellent model organism for studying insect reproductive biology. Although the gene expression profiles of both male and female reproductive organs have been studied in detail, their proteomic profiles and functional characteristics largely remained to be clarified. In this study, we conducted proteome mapping of the male internal reproductive organs using 2-DE. We identified a total of 440 protein components from gels of the male reproductive organs (testis, seminal vesicle, accessory gland, ejaculatory duct, and ejaculatory bulb). A number of proteins associated with odorant/pheromone-binding, lipid metabolism, proteolysis, and antioxidation were expressed tissue specifically in the male reproductive system. Based on our proteomic data set, we constructed reference proteome maps of the reproductive organs, which will provide valuable information toward a comprehensive understanding of Drosophila reproduction.

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.

Deficiency screen identifies a novel role for beta 2 tubulin in salivary gland and myoblast migration in the Drosophila embryo

The Drosophila embryonic salivary gland is an epithelial organ formed by the coordinated invagination and migration of primordial cells. To identify genes that regulate gland migration we performed a deficiency screen of the third chromosome. Here, we report on the analysis of the beta 2 tubulin isoform (beta2t) that maps at 85D15. We show that, in beta2t mutant embryos, salivary glands did not complete their posterior migration and that migration of fusion competent myoblasts and longitudinal visceral muscle founder cells between the gland and circular visceral mesoderm was delayed. We also demonstrate that gland migration defects correlate with reduced betaPS and alphaPS2 integrin expression in the surrounding mesoderm and that beta2t genetically interacts with genes encoding integrin alphaPS1 and alphaPS2 subunits. Our studies reveal for the first time that beta2t is expressed in embryogenesis and that beta2t plays an important role in salivary gland and myoblast migration, possibly through proper regulation of integrin adhesion proteins.

Lentiviral transfection of ependymal primary cultures facilitates the characterisation of kinocilia-specific promoters

Ependymal primary cultures (EPCs) are an established model for studying ependymal cell biochemistry and the biology of kinocilia-bearing cells. However, the difficulty in causing them to express transgenes at high efficiency has been an important drawback of the system. Indeed plasmid-based transfection attempts remain at an efficiency below 1% and fail to elicit reporter gene expression, namely green fluorescent protein (GFP) synthesis, in any of the kinocilia-bearing cells of the cultures. Human immunodeficiency virus pseudotyped with the vesicular stomatitis virus envelope glycoprotein (HIV/VSV-G) and encoding GFP under the control of the ubiquitously recognised promoter of elongation factor 1 alpha (EF1alpha) also does not cause transgene expression in the kinocilia-bearing cells of an EPC when applied at multiplicities of infection (MOIs) of up to 40 and destroys the culture when the MOI is increased further. In contrast, HIV/VSV-G encoding GFP under the control of a promoter specifically active in kinocilia-bearing cells leads to transgene expression in up to 79% of the kinociliated cells of an EPC when applied at an MOI of 20. This has permitted the initial characterisation of the promoter for the gene specifically transcribed in kinocilia-bearing cells, wdr16. The results have identified two regions of 100 nucleotides length each, which are critical for promoter activity and contain putative binding sites for the transcription factors Foxd1, Sox17 and Spz1. It appears that wdr16 is controlled by a bidirectional promoter also responsible for regulating the syntaxin 8 gene.

Secondary mutations correct fitness defects in Toxoplasma gondii with dinitroaniline resistance mutations

Dinitroanilines (oryzalin, trifluralin, ethafluralin) disrupt microtubules in protozoa but not in vertebrate cells, causing selective death of intracellular Toxoplasma gondii parasites without affecting host cells. Parasites containing alpha1-tubulin point mutations are dinitroaniline resistant but show increased rates of aberrant replication relative to wild-type parasites. T. gondii parasites bearing the F52Y mutation were previously demonstrated to spontaneously acquire two intragenic mutations that decrease both resistance levels and replication defects. Parasites bearing the G142S mutation are largely dependent on oryzalin for viable growth in culture. We isolated 46 T. gondii lines that have suppressed microtubule defects associated with the G142S or the F52Y mutations by acquiring secondary mutations. These compensatory mutations were alpha1-tubulin pseudorevertants or extragenic suppressors (the majority alter the beta1-tubulin gene). Many secondary mutations were located in tubulin domains that suggest that they function by destabilizing microtubules. Most strikingly, we identified seven novel mutations that localize to an eight-amino-acid insert that stabilizes the alpha1-tubulin M loop, including one (P364R) that acts as a compensatory mutation in both F52Y and G142S lines. These lines have reduced dinitroaniline resistance but most perform better than parental lines in competition assays, indicating that there is a trade-off between resistance and replication fitness.

Sperm ultrastructure and spermiogenesis of Coniopterygidae (Neuroptera, Insecta)

The spermiogenesis and the sperm ultrastructure of several species of Coniopterygidae have been examined. The spermatozoa consist of a three-layered acrosome, an elongated elliptical nucleus, a long flagellum provided with a 9+9+3 axoneme and two mitochondrial derivatives. No accessory bodies were observed. The axoneme exhibits accessory microtubules provided with 13, rather than 16, protofilaments in their tubular wall; the intertubular material is reduced and distributed differently from that observed in other Neuropterida. Sperm axoneme organization supports the isolated position of the family previously proposed on the basis of morphological data.

Axoneme beta-tubulin sequence determines attachment of outer dynein arms

Axonemes of motile eukaryotic cilia and flagella have a conserved structure of nine doublet microtubules surrounding a central pair of microtubules. Outer and inner dynein arms on the doublets mediate axoneme motility [1]. Outer dynein arms (ODAs) attach to the doublets at specific interfaces [2-5]. However, the molecular contacts of ODA-associated proteins with tubulins of the doublet microtubules are not known. We report here that attachment of ODAs requires glycine 56 in the beta-tubulin internal variable region (IVR). We show that in Drosophila spermatogenesis, a single amino acid change at this position results in sperm axonemes markedly deficient in ODAs. Moreover, we found that axonemal beta-tubulins throughout the phylogeny have invariant glycine 56 and a strongly conserved IVR, whereas nonaxonemal beta-tubulins vary widely in IVR sequences. Our data reveal a deeply conserved physical requirement for assembly of the macromolecular architecture of the motile axoneme. Amino acid 56 projects into the microtubule lumen [6]. Imaging studies of axonemes indicate that several proteins may interact with the doublet-microtubule lumen [3, 4, 7, 8]. This region of beta-tubulin may determine the conformation necessary for correct attachment of ODAs, or there may be sequence-specific interaction between beta-tubulin and a protein involved in ODA attachment or stabilization.

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

New insights into the cell biology of insect axonemes

Insects do not possess ciliated epithelia, and cilia/flagella are present in the sperm tail and--as modified cilia--in mechano- and chemosensory neurons. The core cytoskeletal component of these organelles, the axoneme, is a microtubule-based structure that has been conserved throughout evolution. However, in insects the sperm axoneme exhibits distinctive structural features; moreover, several insect groups are characterized by an unusual sperm axoneme variability. Besides the abundance of morphological data on insect sperm flagella, most of the available molecular information on the insect axoneme comes from genetic studies on Drosophila spermatogenesis, and only recently other insect species have been proposed as useful models. Here, we review the current knowledge on the cell biology of insect axoneme, including contributions from both Drosophila and other model insects.

Conformational analysis of the carboxy-terminal tails of human beta-tubulin isotypes

Several isotypes of the structural protein tubulin have been characterized. Their expression offers a plausible explanation for differences regarding microtubule function. Although sequence variation between tubulin isotypes occurs throughout the entire protein, it is the extreme carboxy-terminal tails (CTTs) that exhibit the greatest concentration of differences. In humans, the CTTs range in length from 9 to 25 residues and because of a considerable number of glutamic acid residues, contain over 1/3 of tubulin's total electrostatic charge. The CTTs are believed to be highly disordered and their precise function has yet to be determined. However, their absence has been shown to result in altered microtubule stability and a reduction in the interaction with several microtubule-associated proteins (MAPs). To characterize the role that CTTs play in microtubule function, we examined the global conformational differences within a set of nine human beta-tubulin isotypes using replica exchange molecular dynamics simulations. Through the analysis of the resulting configuration ensembles, we quantified differences such as the CTTs sequence influence on overall flexibility and average secondary structure. Although only minor variations between each CTT were observed, we suggest that these differences may be significant enough to affect interactions with MAPs, thereby influencing important properties such as microtubule assembly and stability.

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.

Unusual Axonemes of Hexapod Spermatozoa

Hexapod spermatozoa exhibit a great variation in their axoneme structure. The 9+2 pattern organization is present in a few basal taxa and in some derived groups. In most hexapods, a crown of nine accessory microtubules surrounds the 9+2 array, giving rise to the so-called 9+9+2 pattern. This general organization, however, displays a number of modifications in several taxa. In this review, the main variations concerning the number and localization of the accessory tubules, microtubular doublets, central microtubules, dynein arms, and axonemal length are summarized. We discuss the phylogenetic significance of all this structural information as well as the current hypotheses relating the sperm size and sperm polymorphism with reproductive success of some hexapod species. Also described are the biochemical data and the motility patterns which are currently known on some peculiar aberrant axonemes, in light of the contribution these models may give to the comprehension of the general functioning of the conventional 9+2 axoneme. Finally, we summarize methodological developments for the study of axoneme ultrastructure and the new opportunities for the molecular analysis of hexapod axonemes.

Replacement by Drosophila melanogaster protamines and Mst77F of histones during chromatin condensation in late spermatids and role of sesame in the removal of these proteins from the male pronucleus

Chromatin condensation is a typical feature of sperm cells. During mammalian spermiogenesis, histones are first replaced by transition proteins and then by protamines, while little is known for Drosophila melanogaster. Here we characterize three genes in the fly genome, Mst35Ba, Mst35Bb, and Mst77F. The results indicate that Mst35Ba and Mst35Bb encode dProtA and dProtB, respectively. These are considerably larger than mammalian protamines, but, as in mammals, both protamines contain typical cysteine/arginine clusters. Mst77F encodes a linker histone-like protein showing significant similarity to mammalian HILS1 protein. ProtamineA-enhanced green fluorescent protein (eGFP), ProtamineB-eGFP, and Mst77F-eGFP carrying Drosophila lines show that these proteins become the important chromosomal protein components of elongating spermatids, and His2AvDGFP vanishes. Mst77F mutants [ms(3)nc3] are characterized by small round nuclei and are sterile as males. These data suggest the major features of chromatin condensation in Drosophila spermatogenesis correspond to those in mammals. During early fertilization steps, the paternal pronucleus still contains protamines and Mst77F but regains a nucleosomal conformation before zygote formation. In eggs laid by sesame-deficient females, the paternal pronucleus remains in a protamine-based chromatin status but Mst77F-eGFP is removed, suggesting that the sesame gene product is essential for removal of protamines while Mst77F removal is independent of Sesame.

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.

Asymmetric sequence divergence of duplicate genes

Much like humans, gene duplicates may be created equal, but they do not stay that way for long. For four completely sequenced genomes we show that 20%-30% of duplicate gene pairs show asymmetric evolution in the amino acid sequence of their protein products. That is, one of the duplicates evolves much faster than the other. The greater this asymmetry, the greater the ratio Ka/Ks of amino acid substitutions (Ka) to silent substitutions (Ks) in a gene pair. This indicates that most asymmetric divergence may be caused by relaxed selective constraints on one of the duplicates. However, we also find some candidate duplicates where positive (directional) selection of beneficial mutations (Ka/Ks > 1) may play a role in asymmetric divergence. Our analysis rests on a codon-based model of molecular evolution that allows a test for asymmetric divergence in Ka. The method is also more sensitive in detecting positive selection (Ka/Ks > 1) than models relying only on pairwise gene comparisons.

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.

A new translational repression element and unusual transcriptional control regulate expression of don juan during Drosophila spermatogenesis

The Drosophila don juan (dj) gene encodes a basic protein that is expressed solely in the male germline and shows structural similarities to the linker histone H1. Don Juan is located in two different subcellular structures: in the nucleus during the phase of chromatin condensation and later in the mitochondrial derivatives starting with spermatid individualization. The don juan gene is transcribed in primary spermatocytes under the control of 23 bp upstream in combination with downstream sequences. During meiotic stages and in early spermatid stages don juan mRNA is translationally repressed for several days. Analysis of male sterile mutants which fail to undergo meiosis shows that release of dj mRNA from translational repression is independent of meiosis. In gel retardation assays 60 nucleotides at the end of the dj leader form four major complexes with proteins that were extracted from testes but not with protein extracts from ovaries. Transformation studies prove that in vivo 35 bp within that region of the dj mRNA is essential to confer translational repression. UV cross-linking studies show that a 62 kDa protein specifically binds to the same region within the 5' untranslated region. The dj translational repression element, TRE, is distinct from the translational control element, TCE, described earlier for all members of the Mst(3)CGP gene family. Moreover, expression studies in several male sterile mutants reveal that don juan mRNA is translated in earlier developmental stages during sperm morphogenesis than the Mst(3)CGP mRNAs. This proves that translational activation of dormant mRNAs in spermatogenesis occurs at different time-points which are characteristic for each gene, an essential feature for coordinated sperm morphogenesis.

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.

A transient specialization of the microtubule cytoskeleton is required for differentiation of the Drosophila visual system

Drosophila beta3-tubulin is an essential isoform expressed during differentiation of many cell types in embryos and pupae. We report here that during pupal development transient beta3 expression demarcates a unique subset of neurons in the developing adult visual system. beta3 is coassembled into microtubules with beta1, the sole beta-tubulin isoform in the permanent microtubule cytoskeleton of the adult eye and brain. Examination of beta3 mutant phenotypes showed that beta3 is required for axonal patterning and connectivity and for spatial positioning within the optic lobe. Comparison of the phenotypes of beta3 mutations with those that result from disruption of the Hedgehog signaling pathway shows that beta3 functions early in the establishment of the adult visual system. Our data support the hypothesis that beta3 confers specialized properties on the microtubules into which it is incorporated. Thus a transient specialization of the microtubule cytoskeleton during differentiation of a specific subset of the neurons has permanent consequences for later cell function.

Maternally expressed gamma Tub37CD in Drosophila is differentially required for female meiosis and embryonic mitosis

We report functional analysis of gamma Tub37CD, a maternally synthesized gamma-tubulin that is highly expressed during oogenesis and utilized at centrosomes in precellular embryos. Two gamma Tub37CD mutants contained missense mutations that altered residues conserved in all gamma-tubulins and alpha- and/or beta-tubulins. A third gamma Tub37CD missense mutant identified a conserved motif unique to gamma-tubulins. A fourth gamma Tub37CD mutant contained a nonsense mutation and the corresponding premature stop codon generated a protein null allele. Immunofluorescence analysis of laid eggs and activated oocytes derived from the mutants revealed microtubules and meiotic spindles that were close to normal even in the absence of gamma Tub37CD. Eggs lacking the maternal gamma-tubulin were arrested in meiosis, indicative of a deficiency in activation. Analysis of meiosis with in vitro activation techniques showed that the cortical microtubule cytoskeleton of mature wild-type eggs was reorganized upon activation and expressed as transient assembly of cortical asters, and this cortical reorganization was altered in gamma Tub37CD mutants. In precellular embryos of partial loss of function mutants, spindles were frequently abnormal and cell cycle progression was inhibited. Thus, gamma Tub37CD functions differentially in female meiosis and in the early embryo; while involved in oocyte activation, it is apparently not required or plays a subtle role in formation of the female meiotic spindle which is acentriolar, but is essential for assembly of a discrete bipolar mitotic spindle which is directed by centrosomes organized about centrioles.

Drosophila centrosomin protein is required for male meiosis and assembly of the flagellar axoneme

Centrosomes and microtubules play crucial roles during cell division and differentiation. Spermatogenesis is a useful system for studying centrosomal function since it involves both mitosis and meiosis, and also transformation of the centriole into the sperm basal body. Centrosomin is a protein localized to the mitotic centrosomes in Drosophila melanogaster. We have found a novel isoform of centrosomin expressed during spermatogenesis. Additionally, an anticentrosomin antibody labels both the mitotic and meiotic centrosomes as well as the basal body. Mutational analysis shows that centrosomin is required for spindle organization during meiosis and for organization of the sperm axoneme. These results suggest that centrosomin is a necessary component of the meiotic centrosomes and the spermatid basal body.

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.

Evolution of the multi-tubulin hypothesis

Microtubules are organized into diverse cellular structures in multicellular organisms. How is such diversity generated? Although highly conserved overall, variable regions within alpha- and beta-tubulins show divergence from other alpha- and beta-tubulins in the same species, but show conservation among different species. Such conservation raises the question of whether diversity in tubulin structure mediates diversity in microtubule organization. Recent studies probing the function of beta-tubulin isotypes in axonemes of insects suggest that tubulin structure, through interactions with extrinsic proteins, can direct the architecture and supramolecular organization of microtubules.

An essential cell division gene of Drosophila, absent from Saccharomyces, encodes an unusual protein with tubulin-like and myosin-like peptide motifs

Null mutations at the misato locus of Drosophila melanogaster are associated with irregular chromosomal segregation at cell division. The consequences for morphogenesis are that mutant larvae are almost devoid of imaginal disk tissue, have a reduction in brain size, and die before the late third-instar larval stage. To analyze these findings, we isolated cDNAs in and around the misato locus, mapped the breakpoints of chromosomal deficiencies, determined which transcript corresponded to the misato gene, rescued the cell division defects in transgenic organisms, and sequenced the genomic DNA. Database searches revealed that misato codes for a novel protein, the N-terminal half of which contains a mixture of peptide motifs found in alpha-, beta-, and gamma-tubulins, as well as a motif related to part of the myosin heavy chain proteins. The sequence characteristics of misato indicate either that it arose from an ancestral tubulin-like gene, different parts of which underwent convergent evolution to resemble motifs in the conventional tubulins, or that it arose by the capture of motifs from different tubulin genes. The Saccharomyces cerevisiae genome lacks a true homolog of the misato gene, and this finding highlights the emerging problem of assigning functional attributes to orphan genes that occur only in some evolutionary lineages.

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.

Microtubule architecture specified by a beta-tubulin isoform

In Drosophila melanogaster, a testis-specific beta-tubulin (beta2) is required for spermatogenesis. A sequence motif was identified in carboxyl termini of axonemal beta-tubulins in diverse taxa. As a test of whether orthologous beta-tubulins from different species are functionally equivalent, the moth Heliothis virescens beta2 homolog was expressed in Drosophila testes. When coexpressed with beta2, the moth isoform imposed the 16-protofilament structure characteristic of that found in the moth on the corresponding subset of Drosophila microtubules, which normally contain only 13-protofilament microtubules. Thus, the architecture of the microtubule cytoskeleton can be directed by a component beta-tubulin.

The marine red alga Chondrus crispus has a highly divergent beta-tubulin gene with a characteristic 5' intron: functional and evolutionary implications

We characterized a nuclear gene and its corresponding cDNA encoding beta-tubulin (gene TubB1) of the marine red alga Chondrus crispus. The deduced TubB1 protein is the most divergent beta-tubulin so far reported with only 64 to 69% amino acid identity relative to other beta-tubulins from higher and lower eukaryotes. Our analysis reveals that TubB1 has an accelerated evolutionary rate probably due to a release of functional constraints in connexion with a specialization of microtubular structures in rhodophytes. It further indicates that isoform diversity and functional differentiation of tubulins in eukaryotic cells may be controlled by independent selective constraints. TubB1 has a short spliceosomal intron at its 5' end which seems to be a characteristic feature of nuclear protein-coding genes from rhodophytes. The splice junctions of the four known rhodophyte introns comply well with the corresponding consensus sequences of higher plants in agreement with previous suggestions from phylogenetic inference that red algae and green plants may be sister groups. The paucity and asymmetrical location of introns in rhodophyte genes can be explained by differential intron loss due to conversion of genes by homologous recombination with cDNAs corresponding to reverse transcribed mRNAs or partially spliced pre-mRNAs, respectively. The identification of an intron containing TubB1 cDNA in C. crispus confirms that pre-mRNAs can escape both splicing and degradation in the nucleus prior to transport into the cytoplasm. Differential Southern hybridizations under non-stringent conditions with homologous and heterologous probes suggest that C. crispus contains a second degenerate beta-tubulin gene (or pseudogene?) which, however, is only distantly related to TubB1 as it is to the more conserved homologues of other organisms.

Regulation of beta-tubulin function and expression in Drosophila spermatogenesis

In this study we examined two aspects of beta-tubulin function in Drosophila spermatogenesis: 1) beta-tubulin structural requirements for assembly of different categories of microtubules and 2) regulatory requirements for production of the correct tubulin protein level. In normal Drosophila spermatogenesis, the testis-specific beta 2-tubulin isoform supports multiple microtubule functions. Our previous work showed that another Drosophila isoform, beta 3, cannot support spermatogenesis, whereas a carboxyl-truncated form of beta 2, beta 2 delta C, can at least to some extent provide all of beta 2's normal functions, save one: beta 2 delta C cannot support organization of axonemal microtubules into the supramolecular architecture of the axoneme. Here, to test whether beta 2 carboxyl sequences can rescue the functional failure of the beta 3 isoform in spermatogenesis, we constructed a gene encoding a chimeric protein, beta 3 beta 2C, in which beta 3 sequences in the carboxyl region are replaced with those of beta 2. Unlike either beta 3 or beta 2 delta C, beta 3 beta 2C can provide partial function for both assembly of axonemal microtubules and their organization into the supramolecular architecture of the axoneme. In particular, the beta 2 carboxyl sequences mediate morphogenesis of the axoneme doublet tubule complex, including accessory microtubule assembly and attachment of spokes and linkers. However, our data also reveal aspects of beta 2-specific function that require structural features other than the primary sequence of the isotype-defining variable regions, the C terminus and the internal variable region. Tests of fecundity in males that coexpress beta 2 and the chimeric beta 3 beta 2C protein showed that in Drosophila there are differential requirements for sperm motility in the male and in the female reproductive tract. Since some aspects of microtubule function in spermatogenesis are sensitive to the tubulin pool size, we examined the mechanisms for control of tubulin protein levels in the male germ cells. We found that both beta 2-tubulin mRNA accumulation and protein synthesis are dependent on gene dose, and that the level of expression is regulated by 3' noncoding sequences in the beta 2 gene. Our data show that the regulatory mechanisms that control tubulin pool levels in the Drosophila male germ line differ from those observed in cultured animal somatic cells. Finally, expression of transgenic constructs is consistent with early cessation of X chromosome expression in Drosophila spermatogenesis.