Gamma-tubulin37C and gamma-tubulin ring complex protein 75 are essential for bicoid RNA localization during drosophila oogenesis

bicoid (bcd) RNA localization requires the activity of exuperantia and swallow at sequential steps of oogenesis and is microtubule dependent. In a genetic screen, we identified two novel genes essential for bcd RNA localization. They encode maternal gamma-Tubulin37C (gammaTub37C) and gamma-tubulin ring complex protein 75 (Dgrip75), both of which are gamma-tubulin ring complex components. Mutations in these genes specifically affect bcd RNA localization, whereas other microtubule-dependent processes during oogenesis are not impaired. This provides direct evidence that a subset of microtubules organized by the gamma-tubulin ring complex is essential for localization of bcd RNA. At stage 10b, we find gammaTub37C and Dgrip75 anteriorly concentrated and propose the formation of a microtubule-organizing center at the anterior pole of the oocyte.     

GCP-WD is a gamma-tubulin targeting factor required for centrosomal and chromatin-mediated microtubule nucleation

The gamma-tubulin ring complex (gammaTuRC) is a large multi-protein complex that is required for microtubule nucleation from the centrosome. Here, we show that the GCP-WD protein (originally named NEDD1) is the orthologue of the Drosophila Dgrip71WD protein, and is a subunit of the human gammaTuRC. GCP-WD has the properties of an attachment factor for the gammaTuRC: depletion or inhibition of GCP-WD results in loss of the gammaTuRC from the centrosome, abolishing centrosomal microtubule nucleation, although the gammaTuRC is intact and able to bind to microtubules. GCP-WD depletion also blocks mitotic chromatin-mediated microtubule nucleation, resulting in failure of spindle assembly. Mitotic phosphorylation of GCP-WD is required for association of gamma-tubulin with the spindle, separately from association with the centrosome. Our results indicate that GCP-WD broadly mediates targeting of the gammaTuRC to sites of microtubule nucleation and to the mitotic spindle, which is essential for spindle formation.     

The role of Xgrip210 in gamma-tubulin ring complex assembly and centrosome recruitment

The gamma-tubulin ring complex (gammaTuRC), purified from the cytoplasm of vertebrate and invertebrate cells, is a microtubule nucleator in vitro. Structural studies have shown that gammaTuRC is a structure shaped like a lock-washer and topped with a cap. Microtubules are thought to nucleate from the uncapped side of the gammaTuRC. Consequently, the cap structure of the gammaTuRC is distal to the base of the microtubules, giving the end of the microtubule the shape of a pointed cap. Here, we report the cloning and characterization of a new subunit of Xenopus gammaTuRC, Xgrip210. We show that Xgrip210 is a conserved centrosomal protein that is essential for the formation of gammaTuRC. Using immunogold labeling, we found that Xgrip210 is localized to the ends of microtubules nucleated by the gammaTuRC and that its localization is more distal, toward the tip of the gammaTuRC-cap structure, than that of gamma-tubulin. Immunodepletion of Xgrip210 blocks not only the assembly of the gammaTuRC, but also the recruitment of gamma-tubulin and its interacting protein, Xgrip109, to the centrosome. These results suggest that Xgrip210 is a component of the gammaTuRC cap structure that is required for the assembly of the gammaTuRC.     

The Drosophila microtubule-associated protein mini spindles is required for cytoplasmic microtubules in oogenesis

The XMAP215/TOG family of proteins is a closely related set of MAPs (microtubule-associated proteins) found in animals, yeast, and plants . In yeast and animal cells, the XMAP215/TOG proteins are required for both mitosis and meiosis. Although effects of XMAP215/TOG proteins on cytoplasmic microtubules have not previously been shown in animal cells, in plants the Arabidopsis family member MOR1 is required for the organization of cortical microtubule arrays . The Drosophila family member, encoded by the mini spindles (msps) gene, is maternally expressed and loaded into the egg, where it is an essential component of meiotic and mitotic spindles . Here we show that msps is also required during oogenesis for the structure and function of cytoplasmic microtubules. Localization of bicoid (bcd) mRNA in the oocyte is a microtubule-mediated event . We show that bcd RNA localization is defective in msps mutants. We also identify defects in cytoplasmic microtubules in both the germ and follicle cells of mutant ovaries and determine the expression pattern of msps mRNA and protein in developing egg chambers. Our findings reveal a new role for msps in cell patterning and raise the possibility that other family members may perform similar functions.     

Redundant RNA recognition events in bicoid mRNA localization.

A cis-acting signal in the 3' UTR of the Drosophila bicoid mRNA directs both the transport of the mRNA from the nurse cells to the oocyte and its anterior localization within the oocyte. Here we demonstrate that the signal mediates redundant RNA recognition events, A and B, that initiate largely overlapping programs of mRNA localization during oogenesis. Recognition event A requires a region encompassing stem-loops IV/V of the predicted secondary structure, and can be eliminated by a single nucleotide mutation. Localization initiated through event B begins slightly later in oogenesis, and requires sequences that have not been narrowly defined. Using forms of the 3' UTR lacking this RNA recognition redundancy, we reexamine the roles of the swallow, staufen, and exuperantia genes, which are all required for normal bicoid mRNA localization. Our results reveal that exuperantia first becomes essential for localization at a time when well-defined microtubule tracks between the nurse cells and oocyte disappear. Thus, exuperantia may specifically facilitate a form of nurse cell-to-oocyte mRNA transport not dependent on the microtubule tracks.     

Structure of the gamma-tubulin ring complex: a template for microtubule nucleation

The gamma-tubulin ring complex (gammaTuRC) is a protein complex of relative molecular mass approximately 2.2 x 10(6) that nucleates microtubules at the centrosome. Here we use electron-microscopic tomography and metal shadowing to examine the structure of isolated Drosophila gammaTuRCs and the ends of microtubules nucleated by gammaTuRCs and by centrosomes. We show that the gammaTuRC is a lockwasher-like structure made up of repeating subunits, topped asymmetrically with a cap. A similar capped ring is also visible at one end of microtubules grown from isolated gammaTuRCs and from centrosomes. Antibodies against gamma-tubulin label microtubule ends, but not walls, in centrosomes. These data are consistent with a template-mediated mechanism for microtubule nucleation by the gammaTuRC.     

Localization of bicoid mRNA in late oocytes is maintained by continual active transport

Localization of bicoid mRNA to the anterior of the Drosophila oocyte is essential to produce the Bicoid protein gradient that patterns the anterior-posterior axis of the embryo. Previous studies have characterized a microtubule-dependent pathway for bicoid mRNA localization during midoogenesis, when bicoid first accumulates at the anterior. We show that the majority of bicoid is actually localized later in oogenesis, when the only known mechanism for mRNA localization is based on passive trapping. Through live imaging of fluorescently tagged endogenous bicoid mRNA, we identify a temporally distinct pathway for bicoid localization in late oocytes that utilizes a specialized subpopulation of anterior microtubules and dynein. The directional movement of bicoid RNA particles within the oocyte observed here is consistent with dynein-mediated transport. Furthermore, our results indicate that association of bicoid with the anterior oocyte cortex is dynamic and support a model for maintenance of bicoid localization by continual active transport on microtubules.     

The actin cytoskeleton is required for maintenance of posterior pole plasm components in the Drosophila embryo.

Localization of mRNAs is one of many aspects of cellular organization that requires the cytoskeleton. In Drosophila, microtubules are known to be required for correct localization of developmentally important mRNAs and proteins during oogenesis; however, the role of the actin cytoskeleton in localization is less clear. Furthermore, it is not known whether either of these cytoskeletal systems are necessary for maintenance of RNA localization in the early embryo. We have examined the contribution of the actin and microtubule cytoskeletons to maintenance of RNA and protein localization in the early Drosophila embryo. We have found that while microtubules are not necessary, the actin cytoskeleton is needed for stable association of nanos, oskar, germ cell-less and cyclin B mRNAs and Oskar and Vasa proteins at the posterior pole in the early embryo. In contrast, bicoid RNA, which is located at the anterior pole, does not require either cytoskeletal system to remain at the anterior.     

Drosophila melanogaster gamma-TuRC is dispensable for targeting gamma-tubulin to the centrosome and microtubule nucleation

In metazoans, gamma-tubulin acts within two main complexes, gamma-tubulin small complexes (gamma-TuSCs) and gamma-tubulin ring complexes (gamma-TuRCs). In higher eukaryotes, it is assumed that microtubule nucleation at the centrosome depends on gamma-TuRCs, but the role of gamma-TuRC components remains undefined.For the first time, we analyzed the function of all four gamma-TuRC-specific subunits in Drosophila melanogaster: Dgrip75, Dgrip128, Dgrip163, and Dgp71WD. Grip-motif proteins, but not Dgp71WD, appear to be required for gamma-TuRC assembly. Individual depletion of gamma-TuRC components, in cultured cells and in vivo, induces mitotic delay and abnormal spindles. Surprisingly, gamma-TuSCs are recruited to the centrosomes. These defects are less severe than those resulting from the inhibition of gamma-TuSC components and do not appear critical for viability. Simultaneous cosilencing of all gamma-TuRC proteins leads to stronger phenotypes and partial recruitment of gamma-TuSC. In conclusion, gamma-TuRCs are required for assembly of fully functional spindles, but we suggest that gamma-TuSC could be targeted to the centrosomes, which is where basic microtubule assembly activities are maintained.     

The Drosophila gamma-tubulin small complex subunit Dgrip84 is required for structural and functional integrity of the spindle apparatus

Gamma-tubulin, a protein critical for microtubule assembly, functions within multiprotein complexes. However, little is known about the respective role of gamma-tubulin partners in metazoans. For the first time in a multicellular organism, we have investigated the function of Dgrip84, the Drosophila orthologue of the Saccharomyces cerevisiae gamma-tubulin-associated protein Spc97p. Mutant analysis shows that Dgrip84 is essential for viability. Its depletion promotes a moderate increase in the mitotic index, correlated with the appearance of monopolar or unpolarized spindles, impairment of centrosome maturation, and increase of polyploid nuclei. This in vivo study is strengthened by an RNA interference approach in cultured S2 cells. Electron microscopy analysis suggests that monopolar spindles might result from a failure of centrosome separation and an unusual microtubule assembly pathway via centriolar triplets. Moreover, we point to an involvement of Dgrip84 in the spindle checkpoint regulation and in the maintenance of interphase microtubule dynamics. Dgrip84 also seems essential for male meiosis, ensuring spindle bipolarity and correct completion of cytokinesis. These data sustain that Dgrip84 is required in some aspects of microtubule dynamics and organization both in interphase and mitosis. The nature of a minimal gamma-tubulin complex necessary for proper microtubule organization in the metazoans is discussed.     

Characterization and reconstitution of Drosophila gamma-tubulin ring complex subunits

The gamma-tubulin ring complex (gammaTuRC) is important for microtubule nucleation from the centrosome. In addition to gamma-tubulin, the Drosophila gammaTuRC contains at least six subunits, three of which [Drosophila gamma ring proteins (Dgrips) 75/d75p, 84, and 91] have been characterized previously. Dgrips84 and 91 are present in both the small gamma-tubulin complex (gammaTuSC) and the gammaTuRC, while the remaining subunits are found only in the gammaTuRC. To study gammaTuRC assembly and function, we first reconstituted gammaTuSC using the baculovirus expression system. Using the reconstituted gammaTuSC, we showed for the first time that this subcomplex of the gammaTuRC has microtubule binding and capping activities. Next, we characterized two new gammaTuRC subunits, Dgrips128 and 163, and showed that they are centrosomal proteins. Sequence comparisons among all known gammaTuRC subunits revealed two novel sequence motifs, which we named grip motifs 1 and 2. We found that Dgrips128 and 163 can each interact with gammaTuSC. However, this interaction is insufficient for gammaTuRC assembly.     

Immunostructural evidence for the template mechanism of microtubule nucleation

Two opposing models have been proposed to explain how the gamma-tubulin ring complex (gammaTuRC) induces microtubule nucleation. In the 'protofilament' model, the gammaTuRC induces nucleation as a partially or completely straightened protofilament that is incorporated longitudinally into the wall of the nascent microtubule, whereas the 'template' model proposes that the gammaTuRC acts as a helical template that constitutes the base of the newly-formed polymer. Here we appraise these two models, using high-resolution structural and immunolocalization methods. We show that components of the gammaTuRC localize to a narrow zone at the extreme minus end of the microtubule and that these ends terminate in a pointed cap. Together, these results strongly favour the template model of microtubule nucleation.     

A new function for the gamma-tubulin ring complex as a microtubule minus-end cap

Microtubule nucleation from centrosomes involves a lockwasher-shaped protein complex containing gamma-tubulin, named the gamma-tubulin ring complex (gammaTuRC). Here we investigate the mechanism by which the gammaTuRC nucleates microtubules, using a direct labelling method to visualize the behaviour of individual gammaTuRCs. A fluorescently-labelled version of the gammaTuRC binds to the minus ends of microtubules nucleated in vitro. Both gammaTuRC-mediated nucleation and binding of the gammaTuRC to preformed microtubules block further minus-end growth and prevent microtubule depolymerization. The gammaTuRC therefore acts as a minus-end-capping protein, as confirmed by electron-microscopic examination of gold-labelled gammaTuRCs. These data support a nucleation model for gammaTuRC function that involves capping of microtubules.     

The role of γ-tubulin in centrosomal microtubule organization

As part of a multi-subunit ring complex, γ-tubulin has been shown to promote microtubule nucleation both in vitro and in vivo, and the structural properties of the complex suggest that it also seals the minus ends of the polymers with a conical cap. Cells depleted of γ-tubulin, however, still display many microtubules that participate in mitotic spindle assembly, suggesting that γ-tubulin is not absolutely required for microtubule nucleation in vivo, and raising questions about the function of the minus end cap. Here, we assessed the role of γ-tubulin in centrosomal microtubule organisation using three-dimensional reconstructions of γ-tubulin-depleted C. elegans embryos. We found that microtubule minus-end capping and the PCM component SPD-5 are both essential for the proper placement of microtubules in the centrosome. Our results further suggest that γ-tubulin and SPD-5 limit microtubule polymerization within the centrosome core, and we propose a model for how abnormal microtubule organization at the centrosome could indirectly affect centriole structure and daughter centriole replication.     

Reorganization of the cytoskeleton during Drosophila oogenesis: implications for axis specification and intercellular transport.

Inhibitor studies have implicated microtubules in at least three important developmental processes during Drosophila oogenesis: oocyte determination and growth during stages 1 through 6, positioning of the anterior determinant bicoid mRNA during stages 9 through 12, and ooplasmic streaming during stages 10b through 12. We have used fluorescence cytochemistry together with laser scanning confocal microscopy to identify distinct microtubule structures at each of the above three periods that are likely to be involved in these processes. During stages 1 through 7, maternal components synthesized in nurse cells are transported through cytoplasmic bridges to the oocyte. At this time, microtubules that appear to originate in the oocyte pass through these cytoplasmic bridges into the adjacent nurse cells; these microtubules are likely to serve as a polarized scaffold on which maternal RNAs and proteins are transported. During stages 7 and 8, microtubules in the oocyte cortex reorganize to form an anterior-to-posterior gradient, suggesting a role for microtubules in the localization of morphogenetic determinants. Finally, when ooplasmic streaming begins during stage 10 b, it is accompanied by the assembly of subsurface microtubule arrays that spiral around the oocyte; these arrays disassemble as the oocyte matures and streaming stops. During ooplasmic streaming, many vesicles are closely associated with the subsurface microtubules, suggesting that streaming is driven by vesicle translocation along microtubules. We believe that actin plays a secondary role in each of these morphogenetic events, based on our parallel studies of actin organization during each of the above stages of oogenesis.     

In vivo analysis of Drosophila bicoid mRNA localization reveals a novel microtubule-dependent axis specification pathway.

Drosophila bicoid mRNA is synthesized in the nurse cells and transported to the oocyte where microtubules and Exuperantia protein mediate localization to the anterior pole. Fluorescent bicoid mRNA injected into the oocyte displays nonpolar microtubule-dependent transport to the closest cortical surface, and the oocyte microtubule cytoskeleton lacks clear axial asymmetry. Nonetheless, bicoid mRNA injected into the nurse cell cytoplasm, withdrawn, and injected into a second oocyte shows microtubule-dependent transport to the anterior cortex. Nurse cells require microtubules and Exuperantia to support anterior transport of bicoid mRNA, and microtubules are required for bicoid mRNA-Exuperantia particle coassembly. We propose that microtubule-dependent Exuperantia-bicoid mRNA complex formation in the nurse cell cytoplasm allows anterior-specific transport on a grossly nonpolar oocyte microtubule network.     

Microtubules mediate the localization of bicoid RNA during Drosophila oogenesis.

We have examined cytoskeletal requirements for bicoid (bcd) RNA localization during Drosophila oogenesis. bcd is an anterior morphogen whose proper function relies on the localization of its messenger RNA to the anterior cortex of the egg. Drugs that depolymerize microtubules perturb all aspects of bcd RNA localization. During recovery from drug treatment, bcd RNA relocalizes to the oocyte cortex, suggesting that the localization machinery is a component of the cortical cytoskeleton. Taxol, a drug that stabilizes microtubules, also effectively disrupts bcd RNA localization, and the effects of taxol treatments on exuperantia and swallow mutants suggest general roles for these gene products in the multi-step bcd RNA localization process.     

Evidence for common machinery utilized by the early and late RNA localization pathways in Xenopus oocytes

In Xenopus, an early and a late pathway exist for the selective localization of RNAs to the vegetal cortex during oogenesis. Previous work has suggested that distinct cellular mechanisms mediate localization during these pathways. Here, we provide several independent lines of evidence supporting the existence of common machinery for RNA localization during the early and late pathways. Data from RNA microinjection assays show that early and late pathway RNAs compete for common localization factors in vivo, and that the same short RNA sequence motifs are required for localization during both pathways. In addition, quantitative filter binding assays demonstrate that the late localization factor Vg RBP/Vera binds specifically to several early pathway RNA localization elements. Finally, confocal imaging shows that early pathway RNAs associate with a perinuclear microtubule network that connects to the mitochondrial cloud of stage I oocytes suggesting that motor driven transport plays a role during the early pathway as it does during the late pathway. Taken together, our data indicate that common machinery functions during the early and late pathways. Thus, RNA localization to the vegetal cortex may be a regulated process such that differential interactions with basal factors determine when distinct RNAs are localized during oogenesis.     

Localization of nanos RNA controls embryonic polarity.

Anterior-posterior polarity of the Drosophila embryo is initiated during oogenesis through differential maternal RNA localization. The RNA of the anterior morphogen bicoid is localized to the anterior pole of the embryo, where bicoid protein controls head and thorax development. The RNA of the posterior morphogen nanos is localized to the posterior pole, where nanos protein is required for abdomen formation. Here we show that the nanos 3' untranslated region, like that of the bicoid RNA, is sufficient for RNA localization. We have used the bicoid RNA localization signal to mislocalize nanos, producing embryos with two sources of nanos protein. Such embryos form two abdomens with mirror image symmetry. Embryos with nanos RNA localized only to the anterior have greater nanos gene activity than embryos with nanos RNA localized posteriorly. We propose a role for RNA localization in regulating nanos activity.     

Barentsz is essential for the posterior localization of oskar mRNA and colocalizes with it to the posterior pole

The localization of Oskar at the posterior pole of the Drosophila oocyte induces the assembly of the pole plasm and therefore defines where the abdomen and germ cells form in the embryo. This localization is achieved by the targeting of oskar mRNA to the posterior and the localized activation of its translation. oskar mRNA seems likely to be actively transported along microtubules, since its localization requires both an intact microtubule cytoskeleton and the plus end-directed motor kinesin I, but nothing is known about how the RNA is coupled to the motor. Here, we describe barentsz, a novel gene required for the localization of oskar mRNA. In contrast to all other mutations that disrupt this process, barentsz-null mutants completely block the posterior localization of oskar mRNA without affecting bicoid and gurken mRNA localization, the organization of the microtubules, or subsequent steps in pole plasm assembly. Surprisingly, most mutant embryos still form an abdomen, indicating that oskar mRNA localization is partially redundant with the translational control. Barentsz protein colocalizes to the posterior with oskar mRNA, and this localization is oskar mRNA dependent. Thus, Barentsz is essential for the posterior localization of oskar mRNA and behaves as a specific component of the oskar RNA transport complex.