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D St Johnston

Publications and source records attributed to D St Johnston.

34 records · Page 2Linked to original sources
PubMed1998

The Drosophila AP axis is polarised by the cadherin-mediated positioning of the oocyte.

The anterior-posterior axis of Drosophila originates from two symmetry-breaking steps during early oogenesis. First, one of the two pro-oocytes within the cyst of 16 germline cells is selected to become the oocyte. This cell then comes to lie posterior to the other germline cells of the cyst, thereby defining the polarity of the axis. Here we show that the oocyte reaches the posterior of the cyst in two steps. (1) The cyst flattens as it enters region 2b of the germarium to place the two pro-oocytes in the centre of the cyst, where they contact the posterior follicle cells. (2) One cell is selected to become the oocyte and protrudes into the posterior follicle cell layer when the cyst rounds up on entering region 3. During this germ cell rearrangement, the components of the homophilic cadherin adhesion complex, DE-cadherin, Armadillo and alpha-catenin, accumulate along the border between the oocyte and the posterior follicle cells. Furthermore, the positioning of the oocyte requires cadherin-dependent adhesion between these two cell types, since the oocyte is frequently misplaced when DE-cadherin is removed from either the germline or the posterior follicle cells. We conclude that the oocyte reaches the posterior of the germline cyst because it adheres more strongly to the posterior follicle cells than its neighbours during the germ cell rearrangement that occurs as the cyst moves into region 3. The Drosophila anterior-posterior axis therefore becomes polarised by an unusual cadherin-mediated adhesion between a germ cell and mesodermal follicle cells.

Alleles↗
PubMed1997

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.

Amino Acid Sequence↗
PubMed1997

Oocyte determination and the origin of polarity in Drosophila: the role of the spindle genes.

The two main body axes in Drosophila become polarised as a result of a series of symmetry-breaking steps during oogenesis. Two of the sixteen germline cells in each egg chamber develop as pro-oocytes, and the first asymmetry arises when one of these cells is selected to become the oocyte. Anterior-posterior polarity originates when the oocyte then comes to lie posterior to the nurse cells and signals through the Gurken/Egfr pathway to induce the adjacent follicle cells to adopt a posterior fate. This directs the movement of the germinal vesicle and associated gurken mRNA from the posterior to an anterior corner of the oocyte, where Gurken protein signals for a second time to induce the dorsal follicle cells, thereby polarising the dorsal-ventral axis. Here we describe a group of five genes, the spindle loci, which are required for each of these polarising events. spindle mutants inhibit the induction of both the posterior and dorsal follicle cells by disrupting the localisation and translation of gurken mRNA. Moreover, the oocyte often fails to reach the posterior of mutant egg chambers and differentiates abnormally. Finally, double mutants cause both pro-oocytes to develop as oocytes, by delaying the choice between these two cells. Thus, these mutants reveal a novel link between oocyte selection, oocyte positioning and axis formation in Drosophila, leading us to propose that the spindle genes act in a process that is common to several of these events.

Animals↗
PubMed1996

RNA localization and the development of asymmetry during Drosophila oogenesis.

Recent work on axis formation in Drosophila has revealed that polarity arises in several distinct stages during oogenesis. One cell of a germline cyst is selected to become the oocyte, the position of the oocyte determines the posterior of the follicle, and the position of the oocyte nucleus determines the dorsal side. Each of these symmetry-breaking steps involves the asymmetric localization of a unique structure, leading to polarization of the cytoskeleton and the localization of specific mRNAs.

Animals↗
PubMed1995

NMR solution structure of a dsRNA binding domain from Drosophila staufen protein reveals homology to the N-terminal domain of ribosomal protein S5.

The double-stranded RNA binding domain (dsRBD) is an approximately 65 amino acid motif that is found in a variety of proteins that interact with double-stranded (ds) RNA, such as Escherichia coli RNase III and the dsRNA-dependent kinase, PKR. Drosophila staufen protein contains five copies of this motif, and the third of these binds dsRNA in vitro. Using multinuclear/multidimensional NMR methods, we have determined that staufen dsRBD3 forms a compact protein domain with an alpha-beta-beta-beta-alpha structure in which the two alpha-helices lie on one face of a three-stranded anti-parallel beta-sheet. This structure is very similar to that of the N-terminal domain of a prokaryotic ribosomal protein S5. Furthermore, the consensus derived from all known S5p family sequences shares several conserved residues with the dsRBD consensus sequence, indicating that the two domains share a common evolutionary origin. Using in vitro mutagenesis, we have identified several surface residues which are important for the RNA binding of the dsRBD, and these all lie on the same side of the domain. Two residues that are essential for RNA binding, F32 and K50, are also conserved in the S5 protein family, suggesting that the two domains interact with RNA in a similar way.

Amino Acid Sequence↗
PubMed1995

Polarization of both major body axes in Drosophila by gurken-torpedo signalling.

Anterior-posterior polarity in Drosophila arises from the movement of the oocyte to the posterior of the egg chamber, and the subsequent acquisition of posterior fate by the adjacent somatic follicle cells. We demonstrate that gurken is necessary in the oocyte and torpedo/DER in the follicle cells for the induction of posterior fate. As the gurken-torpedo/DER pathway also establishes dorsoventral polarity later in oogenesis, Drosophila uses the same germline to soma signalling pathway to determine both embryonic axes.

Animals↗
PubMed1995

The intracellular localization of messenger RNAs.

As I hope this review has made clear, mRNA localization plays an important role in directing specific proteins to their correct position within a cell. Although the study of this process is still in its infancy, it is already apparent that there are several ways that mRNAs can be targeted to particular subcellular sites. However, the molecular mechanisms responsible for these different localization pathways are still largely obscure, and their elucidation must await the identification of the specific factors that mediate the interactions between the localized mRNAs and more general components such as the cytoskeleton. Most examples of localized mRNAs are likely to share several common features. First, the site of localization will be determined by the preexisting polarity of the cell, and this will most often depend on the organization of the cytoskeleton, either directly, in the case of active transport, or indirectly, when localization is mediated by localized anchoring sites or stability factors. Second, mRNA localization is likely to be tightly coupled to translational control. If it is important for a cell to synthesize a protein in a particular place, then the translation of the mRNA must be repressed until it is localized. Indeed, there are already several examples where the direct linkage between translational control and localization has been demonstrated, and these are discussed in the accompanying review by Curtis et al. (1995).

Animals↗
PubMed1995

Assignment of the backbone 1H,15N,13C NMR resonances and secondary structure of a double-stranded RNA binding domain from the Drosophila protein staufen.

NMR spectroscopy has been used to determine the secondary structure of one of the double-stranded RNA binding domains from the Drosophila protein staufen. The domain has an alpha beta beta beta alpha arrangement of secondary structure, with the beta strands forming an antiparallel beta sheet. The secondary structure differs from that found in the RNP RNA binding domain.

Amino Acid Sequence↗
PubMed1994

Staufen protein associates with the 3'UTR of bicoid mRNA to form particles that move in a microtubule-dependent manner.

Staufen protein is required in order to anchor bicoid (bcd) mRNA at the anterior pole of the Drosophila egg. Here we show that staufen protein colocalizes with bcd mRNA at the anterior, and that this localization depends upon its association with the mRNA. Upon injection into the embryo, bcd transcripts specifically interact with staufen, and we have mapped the sequences required to three regions of the 3'UTR, each of which is predicted to form a long stem-loop. The resulting staufen-bcd 3'UTR complexes form particles that show a microtubule-dependent localization. Since staufen is also transported with oskar (osk) mRNA during oogenesis, staufen associates specifically with both osk and bcd mRNAs to mediate their localizations, but at two distinct stages of development.

Animals↗
PubMed1994

Role of oocyte position in establishment of anterior-posterior polarity in Drosophila.

The polarized microtubule cytoskeleton of the Drosophila oocyte directs the localization of the maternal determinants which establish the anterior-posterior (AP) axis of the embryo. Because the formation of this microtubule array is dependent on signals from the follicle cells that surround the oocyte, it has been proposed that AP polarity originates in the follicle cells. Here it is shown that the movement of the oocyte to the posterior of the egg chamber early in oogenesis determines AP polarity in the follicle cell layer, and also in the oocyte. Moreover, the generation of AP asymmetry requires signaling from the germ line to the soma and back again.

Animals↗
PubMed1994

RNA localization. Getting to the top.

Localizing gurken mRNA dorsally in Drosophila oocytes seems to be the first step in dorsoventral polarity establishment. Gurken, which resembles TGF alpha, may signal directly to dorsal follicle cells through the receptor Top.

Animals↗
PubMed1992

A conserved double-stranded RNA-binding domain.

We have identified a double-stranded (ds)RNA-binding domain in each of two proteins: the product of the Drosophila gene staufen, which is required for the localization of maternal mRNAs, and a protein of unknown function, Xlrbpa, from Xenopus. The amino acid sequences of the binding domains are similar to each other and to additional domains in each protein. Database searches identified similar domains in several other proteins known or thought to bind dsRNA, including human dsRNA-activated inhibitor (DAI), human trans-activating region (TAR)-binding protein, and Escherichia coli RNase III. By analyzing in detail one domain in staufen and one in Xlrbpa, we delimited the minimal region that binds dsRNA. On the basis of the binding studies and computer analysis, we have derived a consensus sequence that defines a 65- to 68-amino acid dsRNA-binding domain.

Amino Acid Sequence↗
PubMed1991

Staufen, a gene required to localize maternal RNAs in the Drosophila egg.

The posterior group gene staufen is required both for the localization of maternal determinants to the posterior pole of the Drosophila egg and for bicoid RNA to localize correctly to the anterior pole. We report the cloning and sequencing of staufen and show that staufen protein is one of the first molecules to localize to the posterior pole of the oocyte, perhaps in association with oskar RNA. Once localized, staufen is found in the polar granules and is required to hold other polar granule components at the posterior pole. By the time the egg is laid, staufen protein is also concentrated at the anterior pole, in the same region as bicoid RNA.

Amino Acid Sequence↗
PubMed1989

Multiple steps in the localization of bicoid RNA to the anterior pole of the Drosophila oocyte.

The anterior region of the Drosophila embryonic pattern is determined by a gradient of the bicoid (bcd) protein. The correct formation of this gradient requires the localization of bcd RNA to the anterior pole of the egg. Here we use a wholemount in situ technique to examine the process of bcd RNA localization during oogenesis and embryogenesis. While bcd protein becomes distributed in a gradient that extends throughout the anterior two thirds of the early embryo, bcd RNA remains restricted to a much smaller region at the anterior pole. The difference between these distributions indicates that the shape of the protein gradient must depend to some extent on the posterior movement of the protein after it has been synthesized. Four distinct phases of bcd RNA localization can be distinguished during oogenesis. Between stages 6 and 9 of oogenesis, the RNA accumulates in a ring at the anterior end of the oocyte. During the second phase, in stage 9-10a follicles, the RNA also localizes to the apical regions of the nurse cells, demonstrating that the nurse cells possess an intrinsic polarity. As the nurse cells contract during stages 10b-11, all of the bcd RNA becomes localized to the cortex at the anterior end of the oocyte. During a final phase that must occur between stage 12 of oogenesis and egg deposition, the RNA becomes localized to a spherical region that occupies a slightly dorsal position at the anterior pole.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗