The Shot CH1 domain recognises a distinct form of F-actin during Drosophila oocyte determination.

In Drosophila, only one cell in a multicellular female germline cyst is specified as an oocyte and a similar process occurs in mammals. The symmetry-breaking cue for oocyte selection is provided by the fusome, a tubular structure connecting all cells in the cyst. The Drosophila spectraplakin Shot localises to the fusome and translates its asymmetry into a polarised microtubule network that is essential for oocyte specification, but how Shot recognises the fusome is unclear. Here, we demonstrate that the actin-binding domain (ABD) of Shot is necessary and sufficient to localise Shot to the fusome and mediates Shot function in oocyte specification together with the microtubule-binding domains. The calponin homology domain 1 of the Shot ABD recognises fusomal F-actin and requires calponin homology domain 2 to distinguish it from other forms of F-actin in the cyst. By contrast, the ABDs of utrophin, Fimbrin, Filamin, Lifeact and F-tractin do not recognise fusomal F-actin. We therefore propose that Shot propagates fusome asymmetry by recognising a specific conformational state of F-actin on the fusome.

User-friendly oblique plane microscopy on a fully functional commercially available microscope base.

In this work we present an oblique plane microscope designed to work seamlessly with a commercially available microscope base. To support all the functionality offered by the microscope base, where the position of the objective lens is not fixed, we adopted a two-mirror scanning geometry that can compensate for changes to the position of the objective lens during routine microscope operation. We showed that within a ± 1 mm displacement range of the 100X, 1.35 NA objective lens away from its designed position, the PSF size increased by <3% and <11% in the lateral and axial dimensions, respectively, while the error in magnification was <0.5% within volumes extending ± 10 µm about the focal plane. Compared to the more traditional scan-lens/galvo-mirror combination, the two-mirror scanning geometry offers higher light efficiency and a more compact footprint, which could be beneficial to all OPM designs regardless of the use of a commercial base or not.

Localised dynactin protects growing microtubules to deliver oskar mRNA to the posterior cortex of the Drosophila oocyte.

The localisation of oskar mRNA to the posterior of the Drosophila oocyte defines where the abdomen and germ cells form in the embryo. Kinesin 1 transports oskar mRNA to the oocyte posterior along a polarised microtubule cytoskeleton that grows from non-centrosomal microtubule organising centres (ncMTOCs) along the anterior/lateral cortex. Here, we show that the formation of this polarised microtubule network also requires the posterior regulation of microtubule growth. A missense mutation in the dynactin Arp1 subunit causes most oskar mRNA to localise in the posterior cytoplasm rather than cortically. oskar mRNA transport and anchoring are normal in this mutant, but the microtubules fail to reach the posterior pole. Thus, dynactin acts as an anti-catastrophe factor that extends microtubule growth posteriorly. Kinesin 1 transports dynactin to the oocyte posterior, creating a positive feedback loop that increases the length and persistence of the posterior microtubules that deliver oskar mRNA to the cortex.

bicoid mRNA localises to the Drosophila oocyte anterior by random Dynein-mediated transport and anchoring.

bicoid mRNA localises to the Drosophila oocyte anterior from stage 9 of oogenesis onwards to provide a local source for Bicoid protein for embryonic patterning. Live imaging at stage 9 reveals that bicoid mRNA particles undergo rapid Dynein-dependent movements near the oocyte anterior, but with no directional bias. Furthermore, bicoid mRNA localises normally in shot2A2, which abolishes the polarised microtubule organisation. FRAP and photo-conversion experiments demonstrate that the RNA is stably anchored at the anterior, independently of microtubules. Thus, bicoid mRNA is localised by random active transport and anterior anchoring. Super-resolution imaging reveals that bicoid mRNA forms 110-120 nm particles with variable RNA content, but constant size. These particles appear to be well-defined structures that package the RNA for transport and anchoring.

Patronin/Shot Cortical Foci Assemble the Noncentrosomal Microtubule Array that Specifies the Drosophila Anterior-Posterior Axis.

Noncentrosomal microtubules play an important role in polarizing differentiated cells, but little is known about how these microtubules are organized. Here we identify the spectraplakin, Short stop (Shot), as the cortical anchor for noncentrosomal microtubule organizing centers (ncMTOCs) in the Drosophila oocyte. Shot interacts with the cortex through its actin-binding domain and recruits the microtubule minus-end-binding protein, Patronin, to form cortical ncMTOCs. Shot/Patronin foci do not co-localize with Î³-tubulin, suggesting that they do not nucleate new microtubules. Instead, they capture and stabilize existing microtubule minus ends, which then template new microtubule growth. Shot/Patronin foci are excluded from the oocyte posterior by the Par-1 polarity kinase to generate the polarized microtubule network that localizes axis determinants. Both proteins also accumulate apically in epithelial cells, where they are required for the formation of apical-basal microtubule arrays. Thus, Shot/Patronin ncMTOCs may provide a general mechanism for organizing noncentrosomal microtubules in differentiated cells.

Anterior-posterior axis specification in Drosophila oocytes: identification of novel bicoid and oskar mRNA localization factors.

The Drosophila melanogaster anterior-posterior axis is established during oogenesis by the localization of bicoid and oskar mRNAs to the anterior and posterior poles of the oocyte. Although genetic screens have identified some trans-acting factors required for the localization of these transcripts, other factors may have been missed because they also function at other stages of oogenesis. To circumvent this problem, we performed a screen for revertants and dominant suppressors of the bicaudal phenotype caused by expressing Miranda-GFP in the female germline. Miranda mislocalizes oskar mRNA/Staufen complexes to the oocyte anterior by coupling them to the bicoid localization pathway, resulting in the formation of an anterior abdomen in place of the head. In one class of revertants, Miranda still binds Staufen/oskar mRNA complexes, but does not localize to the anterior, identifying an anterior targeting domain at the N terminus of Miranda. This has an almost identical sequence to the N terminus of vertebrate RHAMM, which is also a large coiled-coil protein, suggesting that it may be a divergent Miranda ortholog. In addition, we recovered 30 dominant suppressors, including multiple alleles of the spectroplakin, short stop, a lethal complementation group that prevents oskar mRNA anchoring, and a female sterile complementation group that disrupts the anterior localization of bicoid mRNA in late oogenesis. One of the single allele suppressors proved to be a mutation in the actin nucleator, Cappuccino, revealing a previously unrecognized function of Cappuccino in pole plasm anchoring and the induction of actin filaments by Long Oskar protein.

Egalitarian recruitment of localized mRNAs.

Bicaudal-D (Bic-D) and Egalitarian (Egl) are required for the dynein-dependent localization of many mRNAs in Drosophila, but the mRNAs show no obvious sequence similarities, and the RNA-binding proteins that recognize them and link them to dynein are not known. In this issue of Genes & Development, Dienstbier and colleagues (pp. 1546-1558) present evidence that the elusive RNA-binding protein is Egl itself. As well as linking mRNA to dynein, they show that Egl also activates dynein motility by binding Bic-D and the dynein light chain.

Two splicing isoforms of the Y-box protein ctYB-1 appear on the same mRNA molecule.

Y-box proteins constitute an evolutionarily conserved family of DNA- and RNA-binding proteins involved in the regulation of transcription and translation. In the dipteran Chironomus tentans, a homologue to the vertebrate Y-box protein YB-1 was recently characterized and designated ctYB-1. It is transferred from nucleus to cytoplasm bound to mRNA and is likely to affect translation. It appears in two size variants, p40 and p50. We further analysed the two size variants and their interaction with mRNA. Southern blot analysis, in situ hybridization and RT-PCR analysis suggested that there is just one YB-1 gene, and that the two size variants represent splicing isoforms. In a C. tentans epithelial cell line, only p40 is present, whereas both variants appear together in eight tissues from fourth-instar larvae, although in somewhat different proportions. Furthermore, the appearance of the two isoforms was studied in relation to a specific 35-40 kb mRNA transcript in the salivary glands, the Balbiani ring mRNA. Because of their exceptional size, Balbiani ring messenger ribonucleoprotein particles in nucleoplasm and Balbiani ring polysomes in cytoplasm could be identified and selectively studied. We were able to establish that both isoforms are associated with both nuclear and cytoplasmic Balbiani ring mRNA. In addition, a p50-specific antibody coimmunoprecipitated p40 from Balbiani ring polysomes, suggesting that the two splicing isoforms are located along the same Balbiani ring mRNA molecule. The functional significance of the two isoforms is being discussed.