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

Abstract

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.DOI: http://dx.doi.org/10.7554/eLife.17537.001

Localised Dynactin protects growing microtubules to deliveroskarmRNA to the posterior cortex of theDrosophilaoocyte

“…Dynein and Dynactin both fulfil multiple functions during the later stages of oogenesis: they are required to transport RNAs, such as bicoid, gurken and oskar from the nurse cells into the oocyte (Clark et al, 2007;Mische et al, 2007), they transport bicoid mRNA to the anterior of the oocyte (Duncan and Warrior, 2002;Weil et al, 2006;Weil et al, 2008;Trovisco et al, 2016), anchor the nucleus at the dorsal/anterior corner of the oocyte (Swan et al, 1999;Lei and Warrior, 2000;Januschke et al, 2002;Zhao et al, 2012), and localise and anchor gurken mRNA above the nucleus (MacDougall et al, 2003;Delanoue et al, 2007). Both bicoid and oskar mRNAs enter the oocyte normally in arp1 4D2 homozygous germline clones, and bicoid mRNA localises to the anterior cortex as in wild-type ( Figure 1D, Figure 5D-G).…”

“…If this model is correct, one would expect a proportion of oskar mRNA particles to move at the speed of the growing microtubules in wild-type. However, there is no significant peak in the distribution of oskar mRNA velocities that corresponds to the speed of the growing microtubule plus ends, indicating that dynactin does not couple the RNA particles to the plus ends ( Figure 6D; Trovisco et al, 2016). Since Kinesin 1 transports oskar mRNA with an average speed of 0.47µm/sec, while the microtubules grow at 0.23µm/sec, the oskar mRNA/Kinesin1 complexes will continually catch up with the growing plus ends and then fall off ( Figure 6D).…”

Egalitarian binding partners, Dynein Light Chain and Bicaudal-D, act sequentially to link mRNA to the Dynein motor

“…Another class of cargo that might be highly sensitive to disruption of the Egl/Dlc interaction are cargoes that not actively anchored at their site of localization. Both grk and bcd mRNAs have well-defined anchoring mechanisms (Delanoue et al, 2007;Weil et al, 2008;Trovisco et al, 2016). Thus, in the case of these mRNAs, even if transport is not that efficient, the presence of an anchoring mechanism can lessen the severity of the localization defect.…”

The Egalitarian binding partners Dynein light chain and Bicaudal-D act sequentially to link mRNA to the Dynein motor

“…Another class of cargo that might be highly sensitive to disruption of the Egl/Dlc interaction are cargoes that are not actively anchored at their site of localization. Both grk and bcd mRNAs have well-defined anchoring mechanisms (Delanoue et al, 2007;Weil et al, 2008;Trovisco et al, 2016). Thus, in the case of these mRNAs, even if transport is not that efficient, the presence of an anchoring mechanism can lessen the severity of the localization defect.…”