Mechanism of grk mRNA anchoring during Drosophila oogenesis
Abstract
Messenger RNA localization is a widespread mechanism of posttranscriptional
regulation of gene expression in multicellular organisms ranging from yeast to
mammals. In Drosophila oocytes, gurken (grk) mRNA is transported by Dynein to
produce a local secreted signal to the overlying follicle cells. This signal is
responsible for setting up the primary axes in the oocyte. grk mRNA is transcribed in
nurse cells and transported into the oocyte where it localizes at two distinct stages of
oogenesis, thus targeting the translation of Grk/TGFalpha protein, first to the posterior
and later to the dorso-anterior (DA) corner where it is translated. Gurken protein
signals to the overlying follicle cells to establish the dorsal fate of the oocyte. grk
transcripts are transported by Dynein in EM-dense particles on microtubules. These
particles are not associated with vesicles nor membrane-bound and contain many
copies of grk mRNA, Dynein and hnRNP Squid. At the DA corner transport particles
assemble into large EM-dense cytoplasmic anchoring complexes called Sponge
Bodies. In this thesis I present evidence that at their dorso-anterior destination, grk
transcripts are statically anchored by Dynein, independently of functional Egalitarian
and Bicaudal D, which are required for Dynein transport. I show by the disrupting the
protein’s function after it has fulfilled its role in transport that hnRNP Squid is
involved in the formation and maintenance of these Sponge Bodies. I provide
evidence by EM and fluorescent microscopy that Sponge Bodies share many of
components of translational regulation pathways found in Processing Bodies. I show
by small RNA interference experiments and by genetic analysis that the structural role
of Dynein heavy chain is a unique feature of the Sponge Bodies and that such a
function does not occur in Processing Bodies in Drosophila. I show that the
localization and anchoring of RNA in Sponge Bodies is not a unique feature of grk
mRNA but that I factor RNA is also localized to Sponge bodies. The work presented
tries to elucidate the function of Sponge Bodies in translational control of grk mRNA
and illustrates by EM the dynamic nature of the Sponge Body structure during
oogenesis. My results suggest that Sponge Bodies are RNA granules that are similar
to Processing Bodies in a way that they are involved in translational regulation but
unlike Processing Bodies depend on Dynein for their structural integrity. I propose
that Sponge Bodies are RNA dependent granules that form by the recruitment of
proteins involved in the anchoring and translational regulation.