mRNA localisation and cell polarity in the Drosophila embryo
Asymmetric localisation of mRNA transcripts to specific sites within the cytoplasm is a widely employed mechanism for targeting of proteins and generating cell polarity. The mechanism and function of mRNA localisation has been extensively studied in Drosophila melanogaster, where, for example, the Egalitarian/Bicaudal- D/dynein complex mediates transport of mRNA transcripts, towards microtubule minus-ends, during oogenesis and in syncytial blastoderm embryos. However, it is not known whether the Egalitarian/Bicaudal-D/dynein mRNA transport machinery is required to localise mRNAs in somatic cell types in the Drosophila embryo. In this thesis, I show that the Egalitarian/Bicaudal-D/dynein complex is active in embryonic epithelial cells and neuroblasts and mediates asymmetric localisation of inscuteable, wingless and crumbs, but not miranda, mRNA transcripts, indicating that this is a general mechanism for mRNA localisation in Drosophila. I provide preliminary evidence that y-Tubulin mediates asymmetric miranda localisation. I have also explored the role of mRNA localisation in protein targeting in epithelial cells and neuroblasts, and find that asymmetric localisation of inscuteable and wingless, but not crumbs, mRNA transcripts is required to enhance the targeting of their protein products. I find that asymmetric localisation of wingless mRNA and protein is not required to support Wingless function in the embryo, although, Inscuteable activity is significantly reduced when inscuteable mRNA localisation is disrupted, and neuroblasts display defects in apico-basal polarity and metaphase spindle length. In conclusion, mRNA localisation acts to enhance protein targeting and activity in somatic cell types in the Drosophila embryo.