Implications for bcd mRNA localization from spatial distribution of exu protein in Drosophila oogenesis.

Subcellular RNA localization in different cell types leads to asymmetric distribution of proteins in these cells. The localization of bicoid (bcd) messenger RNA to the anterior pole of the developing Drosophila oocyte gives rise in embryogenesis to a steep concentration gradient of the bcd protein, a transcription factor that activates expression of zygotic genes needed for anterior development. The exuperantia (exu) gene is necessary for this localization of bcd mRNA. Here we express a chimaeric gene encoding a fusion between the Acquorea victoria green fluorescent protein (GFP) and the exu protein (Exu) in female germ cells, and find that the fusion protein fluoresces strongly in both live and fixed cells during Drosophila oogenesis. The fusion protein rescues an exu null allele, restoring full fertility to females, and is expressed and localized in a temporal and spatial pattern similar to native Exu. The high sensitivity of the GFP tag provides important new details on the subcellular localization of Exu. The fusion protein is found in particles concentrated at ring canals, where transport occurs between nurse cells and the oocyte. Drugs such as colchicine and taxol that affect microtubule stability alter localization of the particles. We propose that the particles are ribonucleoprotein complexes or vesicles which transport bcd mRNA along microtubules and target it to the anterior oocyte cortex.