Abstract
Some cells are able to fulfill their specialized function because of their asymmetry. The Drosophila melanogaster oocyte is a polarized cell, and its polarity is important for the establishment of the body axes of the future fly. The body axis formation in the oocyte depends on the restricted secretion of
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the protein Gurken, a TGF-alpha-like peptide. For dorso-ventral axis formation this protein is exclusively synthesized at the future dorsal side. In this dorso-anterior (DA) corner, Gurken deposition depends on the exocytic pathway of the oocyte. This is composed of one thousand independently functioning tER-Golgi units. However, only a few of those units are recruited for Gurken secretion. This depends on two mechanisms. The first is efficient sorting. This depends on the transmembrane domain of the newly synthesized Gurken, which restricts its diffusion in the ER, and it depends on the ER cargo receptor Cornichon. This protein then helps the exit of the Gurken protein from the ER into an ER exit site (tER-site) into the nearest Golgi apparatus, from where it is sorted towards the nearest plasma membrane. The second mechanism is mRNA localization. The gurken mRNA is localized at the DA corner. When this mRNA is forced to other regions of the oocyte by the use of mutant fly strains, not only the mRNA, but also the Gurken protein is found in those regions of the oocyte. However, with the development of a protocol that enabled us to perform an in situ hybridization on ultra-thin cryosections, we were able to establish at electron-microscopic level that the localization of gurken mRNA is not in direct relation with the tER-Golgi units; the mRNA was present in a distinct cytoplasmic structure, the sponge bodies. These sponge bodies function to anchor mRNAs. We show that these anchoring structures contain components of the Dynein motor complex and proteins that are involved in RNA translation regulation and repression. These proteins are typically found in eukaryotic processing (P-) bodies, and therefore we propose that the sponge bodies are the oocyte P-bodies. To investigate how mRNAs populate P-bodies, we studied the gurken mRNA in transport phase by RNA injection experiments. We found that the moving mRNA resides in transport particles, while stationary mRNA is anchored in the P-bodies. The balance between transport and anchoring phase was shown to be regulated by the protein Squid, an hnRNPA1 homologue. The arrival and anchoring of mRNAs in the P-bodies was shown to be dependent on the Dynein motor, not only for gurken mRNA, but also dhc mRNA itself. In this research we have identified a novel link between RNA transport, anchoring and translation regulation, which is mediated by a dual role for Dynein, which functions as a motor in transport particles and as a structural component in the oocyte P-bodies.
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