Abstract:

Spatiotemporal regulation of eukaryotic mRNA translation depends upon the coordination of asymmetric mRNA localization and translational regulation within specific subcellular compartments. The early development of many metazoans is driven by localized mRNAs whose translation in the developing egg and embryo underlies a variety of developmental decisions. In the Drosophila ovary, the determination of cell fate and the establishment of polarity depends upon pathways that localize mRNAs within the developing egg chamber and control their on site translation. CPEB/Orb proteins comprise a family of evolutionarily conserved RNA-binding proteins involved in RNA localization and required to activate the translation of localized mRNAs. Moreover, critical to carrying out its functions, Orb protein autoregulates its own synthesis by binding to orb mRNA and activating its translation. In the studies reported here, we have sought to identify proteins that associate with Orb protein and modulate orb autoregulation. Here we show that the Drosophila Fragile-X Mental Retardation (dFMR1) protein (whose human orthologue underlies the etiology of the Fragile-X syndrome) associates with Orb as part of a mRNP complex that controls the localized translation of mRNAs in the developing oocyte. Misexpression of Orb mRNA targets in absence of dfmr1 function indicates that dFMR1 negatively regulates Orb-mediated translational activation. In addition, dFMR1 regulates the distribution of CPEB/Orb protein in developing egg chambers by negatively regulating orb mRNA translation. Here, we also show that Rasputin, the Drosophila homologue of Ras-GAP SH3 Binding Protein (G3BP), is another component of the Orb mRNP complex. G3BP is a multifunctional protein overexpressed in several human tumors and believed to function as a link between Ras signaling and RNA metabolism. Genetic and biochemical analyses indicate that Rasputin associates with Orb and dFMR1 at the oocyte cortex and positively regulates orb autoregulation. Our results suggest a mechanism by which Fragile-X proteins may regulate localized translation in the nervous system and indicate that Rasputin may function as a link between Ras signaling and translational regulation.