The RNA-binding protein Tsunagi interacts with Mago Nashi to establish polarity and localize oskar mRNA during Drosophila oogenesis

In Drosophila melanogaster, formation of the axes and the primordial germ c ells is regulated by interactions between the germ line-derived oocyte and the surrounding somatic follicle cells. This reciprocal signaling results i n the asymmetric localization of mRNAs and proteins critical for these ooge nic processes. Mago Nashi protein interprets the posterior follicle cell-to -oocyte signal to establish the major axes and to determine the fate of the primordial germ cells. Using the yeast two-hybrid system we have identifie d an RNA-binding protein, Tsunagi, that interacts with Mago Nashi protein. The proteins coimmunoprecipitate and colocalize, indicating that they form a complex in vivo. Immunolocalization reveals that Tsunagi protein is local ized within the posterior oocyte cytoplasm during stages 1-5 and 8-9, and t hat this localization is dependent on wild-type mago nashi function. When t sunagi function is removed from the germ line, egg chambers develop in whic h the oocyte nucleus fails to migrate, oskar mRNA is not localized within t he posterior pole, and dorsal-ventral pattern abnormalities are observed. T hese results show that a Mago Nashi-Tsunagi protein complex is required for interpreting the posterior follicle cell-to-oocyte signal to define the ma jor body axes and to localize components necessary for determination of the primordial germ cells.