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 cells is regulated by interactions between the germ line-derived oocyte and the surrounding somatic follicle cells. This reciprocal signaling results in the asymmetric localization of mRNAs and proteins critical for these oogenic 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 identified 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 localized within the posterior oocyte cytoplasm during stages 1-5 and 8-9, and that this localization is dependent on wild-type mago nashi function. When tsunagi function is removed from the germ line, egg chambers develop in which the oocyte nucleus fails to migrate, oskar mRNA is not localized within the posterior pole, and dorsal-ventral pattern abnormalities are observed. These results show that a Mago Nashi-Tsunagi protein complex is required for interpreting the posterior follicle cell-to-oocyte signal to define the major body axes and to localize components necessary for determination of the primordial germ cells.