Isolation of a ribonucleoprotein complex involved in mRNA localization in Drosophila oocytes

Localization of bicoid (bcd) mRNA to the anterior and oskar (osk) mRNA to t he posterior of the Drosophila oocyte is critical for embryonic patterning. Previous genetic studies implicated exuperantia (exu) in bcd mRNA localiza tion, but its role in this process is not understood. We have biochemically isolated Exu and show that it is part of a large RNase-sensitive complex t hat contains at least seven other proteins. One of these proteins was ident ified as the cold shock domain RNA-binding protein Ypsilon Schachtel (Yps), which we show binds directly to Exu and colocalizes with Exu in both the o ocyte and nurse cells of the Drosophila egg chamber. Surprisingly, the Exu- Yps complex contains osk mRNA. This biochemical result led us to reexamine the role of Exu in the localization of osk mRNA. We discovered that exu-nul l mutants are defective in osk mRNA localization in both nurse cells and th e oocyte. Furthermore, both Exu/Yps particles and osk mRNA follow a similar temporal pattern of localization in which they transiently accumulate at t he oocyte anterior and subsequently localize to the posterior pole. We prop ose that Exu is a core component of a large protein complex involved in loc alizing mRNAs both within nurse cells and the developing oocyte.