Localization of bicoid (bcd) mRNA to the anterior and oskar (osk) mRNA to the 
posterior of the Drosophila oocyte is critical for embryonic patterning. 
Previous genetic studies implicated exuperantia (exu) in bcd mRNA localization, 
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 that contains at 
least seven other proteins. One of these proteins was identified 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 oocyte 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-null mutants are defective in osk mRNA 
localization in both nurse cells and the oocyte. Furthermore, both Exu/Yps 
particles and osk mRNA follow a similar temporal pattern of localization in 
which they transiently accumulate at the oocyte anterior and subsequently 
localize to the posterior pole. We propose that Exu is a core component of a 
large protein complex involved in localizing mRNAs both within nurse cells and 
the developing oocyte.