Abstract
The mRNAs encoded by the Drosophila Wnt-1 homologue, wingless, are localized to a small region of the apical cytoplasm in early embryonic epithelial cells. This requires the activity of cis-acting sequences within the 3' untranslated region called wingless localization elements (WLEs). One of these, WLE2, was identified initially as a cis-acting sequence that directed apical localization of in vivo expressed mRNAs in four to five hour Drosophila embryos. To determine the precise activity of WLE2 during wingless apical localization, we performed an in-depth mutagenesis of this sequence. Using both direct-injection and in vivo transgenes we determined that WLE2, by itself, is not sufficient to direct binding of trans-acting factors that mediate apical RNA transport in embryonic cells. Rather, RNAs containing only WLE2 are transported apically only in the presence of a second RNA containing other cis-acting localization sequences of the wingless 3' untranslated region. This co-requirement for WLE2 and additional wingless 3'UTR sequences is likely a required function during apical localization, as mutations within WLE2 can modify the apical localization activity of RNA molecules that contain other wingless localization signals. This suggests that, similar to other localized mRNAs like bicoid, wingless localization may require specific RNA-RNA interactions as well as recognition by cytoplasmic RNA localization complexes.
