Squid is required for efficient posterior localization of oskar mRNA during Drosophila oogenesis

The nuclear-cytoplasmic shuttling heterogeneous nuclear RNA-binding protein (hnRNP) Squid (Sqd) is required during Drosophila melanogaster oogenesis, where it plays a critical role in the regulation of the TGFalpha-like molecule Gurken (Grk). Three Sqd isoforms have been described, SqdA, S and B, and two of these, SqdA and SqdS, differentially function in grk mRNA nuclear export, cytoplasmic transport and translational control during oogenesis. Here, we report that Sqd is also required for the regulation of oskar (osk) mRNA, functioning in the cytoplasmic localization of the osk transcript. In oocytes from sqd females, osk mRNA is not efficiently localized to the posterior pole, but rather accumulates at the anterior cortex. Furthermore, anterior patterning defects observed in embryos from sqd females expressing only the SqdS protein isoform suggest that Sqd may also play a role in the translational regulation of the mislocalized osk mRNA. These findings provide additional support for models of mRNA regulation in which cytoplasmic events, such as localization and translational regulation, are coupled. These results also place Sqd among an emerging class of proteins, including such other members as Bruno (Bru) and Hrb27C/Hrp48, which function in multiple aspects of both grk and osk mRNA regulation during Drosophila oogenesis.