Egalitarian feeds forward to Staufen to inhibit Dynein during mRNP transport

The regulated recruitment and activity of motor proteins is crucial for intracellular transport of cargoes, including ribonucleoprotein complexes (RNP). Here we show that regulation of mRNP transport by the minus end-directed dynein motor complex in the Drosophila germline relies on the interplay of two double-stranded RNA binding proteins, Staufen and the dynein adaptor Egalitarian. Our quantitative in situ analysis shows that, in the nurse cells, Egalitarian associates with bicoid, oskar and staufen mRNAs, which consequently enrich in the oocyte. This results in ooplasmic accumulation of Staufen and its recruitment to the transcripts. We demonstrate that Staufen levels scale proportionately with RNA content in oskar mRNPs, and to a lesser extent in bicoid mRNPs. We show that enhanced recruitment of Staufen to oskar mRNPs results in dissociation of Egalitarian and a reduction in minus end-directed transport, although dynein remains associated with the RNPs. The downregulation of dynein activity prevents anterior accumulation of oskar mRNA, whose transport to the posterior is essential for localized production of Oskar protein. Our observations identify a feed-forward loop, whereby staufen mRNA localization and protein accumulation in the oocyte enable motor switching, promoting oskar mRNA localization to the posterior pole.