Distinct roles of two conserved Staufen domains in oskar mRNA localizationand translation

Drosophila Staufen protein is required for the localization of oskar mRNA t o the posterior of the oocyte, the anterior anchoring of bicoid mRNA and th e basal localization of prospero mRNA in dividing neuroblasts, The only reg ions of Staufen that have been conserved throughout animal evolution are fi ve double-stranded (ds)RNA-binding domains (dsRBDs) and a short region with in an insertion that splits dsRBD2 into two halves. dsRBDs 1, 3 and 4 bind dsRNA in vitro, but dsRBDs 2 and 5 do not, although dsRBD2 does bind dsRNA when the insertion is removed. Full-length Staufen protein lacking this ins ertion is able to associate with oskar mRNA and activate its translation, b ut fails to localize the RNA to the posterior, In contrast, Staufen lacking dsRBD5 localizes oskar mRNA normally, but does not activate its translatio n. Thus, dsRBD2 is required for the microtubule-dependent localization of o sk mRNA, and dsRBD5 for the derepression of oskar mRNA translation, once lo calized, Since dsRBD5 has been shown to direct the actin-dependent localiza tion of prospero mRNA, distinct domains of Staufen mediate microtubule- and actin-based mRNA transport.