Background: Asymmetric localization of mRNAs within cells promotes precise spatio-temporal control of protein synthesis. Although cytoskeletal transport-based localization during Drosophila oogenesis is well characterized, little is known about the mechanisms that operate to localize maternal RNAs in the early embryo. One such mechanism-termed "degradation/protection"-acts on maternal Hsp83 transcripts, removing them from the bulk cytoplasm while protecting them in the posterior pole plasm.
Results: Here, we identify the RNA binding protein, Smaug, previously known as a translational repressor of nanos, as a key regulator of degradation/protection-based transcript localization. In smaug mutants, degradation of Hsp83 transcripts is not triggered, and, thus, localization does not occur. Hsp83 transcripts are in an mRNP complex containing Smaug, but Smaug does not translationally repress Hsp83 mRNA. Rather, Smaug physically interacts with the CCR4/POP2/NOT deadenylase, recruiting it to Hsp83 mRNA to trigger transcript deadenylation and degradation. When Smaug is targeted to heterologous stable reporter transcripts in vivo, these are deadenylated and destabilized. A deletion that removes the gene encoding CCR4 exhibits dose-sensitive interactions with Smaug in both a loss-of-function and a gain-of-function context. Reduction of CCR4 protein levels compromises Hsp83 transcript destabilization.
Conclusions: Smaug triggers destabilization and localization of specific maternal transcripts through recruitment of the CCR4/POP2/NOT deadenylase. In contrast, Smaug-mediated translational repression is accomplished via an indirect interaction between Smaug and eIF4E, a component of the basic translation machinery. Thus, Smaug is a multifunctional posttranscriptional regulator that employs distinct mechanisms to repress translation and to induce degradation of target transcripts.