The Drosophila Bruno paralogue Bru-3 specifically binds the EDEN translational repression element

We reported in our previous work that the EDEN-dependent translational repression of maternal mRNAs was conserved between Drosophila and Xenopus. In Xenopus, this repression is achieved through the binding of EDEN to the Bruno-like factor, EDEN-BP. We show in the present work that the Drosophila Bruno paralogue, the 45 kDa Bru-3 protein (p45), binds specifically to the EDEN element and acts as a homodimer. We describe for the first time a previously undetected 67 amino acid domain, found in the divergent linker region, the lsm domain (lsm stands for linker-specific motif). We propose that the presence of this domain in a subset of the Bruno-like proteins, including Bru-3, EDEN-BP and CUG-BP but not Bruno nor its other paralogue Bru-2, might be responsible for specific RNA recognition. Interestingly, comparative structural analyses using threaders and molecular modelling suggest that the new domain might be distantly related to the first RNA recognition motif of the Drosophila sex-lethal protein (sxl). The phylogenetic analyses and the experimental data based on its specific binding to the EDEN element support the conclusion that Bru-3 is an EDEN-BP/CUG-BP orthologue.