Translational repression of msl-2 mRNA in females of Drosophila melanogaster is an essential step in the regulation of X-chromosome dosage compensation. Repression is orchestrated by Sex-lethal (SXL), which binds to both untranslated regions (UTRs) of msl-2 and inhibits translation initiation by poorly understood mechanisms. Here we identify Hrp48 as a SXL co-factor. Hrp48 binds to the 3' UTR of msl-2 and is required for optimal repression by SXL. Hrp48 interacts with eIF3d, a subunit of the eIF3 translation initiation complex. Reporter and RNA chromatography assays showed that eIF3d binds to msl-2 5' UTR, and is required for efficient translation and translational repression of msl-2 mRNA. In line with these results, eIF3d depletion -but not depletion of other eIF3 subunits- de-represses msl-2 expression in female flies. These data are consistent with a model where Hrp48 inhibits msl-2 translation by targeting eIF3d. Our results uncover an important step in the mechanism of msl-2 translation regulation, and illustrate how general translation initiation factors can be co-opted by RNA binding proteins to achieve mRNA-specific control.

Spanish Ministry of Economy and Competitiveness MINECO and the European Regional Development Fund (ERDF) [BFU2012-37135, BFU2015-68741, Consolider CSD2009-00080]; Spanish Ministry of Economy and Competitiveness, ‘Centro de Excelencia Severo Ochoa 2013–2017’ [SEV-2012-0208]; La Caixa Foundation (to M.G.-B.); Fondation pour la Recherche Médicale (FRM) (to A.G.). Funding for open access charge: Spanish Ministry of Economy and Competitiveness (MINECO) [BFU2015-68741].