Interaction of Staufen1 with the 5' end of mRNA facilitates translation of these RNAs

Staufen1 is a component of transported ribonucleoprotein complexes. Genetic work in Drosophila has suggested that Staufen plays a role in the de-repression of translation of oskar mRNA following localization. To determine whether Staufen1 can play a similar role in mammals, we studied translation of transcripts in the presence or in the absence of Staufen1. Translationally repressed mRNAs were generated by fusing the structured human immunodeficiency virus type 1 trans-activating response (TAR) element to the 5' end of a reporter transcript. In rabbit reticulocyte lysates and in mammalian cultured cells, the addition of Staufen1 resulted in the up-regulation of reporter activity when translation was driven by the TAR-bearing RNA. In contrast, Staufen1 had no effect on translation of efficiently translated mRNAs lacking an apparent structured 5' end, suggesting that Staufen1-binding to the 5' end is required for enhanced translation. Consistently, Staufen1 RNA-binding activity is necessary for this translational effect. In addition, similar up-regulation of translation was observed when Staufen1 was tethered to the 5' end of mRNAs via other structured RNAs, the highest level of translational increase being obtained with the bona fide Staufen1-binding site of the Arf1 transcript. The expression of Staufen1 promoted polysomal loading of TAR-luciferase transcripts resulting in enhanced translation. Our results support a model in which the expression of Staufen1 and its interaction with the 5' end of RNA and ribosomes facilitate translation initiation.