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Abstract

miRNAs are genome-encoded 22 nucleotide-long RNAs that silence gene expression by binding 3¢ untranslated regions of target mRNAs. They play important roles in a broad range of biological processes including development, cellular differentiation and apoptosis. Emerging evidence also implicates miRNAs in the pathogenesis of human dis- eases such as cancer and metabolic disorders. Although recent years amassed a wealth of information about miRNA biogenesis, the mecha- nisms allowing miRNAs to silence gene expression in animal cells are still under debate. To elucidate how silencing is accomplished, we screened an RNA interference library for suppressors of miRNA-medi- ated regulation in Drosophila melanogaster cells. In addition to proteins known to be required for miRNA biogenesis and function (i.e. Drosha, Dicer-1 and AGO1), the screen identified the P-body component GW182 as being required for silencing by miRNAs. Depleting GW182 suppresses silencing of all reporters tested, demonstrating an essential role for GW182 in the miRNA pathway. We further show that the N-terminal GW-repeats of GW182 interact with the PIWI domain of AGO1. Together our findings indicate that miRNA function is effected by AGO1-GW182 complexes, irrespective of whether silencing occurs at the level of translation or of mRNA decay.