Trafficking of siRNA precursors by the dsRBD protein Blanks in Drosophila

RNA interference targets aberrant transcripts with cognate small interfering RNAs, which are derived from double-stranded RNA precursors. Several functional screens have identified Drosophila Blanks as a facilitator of RNAi, yet its molecular function has remained unknown - potentially because of its restricted expression pattern. We demonstrate that Blanks selectively boosts RNAi phenomena triggered by dsRNA of nuclear origin . Blanks binds dsRNA, shuttles between nucleus and cytoplasm and the abundance of certain endogenous siRNAs is reduced in Blanks mutant testes. The fertility defect of blanks mutant males is unrelated to the RNA binding activity of blanks. In contrast, increased nuclear retention of Blanks reduces fertility and distorts the small RNA profile. However, this only occurs if Blanks is competent for RNA binding. Since Blanks protein with increased nuclear retention reduces endogenous siRNA abundance, we propose that Blanks facilitates the export of dsRNA to the cytoplasm for further processing by the RNAi machinery. Accordingly, we identified a set of blanks dependent small RNAs and refer to them as “Blanks exported endogenous siRNAs” or bepsiRNAs. While Blanks considerably augments nuclear-triggered interference in S2-cells, it is not per se essential for RNAi. This is consistent with the restricted expression pattern of blanks (testes and certain cell culture lines) and indicates redundancy and/or alternative backup mechanisms.