Proteins that interact physically with Drosophila blanks

RNA interference targets aberrant transcripts with cognate small interfering RNAs, which derive from double-stranded RNA precursors through nucleolytic processing. Several functional screens have identified Drosophila blanks/lump (CG10630) as a facilitator of RNAi in cultured cells, yet its molecular function has remained unknown. The protein carries two dsRNA binding domains (dsRBD) and blanks mutant males have a spermatogenesis defect. We demonstrate that blanks selectively boosts RNAi triggered by dsRNA of nuclear origin. Blanks binds dsRNA in vitro, shuttles between nucleus and cytoplasm and the abundance of certain endogenous siRNAs is reduced in blanks mutant testes. Transgenic expression of a Blanks protein variant with increased nuclear retention reduces endogenous siRNA abundance unless the protein also carries a mutation that prevents binding of dsRNA to the second dsRBD. Blanks thus facilitates the export of dsRNA to the cytoplasm for further processing by the canonical RNAi machinery. In contrast, rescue of male fertility in blanks mutant animals was independent of RNA binding to the second dsRBD, consistent with a previous report that linked fertility to the first dsRBD of Blanks. The role of blanks in spermatogenesis appears thus unrelated to its role in dsRNA export.