Schematic diagram of the Siwi shuttling model between nuage and piP-bodies. At steady state, wild-type Siwi is required for the piP-bodies assembly and physically interacts with the BmSpnE/BmQin heterodimer. At the same time, Siwi is enriched in nuage, colocalizing with BmVasa. This suggests that Siwi is likely to shuttle between nuage and piP-bodies in a transient manner. Catalytic mutant Siwi-D670A is instead retained in piP-bodies and seldom localizes in nuage, suggesting that slicer activity of Siwi is required for this shuttling. Co-expressing Siwi-D670A and BmVasa-E339Q result in enormous cytoplasmic aggregates, suggesting that BmVasa-E339Q functions as a dominant-negative mutant that masks the ATP-dependent RNPase activity of wild-type BmVasa. Therefore, both activities of BmVasa ATPase and Siwi slicer are required for Siwi shuttling. 