Cell 157, 1698–1711 (2014)

Piwi-interacting RNAs (piRNAs) act with the Piwi class of Argonaute (Ago) proteins to suppress transposons in germline cells. Primary piRNAs are derived from genomic piRNA clusters and direct Piwi endonuclease activity to expressed transposons. The anti-transposon response is amplified by the 'ping-pong' mechanism, which uses sliced transposon RNA as guides to generate secondary piRNAs. Xiol et al. now identify key components of the piRNA amplifier complex (PAC) that mediates the ping-pong cycle. Comparative transcriptomic analysis of silkworms, flies and mice identified Vasa, a conserved DEAD-box RNA helicase, as a likely component of the PAC. Proteomics using a DQAD mutant of Vasa, which lacks ATPase activity, revealed that the two Agos, Siwi and Ago3, and Qin/Kumo, a Tudor domain–containing protein, interact with Vasa to form the PAC, which is localized in cytoplasmic granules called nuage. Immunoprecipitation and deep sequencing showed that the PAC also contains antisense piRNA intermediates normally associated with Siwi as well as sense transposon RNA. On the basis of surface plasmon resonance characterization of Vasa-RNA binding and crystallographic analysis of the Vasa DQAD mutant, the authors propose a model in which Vasa acts as an ATP-dependent clamp that captures Siwi–piRNA–transposon complexes, induces PAC assembly by recruiting Ago3 and Qin/Kumo and facilitates RNA intermediate transfer from Siwi to Ago3 in an ATP hydrolysis–dependent step. Taken together, this molecular proposal for strand transfer offers a useful model for future investigations of piRNA amplification mechanisms.