RNA clamping by Vasa assembles a piRNA Amplifier complex on transposon transcripts

Germline-specific Piwi-interacting RNAs (piRNAs) protect the genome against selfish genetic elements and are essential for fertility in animals. piRNAs targeting active transposons are amplified by a feed-forward loop known as the Ping-pong cycle, which links endonucleolytic slicing of target RNAs by Piwi proteins to biogenesis of new piRNAs. However, the biochemical framework for this pathway remains elusive. Here, we describe the identification of a transient Amplifier complex mediating the biogenesis of secondary piRNAs in insect cells. This complex is nucleated by the RNA helicase Vasa and contains the two Piwi proteins participating in the Ping-pong loop, the Tudor protein Qin/Kumo and antisense piRNA guides. These components assemble on the surface of Vasa's helicase domain, which functions as an RNA clamp to anchor Amplifier onto transposon transcripts. We show that ATP-dependent RNP remodelling by Vasa facilitates the transfer of 5'-sliced piRNA precursors between the Ping-pong partners, and failure to achieve this results in Drosophila female sterility. Overall design: Immunoprecipitated small RNA were purified from untransfected or transfected Bmn4 cells for preparation of high-throughput sequencing libraries.