Roles for the Yb body components Armitage and Yb in primary piRNA biogenesis in Drosophila

Abstract

PIWI-interacting RNAs (piRNAs) protect genome integrity from transposons. In Drosophila ovarian somas, primary piRNAs are produced and loaded onto Piwi. Here, we describe roles for the cytoplasmic Yb body components Armitage and Yb in somatic primary piRNA biogenesis. Armitage binds to Piwi and is required for localizing Piwi into Yb bodies. Without Armitage or Yb, Piwi is freed from the piRNAs and does not enter the nucleus. Thus, piRNA loading is required for Piwi nuclear entry. We propose that a functional Piwi–piRNA complex is formed and inspected in Yb bodies before its nuclear entry to exert transposon silencing.

The RNA helicase MOV10L1 binds piRNA precursors to initiate piRNA processing

“…MOV10L1 does not interact with Tdrkh (Saxe et al 2013), which is required for 3 ′ end maturation. Experiments in Drosophila are consistent with our model; RNAs isolated from Armitage by standard immunoprecipitation conditions show characteristics similar to those of piRNA precursor fragments (Saito et al 2010). A significant number of Armitage-bound RNA fragments mapped within the flamenco locus, which gives rise to long piRNA precursor transcripts (Malone et al 2009), overlapping with piRNA sequences (Saito et al 2010).…”

“…There is substantial evidence from genetic and cell culture experiments that Zuc is essential for primary piRNA biogenesis (Pane et al 2007;Malone et al 2009;Haase et al 2010;Saito et al 2010;Watanabe et al 2011), and it was recently shown both structurally and biochemically that Zuc is an endonuclease that cleaves ssRNA and leaves a 5 ′ phosphate end (Ipsaro et al 2012;Nishimasu et al 2012;Voigt et al 2012). Therefore, Zuc fits the description for the endonuclease that generates the 5 ′ ends of piRNA intermediates, but direct proof of this is lacking.…”

“…MOV10L1 is a predicted RNA helicase that is essential for primary piRNA biogenesis; it is localized in ICM, and its expression starts in prenatal gonocytes, peaks in pachytene spermatocytes, and terminates in post-meiotic spermatids (Frost et al 2010;Zheng et al 2010;Zheng and Wang 2012). The Drosophila homolog of MOV10L1, Armitage, is localized to the nuage and is essential for primary piRNA generation (Klattenhoff et al 2007;Haase et al 2010;Saito et al 2010). Furthermore, mice with postnatal deletion of Mov10l1 lack pachytene piRNAs and provide an ideal system to study their functions (Zheng and Wang 2012).…”

Drosophila Piwi functions downstream of piRNA production mediating a chromatin-based transposon silencing mechanism in female germ line

“…Thus, the signals detected in the latter experiments probably reflect functions of somatic Piwi. Using an ovarian somatic cell line (OSC), Saito et al have shown that Piwi is required to maintain normal piRNA levels in these cells (17). Although the mechanism remains unclear, their results in soma taken together with our observations in germ line highlight a distinction in Piwi function between the two tissues.…”

“…In the germ line, piRNA biogenesis involves both primary processing and a secondary amplification pathway (ping-pong amplification) (8,9,16), whereas piRNAs in the soma are generated solely from primary transcripts (primary pathway) (14,15,17).…”