Drosophila Gtsf1 is an essential component of the Piwi-mediated transcriptional silencing complex

Abstract

The PIWI-interacting RNA (piRNA) pathway is a small RNA silencing system that keeps selfish genetic elements such as transposons under control in animal gonads. Several lines of evidence indicate that nuclear PIWI family proteins guide transcriptional silencing of their targets, yet the composition of the underlying silencing complex is unknown. Here we demonstrate that the double CHHC zinc finger protein gametocyte-specific factor 1 (Gtsf1) is an essential factor for Piwi-mediated transcriptional repression in Drosophila. Cells lacking Gtsf1 contain nuclear Piwi loaded with piRNAs, yet Piwi's silencing capacity is ablated. Gtsf1 interacts directly with a small subpool of nuclear Piwi, and loss of Gtsf1 phenocopies loss of Piwi in terms of deregulation of transposons, loss of H3K9 trimethylation (H3K9me3) marks at euchromatic transposon insertions, and deregulation of genes in proximity to repressed transposons. We propose that only a small fraction of nuclear Piwi is actively engaged in target silencing and that Gtsf1 is an essential component of the underlying Piwi-centered silencing complex.[Keywords: transposon control; piRNA pathway; Piwi; Gtsf1; transcriptional silencing; heterochromatin formation] Supplemental material is available for this article. The nuclear Piwi-piRNA complex inhibits transcription of TEs by an unknown mechanism. Target silencing is accompanied by histone H3 Lys 9 trimethylation (H3K9me3) and appears to require the HP1 family member Su(var)205 (Shpiz et al. 2011;Wang and Elgin 2011;Sienski et al. 2012;Le Thomas et al. 2013;Rozhkov et al. 2013). The piRNA pathway factor Maelstrom (Mael) is required for Piwi-guided transcriptional silencing, yet loss of Mael (in contrast to loss of Piwi) does not result in significant loss of H3K9me3 marks at repressed TE insertions (Sienski et al. 2012). Whether H3K9me3, a mark for heterochromatin, is required for transcriptional silencing or accompanies it is therefore not fully understood.Conceptually, TE silencing by Piwi might resemble the much better understood process of RNAi-induced heterochromatin formation in Schizosaccharomyces pombe (Moazed 2009;Grewal 2010;Castel and Martienssen 2013). In both systems, a nuclear Argonaute protein guides the transcriptional silencing of targets, and this is accompanied by H3K9 methylation. Fission yeast AGO1 assembles in the trimeric RNA-induced transcriptional silencing (RITS) complex and recruits the histone methyltransferase complex CLRC that mediates H3K9 methylation. In analogy, vertebrate and invertebrate nuclear PIWI proteins are likely to assemble a silencing complex that interacts with nascent RNA at the target locus and induces transcriptional silencing by recruiting factors impacting RNA polymerase II (Pol II) recruitment and/or the chromatin status. However, besides Mael and possibly Su(var)205, no factor is known to be required for

Setdb1 is required for germline development and silencing of H3K9me3-marked endogenous retroviruses in primordial germ cells

“…Conversely, depletion of Piwi or its binding partner, Gtsf1, leads to a rapid loss of H3K9me3 and derepression of specific LTR elements, indicating that the Drosophila piRNA pathway impinges on this histone mark (Donertas et al 2013). Similarly, the AGO1 silencing complex RITS interacts with nascent target RNAs to guide H3K9me3 at repetitive regions in Schizosaccharomyces pombe (Castel and Martienssen 2013).…”

piRNAs: from biogenesis to function

“…2A). piRISC-induced TGS also requires the zinc-finger protein GTSF-1/Asterix, which likely directly interacts with Piwi and is required for establishment of H3K9 methylation (Donertas et al, 2013;Handler et al, 2013;Muerdter et al, 2013). Furthermore, histone methylation may not be the final silencing mark; the high mobility group protein Maelstrom, like Piwi, is required for POL II inhibition; however, changes in H3K9me3 methylation following mael knockdown are modest compared with the effects seen upon piwi knockdown, indicating that Mael acts downstream of Piwi and histone methylation (Sienski et al, 2012).…”

Small RNA profiling and characterization of piRNA clusters in the adult testes of the common marmoset, a model primate

“…Although miRNAs and endo-siRNAs are expressed in both animals and plants, piRNA expression is largely restricted to animal gonads, where they primarily mediate TE silencing for genome integrity and germ cell development (Thomson and Lin 2009;Siomi et al 2011). piRNAs act as guides for the cleavage of TE transcripts by PIWI protein Slicer activity in the cytoplasm, which often couples with the ping-pong cycle, or for Slicer-independent heterochromatin formation at TE loci in the nucleus (Sienski et al 2012;Dönertas et al 2013;Huang et al 2013;Le Thomas et al 2013;Muerdter et al 2013;Ohtani et al 2013).…”