Dimerisation of the PICTS complex via LC8/Cut-up drives co-transcriptional transposon silencing inDrosophila

Abstract

In animal gonads, the PIWI-interacting RNA (piRNA) pathway guards genome integrity in part through the co-transcriptional gene silencing of transposon insertions. In Drosophila ovaries, piRNA-loaded Piwi detects nascent transposon transcripts and instructs heterochromatin formation through the Panoramix-induced co-transcriptional silencing (PICTS) complex, containing Panoramix, Nxf2 and Nxt1. Here, we report that the highly conserved dynein light chain LC8/Cut-up (Ctp) is an essential component of the PICTS complex. Loss of Ctp results in transposon de-repression and a reduction in repressive chromatin marks specifically at transposon loci. In turn, Ctp can enforce transcriptional silencing when artificially recruited to RNA and DNA reporters. We show that Ctp drives dimerisation of the PICTS complex through its interaction with conserved motifs within Panoramix. Artificial dimerisation of Panoramix bypasses the necessity for its interaction with Ctp, demonstrating that conscription of a protein from a ubiquitous cellular machinery has fulfilled a fundamental requirement for a transposon silencing complex.

Piwi–piRNA complexes induce stepwise changes in nuclear architecture at target loci

“…Within the nucleus, Piwi–piRNA functions together with Panx‐Nxf2‐p15 at the nascent transcript, in order to silence target TEs (Batki et al , 2019; Fabry et al , 2019; Murano et al , 2019; Zhao et al , 2019). Recent studies have suggested that Panx‐Nxf2‐p15 complex forms condensates upon Piwi–piRNA‐mediated silencing (Eastwood et al , 2021; Schnabl et al , 2021). Condensate formation is coupled to dimerization of Panx‐Nxf2‐p15 complex, and piRNA target regions may therefore be spatially regulated upon silencing.…”

“…Moreover, whether Piwi–piRNA and its silencing complex, which associate with nascent RNA rather than chromatin, regulate chromatin conformation to trigger heterochromatin formation is unknown. Recent studies indicate that silencing complexes formed by Panx–Nxf2–p15 further associate with the dynein light chain protein, LC8, and form condensates in a dimer‐dependent manner to induce co‐transcriptional silencing (Eastwood et al , 2021; Schnabl et al , 2021). However, whether Piwi–piRNA regulation impacts nuclear spatial organization in vivo is still unrevealed.…”

Rapid evolution of promoters from germline-specifically expressed genes including transposon silencing factors

“…4A and B). Next, using RNA-seq data from ovarian somatic cells (OSCs) (GSE160860) (41), we further filtered the soma-enriched genes that are highly expressed in OSCs.…”

“…To stringently define the germline genes, we additionally used processed RNA-seq data from early fly embryos (0-2hr, modENCODE; mRNA-seq fly embryo 0-2hr; RPKM; mE_mRNA_em0-2hr; FBlc0000086) to further filter the germline-enriched genes using >9 RPKM parameter. To stringently define the somatic-specific genes, we additionally used processed RNA-seq data from ovarian somatic cells (OSCs) (n=4, GSE160860) (41) and filtered the soma-enriched genes using >20 RPKM to select genes that are expressed in OSCs as well as parameter <2 RPKM for early fly embryo (0-2hr, modENCODE) to filter out genes that are expressed in early embryos. This analysis generated a list of 107 germ cell-specifically expressed genes and of 59 somatic cell-specifically expressed genes (Fig.…”

piRNA‐ and siRNA‐mediated transcriptional repression in Drosophila , mice, and yeast: new insights and biodiversity

“…Cutup/LC8 (Ctp) has recently been found to be the fourth component of the PICTS/PPNP/SFiNX/Pandas complex (Eastwood et al , 2021; Schnabl et al , 2021). Ctp is ubiquitous and functions in numerous biological processes by dimerizing its interacting complexes to stabilize them (Jespersen & Barbar, 2020).…”