Molecular principles of Piwi-mediated cotranscriptional silencing through the dimeric SFiNX complex

Abstract

Nuclear Argonaute proteins, guided by their bound small RNAs to nascent target transcripts, mediate cotranscriptional silencing of transposons and repetitive genomic loci through heterochromatin formation. The molecular mechanisms involved in this process are incompletely understood. Here, we show that the SFiNX complex, a silencing mediator downstream from nuclear Piwi-piRNA complexes in Drosophila, facilitates cotranscriptional silencing as a homodimer. The dynein light chain protein Cut up/LC8 mediates SFiNX dimerization, and its function can be bypassed by a heterologous dimerization domain, arguing for a constitutive SFiNX dimer. Dimeric, but not monomeric SFiNX, is capable of forming molecular condensates in a nucleic acid-stimulated manner. Mutations that prevent SFiNX dimerization result in loss of condensate formation in vitro and the inability of Piwi to initiate heterochromatin formation and silence transposons in vivo. We propose that multivalent SFiNX-nucleic acid interactions are critical for heterochromatin establishment at piRNA target loci in a cotranscriptional manner.

Dynein light chain-dependent dimerization of Egalitarian is essential for maintaining oocyte fate in Drosophila

“…Artificial dimerization of a panx mutant that could no longer interact with Dlc (using the same GCN4 leucine zipper) restored the function of panx (Eastwood et al, 2021;Schnabl et al, 2021). Similar to our findings, the rescue of certain panx mutant phenotypes was incomplete (Schnabl et al, 2021).…”

“…Recently, Panoramix (Panx), a protein that functions in the piRNA pathway was also shown to dimerize in a Dlc-dependent manner (Eastwood et al, 2021;Schnabl et al, 2021). Artificial dimerization of a panx mutant that could no longer interact with Dlc (using the same GCN4 leucine zipper) restored the function of panx (Eastwood et al, 2021;Schnabl et al, 2021).…”

Rewiring of chromatin state and gene expression by transposable elements

“…(b) Hypothetical model showing the condensation of multiple Piwi–piRNA silencing complexes. Possible formation of LLPS is suggested in recent studies (Eastwood et al., 2021; Schnabl et al., 2021)…”

“…Nxf2 associates with the nascent transcript of target TEs to form a silencing complex. Recent studies have suggested that Panx can further associate with Cut‐up/LC8 and form a dimerized structure, suggesting the possibility that the Piwi–piRNA silencing complex can form a liquid‐liquid phase separation (LLPS) (Eastwood et al., 2021; Schnabl et al., 2021) (Figure 4b). HP1a, which is recruited to H3K9me, and the linker histone H1 are further associated with the piRNA target regions to induce a stable heterochromatin structure (Huang et al., 2013; Iwasaki et al., 2016; Le Thomas et al., 2013; Rozhkov et al., 2013).…”

“…Similarly, in the case of Piwi–piRNA‐mediated silencing, chromatin regulation upon heterochromatin formation has been shown to spread for several tens of kilobases from target TEs (Sienski et al., 2012). Recent studies have suggested the possible formation of an LLPS upon Piwi–piRNA‐mediated silencing of TEs (Eastwood et al., 2021; Schnabl et al., 2021). Therefore, piRNA target regions and their silencing machinery may form a spatial complex to silence their targets.…”