No abstract

40The repression of transposons by the Piwi-interacting RNA (piRNA) pathway is essential to 41 protect animal germ cells. In Drosophila ovaries, Panoramix (Panx) enforces transcriptional 42 silencing by binding to the target-engaged Piwi-piRNA complex, although the precise 43 mechanisms by which this occur remain elusive. Here, we show that Panx functions 44 together with a germline specific paralogue of a nuclear export factor, dNxf2, and its 45 cofactor dNxt1 (p15) as a ternary complex to suppress transposon expression. Structural 46 and functional analysis demonstrate that dNxf2 binds Panx via its UBA domain and play an 47 important role in transposon silencing through binding to transposon transcripts directly. 48Unexpectedly, dNxf2 interacts directly with dNxf1 (TAP), which is also essential for 49 transposon silencing. Transient tethering of dNxf2 to nascent transcripts leads to their 50 nuclear retention. Therefore, we propose that dNxf2 may function as a Pandas (Panoramix-51 dNxf2 dependent TAP/p15 silencing) complex, which counteracts the canonical RNA 52 exporting machinery (TAP/p15) and restricts transposons to nuclear peripheries. Our 53 findings may have broader implications for understanding how RNA metabolism modulates 54 epigenetic gene silencing and heterochromatin formation. 55 56Transposons are highly abundant in eukaryotes, which make up nearly half of human genome. To 57 maintain eukaryotic genome integrity, nascent transcripts of transposons are often targeted by 58 nuclear Argonaute proteins for transcriptional gene silencing (TGS) 1-5 . In animal gonads, the PIWI-59 clade Argonautes guided by piRNAs (PIWI-interacting RNA) are thought to recognize nascent 60 transposon transcripts and direct sequence-specific heterochromatin formation 3 . As a critical 61 cofactor of Drosophila nuclear Piwi, Panoramix (Panx, also known as Silencio) links the target-62 engaged Piwi-piRNA complex to the general silencing machinery 6,7 . Enforced tethering of Panx to 63 nascent transcripts leads to cotranscriptional silencing and correlates with deposition of histone H3 64 lysine 9 trimethylation (H3K9me3) marks 6,7 . However, the mechanism by which Panx mediates the 65 repression remains unknown. An equally important question is why H3K9me3 marks are not always 66 sufficient for transposon silencing 8 . 67 68To understand this fundamental question, we cross-examined proteins that co-69 immunoprecipitated with Panx (Extended Data Fig. 1a) with piRNA pathway candidate genes from 70 RNA interference (RNAi) screens 9-12 . Unexpectedly, dNxf2 was identified as a potential cofactor of 71 Panx (Extended Data Fig.1a-c). dNxf2 belongs to an evolutionarily conserved NXF (nuclear export 72 factor) family of proteins, yet depletion of dNxf2 had no effect on polyadenylated mRNA export 13,14 . 73Instead, dNxf2 and its cofactor dNxt1 (also known as p15) were both identified in two published 74RNAi screens as being essential for transposon silencing 9,10 . Similar to Panx, dNxf2 is specifically 75 expressed in female gonads...