The nascent RNA binding complex SFiNX licenses piRNA-guided heterochromatin formation

The PIWI-interacting RNA (piRNA) pathway protects animal genome integrity in part through establishing repressive heterochromatin at transposon loci. Silencing requires piRNA- guided targeting of nuclear PIWI proteins to nascent transposon transcripts, yet the subsequent molecular events are not understood. Here, we identify SFiNX (Silencing Factor interacting Nuclear eXport variant), an interdependent protein complex required for Piwi-mediated co-transcriptional silencing in Drosophila. SFiNX consists of Nxf2-Nxt1, a gonad- specific variant of the heterodimeric mRNA export receptor Nxf1-Nxt1, and the Piwi-associated protein Panoramix. SFiNX mutant flies are sterile and exhibit transposon de-repression because piRNA-loaded Piwi is unable to establish heterochromatin. Within SFiNX, Panoramix recruits the cellular heterochromatin machinery, while Nxf2 binds the nascent target RNA and licenses co-transcriptional silencing. Our results reveal an unexpected, RNA-export independent role for Nxf2 in nuclear small RNA biology and suggest that NXF-variants, which have largely unknown function in animals, are functionally linked to the genome-transposon conflict. RNA-Seq analysis of total RNAs (with rRNA depletion or polyA selection) from ovaries and ovarian stem cells (OSCs) of Nxf2, Panx, Piwi mutants.