Specialization of theDrosophilanuclear export family protein Nxf3 for piRNA precursor export

Abstract

The PIWI-interacting RNA (piRNA) pathway is a conserved small RNA-based immune system that protects animal germ cell genomes from the harmful effects of transposon mobilization. In Drosophila ovaries, most piRNAs originate from dual-strand clusters, which generate piRNAs from both genomic strands. Dual-strand clusters use noncanonical transcription mechanisms. Although transcribed by RNA polymerase II, cluster transcripts lack splicing signatures and poly(A) tails. mRNA processing is important for general mRNA export mediated by nuclear export factor 1 (Nxf1). Although UAP56, a component of the transcription and export complex, has been implicated in piRNA precursor export, it remains unknown how dual-strand cluster transcripts are specifically targeted for piRNA biogenesis by export from the nucleus to cytoplasmic processing centers. Here we report that dual-strand cluster transcript export requires CG13741/Bootlegger and the Drosophila nuclear export factor family protein Nxf3. Bootlegger is specifically recruited to piRNA clusters and in turn brings Nxf3. We found that Nxf3 specifically binds to piRNA precursors and is essential for their export to piRNA biogenesis sites, a process that is critical for germline transposon silencing. Our data shed light on how dual-strand clusters compensate for a lack of canonical features of mature mRNAs to be specifically exported via Nxf3, ensuring proper piRNA production.

Nxf3: a middleman with the right connections for unspliced piRNA precursor export

“…Instead, the human Nxf3 is shown to depend on the Crm1-mediated protein export pathway to transport an Nxf3-tethered RNA to the cytoplasm (Yang et al 2001). Kneuss et al (2019) reach a similar conclusion by demonstrating that nucleo-cytoplasmic shuttling of fly Nxf3 protein is also Crm1-dependent. Crm1 recognizes nuclear export signals (NESs) on cargo proteins to facilitate their passage into the cytoplasm.…”

“…The same screen also identified another factor, CG13741. Kneuss et al (2019) now demonstrate that fly mutants of Nxf3 and CG13741 (renamed Bootlegger [Boot]) derepress germline-expressed transposons, leading to infertility. They determined that transposon derepression is due to a piRNA biogenesis defect specifically affecting dual-strand clusters.…”

“…So, what distinguishes the cluster transcripts from spliced RNAs? Kneuss et al (2019) and ElMaghraby et al (2019) now place Nxf3 and Boot as novel germline-specific components within this noncanonical RNA export cargo. Using knockdown experiments, they provide evidence for a sequence of events in which Rhi (via Del) first recruits Boot, which in turn recruits Nxf3 and UAP56 to the transcription loci to bind the nascent RNA.…”

“…protein export pathway. In this issue of Genes & Development, Kneuss et al (2019) describe yet another novel exception in the context of biogenesis of Piwi-interacting RNAs (piRNAs), specialized small RNAs that repress transposable elements in animal gonads.…”

Assembly and Function of Gonad-Specific Non-Membranous Organelles in Drosophila piRNA Biogenesis

“…In contrast, the dual-strand clusters require a quite unique system for their transcription, involving germ-specific factors such as Rhino, Deadlock, Cutoff, and Moonshiner [50][51][52][53][54][55]. The resultant RNA transcripts are then exported to the cytoplasm in an Nxf3-and Bootlegger-dependent manner [56,57].…”