Specialization of the Drosophila nuclear export family protein Nxf3 for piRNA precursor export.

Emma Kneuss,Undine-Sophie Deumer,Jonathan B Preall,Gregory J Hannon,Benjamin Czech,Evelyn L Eastwood,Marzia Munafò

doi:10.1101/gad.328690.119

Emma Kneuss, Undine-Sophie Deumer + Show 5 more

Open Access

https://doi.org/10.1101/gad.328690.119

Copy DOI

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.

Highlights

Transposable elements represent a major threat to genome integrity
Transposon mobilization is prevented by a conserved small RNA-based immune system, the PIWI-interacting RNA pathway. piRNAs are 23- to 30-nucleotide small RNAs that associate with Argonaute proteins of the PIWI clade (Czech et al 2018; Ozata et al 2019)
Since proteins involved in dual-strand piRNA cluster biology will compromise transposon silencing in germ cells, we focused on two uncharacterized germline-specific screen hits: CG13741 and Nxf3 (Czech et al 2013)

Summary

Introduction

Transposable elements represent a major threat to genome integrity. In the animal germline, transposon mobilization is prevented by a conserved small RNA-based immune system, the PIWI-interacting RNA (piRNA) pathway. piRNAs are 23- to 30-nucleotide (nt) small RNAs that associate with Argonaute proteins of the PIWI clade (Czech et al 2018; Ozata et al 2019). Completion of mRNA processing leads to the recruitment of the transcription and export complex (TREX), which is a prerequisite for association with the Nxf1–Nxt heterodimer Unistrand clusters, such as flamenco ( flam), are capped, spliced, and polyadenylated and follow canonical. Cuff suppresses splicing and Pol II termination and is thought to protect dual-strand piRNA cluster transcripts from degradation by the exonuclease Rat (Zhang et al 2014; Chen et al 2016) This Rhi-anchored complex generates transcripts that lack signatures of canonical mRNAs. UAP56, a DEAD-box helicase involved in canonical mRNA splicing and export, was shown to associate with piRNA precursor transcripts (Zhang et al 2012, 2018). Our data shed light on how noncanonical piRNA precursor transcripts are exported from their heterochromatic source loci to the processing sites in the cytoplasm and how specific components of the nuclear export machinery are adapted to facilitate this

Methods

Results

Conclusion