Bombyx Vasa sequesters transposon mRNAs in nuage via phase separation requiring RNA binding and self-association

Bombyx Vasa assembles Vasa bodies, the site of transposon silencing by Siwi and Ago3-piRISC formation. Here, the authors show Vasa sequesters transposon mRNAs in Vasa bodies via phase separation requiring RNA binding and self-association of Vasa. Bombyx Vasa (BmVasa) assembles non-membranous organelle, nuage or Vasa bodies, in germ cells, known as the center for Siwi-dependent transposon silencing and concomitant Ago3-piRISC biogenesis. However, details of the body assembly remain unclear. Here, we show that the N-terminal intrinsically disordered region (N-IDR) and RNA helicase domain of BmVasa are responsible for self-association and RNA binding, respectively, but N-IDR is also required for full RNA-binding activity. Both domains are essential for Vasa body assembly in vivo and droplet formation in vitro via phase separation. FAST-iCLIP reveals that BmVasa preferentially binds transposon mRNAs. Loss of Siwi function derepresses transposons but has marginal effects on BmVasa-RNA binding. This study shows that BmVasa assembles nuage by phase separation via its ability to self-associate and bind newly exported transposon mRNAs. This unique property of BmVasa allows transposon mRNAs to be sequestered and enriched in nuage, resulting in effective Siwi-dependent transposon repression and Ago3-piRISC biogenesis.

Automated summary

Bombyx Vasa assembles Vasa bodies in germ cells, which are responsible for transposon silencing and Ago3-piRISC biogenesis. It is found that the N-terminal intrinsically disordered region (N-IDR) and RNA helicase domain of BmVasa play important roles in self-association and RNA binding, respectively, for Vasa body assembly and droplet formation via phase separation. BmVasa preferentially binds transposon mRNAs, enabling their sequestration and enrichment in Vasa bodies, leading to effective Siwi-dependent transposon repression and Ago3-piRISC biogenesis.