Remnants of the Balbiani Body Are Required for Formation of RNA Transport Granules in Xenopus Oocytes

Balbiani body (Bb) is a conserved structure in oocytes that is characterized by mitochondria, ER membranes and germ plasm RNAs. In Xenopus oocytes, Bb has a role in localizing maternal mRNAs to the vegetal cortex during early oogenesis. Confocal imaging revealed that the fragmentation of the Bb engenders a region extending from the nucleus towards the vegetal pole that is enriched in mitochondria. Vg1 RNA, one of the vegetal localized mRNAs during midoogenesis, later occupies this area, primarily at the perinuclear site, the region RNA get trapped in after exiting the nucleus. Inhibition of mitochondrial ATP synthesis prevents the formation of these RNA transport granules at the perinuclear site, which can be reversed by the nonhydrolyzable ATP analog, adenosine 5'-(b,g-imido) triphosphate. Furthermore, the transport granules are sensitive to hexanediol, a chemical that can dissolve dynamic, liquid-like phase separated assemblies. In addition, biotinylated ribonucleoprotein (RNP) pull-down coupled with label-free quantitative proteomics (LFQ) was utilized to determine the full inventory of protein factors comprising the localization complex and identified multiple liquid-phase separation driving proteins. It is thus proposed that during midoogenesis, the remnant of Bb produces an elevated amount of ATP that acts as a hydrotrope to support the perinuclear liquid-phase transition leading to granule formation.