Conserved RNA helicase Vasa regulates ribonucleoprotein condensates through protein interaction and mRNA recruitment

Abstract

Germlines contain ribonucleoprotein condensates known as germ granules, which concentrate proteins and mRNAs essential for animal development. Vasa, a conserved DEAD-box RNA helicase, is a core and highly concentrated constituent of germ granules. However, its roles within these structures remain poorly understood. Here, we use Drosophila germ granules as a model system to address this question. Applying in vivo and human cell systems, we found that condensation of Oskar (Osk) protein, which nucleates germ granules, occurred independently of Vasa. However, in oocytes lacking detectable Vasa protein, Osk formed aggregated condensates regardless of the eGFP tag. Furthermore, Osk-eGFP showed minimal recovery as measured by fluorescence recovery after photobleaching (FRAP) indicating that its exchange between condensates and their surroundings was greatly reduced in the absence of Vasa. Supporting this result, co-expression of Vasa increased the FRAP recovery of Osk-eGFP condensates and reduced Osk-eGFP partitioning into them in cells. This effect required the interaction between Vasa and Osk, suggesting that by binding Osk, Vasa modulates Osks phase behavior and its condensate material properties. Super-resolution microscopy further revealed that Vasa is required for germ granule mRNA localization to Osk condensates in vivo. Co-expression of Vasa with Osk-eGFP is necessary and sufficient to recruit germ granule mRNAs to condensates in cells. Although this activity depends on Vasa-Osk interaction, the interaction itself is not sufficient. Notably, localization of a conserved germ granule mRNA nanos reduced the FRAP recovery of Osk-eGFP condensates in cells, partially counteracting Vasas effect. Collectively, our findings uncovered a novel function of the DEAD-box RNA helicase Vasa in regulating the material properties of Osk condensates through coordinated protein-protein and protein-mRNA interactions. Four highlightsO_LIVasa modulates the material properties of Oskar ribonucleoprotein condensates in fly oocytes and human cells. C_LIO_LIVasa modulates condensate material properties through its interaction with Oskar. C_LIO_LImRNA localization to Oskar condensates requires Vasa and Vasa-Oskar interaction. C_LIO_LInanos mRNA localization counteracts Vasas enhancement of Oskar exchange between condensates and their surroundings. C_LI