THE DYNAMIC FUNCTIONS OF GERM GRANULES AND THEIR COMPONENTS DURING GERM CELL DEVELOPMENT

Abstract

Post-transcriptional regulation has an essential role in regulating gene expression and is mediated by RNA binding proteins that have important functions throughout the mRNA life cycle. Compartmentalization of RNAs and RNA binding proteins into membraneless structures called granules is a ubiquitous mechanism for organizing and regulating cohorts of RNAs. One class of granules, the germ granules, are ribonucleoprotein assemblies required for germline development across the animal kingdom, but their regulatory roles in germ cells are not fully understood. Here, we analyze the functions of Drosophila germ granules and their core protein components after germline specification. We show that in the primordial germ cells, called the pole cells, Drosophila germ granules enlarge through fusion and this growth is accompanied by a shift in function. Whereas germ granules initially protect their constituent mRNAs from degradation, they subsequently target a subset of these mRNAs for degradation while maintaining protection of others. This functional shift occurs through the recruitment of decapping and degradation factors to the germ granules, which is promoted by decapping activators and renders these structures P body-like. Disrupting either the mRNA protection or degradation function results in germ cell migration defects. These findings reveal an unexpected plasticity and complexity in germ granule function that allows them to be repurposed throughout development and differentially regulate RNAs within the same granule. Furthermore, we adapted the auxin inducible degradation system to degrade two core germ granule proteins, Oskar and Vasa, specifically in the pole cells. Our results reveal that Vasa is not necessary for germ granule maintenance or RNA stability, but is required for proper pole cell migration. By contrast, Osk is necessary for pole cell survival, demonstrating that different protein components of the germ granules have distinct roles during germ cell development. Our findings provide insights into the complex and dynamic roles of RNP granules throughout development and how individual protein components can contribute to these functions.