Proteins rather than mRNAs regulate nucleation and persistence of Oskar germ granules in Drosophila

RNA granules are membraneless condensates that provide functional compartmentalization within cells. The mechanisms by which RNA granules form are under intense investigation. Here, we characterize the role of mRNAs and proteins in the formation of germ granules in Drosophila. Super-resolution microscopy reveals that the number, size, and distribution of germ granules is precisely controlled. Surprisingly, germ granule mRNAs are not required for the nucleation or the persistence of germ granules but instead control their size and composition. Using an RNAi screen, we determine that RNA regulators, helicases, and mitochondrial proteins regulate germ granule number and size, while the proteins of the endoplasmic reticulum, nuclear pore complex, and cytoskeleton control their distribution. Therefore, the protein-driven formation of Drosophila germ granules is mechanistically distinct from the RNA-dependent condensation observed for other RNA granules such as stress granules and P-bodies. [Display omitted] •The number, size, and distribution of germ granules is precisely regulated•Oskar germ granules do not require RNA for their nucleation and stability•RNA regulators and mitochondrial proteins control germ granule number and size•ER, nuclear pore, and cytoskeletal proteins regulate germ granule distribution Curnutte et al. observe that germ granule proteins rather than mRNAs promote the nucleation and stability of Oskar germ granules in Drosophila. While RNA regulators and mitochondrial proteins control the number and size of germ granules, the proteins of the ER, nuclear pores, and cytoskeleton control their distribution.