The assembly of large RNA-protein granules occurs in germ cells of many animals and these germ granules have provided a paradigm to study structure-functional aspects of similar structures in different cells. Germ granules in Drosophila oocyte's posterior pole (polar granules) are composed of RNA, in the form of homotypic clusters, and proteins required for germline development. In the granules, Piwi protein Aubergine binds to a scaffold protein Tudor, which contains 11 Tudor domains. Using a super-resolution microscopy, we show that surprisingly, Aubergine and Tudor form distinct clusters within the same polar granules in early Drosophila embryos. These clusters partially overlap and, after germ cells form, they transition into spherical granules with the structural organization unexpected from these interacting proteins: Aubergine shell around the Tudor core. Consistent with the formation of distinct clusters, we show that Aubergine forms homo-oligomers and using all purified Tudor domains, we demonstrate that multiple domains, distributed along the entire Tudor structure, interact with Aubergine. Our data suggest that in polar granules, Aubergine and Tudor are assembled into distinct phases, partially mixed at their "interaction hubs", and that association of distinct protein clusters may be an evolutionarily conserved mechanism for the assembly of germ granules.

In Drosophila melanogaster there are two genes encoding ribosomal protein S5, RpS5a and RpS5b. Here, we demonstrate that RpS5b is required for oogenesis. Females lacking RpS5b produce ovaries with numerous developmental defects that undergo widespread apoptosis in mid-oogenesis. Females lacking germline RpS5a are fully fertile, but germline expression of interfering RNA targeting germline RpS5a in an RpS5b mutant background worsened the RpS5b phenotype and blocked oogenesis before egg chambers form. A broad spectrum of mRNAs co-purified in immunoprecipitations with RpS5a, while RpS5b-associated mRNAs were specifically enriched for GO terms related to mitochondrial electron transport and cellular metabolic processes. Consistent with this, RpS5b mitochondrial fractions are depleted for proteins linked to oxidative phosphorylation and mitochondrial respiration, and RpS5b mitochondria tended to form large clusters and had more heterogeneous morphology than those from controls. We conclude that RpS5b-containing ribosomes preferentially associate with particular mRNAs and serve an essential function in oogenesis.