Abstract
Germ granules are non-membranous ribonucleoprotein granules deemed the hubs for post-transcriptional gene regulation and functionally linked to germ cell fate across species. Little is known about the physical properties of germ granules and how these relate to germ cell function. Here we study two types of germ granules in the Drosophila embryo: cytoplasmic germ granules that instruct primordial germ cells (PGCs) formation and nuclear germ granules within early PGCs with unknown function. We show that cytoplasmic and nuclear germ granules are phase transitioned condensates nucleated by Oskar protein that display liquid as well as hydrogel-like properties. Focusing on nuclear granules, we find that Oskar drives their formation in heterologous cell systems. Multiple, independent Oskar protein domains synergize to promote granule phase separation. Deletion of Oskar's nuclear localization sequence specifically ablates nuclear granules in cell systems. In the embryo, nuclear germ granules promote germ cell divisions thereby increasing PGC number for the next generation.
Highlights
Specialized ribonucleoprotein (RNP) granules are a hallmark of all germ cells
We focused on known germ granule components: Vasa, a DEAD-box RNA helicase (Hay et al, 1988) and two Piwi family RNA-binding proteins Piwi and Aub (Harris and Macdonald, 2001; Juliano et al, 2011), as well as several proteins highly abundant in short-Osk IP: DCP1, an mRNA decapping protein involved in mRNA turnover (Beelman et al, 1996; Lin et al, 2006), Kinesin heavy chain protein Khc, a microtubule motor protein that functions in the long-distance transport of cytoplasmic cargoes such as osk mRNA (Brendza et al, 2000) and a protein of unknown biological function encoded by the CG5726 gene
Similar to the measurements obtained on nuclear germ granules formed in primordial germ cells (PGCs) (Figure 2H), we found that the majority of the Short mCherry:Osk fluorescence was located in granules rather than in the nucleoplasm (Figure 4A, Figure 4—figure supplement 1A)
Summary
Specialized ribonucleoprotein (RNP) granules are a hallmark of all germ cells. Throughout the animal kingdom, these granules share germline-specific proteins such as the ATP-dependent RNA helicase Vasa and are present during different stages of germ cell development. Vasa is recruited to these granules by Oskar and we term these condensates ‘nuclear germ granules’ We show that these are phase transitioned granules that display liquid-like and hydrogel-like properties; they can fuse, dissolve, condense and exchange their protein content with the granule environment while they are stable when purified. Our studies reveal a new function for Oskar protein as the nucleator of phase separated granules in the germ cell nuclei These results provide new insight into how early PGCs division may be regulated independently of the cell cycle timing mechanisms that exist in the syncytial environment of the early embryo
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