| Publication note | We find that Short Oskar, which nucleates the formation of cytoplasmic germ plasm and germ granules, also induces the formation of nuclear granules in the PGCs. Upon Short but not Long Oskar protein expression in heterologous systems such as Drosophila S2R + and mammalian HEK293 cell lines, nuclear granules assemble independently of other germ plasm factors. Vasa is recruited to these granules by Oskar and hence we term these condensates ‘nuclear germ granules’. (Organism: Drosophila melanogaster; Cell line: S2R+ cells) |
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| Material state | liquid,hydrogel : 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. | ||
| LLPS region | 139-606 | Key domains | linker region 1(L1),linker region 2(L2),LOTUS,SGNH : multiple independent Osk protein domains synergize to promote efficient phase separation of Short Osk in S2R+ cells. |
| Protein partner | Vasa ( P09052 ) |
nuclear germ granules are round and membraneless structures, populated by Oskar and Vasa |
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| RNA partner | _ |
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| Others | _ |
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| PTM | _ |
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| Mutation | ΔLOTUS,ΔL1,ΔL2ΔSGNH |
Osk-ΔLOTUS failed to localize Vasa to nuclei and instead Vasa took on a diffuse and non-granular distribution in S2R+ cells that resembled its distribution in the absence of Oskar. Osk-ΔL1 protein did not localize to the nucleus, but instead accumulated in cytoplasmic granules and Osk-ΔL2 granules were found in the cytoplasm and the nucleus. Surprisingly, deletion of L2 prevented accumulation and co-localization of Vasa:GFP with Osk-ΔL2 protein specifically in the nucleus, while it remained co-localized, albeit faintly, with Osk-ΔL2 in cytoplasmic foci. The size of nuclear granules formed by Osk-ΔLOTUS, Osk-ΔL1 and Osk-ΔL2 mutants was similar to those formed by full length Short Osk, while deletion of the SGNH domain lead to the formation of much larger nuclear granules, which lacked the appearance of a hollow core. |
| Alternative splicing | Translation of osk mRNA produces two isoforms, long and short (Long Osk, Short Osk, respectively) that differ in the first 138 N-terminal aa. Only Short, but not Long Osk is necessary and sufficient to instruct the formation of cytoplasmic germ granules. Long Oskar does not form granules in S2R+ cells. | |
| Repeat | _ | |
| Oligomerization | The ability of Short Osk to homo-dimerize, interact with Vasa, Valois and LASP or bind RNA is dispensable for nuclear granule formation, nor were the LC sequence or the IDR necessary. | |