Germ granule compartments coordinate specialized small RNA production

Germ granules are biomolecular condensates present in most animal germ cells. One function of germ granules is to help maintain germ cell totipotency by organizing mRNA regulatory machinery, including small RNA-based gene regulatory pathways. The C. elegans germ granule is compartmentalized into multiple subcompartments whose biological functions are largely unknown. Here, we identify an uncharted subcompartment of the C. elegans germ granule, which we term the E granule. The E granule is nonrandomly positioned within the germ granule. We identify five proteins that localize to the E granule, including the RNA-dependent RNA polymerase (RdRP) EGO-1, the Dicer-related helicase DRH-3, the Tudor domain-containing protein EKL-1, and two intrinsically disordered proteins, EGC-1 and ELLI-1. Localization of EGO-1 to the E granule enables synthesis of a specialized class of 22G RNAs,which derive exclusively from 5’ regions of a subset of germline-expressed mRNAs. Defects in E granule assembly elicit disordered production of endogenous siRNAs, which disturbs fertility and the RNAi response. Our results define a distinct subcompartment of the C. elegans germ granule and suggest that one function of germ granule compartmentalization is to facilitate the localized production of specialized classes of small regulatory RNAs.

生殖颗粒（germ granule）是存在于绝大多数动物生殖细胞中的生物分子缩合物。其功能之一是通过组装mRNA调控系统（涵盖基于小RNA的基因调控通路），维持生殖细胞的全能性。秀丽隐杆线虫（C. elegans）的生殖颗粒被划分为多个亚区室，但其生物学功能大多尚未明确。本研究中，我们鉴定出秀丽隐杆线虫生殖颗粒中一个未被探索过的亚区室，并将其命名为E颗粒。E颗粒在生殖颗粒内的定位并非随机。我们鉴定出五种定位于E颗粒的蛋白质，包括RNA依赖的RNA聚合酶（RNA-dependent RNA polymerase，RdRP）EGO-1、Dicer相关解旋酶DRH-3、含Tudor结构域的蛋白质EKL-1，以及两种内在无序蛋白质EGC-1和ELLI-1。EGO-1定位于E颗粒这一过程，可介导一类特殊22G RNA的合成，这类RNA仅来源于部分生殖系表达mRNA的5’端区域。E颗粒组装缺陷会引发内源性小干扰RNA（siRNA）的产生紊乱，进而干扰生殖能力与RNA干扰（RNAi）应答。本研究结果明确了秀丽隐杆线虫生殖颗粒中一个独特的亚区室，并提示生殖颗粒的区室化功能之一，是促进特殊类别小调控RNA的定位合成。