The DUF3715 domain has a conserved role in RNA-directed transposon silencing

RNA-directed transposon silencing operates in the mammalian soma and germline tosafeguard genomic integrity. The piRNA pathway and the HUSHcomplex identify activetransposons through recognition of their nascent transcripts, but mechanisticunderstanding of how these distinct pathways evolved is lacking. TASOR is anessential component of the HUSH complex. TASOR’s DUF3715 domain adopts apseudo-PARP structure and is required for transposon silencing in a mannerindependent of complex assembly. TEX15, an essential piRNA pathway factor, alsocontains the DUF3715 domain. Here, we show that TASOR’s and TEX15’s DUF3715domain share extensive structural homology. We found that the DUF3715 domain arosein early eukaryotes and that in vertebrates it is restricted to TEX15, TASOR, andTASORB orthologues. While TASOR-like proteins are found throughout metazoa,TEX15 is vertebrate specific. The branchingof TEX15 and TASOR-like DUF3715 domainlikely occurred in early metazoan evolution. Remarkably, despite this vast evolutionarydistance, the DUF3715 domain from divergent TEX15 sequences can functionallysubstitute for the same domain in TASOR and mediates transposon silencing. We havethus termed this domain of unknown function as the RNA-directed pseudo-PARPtransposon silencing (RDTS) domain. In summary, we show an unexpected functionallink between these critical transposon silencing pathways. To investigate the conservation of the DUF3715 domain between TEX15 and TASOR we established Tasor-deficient mouse ES cell lines which were subsequently reconstituted with mutant and chimeric TASOR constructs.

RNA指导的转座子沉默（RNA-directed transposon silencing）在哺乳动物体细胞与生殖系中发挥功能，以维持基因组完整性。piRNA通路（piRNA pathway）与HUSH复合物（HUSH complex）可通过识别活性转座子的新生转录本实现靶向识别，但目前学界对这两条不同通路的演化机制仍缺乏清晰认知。TASOR是HUSH复合物的核心必需组分。TASOR的DUF3715结构域呈假PARP（pseudo-PARP）折叠构象，其介导转座子沉默的功能不依赖于复合物组装。作为piRNA通路的关键必需因子，TEX15同样携带有DUF3715结构域。本研究发现，TASOR与TEX15的DUF3715结构域存在广泛的结构同源性。实验结果显示，DUF3715结构域起源于早期真核生物，在脊椎动物中仅存在于TEX15、TASOR及TASORB同源物（orthologues）中。尽管类TASOR蛋白广泛分布于后生动物（metazoa）中，但TEX15为脊椎动物所特有。TEX15与类TASOR的DUF3715结构域的分化事件可能发生在早期后生动物演化阶段。值得注意的是，尽管二者演化分歧极大，不同演化分支的TEX15来源的DUF3715结构域仍可在功能上替代TASOR中的同源结构域，介导转座子沉默过程。据此，我们将这一功能未知的结构域命名为RNA指导的假PARP转座子沉默结构域（RNA-directed pseudo-PARP transposon silencing domain，简称RDTS结构域）。综上，本研究揭示了这两条关键转座子沉默通路之间此前未被发现的功能关联。为探究TEX15与TASOR的DUF3715结构域的保守性，我们构建了TASOR缺陷型小鼠胚胎干细胞（embryonic stem，ES）细胞系，并通过携带突变型或嵌合型序列的TASOR重组载体对该细胞系进行回补重构。