Department of Molecular Biology and Genetics, Aarhus University, C.F. Møllers Allé 3, Building 1130, 8000 Aarhus C, Denmark.
The Bioinformatics Centre, Department of Biology and Biotech Research and Innovation Centre, University of Copenhagen, Ole Maaloes Vej 5, 2200 Copenhagen, Denmark.
Cell Rep. 2020 Feb 18;30(7):2387-2401.e5. doi: 10.1016/j.celrep.2020.01.068.
Degradation of transcripts in human nuclei is primarily facilitated by the RNA exosome. To obtain substrate specificity, the exosome is aided by adaptors; in the nucleoplasm, those adaptors are the nuclear exosome-targeting (NEXT) complex and the poly(A) (pA) exosome-targeting (PAXT) connection. How these adaptors guide exosome targeting remains enigmatic. Employing high-resolution 3' end sequencing, we demonstrate that NEXT substrates arise from heterogenous and predominantly pA 3' ends often covering kilobase-wide genomic regions. In contrast, PAXT targets harbor well-defined pA 3' ends defined by canonical pA site use. Irrespective of this clear division, NEXT and PAXT act redundantly in two ways: (1) regional redundancy, where the majority of exosome-targeted transcription units produce NEXT- and PAXT-sensitive RNA isoforms, and (2) isoform redundancy, where the PAXT connection ensures fail-safe decay of post-transcriptionally polyadenylated NEXT targets. In conjunction, this provides a two-layered targeting mechanism for efficient nuclear sorting of the human transcriptome.
人类核内转录本的降解主要由 RNA 外切体来完成。为了获得底物特异性，外切体需要衔接子的辅助；在核质中，这些衔接子是核外切体靶向（NEXT）复合物和多聚腺苷酸（pA）外切体靶向（PAXT）连接。然而，这些衔接子如何指导外切体靶向仍然是一个谜。我们采用高分辨率 3' 端测序技术，证明了 NEXT 底物来源于异质性的、主要带有 pA 3' 末端的转录本，这些转录本通常覆盖了千碱基大小的基因组区域。相比之下，PAXT 的靶标则带有明确的 pA 3' 末端，由规范的 pA 位点使用所定义。尽管存在这种明显的划分，但 NEXT 和 PAXT 以两种方式冗余地发挥作用：（1）区域冗余，大多数被外切体靶向的转录单位产生 NEXT 和 PAXT 敏感的 RNA 异构体；（2）异构体冗余，其中 PAXT 连接确保了转录后多聚腺苷酸化的 NEXT 靶标的可靠降解。总之，这为人类转录组的高效核分拣提供了一种双层靶向机制。
