Quality Control of Gene Expression / RNA Surveillance

外泌体是一个具有外切酶活性的大的多亚基复合体。该核酸酶是真核细胞中主要的3‘-5’外切酶。它由含有10种不同蛋白质的核心组成，这些蛋白质与核和细胞质的特定亚基相关。外泌体与大多数涉及去除异常和无功能RNA分子的衰变途径有关。其中包括消除隐性不稳定转录（CUTs）、异常trna、非剪接前mrna以及一些snrna和rrna。在细胞质中，外泌体已被证明可以降解含有过早停止密码子（NMD）、缺乏停止密码子（NSD）或引发核糖体停滞（No-Go衰变）的无功能mrna。因此，外泌体在RNA质量的控制中起着核心和重要的作用。外泌体必须在不攻击功能分子的情况下靶向并降解异常rna。这种歧视部分是由于结合和/或修饰生物学相关底物并使其降解的因素的作用。这些因子包括细胞核中的Trf/Air聚(A)聚合酶和Mtr4 DEAD盒解旋酶以及细胞质中的Ski7 GTpase和Ski2/3/8复合物。如果这些途径中涉及的酶和因子是已知的，那么导致它们选择性地降解异常RNA分子的机制仍然不清楚。我们在此建议对外泌体在RNA质量控制途径中的作用进行多学科研究。该分析将结合分子生物学和生物化学方法、天然质谱法、电子显微镜结构分析和小角度x射线散射来提供一个结构和功能框架，从而了解外泌体在RNA监测中的作用。我们设想，在这个涉及互补领域专家的合作项目中开发的方法，将有助于未来研究其他基于RNA的过程和其他分子机制。

The exosome is a large multisubunit complex endowed with exonuclease activity. This nuclease is the major 3'-5' exonuclease of eukaryotic cells. It is constituted of a core containing 10 different proteins to which associate nuclear and cytoplasmic specific subunits. The exosome has been implicated in most decay pathways involved in the removal of aberrant and non-functional RNA molecules. Those include elimination of Cryptic Unstable Transcrips (CUTs), aberrant tRNAs, non-spliced pre-mRNA as well as some snRNAs and rRNAs in the nucleus. In the cytoplasm, the exosome has been show to degrade non-functional mRNAs containing a premature stop codon (NMD), those lacking a stop codon (NSD) or those provoking ribosome stalling (No-Go decay). The exosome thus plays a central and essential role in the control of RNA quality. The exosome has to target and degrade aberrant RNAs without attacking functional molecules. This discrimination results in part from the action of factors that will bind and/or modify biologically relevant substrates and target them to degradation. Such factors include the Trf/Air poly(A) polymerase and Mtr4 DEAD box helicase in the nucleus and the Ski7 GTpase and Ski2/3/8 complex in the cytoplasm. If the enzymes and factors involved in these pathways are known, the mechanisms leading them to selectively degrade aberrant RNA molecules remain unclear. We propose here a multidisciplinary study of the role of the exosome in RNA quality control pathways. This analysis will combine molecular biology and biochemical approaches, native mass spectrometry, structural analyses by electron microscopy and small angle X-ray scattering to provide a structural and functional framework allowing the understanding of the role of the exosome in RNA surveillance. We envision that the methodology, developed in this collaborative project involving experts in complementary fields, will be useful to study other RNA based processes and other molecular mechanisms in the future.