Institut de Biologie Moléculaire des Plantes du Centre National de la Recherche Scientifique, Université de Strasbourg, 12 rue du Général Zimmer, 67084 Strasbourg Cedex, France.
Plant J. 2011 Oct;68(1):51-63. doi: 10.1111/j.1365-313X.2011.04675.x. Epub 2011 Jul 25.
The exosome is a conserved protein complex that is responsible for essential 3'→5' RNA degradation in both the nucleus and the cytosol. It is composed of a nine-subunit core complex to which co-factors confer both RNA substrate recognition and ribonucleolytic activities. Very few exosome co-factors have been identified in plants. Here, we have characterized a putative RNA helicase, AtMTR4, that is involved in the degradation of several nucleolar exosome substrates in Arabidopsis thaliana. We show that AtMTR4, rather than its closely related protein HEN2, is required for proper rRNA biogenesis in Arabidopsis. AtMTR4 is mostly localized in the nucleolus, a subcellular compartmentalization that is shared with another exosome co-factor, RRP6L2. AtMTR4 and RRP6L2 cooperate in several steps of rRNA maturation and surveillance, such as processing the 5.8S rRNA and removal of rRNA maturation by-products. Interestingly, degradation of the Arabidopsis 5' external transcribed spacer (5' ETS) requires cooperation of both the 5'→3' and 3'→5' exoribonucleolytic pathways. Accumulating AtMTR4 targets give rise to illegitimate small RNAs; however, these do not affect rRNA metabolism or contribute to the phenotype of mtr4 mutants. Plants lacking AtMTR4 are viable but show several developmental defects, including aberrant vein patterning and pointed first leaves. The mtr4 phenotype resembles that of several ribosomal protein and nucleolin mutants, and may be explained by delayed ribosome biogenesis, as we observed a reduced rate of rRNA accumulation in mtr4 mutants. Taken together, these data link AtMTR4 with rRNA biogenesis and development in Arabidopsis.
外切体是一种保守的蛋白质复合物，负责细胞核和细胞质中必需的 3'→5' RNA 降解。它由一个九亚基核心复合物组成，协同因子赋予 RNA 底物识别和核糖核酸酶活性。在植物中，很少有外切体协同因子被鉴定出来。在这里，我们鉴定了一个假定的 RNA 解旋酶 AtMTR4，它参与了拟南芥核仁外切体底物的降解。我们表明，AtMTR4 而不是其密切相关的蛋白 HEN2，是拟南芥 rRNA 生物发生所必需的。AtMTR4 主要定位于核仁，这是与另一个外切体协同因子 RRP6L2 共享的亚细胞区室化。AtMTR4 和 RRP6L2 在 rRNA 成熟和监测的几个步骤中合作，例如加工 5.8S rRNA 和去除 rRNA 成熟的副产物。有趣的是，拟南芥 5' 外部转录间隔区 (5' ETS) 的降解需要 5'→3' 和 3'→5' 外切核糖核酸酶途径的合作。积累的 AtMTR4 靶标会产生非规范的小 RNA；然而，这些不会影响 rRNA 代谢或导致 mtr4 突变体的表型。缺乏 AtMTR4 的植物是可行的，但表现出几种发育缺陷，包括异常叶脉模式和尖首叶。mtr4 表型类似于几个核糖体蛋白和核仁素突变体，可能是由于核糖体生物发生延迟，因为我们观察到 mtr4 突变体中 rRNA 积累的速度降低。总之，这些数据将 AtMTR4 与拟南芥的 rRNA 生物发生和发育联系起来。
