State Key Laboratory of Rice Biology, Institute of Biotechnology, Zhejiang University, Zhejiang 310029, China.
Mol Cell. 2012 Jun 29;46(6):859-70. doi: 10.1016/j.molcel.2012.04.013. Epub 2012 May 17.
In plants, DNA methylation can be mediated by a class of Argonaute4 (AGO4)-associated heterochromatic siRNAs (hc-siRNAs), through a pathway termed RNA-directed DNA methylation (RdDM). It has been thought that RdDM is solely a nuclear process, as both the biogenesis and functioning of hc-siRNAs take place in the nucleus. In this study, we unexpectedly found that hc-siRNAs are predominantly present in the cytoplasm. We demonstrated that AGO4 is loaded with hc-siRNAs in the cytoplasm and the formation of mature AGO4/siRNA complexes requires HSP90 and the cleavage activity of AGO4. Intriguingly, siRNA binding facilitates the redistribution of AGO4 into the nucleus, likely through inducing conformational change that leads to the exposure of the nuclear localization signal (NLS). Our findings reveal an unsuspected cytoplasmic step in the RdDM pathway. We propose that selective nuclear import of mature AGO4/siRNA complexes is a key regulatory point prior to the effector stage of RdDM.
在植物中，DNA 甲基化可以通过一类 Argonaute4（AGO4）相关的异染色质小干扰 RNA（hc-siRNA）介导，通过一种称为 RNA 指导的 DNA 甲基化（RdDM）的途径。人们一直认为 RdDM 仅仅是一个核过程，因为 hc-siRNA 的生物发生和功能都发生在核内。在这项研究中，我们出人意料地发现 hc-siRNA 主要存在于细胞质中。我们证明 AGO4 在细胞质中与 hc-siRNA 一起被加载，并且成熟的 AGO4/siRNA 复合物的形成需要 HSP90 和 AGO4 的切割活性。有趣的是，siRNA 结合促进 AGO4 向核内重新分布，可能是通过诱导导致核定位信号（NLS）暴露的构象变化。我们的发现揭示了 RdDM 途径中一个出人意料的细胞质步骤。我们提出，成熟的 AGO4/siRNA 复合物的选择性核输入是 RdDM 效应阶段之前的一个关键调控点。
