Max Planck Institute of Biophysical Chemistry, Department of Cellular Biochemistry, Am Fassberg 11, 37077 Goettingen, Germany.
Curr Top Med Chem. 2012;12(2):79-88. doi: 10.2174/156802612798919132.
Several different pathways, generally termed RNA silencing pathways, utilize small RNA molecules guiding sequence-specific silencing effects of ribonucleoprotein effector complexes, traditionally termed RNA-induced silencing complex (RISC). Three RNA silencing pathways were recognized in mammalian cells: RNA interference (RNAi), where short RNAs produced from long double-stranded RNA guide cleavage of cognate mRNAs, microRNA (miRNA) pathway, where endogenously-encoded miRNAs typically induce translational repression, and piRNA pathway, where piRNAs (PIWI-associated RNAs) guide repression of repetitive sequences in the germline. Originally, RNAi and miRNA pathways were thought to act in the cytoplasm, however, there is a growing body of evidence that these pathways also have a nuclear component. This text reviews the current evidence concerning nuclear localization and function of miRNA and RNAi pathway components. We provide evidence that TRBP, Dicer and AGO2, proteins found in the RISC-loading complex (RLC) and RISC itself, are present in the nucleus. Nonetheless, fully functional RLC is not found in the nuclear compartment which is consistent with the recent findings obtained by Fluorescence Cross-Correlation Spectroscopy experiments illustrating that RISC is specifically loaded within the cytoplasm and shuttles subsequently between the nuclear and cytoplasmic compartment, thereby allowing small RNA gene regulation in both compartments. The function of nuclear TRBP and Dicer proteins remains elusive. We also discuss the consequences of nucleotide analogs introduced into siRNAs which can severely interfere with the natural cytoplasmic localization mediated by Exportin-5 which is required for efficient RISC loading in the cytoplasm.
几种不同的途径，通常称为 RNA 沉默途径，利用小 RNA 分子引导核糖核蛋白效应复合物的序列特异性沉默效应，传统上称为 RNA 诱导沉默复合物（RISC）。哺乳动物细胞中识别出三种 RNA 沉默途径：RNA 干扰（RNAi），其中来自长双链 RNA 的短 RNA 指导同源 mRNA 的切割，miRNA（miRNA）途径，其中内源性编码的 miRNA 通常诱导翻译抑制，以及 piRNA 途径，其中 piRNA（PIWI 相关 RNA）指导生殖系中重复序列的抑制。最初，RNAi 和 miRNA 途径被认为在细胞质中起作用，然而，越来越多的证据表明这些途径也具有核成分。本文综述了有关 miRNA 和 RNAi 途径成分的核定位和功能的当前证据。我们提供的证据表明，TRBP、Dicer 和 AGO2 蛋白存在于 RISC 加载复合物（RLC）和 RISC 本身中，它们是 RISC 加载复合物的组成部分。尽管如此，完整功能的 RLC 并未在核区室中发现，这与荧光相关光谱实验的最新发现一致，该实验表明 RISC 专门在细胞质中加载，并随后在核和细胞质区室之间穿梭，从而允许在两个区室中进行小 RNA 基因调控。核 TRBP 和 Dicer 蛋白的功能仍然难以捉摸。我们还讨论了核苷酸类似物引入 siRNA 中可能严重干扰由 Exportin-5 介导的天然细胞质定位的后果，Exportin-5 是在细胞质中有效加载 RISC 所必需的。
