MicroRNA(miRNA)是一类长度仅约为22个碱基的非编码小RNA，它参与调控真核生物超过三分之一以上基因的表达，具有非常重要的生物学功能。miRNA不仅能抑制靶基因的翻译，还可引起mRNA的降解。目前已知在哺乳动物中miRNA通过加速靶标mRNA 3’端poly(A)的脱腺苷酸化导致整个mRNA被迅速降解，其中脱腺苷酸化是mRNA降解过程的关键和限速步骤。但在哺乳动物中miRNA导致mRNA加速脱腺苷酸化的分子机制仍不清楚。
1月11日，《分子细胞生物学(Molecular and Cellular Biology)》杂志在线发表了miRNA导致mRNA降解机制的最新研究成果。该研究工作由上海生命科学研究院生物化学与细胞生物学研究所吴立刚课题组和美国纽约大学Joel Belasco实验室共同合作完成，生化与细胞所研究生朴香花等研究人员承担了主要研究工作。
本项工作中，通过运用无功能突变子竞争抑制和RNA干扰技术等方法，寻找并证实了CCR4-NOT复合体的成员CAF1和POP2在miRNA引发的mRNA脱腺苷酸化过程中起到了关键性的作用。研究还首次证实了CAF1在siRNA引起的off-target现象中也起到了重要作用。进一步研究发现，CAF1并不是通过RISC的直接招募作用而发挥功能，而可能通过miRNA和RISC改变mRNP的结构，从而促进CAF1对mRNA的降解。该研究成果为进一步揭示哺乳动物细胞内大量mRNA分子命运如何被决定奠定了基础。
英文原文原始出处及摘要：
Mol. Cell. Biol. doi:10.1128/MCB.01481-09
CCR4-NOT deadenylates RISC-associated mRNA in human cells
Xianghua Piao, Xue Zhang, Ligang Wu*, and Joel G. Belasco*
State Key Laboratory of Molecular Biology, Institute of Biochemistry and Cell Biology, Shanghai Institutes for Biological Sciences, Chinese Academy of Sciences, Shanghai, China; Kimmel Center for Biology and Medicine at the Skirball Institute, and Department of Microbiology, New York University School of Medicine, New York, NY, USA
MicroRNAs repress gene expression post-transcriptionally by inhibiting translation and by expediting deadenylation so as to trigger rapid mRNA decay. Their regulatory influence is mediated by the protein components of the RNA-induced silencing complex (RISC), which deliver miRNAs and siRNAs to their mRNA targets. Here we present evidence that CCR4-NOT is the deadenylase that removes poly(A) from messages destabilized by miRNAs in human cells. Overproducing a mutationally inactivated form of either of the catalytic subunits of this deadenylase (CCR4 or CAF1/POP2) significantly impedes the deadenylation and decay of mRNA targeted by a partially complementary miRNA. The same deadenylase initiates the degradation of "off-target" mRNAs that are bound by an imperfectly complementary siRNA introduced by transfection. The greater inhibitory effect of inactive CAF1 or POP2 (versus inactive CCR4) suggests a predominant role for this catalytic subunit of CCR4-NOT in mi/siRNA-mediated deadenylation. These effects of mi/siRNAs and CCR4-NOT can be fully reproduced by directly tethering RISC to mRNA without the guidance of a small RNA, indicating that the ability of RISC to accelerate deadenylation is independent of RNA base pairing. Despite its importance for mi/siRNA-mediated deadenylation, CCR4-NOT appears not to associate significantly with RISC, as judged by the failure of CAF1 and POP2 to co-immunoprecipitate detectably with either the Ago or TNRC6 subunit of RISC, a finding at odds with deadenylase recruitment as the mechanism by which RISC accelerates poly(A) removal.
