牵牛花与RNA干扰

RNAi现象的首次发现，是在外源的多拷贝转基因被引入牵牛花后所激起的一个特定序列的基因沉默响应。RNAi导致了封面上的花的双色彩图案。Ａｒｇｏｎａｕｔｅ是RNAi效应复合体中的标志组分，本期封面上登载了它的晶体结构，其中ＰＡＺ域用蓝色显示，ＰＩＷＩ域用紫色显示。本期《科学》还刊登了曾于７月29日发表的两篇有关的研究文章，以及一篇相关的研究评述。 Argonaute Journeys into the Heart of RISC

Erik J. Sontheimer and Richard W. Carthew

　　Identifying the "slicer" component of the RNA-induced silencing complex (RISC) complex has been a major goal of those interested in the RNA interference pathway of gene silencing. In their Perspective, Sontheimer and Carthew discuss a pair of recent studies (Liu et al., Song et al.) that identify the protein Argonaute2 as the slicer element of RISC that cleaves target messenger RNAs. Argonaute2 Is the Catalytic Engine of Mammalian RNAi

Jidong Liu,1* Michelle A. Carmell,1,2* Fabiola V. Rivas,1 Carolyn G. Marsden,1 J. Michael Thomson,3 Ji-Joon Song,1 Scott M. Hammond,3 Leemor Joshua-Tor,1 Gregory J. Hannon1

Gene silencing through RNA interference (RNAi) is carried out by RISC, the RNA-induced silencing complex. RISC contains two signature components, small interfering RNAs (siRNAs) and Argonaute family proteins. Here, we show that the multiple Argonaute proteins present in mammals are both biologically and biochemically distinct, with a single mammalian family member, Argonaute2, being responsible for messenger RNA cleavage activity. This protein is essential for mouse development, and cells lacking Argonaute2 are unable to mount an experimental response to siRNAs. Mutations within a cryptic ribonuclease H domain within Argonaute2, as identified by comparison with the structure of an archeal Argonaute protein, inactivate RISC. Thus, our evidence supports a model in which Argonaute contributes "Slicer" activity to RISC, providing the catalytic engine for RNAi.

1 Cold Spring Harbor Laboratory, Watson School of Biological Sciences, 1 Bungtown Road, Cold Spring Harbor, NY 11724, USA.

2 Program in Genetics, Stony Brook University, Stony Brook, NY 11794, USA.

3 Department of Cell and Developmental Biology, University of North Carolina, Chapel Hill, NC 27599, USA. Crystal Structure of Argonaute and Its Implications for RISC Slicer Activity

Ji-Joon Song,1,2 Stephanie K. Smith,2 Gregory J. Hannon,1 Leemor Joshua-Tor1,2*

　　Argonaute proteins and small interfering RNAs (siRNAs) are the known signature components of the RNA interference effector complex RNA-induced silencing complex (RISC). However, the identity of "Slicer," the enzyme that cleaves the messenger RNA (mRNA) as directed by the siRNA, has not been resolved. Here, we report the crystal structure of the Argonaute protein from Pyrococcus furiosus at 2.25 angstrom resolution. The structure reveals a crescent-shaped base made up of the amino-terminal, middle, and PIWI domains. The Piwi Argonaute Zwille (PAZ) domain is held above the base by a "stalk"-like region. The PIWI domain (named for the protein piwi) is similar to ribonuclease H, with a conserved active site aspartate-aspartate-glutamate motif, strongly implicating Argonaute as "Slicer." The architecture of the molecule and the placement of the PAZ and PIWI domains define a groove for substrate binding and suggest a mechanism for siRNA-guided mRNA cleavage. 1 Watson School of Biological Sciences, 1 Bungtown Road, Cold Spring Harbor, NY 11724, USA.

2 Keck Structural Biology Laboratory, Cold Spring Harbor Laboratory, 1 Bungtown Road, Cold Spring Harbor, NY 11724, USA.