宋海威《Cell》文章发现蛋白翻译新机制

　　

生物通报道：来自美国霍德华休斯医学院，Health Research, Inc.，新加坡国立大学分子与细胞生物学研究院，密歇根大学医学院，瑞典乌普萨拉大学等处的研究人员利用生物结构学分析发现蛋白翻译II型释放因子(class II release factor，RF3)可以诱导核糖体构象变化，导致I型释放因子(RF)的分裂释放，这对于更加深入了解蛋白翻译过程提供了重要资料。这一研究成果公布在《Cell》杂志上。

文章的通讯作者之一是宋海威（Haiwei Song，音译）教授，其早年毕业于河南大学，后于中科院生物物理研究院获得硕士学位，目前主要从事蛋白翻译过程研究。

原文摘要：
Cell, Vol 129, 929-941, 01 June 2007
RF3 Induces Ribosomal Conformational Changes Responsible for Dissociation of Class I Release Factors
[Abstract]

蛋白合成过程中，mRNA穿过核膜进入胞质后，多个核糖体附着其上形成多核糖体。作为原料的各种氨基酸在tRNA的携带下，在多核糖体上以肽键互相结合，生成具有一定氨基酸序列的特定多肽链。

在这个过程中，II型释放因子(class II release factor，RF3)可以结合到核糖体上，以一种依赖于GTP的方式启动I型释放因子(RF)，参与翻译终止的过程。在这篇研究报告中，研究人员获得了大肠杆菌RF3•GDP的结晶结构，并从中发现了一个与EF-Tu•GTP结构极其相似的三结构域构架，从而发现了蛋白翻译终止过程中的一些新机制。

研究人员通过对RF3突变进行生化分析，结果显示在II结构域和III结构域上的一个表面区域对于RF3活性循环的不同步骤意义重大。而且在对结合在GTP形式RF3上的终止后核糖体（posttermination ribosome）结构进行了冷冻电子显微学（cryo-electron microscopy，cryo-EM）分析后，研究人员提出RF3•GTP结合可以诱导核糖体大型构象的变化，从而破坏RF与编码中心，以及核糖体GTPase关联中心之间的相互作用，导致RF释放。
（生物通：张迪）

附：
Haiwei SONG

1987年，河南大学化学系；
1990年，中科院生物物理研究院蛋白结晶学获硕士学位；
1998年，英国利兹大学分子生物学系博士学位；

研究领域：

Dr. Song’s research focuses on two closely related areas: the mechanism of translation termination in protein biosynthesis and the mechanism of eukaryotic post-transcriptional regulation of gene expression at level of mRNA stability.

Translation Termination
The process of translation termination is mediated by polypeptide release factors (RFs) and GTP. In prokaryotes, two similar proteins, RF1 and RF2, function as class I release factors, whereas a structurally unrelated protein, RF3, is identified as the class II release factor. In eukaryotes, eRF1 (class I) functions as an omnipotent release factor; eRF3 (class II) is a GTPase and essential for cell growth and forms a stable complex with eRF1. Structural studies on individual release factor and eRF1/eRF3 complex will provide vital information on the molecular mechanism of translation termination. Sup35 (eRF3 in yeast) not only play an important role in yeast translation termination process but also share many of the characteristics of prion proteins responsible for the mammalian prion diseases including BSE and human CJD. The protein undergoes a conformational change from a soluble form to an aggregated fibre-like form in auto- catalytic process. Structure studies of Sup35 protein in both soluble and aggregated fibre-like forms will provide information concerning this process and also on the related processed associated with the mammalian prions. 

mRNA decay
Modulation of mRNA stability plays important roles in regulation of gene expression, quality control of mRNA biogenesis by recognizing and degrading the aberrant mRNAs, and antiviral defenses by RNA interference (RNAi). In eukaryotes, two general pathways of mRNA decay have been identified. Both pathways are initiated with deadenylation of the 3'-poly(A) tail of mRNAs. In the 5’ to 3’ decay pathways, the 5’ cap structure can be removed by the Dcp1p/Dcp2p complex following deadenylation, thus exposing the 5’ end to 5’-3’ exoribonuclease activities. In the 3’-5’ decay pathways, deadenylation is followed by exosome-dependent 3’-5’ degradation of mRNA body. In addition, two specialized mRNA decay pathways exist to recognize and degrade aberrant mRNAs. In a process referred to as nonsense-mediated mRNA decay (NMD), transcripts with premature translation termination codons are degraded either by deadenylation-independent decapping (5’-to-3’ NMD), or by accelerated deadenylation and 3’-5’ exonucleolytic digestion by the exosome (3’-to-5’ NMD). There are around 200 human genetic diseases that result from the premature translation termination. Studies of the proteins required for NMD may lead to rational approaches for the treatment of these genetic disorders. Our lab uses X-ray crystallography in conjunction with biochemical, biophysical and molecular biology methods to elucidate structure and function of the proteins and complexes involved in the mRNA decay pathways. 



发表文章：
1. Song_HW, Mugnier_P, Das_AK, Webb_HM, Evans_DR, Tuite_MF, Hemmings_BA, Barford_D. (2000) The Crystal Structure of Human Eukaryotic Release Factor eRF1-Mechanism of Stop Codon Recognition and Peptidyl-tRNA Hydrolysis. Cell, 100, 311-321. 

2. She, M., Decker, C.J., Sundramurthy, K., Liu, Y., Chen, N., Parker, R., and Song, H. (2004). Crystal structure of Dcp1p and its functional implications in mRNA decapping. Nat. Struct. & Mol. Biol. 11, 249-56. 

3. Parker, R. and Song, H. (2004). The enzymes and control of eukaryotic mRNA turnover. Nat. Struct. & Mol. Biol. 11, 121-7.

4. Wu, M., Lu, L., Hong, W. and Song, H. (2004). Structural basis for recruitment of GRIP domain golgin-245 by small GTPase Arl1. Nat. Struct. & Mol. Biol. 11, 86-94. 

5. Kong, C.G., Ito, K., Walsh, A. M., Wada, M., Liu, Y.Y., Kumar, S., Barford, D., Nakamura, Y. and Song, H.W. (2004). Crystal Structure and Functional Analysis of the Eukaryotic Class II Release Factor eRF3 from S. pombe. Mol. Cell 14, 233-245.

6. Cheng, Z.H., Liu, Y.Y., Wang, C.H., Parker, R. and Song, H.W. (2004). Crystal structure of Ski8p, a WD-repeat protein with dual roles in mRNA metabolism and meiotic recombination. Protein Sci. 13, 2673-2684.

7. Chen N, Walsh MA, Liu Y, Parker R, Song H. (2005). Crystal structures of human DcpS in ligand-free and m7GDP-bound forms suggest a dynamic mechanism for scavenger mRNA decapping. J Mol Biol. 347, 707-18.

8. Wu, M., Wang T., Loh E., Hong W. and Song H. (2005). Structural basis for recruitment of RILP by small GTPase Rab7. EMBO J. 24, 1491-1501. 

9. Cheng Z.H., Coller J., Parker R. and Song H. (2005). Crystal structure and functional analysis of DEAD-box protein Dhh1p, RNA in press.