Crystal structure of the β2 adrenergic receptor-Gs protein complex

G protein-coupled receptors (GPCRs) are responsible for the majority of cellular responses to hormones and neurotransmitters as well as the senses of sight, olfaction and taste. The paradigm of GPCR signalling is the activation of a heterotrimeric GTP binding protein (G protein) by an agonist-occupied receptor. The β(2) adrenergic receptor (β(2)AR) activation of Gs, the stimulatory G protein for adenylyl cyclase, has long been a model system for GPCR signalling. Here we present the crystal structure of the active state ternary complex composed of agonist-occupied monomeric β(2)AR and nucleotide-free Gs heterotrimer. The principal interactions between the β(2)AR and Gs involve the amino- and carboxy-terminal α-helices of Gs, with conformational changes propagating to the nucleotide-binding pocket. The largest conformational changes in the β(2)AR include a 14 ? outward movement at the cytoplasmic end of transmembrane segment 6 (TM6) and an α-helical extension of the cytoplasmic end of TM5. The most surprising observation is a major displacement of the α-helical domain of Gαs relative to the Ras-like GTPase domain. This crystal structure represents the first high-resolution view of transmembrane signalling by a GPCR.     

ATP-induced helicase slippage reveals highly coordinated subunits

Helicases are vital enzymes that carry out strand separation of duplex nucleic acids during replication, repair and recombination. Bacteriophage T7 gene product 4 is a model hexameric helicase that has been observed to use dTTP, but not ATP, to unwind double-stranded (ds)DNA as it translocates from 5' to 3' along single-stranded (ss)DNA. Whether and how different subunits of the helicase coordinate their chemo-mechanical activities and DNA binding during translocation is still under debate. Here we address this question using a single-molecule approach to monitor helicase unwinding. We found that T7 helicase does in fact unwind dsDNA in the presence of ATP and that the unwinding rate is even faster than that with dTTP. However, unwinding traces showed a remarkable sawtooth pattern where processive unwinding was repeatedly interrupted by sudden slippage events, ultimately preventing unwinding over a substantial distance. This behaviour was not observed with dTTP alone and was greatly reduced when ATP solution was supplemented with a small amount of dTTP. These findings presented an opportunity to use nucleotide mixtures to investigate helicase subunit coordination. We found that T7 helicase binds and hydrolyses ATP and dTTP by competitive kinetics such that the unwinding rate is dictated simply by their respective maximum rates V(max), Michaelis constants K(M) and concentrations. In contrast, processivity does not follow a simple competitive behaviour and shows a cooperative dependence on nucleotide concentrations. This does not agree with an uncoordinated mechanism where each subunit functions independently, but supports a model where nearly all subunits coordinate their chemo-mechanical activities and DNA binding. Our data indicate that only one subunit at a time can accept a nucleotide while other subunits are nucleotide-ligated and thus they interact with the DNA to ensure processivity. Such subunit coordination may be general to many ring-shaped helicases and reveals a potential mechanism for regulation of DNA unwinding during replication.     

Mixed aromatic-aliphatic organic nanoparticles as carriers of unidentified infrared emission features

Unidentified infrared emission bands at wavelengths of 3-20 micrometres are widely observed in a range of environments in our Galaxy and in others. Some features have been identified as the stretching and bending modes of aromatic compounds, and are commonly attributed to polycyclic aromatic hydrocarbon molecules. The central argument supporting this attribution is that single-photon excitation of the molecule can account for the unidentified infrared emission features observed in 'cirrus' clouds in the diffuse interstellar medium. Of the more than 160 molecules identified in the circumstellar and interstellar environments, however, not one is a polycyclic aromatic hydrocarbon molecule. The detections of discrete and broad aliphatic spectral features suggest that the carrier of the unidentified infrared emission features cannot be a pure aromatic compound. Here we report an analysis of archival spectroscopic observations and demonstrate that the data are most consistent with the carriers being amorphous organic solids with a mixed aromatic-aliphatic structure. This structure is similar to that of the organic materials found in meteorites, as would be expected if the Solar System had inherited these organic materials from interstellar sources.     

马铃薯交联-羧甲基淀粉的结构与性能

以马铃薯淀粉为原料,环氧氯丙烷为交联剂,制备交联-羧甲基淀粉(CCMS).对比了CCMS与马铃薯原淀粉、交联淀粉(CS)、羧甲基淀粉(CMS)和羧甲基纤维素(CMC)的性质,结果表明CCMS糊液具有良好的抗老化能力、热稳定性、酸碱稳定性及抗剪切稳定性.采用傅里叶变换红外光谱(FT-IR)、环境扫描电镜(ESEM)和X射...     

Role of Tip60 tumor suppressor in DNA repair pathway

To prevent the damage caused by DNA strand breaks, eukaryotic cells have evolved a series of highly conserved DNA repair mechanisms. The ubiquitously expressed acetyltransferase, Tip60, plays a central role in ATM (ataxia-telangiectasia mutated) activation which is involved in DNA repair. Recent work uncovered a new mechanism of ATM activation mediated by Tip60 and demonstrated that histone methylation, specifically, trimethylation of histone H3, is a key factor in the process. Here, we review the current understanding of how Tip60 is activated and how it activates ATM in response to DNA damage.     

表达长喙田菁SrPCS4烟草的获得及Cd抗性研究

植物螯合肽(phytochelatins,PCs)在植物解除重金属的毒性方面具有重要作用,是以谷胱甘肽为底物,在植物螯肽合成酶(phytochelatin synthase,PCS)催化下合成的.作者已经克隆得到的长喙田菁(Sesba-nia rostrata)植物螯合肽合成酶SrPCS4 cDNA长为1035 bp,其ORF编码177个氨基酸,以pHANNIBAL及pART27为基础,构建了CaMV35S启动子驱动的SrPCS4基因植物表达载体pAM25,采用电击转化方法将pAM25导入根癌农杆菌EHA105,并通过改良叶盘转化方法用该菌株对烟草进行了转化,对转基因烟草进行了PCR与northern-blot检测,研究结果表明得到了表达该基因的烟草,但表达该基因的烟草不能够提高对Cd的抗性.     

地铁深基坑施工安全评估FDEMATEL-TOPSIS模型及应用

基坑施工过程中风险因素众多且相互关联,现有研究中缺乏对模糊环境下风险因素间相互关系的考虑,导致风险因素权重的计算结果不够合理.为此提出一种基于模糊决策与试验评价实验室(fuzzy decision-making trial and evaluation labora-tory,FDEMATEL)与优劣解距离法(tech...     

基于nested-loop的大数据集快速离群点检测算法

针对已有的多数离群点检测算法存在扩展性差,不能有效应用于大数据集的问题,在已有的基于距离的离群点检测算法的基础上,设计模信息表存储结构,利用向量内积不等式关系以及合理的存储分配和调度策略,提出一种高效离群点检测算法DBoda.该算法通过在预处理中存储每个点的模信息,减少点间距离的计算量,并对嵌套循环方法进行优化,进一步减少I/O的开销.理论分析和试验结果表明,所提算法具有时间消耗小和适用于处理大数据集的特点,可以有效地解决离群点检测中的算法时间复杂性和算法扩展性问题.     

支持地铁车体开发的工程数据库系统

在车体产品数据模块和数据库功能分析的基础上,建立以SQL Server 2000为后台数据库,以Java为W eb Service开发工具的工程数据库系统,完成前台功能化查询、可视化查询和参数化查询的功能设计.数据库内存储所有已建CAD模型和CAE模型的一般信息、分析结果信息及模型指针.该数据库系统管理设计过程、图纸文件、设计结果和分析结果,支持产品的设计变更、审批和发布,实现快速设计信息的检索与查询.实际应用表明,该系统能有效地管理车体开发中的产品数据.     

Lawson-Mislove问题的一个必要条件

借助于Dcpo上的Scott拓扑,引进Scott吸收Dcpo的概念,并证明了函数空间上Scott拓扑与Isbell拓扑一致的必要条件是该函数空间的值域Dcpo是Scott吸收的.结果表明,Scott吸收性是Lawson-Mislove问题的一个必要性刻画.