ClC chloride channels viewed through a transporter lens

Since its discovery, the ClC family of chloride channels has presented biophysicists with unexpected behaviours and unusual surprises. The latest of these is the realization that not only does the family feature genuine chloride channels, it also includes proton-coupled chloride transporters, which move chloride ions and protons across the membrane in opposite directions. The crystal structure of such a transporter serves as a useful platform for understanding ClC channels, and features of chloride/proton exchange-transport may provide a key for comprehending voltage-dependent gating of the channels.     

Crystal structure of a potassium ion transporter, TrkH

The TrkH/TrkG/KtrB proteins mediate K(+) uptake in bacteria and probably evolved from simple K(+) channels by multiple gene duplications or fusions. Here we present the crystal structure of a TrkH from Vibrio parahaemolyticus. TrkH is a homodimer, and each protomer contains an ion permeation pathway. A selectivity filter, similar in architecture to those of K(+) channels but significantly shorter, is lined by backbone and side-chain oxygen atoms. Functional studies showed that TrkH is selective for permeation of K(+) and Rb(+) over smaller ions such as Na(+) or Li(+). Immediately intracellular to the selectivity filter are an intramembrane loop and an arginine residue, both highly conserved, which constrict the permeation pathway. Substituting the arginine with an alanine significantly increases the rate of K(+) flux. These results reveal the molecular basis of K(+) selectivity and suggest a novel gating mechanism for this large and important family of membrane transport proteins.     

Crystal structure of a phosphorylation-coupled saccharide transporter

Saccharides have a central role in the nutrition of all living organisms. Whereas several saccharide uptake systems are shared between the different phylogenetic kingdoms, the phosphoenolpyruvate-dependent phosphotransferase system exists almost exclusively in bacteria. This multi-component system includes an integral membrane protein EIIC that transports saccharides and assists in their phosphorylation. Here we present the crystal structure of an EIIC from Bacillus cereus that transports diacetylchitobiose. The EIIC is a homodimer, with an expansive interface formed between the amino-terminal halves of the two protomers. The carboxy-terminal half of each protomer has a large binding pocket that contains a diacetylchitobiose, which is occluded from both sides of the membrane with its site of phosphorylation near the conserved His250 and Glu334 residues. The structure shows the architecture of this important class of transporters, identifies the determinants of substrate binding and phosphorylation, and provides a framework for understanding the mechanism of sugar translocation.     

X-ray structure of a ClC chloride channel at 3.0 A reveals the molecular basis of anion selectivity.

The ClC chloride channels catalyse the selective flow of Cl- ions across cell membranes, thereby regulating electrical excitation in skeletal muscle and the flow of salt and water across epithelial barriers. Genetic defects in ClC Cl- channels underlie several familial muscle and kidney diseases. Here we present the X-ray structures of two prokaryotic ClC Cl- channels from Salmonella enterica serovar typhimurium and Escherichia coli at 3.0 and 3.5 A, respectively. Both structures reveal two identical pores, each pore being formed by a separate subunit contained within a homodimeric membrane protein. Individual subunits are composed of two roughly repeated halves that span the membrane with opposite orientations. This antiparallel architecture defines a selectivity filter in which a Cl- ion is stabilized by electrostatic interactions with alpha-helix dipoles and by chemical coordination with nitrogen atoms and hydroxyl groups. These findings provide a structural basis for further understanding the function of ClC Cl- channels, and establish the physical and chemical basis of their anion selectivity.     

Crystal structure of a catalytic intermediate of the maltose transporter

The maltose uptake system of Escherichia coli is a well-characterized member of the ATP-binding cassette transporter superfamily. Here we present the 2.8-A crystal structure of the intact maltose transporter in complex with the maltose-binding protein, maltose and ATP. This structure, stabilized by a mutation that prevents ATP hydrolysis, captures the ATP-binding cassette dimer in a closed, ATP-bound conformation. Maltose is occluded within a solvent-filled cavity at the interface of the two transmembrane subunits, about halfway into the lipid bilayer. The binding protein docks onto the entrance of the cavity in an open conformation and serves as a cap to ensure unidirectional translocation of the sugar molecule. These results provide direct evidence for a concerted mechanism of transport in which solute is transferred from the binding protein to the transmembrane subunits when the cassette dimer closes to hydrolyse ATP.     

SARS医用防护服结构的功能性设计

在对以往医用防护服的设计经验总结的基础上，对抗击SARS医用防护服的结构和功能设计有所新突破，为医用防护服的设计提供了新的研究思路及空间，在设计过程中，为做到比传统防护服更为理想、针对性更强的防护目的，对医用防护服的造型功能设计要点以及材料安全功能都做出了科学的要求，并进行设计、制作、试穿、修改，达到了比较满意的效果，为进一步深入研究医用防护服的结构功能起到积极的推动作用。     

Secondary active transport mediated by a prokaryotic homologue of ClC Cl- channels

ClC Cl- channels make up a large molecular family, ubiquitous with respect to both organisms and cell types. In eukaryotes, these channels fulfill numerous biological roles requiring gated anion conductance, from regulating skeletal muscle excitability to facilitating endosomal acidification by (H+)ATPases. In prokaryotes, ClC functions are unknown except in Escherichia coli, where the ClC-ec1 protein promotes H+ extrusion activated in the extreme acid-resistance response common to enteric bacteria. Recently, the high-resolution structure of ClC-ec1 was solved by X-ray crystallography. This primal prokaryotic ClC structure has productively guided understanding of gating and anion permeation in the extensively studied eukaryotic ClC channels. We now show that this bacterial homologue is not an ion channel, but rather a H+-Cl- exchange transporter. As the same molecular architecture can support two fundamentally different transport mechanisms, it seems that the structural boundary separating channels and transporters is not as clear cut as generally thought.     

Crystal structure of bacterial multidrug efflux transporter AcrB

AcrB is a major multidrug exporter in Escherichia coli. It cooperates with a membrane fusion protein, AcrA, and an outer membrane channel, TolC. We have determined the crystal structure of AcrB at 3.5 A resolution. Three AcrB protomers are organized as a homotrimer in the shape of a jellyfish. Each protomer is composed of a transmembrane region 50 A thick and a 70 A protruding headpiece. The top of the headpiece opens like a funnel, where TolC might directly dock into AcrB. A pore formed by three alpha-helices connects the funnel with a central cavity located at the bottom of the headpiece. The cavity has three vestibules at the side of the headpiece which lead into the periplasm. In the transmembrane region, each protomer has twelve transmembrane alpha-helices. The structure implies that substrates translocated from the cell interior through the transmembrane region and from the periplasm through the vestibules are collected in the central cavity and then actively transported through the pore into the TolC tunnel.     

社会化标签系统的结构、演化和功能

从复杂性科学角度总结了社会化标签系统的结构、演化和功能问题.包括多类异质性节点和超图模型的网络结构;基于标注行为的网络演化模型;基于标签的个性化推荐系统.系统地总结和比较了当前几种代表性的模型和推荐算法,并指出各方法的优缺点和适用范围.有助于更深层次理解和解决社会化标签系统中的理论和应用问题.     

Crystal structure of the MgtE Mg2+ transporter

The magnesium ion Mg2+ is a vital element involved in numerous physiological processes. Mg2+ has the largest hydrated radius among all cations, whereas its ionic radius is the smallest. It remains obscure how Mg2+ transporters selectively recognize and dehydrate the large, fully hydrated Mg2+ cation for transport. Recently the crystal structures of the CorA Mg2+ transporter were reported. The MgtE family of Mg2+ transporters is ubiquitously distributed in all phylogenetic domains, and human homologues have been functionally characterized and suggested to be involved in magnesium homeostasis. However, the MgtE transporters have not been thoroughly characterized. Here we determine the crystal structures of the full-length Thermus thermophilus MgtE at 3.5 A resolution, and of the cytosolic domain in the presence and absence of Mg2+ at 2.3 A and 3.9 A resolutions, respectively. The transporter adopts a homodimeric architecture, consisting of the carboxy-terminal five transmembrane domains and the amino-terminal cytosolic domains, which are composed of the superhelical N domain and tandemly repeated cystathionine-beta-synthase domains. A solvent-accessible pore nearly traverses the transmembrane domains, with one potential Mg2+ bound to the conserved Asp 432 within the pore. The transmembrane (TM)5 helices from both subunits close the pore through interactions with the 'connecting helices', which connect the cystathionine-beta-synthase and transmembrane domains. Four putative Mg2+ ions are bound at the interface between the connecting helices and the other domains, and this may lock the closed conformation of the pore. A structural comparison of the two states of the cytosolic domains showed the Mg2+-dependent movement of the connecting helices, which might reorganize the transmembrane helices to open the pore. These findings suggest a homeostasis mechanism, in which Mg2+ bound between cytosolic domains regulates Mg2+ flux by sensing the intracellular Mg2+ concentration. Whether this presumed regulation controls gating of an ion channel or opening of a secondary active transporter remains to be determined.     

Crystal structure of the CorA Mg2+ transporter

The magnesium ion, Mg2+, is essential for myriad biochemical processes and remains the only major biological ion whose transport mechanisms remain unknown. The CorA family of magnesium transporters is the primary Mg2+ uptake system of most prokaryotes and a functional homologue of the eukaryotic mitochondrial magnesium transporter. Here we determine crystal structures of the full-length Thermotoga maritima CorA in an apparent closed state and its isolated cytoplasmic domain at 3.9 A and 1.85 A resolution, respectively. The transporter is a funnel-shaped homopentamer with two transmembrane helices per monomer. The channel is formed by an inner group of five helices and putatively gated by bulky hydrophobic residues. The large cytoplasmic domain forms a funnel whose wide mouth points into the cell and whose walls are formed by five long helices that are extensions of the transmembrane helices. The cytoplasmic neck of the pore is surrounded, on the outside of the funnel, by a ring of highly conserved positively charged residues. Two negatively charged helices in the cytoplasmic domain extend back towards the membrane on the outside of the funnel and abut the ring of positive charge. An apparent Mg2+ ion was bound between monomers at a conserved site in the cytoplasmic domain, suggesting a mechanism to link gating of the pore to the intracellular concentration of Mg2+.     

多功能可编程电子凸轮的研制

针对船舶和港口自动化设备中，机械凸轮的有触点、易损坏及功能固定等降低自动化设备可靠性的缺陷，研制了电子凸轮，即一种基于可编程序控制器的自动化元件．该电子凸轮具有多功能、可编程、可在线修改等优点，电子凸轮的使用使船舶和港口自动化设备的控制精度提高、故障率降低，提高了可靠性，简化了系统的硬件设备，充分利用了资源，大大降低了生产和维修成本。     

仿生跳跃机器人弹跳结构设计

以蚂蚱弹跳为依据,设计了一种仿蚂蚱跳跃功能结构,利用Solidworks以及ADAMS完成结构动态仿真,并按照仿真结构进行零部件的装配及模型跳跃实验。实验证明,弹跳高度可以达到预期要求。     

基于折叠迷宫环的共面左手结构优化设计

左手材料在微波段的应用越来越广泛,集成化、一体化是其发展趋势.本文提出一种共面紧凑型左手结构——折叠方形迷宫环.利用三维电磁场仿真软件计算其在5.8 GHz的S参数,提取了该结构的等效参数,发现在5.8 GHz附近等效介电常数和等效磁导率同时为负.将该迷宫环以阵列形式排布于棱镜形状的基板上,通过数值实验,观察到了负折射现象和近似零折射现象,该结构可应用于天线以提高辐射性能.     

Design and experimental study of a C-band traveling-wave accelerating structure

Research and design of a C-band (5712 MHz) high-gradient traveling-wave accelerating structure is being carried out at Shanghai Institute of Applied Physics, Chinese Academy of Sciences. The structure consists of 53 regular disk-loaded cells and two waveguide couplers, and its length is about 1 m. This paper presents a design method for the accelerating structure, an experimental model and the preliminary results of an RF cold test of the model structure.