Structural basis of water-specific transport through the AQP1 water channel.

Water channels facilitate the rapid transport of water across cell membranes in response to osmotic gradients. These channels are believed to be involved in many physiological processes that include renal water conservation, neuro-homeostasis, digestion, regulation of body temperature and reproduction. Members of the water channel superfamily have been found in a range of cell types from bacteria to human. In mammals, there are currently 10 families of water channels, referred to as aquaporins (AQP): AQP0-AQP9. Here we report the structure of the aquaporin 1 (AQP1) water channel to 2.2 A resolution. The channel consists of three topological elements, an extracellular and a cytoplasmic vestibule connected by an extended narrow pore or selectivity filter. Within the selectivity filter, four bound waters are localized along three hydrophilic nodes, which punctuate an otherwise extremely hydrophobic pore segment. This unusual combination of a long hydrophobic pore and a minimal number of solute binding sites facilitates rapid water transport. Residues of the constriction region, in particular histidine 182, which is conserved among all known water-specific channels, are critical in establishing water specificity. Our analysis of the AQP1 pore also indicates that the transport of protons through this channel is highly energetically unfavourable.     

Importance of NPA motifs in the expression and function of water channel aquaporin-1

The asparagine-proline-alanine sequences (NPA motifs) are highly conserved in aquaporin water channel family. Crystallographic studies of AQP1 structure demonstrated that the two NPA motifs are in the narrow central constriction of the channel, serving to bind water molecules for selective and effi-cient water passage. To investigate the importance of the two NPA motifs in the structure, function and biogenesis of aquaporin water channels, we generated AQP1 mutations with NPA1 deletion, NPA2 de-letion and NPA1,2 double deletion. The coding sequences of the three mutated cDNAs were subcloned into the mammalian expression vector pcDNA3.1 to form expression plasmids. We established stably transfected CHO cell lines expressing these AQP1 mutants. Immunofluorescence indicated that all the three mutated AQP1 proteins are expressed normally on the plasma membrane of stably transfected CHO cells, suggesting that deletion of NPA motifs does not influence the expression and intracellular processing of AQP1. Functional analysis demonstrated that NPA1 or NPA2 deletion reduced AQP1 water permeability by 49.6% and 46.7%, respectively, while NPA1,2 double deletion had little effect on AQP1 water permeability. These results provide evidence that NPA motifs are important for water per-meation but not essential for the expression, intracellular processing and the basic structure of AQP1 water channel.     

Mechanism of ion permeation through calcium channels

Calcium channels carry out vital functions in a wide variety of excitable cells but they also face special challenges. In the medium outside the channel, Ca2+ ions are vastly outnumbered by other ions. Thus, the calcium channel must be extremely selective if it is to allow Ca2+ influx rather than a general cation influx. In fact, calcium channels show a much greater selectivity for Ca2+ than sodium channels do for Na+ despite the high flux that open Ca channels can support. Relatively little is known about the mechanism of ion permeation through Ca channels. Earlier models assumed ion independence or single-ion occupancy. Here we present evidence for a novel hypothesis of ion movement through Ca channels, based on measurements of Ca channel activity at the level of single cells or single channels. Our results indicate that under physiological conditions, the channel is occupied almost continually by one or more Ca2+ ions which, by electrostatic repulsion, guard the channel against permeation by other ions. On the other hand, repulsion between Ca2+ ions allows high throughput rates and tends to prevent saturation with calcium.     

Structural determinants for generating centromeric chromatin

Mammalian centromeres are not defined by a consensus DNA sequence. In all eukaryotes a hallmark of functional centromeres--both normal ones and those formed aberrantly at atypical loci--is the accumulation of centromere protein A (CENP-A), a histone variant that replaces H3 in centromeric nucleosomes. Here we show using deuterium exchange/mass spectrometry coupled with hydrodynamic measures that CENP-A and histone H4 form sub-nucleosomal tetramers that are more compact and conformationally more rigid than the corresponding tetramers of histones H3 and H4. Substitution into histone H3 of the domain of CENP-A responsible for compaction is sufficient to direct it to centromeres. Thus, the centromere-targeting domain of CENP-A confers a unique structural rigidity to the nucleosomes into which it assembles, and is likely to have a role in maintaining centromere identity.     

Ion permeation through the Na+,K+-ATPase

P-type ATPase pumps generate concentration gradients of cations across membranes in nearly all cells. They provide a polar transmembrane pathway, to which access is strictly controlled by coupled gates that are constrained to open alternately, thereby enabling thermodynamically uphill ion transport (for example, see ref. 1). Here we examine the ion pathway through the Na+,K+-ATPase, a representative P-type pump, after uncoupling its extra- and intracellular gates with the marine toxin palytoxin. We use small hydrophilic thiol-specific reagents as extracellular probes and we monitor their reactions, and the consequences, with cysteine residues introduced along the anticipated cation pathway through the pump. The distinct effects of differently charged reagents indicate that a wide outer vestibule penetrates deep into the Na+,K+-ATPase, where the pathway narrows and leads to a charge-selectivity filter. Acidic residues in this region, which are conserved to coordinate pumped ions, allow the approach of cations but exclude anions. Reversing the charge at just one of those positions converts the pathway from cation selective to anion selective. Close structural homology among the catalytic subunits of Ca2+-, Na+,K+- and H+,K+-ATPases argues that their extracytosolic cation exchange pathways all share these physical characteristics.     

柔性蒙皮材料氦气渗透的细观机制

该文研究了由聚芳脂纤维(vectran)平纹织物为承力层、PVF膜(tedlar)为阻氦层、聚酯薄膜(mylar)为防护层的浮空器蒙皮材料氦气渗透的细观机制。首先,根据Hagen-Poiseuille定律,推导得到了承力层等效渗透率。然后,通过氦气溶解扩散模型和多层复合结构渗透模型,理论分析了各层对氦气密封性能的贡献比例,讨论了每层不同厚度对整体蒙皮材料氦气渗透率的影响,理论预测了蒙皮材料的整体氦气渗透率。最后,将实验结果与理论所得的数值进行了对比,验证了多层渗透模型的正确性。     

界面传质阻力对沸石膜中组分渗透的影响

针对沸石膜中典型的Langmuir吸附以及强、弱受限扩散两种情形,利用Maxwell Stefan（MS）模型描述膜内扩散,建立了计及界面非平衡条件下吸、脱附阻力的单组分气体渗透膜模型,得到表征界面阻力的解析解。针对强受限和弱受限扩散分子（CF₄和CH₄）在MFI沸石膜中的扩散,讨论了影响界面阻力的主要因素。结果表明：界面阻力分率随膜厚的增加而减小;对于弱受限的CH₄分子,进料端和通透端压力的改变不会影响界面阻力分率的改变;对于强受限的CF₄分子,界面阻力分率受操作压力的影响较大。本文方法可方便地讨论界面阻力的影响,从而指导沸石膜材料合成以及膜组件的设计。     

Structural determinants for GoLoco-induced inhibition of nucleotide release by Galpha subunits

Heterotrimeric G-proteins bind to cell-surface receptors and are integral in transmission of signals from outside the cell. Upon activation of the Galpha subunit by binding of GTP, the Galpha and Gbetagamma subunits dissociate and interact with effector proteins for signal transduction. Regulatory proteins with the 19-amino-acid GoLoco motif can bind to Galpha subunits and maintain G-protein subunit dissociation in the absence of Galpha activation. Here we describe the structural determinants of GoLoco activity as revealed by the crystal structure of Galpha(i1) GDP bound to the GoLoco region of the 'regulator of G-protein signalling' protein RGS14. Key contacts are described between the GoLoco motif and Galpha protein, including the extension of GoLoco's highly conserved Asp/Glu-Gln-Arg triad into the nucleotide-binding pocket of Galpha to make direct contact with the GDP alpha- and beta-phosphates. The structural organization of the GoLoco Galpha(i1) complex, when combined with supporting data from domain-swapping experiments, suggests that the Galpha all-helical domain and GoLoco-region carboxy-terminal residues control the specificity of GoLoco Galpha interactions.     

Impairment of angiogenesis and cell migration by targeted aquaporin-1 gene disruption

Aquaporin-1 (AQP1) is a water channel protein expressed widely in vascular endothelia, where it increases cell membrane water permeability. The role of AQP1 in endothelial cell function is unknown. Here we show remarkably impaired tumour growth in AQP1-null mice after subcutaneous or intracranial tumour cell implantation, with reduced tumour vascularity and extensive necrosis. A new mechanism for the impaired angiogenesis was established from cell culture studies. Although adhesion and proliferation were similar in primary cultures of aortic endothelia from wild-type and from AQP1-null mice, cell migration was greatly impaired in AQP1-deficient cells, with abnormal vessel formation in vitro. Stable transfection of non-endothelial cells with AQP1 or with a structurally different water-selective transporter (AQP4) accelerated cell migration and wound healing in vitro. Motile AQP1-expressing cells had prominent membrane ruffles at the leading edge with polarization of AQP1 protein to lamellipodia, where rapid water fluxes occur. Our findings support a fundamental role of water channels in cell migration, which is central to diverse biological phenomena including angiogenesis, wound healing, tumour spread and organ regeneration.     

河流水资源结构分析研究

针对河流中水体的功能和作用,从生态保护、兴利运用和防洪减灾的角度出发,提出了河流水资源结构分析的思想和方法.河流水资源可划分为生态水量、安全水量、风险水量和灾害水量四部分,各部分水量由特定的流量来界定.以北方河流为背景,在对4种水量的概念及其功能与作用深入分析的基础上,提出了各类水量的计算方法;研究了松花江哈尔滨水文断面的水资源结构划分和各类水量分配规律.结果表明,所提出的方法可以对河流水资源进行定量分析,进而实现分类管理、高效利用.     

乙炔、水蒸气在聚砜膜中渗透行为的研究

采用"时间滞后"法测定了乙炔、水蒸气在聚砜膜中的渗透系数、扩散系数和溶解度系数,结果表明:乙炔在膜中的渗透符合双方式模型,渗透活化能为负值;水蒸气在膜中的渗透行为比较特殊,存在溶胀和成簇迁移,渗透活化能为正值;乙炔在膜中的扩散系数和溶解度系数均比水蒸气小,聚砜膜中H2O C2H2选择性在300以上.     

Expression of aquaporin-1 in SMMC-7221 liver carcinoma cells promotes cell migration

Aquaporins (AQPs) are membrane water channels that play pivotal roles in physiological and pathophysi- ological processes in diverse mammalian organs[1―3]. Recent studies indicated a novel role of AQPs in cell migration. Mice lacking AQP1, the endothelia…     

Structural determinants for regulation of phosphodiesterase by a G protein at 2.0 A

A multitude of heptahelical receptors use heterotrimeric G proteins to transduce signals to specific effector target molecules. The G protein transducin, Gt, couples photon-activated rhodopsin with the effector cyclic GMP phosophodiesterase (PDE) in the vertebrate phototransduction cascade. The interactions of the Gt alpha-subunit (alpha(t)) with the inhibitory PDE gamma-subunit (PDEgamma) are central to effector activation, and also enhance visual recovery in cooperation with the GTPase-activating protein regulator of G-protein signalling (RGS)-9 (refs 1-3). Here we describe the crystal structure at 2.0 A of rod transducin alpha x GDP x AlF4- in complex with the effector molecule PDEgamma and the GTPase-activating protein RGS9. In addition, we present the independently solved crystal structures of the RGS9 RGS domain both alone and in complex with alpha(t/i1) x GDP x AlF4-. These structures reveal insights into effector activation, synergistic GTPase acceleration, RGS9 specificity and RGS activity. Effector binding to a nucleotide-dependent site on alpha(t) sequesters PDEgamma residues implicated in PDE inhibition, and potentiates recruitment of RGS9 for hydrolytic transition state stabilization and concomitant signal termination.     

冷冻法净化苦咸水的实验

利用调温冰柜在-4℃,-8℃,-10℃和-12℃的条件下对祖厉河水样进行实验研究,实验结果表明:冷冻法对苦咸水中超标离子(阴阳离子)的脱盐效果较显著;碎冰过滤可以进一步提高冰晶质量,使各超标离子平均总脱盐率都在95%以上;脱盐效果受冷冻温度和溶液质量浓度的影响,随着冷冻温度的降低或苦咸水质量浓度的增加呈现下降趋势.通过研究,可以充分利用甘肃省冬季的气候条件,丰富的咸水资源,采用冷冻法来淡化本地区的苦咸水,从而解决祖厉河流域上的饮水问题.     

不同气体混合体系在聚二甲基硅氧烷膜中渗透过程的模拟与对比

分别对氮气-甲烷以及氮气-正戊烷两种混合气在聚二甲基硅氧烷（PDMS）膜中的渗透过程进行了实验,并且分别利用Maxwell-Stefans (MS)方程与UNIQUAC方程的组合模型和Fick定律与Henry定律的组合模型模拟了该渗透过程。实验数据与模拟数据的对比结果表明：对于不凝性气体混合物(如N₂、CH₄等)的渗透过程,两种模型均具有较好的模拟效果;对于含有可凝性气体(如C₅H₁₂)的混合物,则只有MS-UNIQUAC模型具有较好的模拟效果。     

Structural relaxation in supercooled water by time-resolved spectroscopy

Water has many kinetic and thermodynamic properties that exhibit an anomalous dependence on temperature, in particular in the supercooled phase. These anomalies have long been interpreted in terms of underlying structural causes, and their experimental characterization points to the existence of a singularity at a temperature of about 225 K. Further insights into the nature and origin of this singularity might be gained by completely characterizing the structural relaxation in supercooled water. But until now, such a characterization has only been realized in simulations that agree with the predictions of simple mode-coupling theory; unambiguous experimental support for this surprising conclusion is, however, not yet available. Here we report time-resolved optical Kerr effect measurements that unambiguously demonstrate that the structural relaxation of liquid and weakly supercooled water follows the behaviour predicted by simple mode-coupling theory. Our findings thus support the interpretation of the singularity as a purely dynamical transition. That is, the anomalous behaviour of weakly supercooled water can be explained using a fully dynamic model and without needing to invoke a thermodynamic origin. In this regard, water behaves like many other, normal molecular liquids that are fragile glass-formers.     

建立清洁开发机制 促进经济结构调整

阐述了CDM（清洁开发机制）的含义与作用，分析了CDM对山西解决环境污染问题的重要性及实现的可能性，指出在山西建立CDM，是有效改善山西环境和搞好经济结构调整的最佳途径。     

Structural basis for semaphorin signalling through the plexin receptor

Semaphorins and their receptor plexins constitute a pleiotropic cell-signalling system that is used in a wide variety of biological processes, and both protein families have been implicated in numerous human diseases. The binding of soluble or membrane-anchored semaphorins to the membrane-distal region of the plexin ectodomain activates plexin's intrinsic GTPase-activating protein (GAP) at the cytoplasmic region, ultimately modulating cellular adhesion behaviour. However, the structural mechanism underlying the receptor activation remains largely unknown. Here we report the crystal structures of the semaphorin 6A (Sema6A) receptor-binding fragment and the plexin A2 (PlxnA2) ligand-binding fragment in both their pre-signalling (that is, before binding) and signalling (after complex formation) states. Before binding, the Sema6A ectodomain was in the expected 'face-to-face' homodimer arrangement, similar to that adopted by Sema3A and Sema4D, whereas PlxnA2 was in an unexpected 'head-on' homodimer arrangement. In contrast, the structure of the Sema6A-PlxnA2 signalling complex revealed a 2:2 heterotetramer in which the two PlxnA2 monomers dissociated from one another and docked onto the top face of the Sema6A homodimer using the same interface as the head-on homodimer, indicating that plexins undergo 'partner exchange'. Cell-based activity measurements using mutant ligands/receptors confirmed that the Sema6A face-to-face dimer arrangement is physiologically relevant and is maintained throughout signalling events. Thus, homodimer-to-heterodimer transitions of cell-surface plexin that result in a specific orientation of its molecular axis relative to the membrane may constitute the structural mechanism by which the ligand-binding 'signal' is transmitted to the cytoplasmic region, inducing GAP domain rearrangements and activation.