Association of dystrophin and an integral membrane glycoprotein

Duchenne muscular dystrophy (DMD) is caused by a defective gene found on the X-chromosome. Dystrophin is encoded by the DMD gene and represents about 0.002% of total muscle protein. Immunochemical studies have shown that dystrophin is localized to the sarcolemma in normal muscle but is absent in muscle from DMD patients. Many features of the predicted primary structure of dystrophin are shared with membrane cytoskeletal proteins, but the precise function of dystrophin in muscle is unknown. Here we report the first isolation of dystrophin from digitonin-solubilized skeletal muscle membranes using wheat germ agglutinin (WGA)-Sepharose. We find that dystrophin is not a glycoprotein but binds to WGA-Sepharose because of its tight association with a WGA-binding glycoprotein. The association of dystrophin with this glycoprotein is disrupted by agents that dissociate cytoskeletal proteins from membranes. We conclude that dystrophin is linked to an integral membrane glycoprotein in the sarcolemma. Our results indicate that the function of dystrophin could be to link this glycoprotein to the underlying cytoskeleton and thus help either to preserve membrane stability or to keep the glycoprotein non-uniformly distributed in the sarcolemma.     

Structural architecture of an outer membrane channel as determined by electron crystallography.

Porins are a family of membrane channels commonly found in the outer membranes of Gram-negative bacteria where they serve as diffusional pathways for waste products, nutrients and antibiotics, and can also be receptors for bacteriophages. Porin channels have been shown in vitro to be voltage-gated. They can exhibit slight selectivities for certain solutes; for example PhoE porin has some selectivity for anionic and phosphate-containing compounds. Unlike many known membrane proteins which often contain long stretches of hydrophobic segments that are believed to traverse the membrane in a helical conformation, porins are found to have charged residues distributed almost uniformly along their primary sequences and have most of their secondary structure in a beta-sheet conformation. We have made crystalline patches of PhoE porin embedded in a lipid bilayer and have used these to determine the structure of PhoE porin by electron crystallography to a resolution of 6A. The basic structure consists of a trimer of elliptically shaped, cylindrical walls of beta sheet. Each cylinder has an inner lining, formed by parts of the polypeptide, that defines the channel size. The structure provides a clue as to how deletions of segments of polypeptide, which are found in certain mutants, can result in an actual increase in the channel size.     

Structural evidence for gene duplication in the evolution of the acid proteases.

X-ray studies of acid proteases indicate a bilobal structure with a well defined active site cleft. An intramolecular twofold symmetry axis relates two topologically similar domains and the active site residues. A possible mechanism for evolution by gene duplication, divergence and gene fusion is presented.     

Structural basis for the activation of anthrax adenylyl cyclase exotoxin by calmodulin.

Oedema factor, a calmodulin-activated adenylyl cyclase, is important in the pathogenesis of anthrax. Here we report the X-ray structures of oedema factor with and without bound calmodulin. Oedema factor shares no significant structural homology with mammalian adenylyl cyclases or other proteins. In the active site, 3'-deoxy-ATP and a single metal ion are well positioned for catalysis with histidine 351 as the catalytic base. This mechanism differs from the mechanism of two-metal-ion catalysis proposed for mammalian adenylyl cyclases. Four discrete regions of oedema factor form a surface that recognizes an extended conformation of calmodulin, which is very different from the collapsed conformation observed in other structures of calmodulin bound to effector peptides. On calmodulin binding, an oedema factor helical domain of relative molecular mass 15,000 undergoes a 15 A translation and a 30 degrees rotation away from the oedema factor catalytic core, which stabilizes a disordered loop and leads to enzyme activation. These allosteric changes provide the first molecular details of how calmodulin modulates one of its targets.     

Karyopherin-mediated import of integral inner nuclear membrane proteins

Targeting of newly synthesized integral membrane proteins to the appropriate cellular compartment is specified by discrete sequence elements, many of which have been well characterized. An understanding of the signals required to direct integral membrane proteins to the inner nuclear membrane (INM) remains a notable exception. Here we show that integral INM proteins possess basic sequence motifs that resemble 'classical' nuclear localization signals. These sequences can mediate direct binding to karyopherin-alpha and are essential for the passage of integral membrane proteins to the INM. Furthermore, karyopherin-alpha, karyopherin-beta1 and the Ran GTPase cycle are required for INM targeting, underscoring parallels between mechanisms governing the targeting of integral INM proteins and soluble nuclear transport. We also provide evidence that specific nuclear pore complex proteins contribute to this process, suggesting a role for signal-mediated alterations in the nuclear pore complex to allow for passage of INM proteins along the pore membrane.     

一类积分算子族的正性

本文在L2 空间研究迁移理论中出现的一类积分算子———peierls积分算子的正性 ,研究表明 :存在λ≤ 0 ,使得当λ≥λ0 时 ,peierls积分算子是正算子 ,当λ<λ0 时 ,peierls积分算子是不定的。     

Cell-surface labelling reveals no evidence for membrane assembly and disassembly during fibroblast locomotion.

As a cultured fibroblast moves, particles or pieces of debris attached to its surface move backwards with respect to both the cell and the substrate1,2, as also do concanavalin A (Con A) receptors in the membrane of the leading lamella3. One suggested explanation is that membrane components from the cytoplasm are assembled and introduced into the surface membrane of the moving cell close to its leading edge and flow backwards to a 'sink'; there the membrane is disassembled, and its components enter the cytoplasm and flow forward within the cell, to be re-introduced later into the surface membrane1,4. However, because cell-surface receptors can be redistributed as a result of cross-linking by external ligands5,6 it has been proposed that the backward flow of Con A receptors and particles may result from such a cross-linking rather than from a flow of intact cell membrane7,8. To investigate these alternatives, I have studied moving fibroblasts by means of a cell membrane label that does not induce the redistribution of its receptors. My results do not seem compatible with the membrane flow model.     

Structural basis for vinculin activation at sites of cell adhesion

Vinculin is a highly conserved intracellular protein with a crucial role in the maintenance and regulation of cell adhesion and migration. In the cytosol, vinculin adopts a default autoinhibited conformation. On recruitment to cell-cell and cell-matrix adherens-type junctions, vinculin becomes activated and mediates various protein-protein interactions that regulate the links between F-actin and the cadherin and integrin families of cell-adhesion molecules. Here we describe the crystal structure of the full-length vinculin molecule (1,066 amino acids), which shows a five-domain autoinhibited conformation in which the carboxy-terminal tail domain is held pincer-like by the vinculin head, and ligand binding is regulated both sterically and allosterically. We show that conformational changes in the head, tail and proline-rich domains are linked structurally and thermodynamically, and propose a combinatorial pathway to activation that ensures that vinculin is activated only at sites of cell adhesion when two or more of its binding partners are brought into apposition.     

Structural and serological evidence for a novel mechanism of antigenic variation in foot-and-mouth disease virus.

Changes resulting in altered antigenic properties of viruses nearly always occur on their surface and have been attributed to the substitution of residues directly involved in binding antibody. To investigate the mechanism of antigenic variation in foot-and-mouth disease virus (FMDV), variants that escape neutralization by a monoclonal antibody have been compared crystallographically and serologically with parental virus. FMDVs form one of the four genera of the Picornaviridae. The unenveloped icosahedral shell comprises 60 copies each of four structural proteins VP1-4. Representatives from each of the genera have similar overall structure, but differences in the external features. For example, human rhinovirus has a pronounced 'canyon' that is proposed to contain the cell attachment site, whereas elements of the attachment site for FMDV, which involves the G-H loop (residues 134-160) and C-terminus (200-213) of VP1, are exposed on the surface. Moreover, this G-H loop, which is a major antigenic site of FMDV, forms a prominent, highly accessible protrusion, a feature not seen in other picornaviruses. It is this loop that is perturbed in the variant viruses that we have studied. The amino acid mutations characterizing the variants are not at positions directly involved in antibody binding, but result in far-reaching perturbations of the surface structure of the virus. Thus, this virus seems to use a novel escape mechanism whereby an induced conformational change in a major antigenic loop destroys the integrity of the epitope.     

基于整体结构的纳米晶体材料力学模型

为了系统地分析晶粒尺寸、应变速率和缺陷对纳米晶体材料的影响,提出了1个新的本构模型,运用能量法描述纳米晶体的变形机理、微结构演变和力学行为.在模型中,晶粒和晶界作为一个整体共同承担位错和晶界滑移.通过对纳米晶体Ni在不同应变速率和晶粒尺寸条件下的实验,验证改进模型的可行性.与实验数据对比发现,预测的模型可以用来描述纳米晶体材料的力学性能.     

Electron microscope evidence for an 80 A unit in collagen fibrils.

Connective tissues are composite structures containing collagen, elastin, glycosaminoglycans, minerals, water and other minor components. In all cases the collagen exists predominantly in fibrillar form. The size distribution of the fibrils does, however, vary markedly with both age and the mechanical requirements of the tissue. Little is known about the mechanism of fibril formation in vivo, although some information is now available form in vitro polymerisation studies. We have now collected new and extensive electron microscope data on the size of collagen fibrils from tendon, skin, cornea and other tissues from both fetal and immature animals. The results reported here show that the diameters of the collagen fibrils thus measured lie close to a multiple of 80 A, a result which may be simply and readily interpreted in terms of the collagen microfibril and its mode of packing.     

弹性支承柱对整体张拉膜结构的影响

索膜结构分析中将膜边界视为固接,未考虑支承结构与膜结构之间的相互作用以及支承结构在该节点的位移,使找形结果与实际情况产生一定差异,即缺少整体张拉模型的分析.考虑弹性支承结构对膜结构的应力以及自振频率的影响,采用ANSYS模拟分析弹性支承柱与膜结构相互作用过程.结果表明,膜结构在支座提升(施加预应力)阶段基本符合"膜面预应力处处相等"的原则,由"边缘效应"产生的膜面应力不均匀分布也可以通过支承柱的设置来给予适当的调整.     

一个反常积分收敛性的剖析

反常积分是数学分析课程中一个比较难掌握的内容,在<数学分析>教材(华东师范大学数学系编,第三版)中有一道反常积分的题目,在一些数学分析解题参考书中都给出了含有错误的解法,本文对此给予详细讨论,以期能给学生一个正确的指导.     

开口弧上一类奇异积分方程的解关于积分曲线摄动的稳定性

讨论了开口弧上的一类奇异积分方程的解在积分曲线L发生光滑摄动时的稳定性.借助于Cauchy核奇异积分,证明了当指标不小于零时,方程的解是稳定的,当指标小于零时,给出拟解的概念并讨论了拟解的稳定性.     

纤维素在炭化和活化过程中的结构变化

以纤维素为原料,通过在氮气氛下炭化和水蒸气活化得到纤维素基炭.采用热分析、傅里叶红外光谱、X射线衍射及低温N2吸附测试手段研究了纤维素的炭化和活化过程以及过程中炭微晶结构和比表面积的变化.纤维素分子结构中的C—OH、C—O—C、C—H等基团在280~380℃之间大量分解,380℃后少量裂解产生的小分子碎片或基团持续分解,同时碳元素发生结构重排,形成石墨微晶.炭化温度是影响纤维素基活性炭微晶结构及孔结构的关键因素,随炭化温度的升高,石墨微晶尺寸变大,孔结构得到发育,但活性炭的比表面积则呈先增加后下降趋势,当炭化温度为600℃时所得活性炭比表面积最大;炭化时间对炭微晶结构及比表面积的影响不显著;随着活化时间的延长,先是炭结构中的非微晶碳被氧化,比表面积及总孔容积变大,然后微晶碳被氧化,微晶结构被破坏,炭中部分微孔变成中孔或大孔,导致比表面积及总孔容积变小,当微晶间的非微晶碳被充分氧化而又不破坏原微晶结构时得到的炭孔隙最丰富.