Processive movement of MreB-associated cell wall biosynthetic complexes in bacteria

The peptidoglycan cell wall and the actin-like MreB cytoskeleton are major determinants of cell shape in rod-shaped bacteria. The prevailing model postulates that helical, membrane-associated MreB filaments organize elongation-specific peptidoglycan-synthesizing complexes along sidewalls. We used total internal reflection fluorescence microscopy to visualize the dynamic relation between MreB isoforms and cell wall synthesis in live Bacillus subtilis cells. During exponential growth, MreB proteins did not form helical structures. Instead, together with other morphogenetic factors, they assembled into discrete patches that moved processively along peripheral tracks perpendicular to the cell axis. Patch motility was largely powered by cell wall synthesis, and MreB polymers restricted diffusion of patch components in the membrane and oriented patch motion.     

Magnetosomes are cell membrane invaginations organized by the actin-like protein MamK

Magnetosomes are membranous bacterial organelles sharing many features of eukaryotic organelles. Using electron cryotomography, we found that magnetosomes are invaginations of the cell membrane flanked by a network of cytoskeletal filaments. The filaments appeared to be composed of MamK, a homolog of the bacterial actin-like protein MreB, which formed filaments in vivo. In a mamK deletion strain, the magnetosome-associated cytoskeleton was absent and individual magnetosomes were no longer organized into chains. Thus, it seems that prokaryotes can use cytoskeletal filaments to position organelles within the cell.     

Cryo-electron tomography reveals the cytoskeletal structure of Spiroplasma melliferum

Evidence has accumulated recently that not only eukaryotes but also bacteria can have a cytoskeleton. We used cryo-electron tomography to study the three-dimensional structure of Spiroplasma melliferum cells in a close-to-native state at approximately 4-nanometer resolution. We showed that these cells possess two types of filaments arranged in three parallel ribbons underneath the cell membrane. These two filamentous structures are built of the fibril protein and possibly the actin-like protein MreB. On the basis of our structural data, we could model the motility modes of these cells and explain how helical Mollicutes can propel themselves by means of coordinated length changes of their cytoskeletal ribbons.     

Rotational movement of the formin mDia1 along the double helical strand of an actin filament

Formin homology proteins (formins) elongate actin filaments (F-actin) by continuously associating with filament tips, potentially harnessing actin-generated pushing forces. During this processive elongation, formins are predicted to rotate along the axis of the double helical F-actin structure (referred to here as helical rotation), although this has not yet been definitively shown. We demonstrated helical rotation of the formin mDia1 by single-molecule fluorescence polarization (FL(P)). FL(P) of labeled F-actin, both elongating and depolymerizing from immobilized mDia1, oscillated with a periodicity corresponding to that of the F-actin long-pitch helix, and this was not altered by actin-bound nucleotides or the actin-binding protein profilin. Thus, helical rotation is an intrinsic property of formins. To harness pushing forces from growing F-actin, formins must be anchored flexibly to cell structures.     

The machinery of mitochondrial inheritance and behavior

The distribution of mitochondria to daughter cells during cell division is an essential feature of cell proliferation. Until recently, it was commonly believed that inheritance of mitochondria and other organelles was a passive process, a consequence of their random diffusion throughout the cytoplasm. A growing recognition of the reticular morphology of mitochondria in many living cells, the association of mitochondria with the cytoskeleton, and the coordinated movements of mitochondria during cellular division and differentiation has illuminated the necessity for a cellular machinery that mediates mitochondrial behavior. Characterization of the underlying molecular components of this machinery is providing insight into mechanisms regulating mitochondrial morphology and distribution.     

棉花纤维品质改良相关基因研究进展

棉纤维是优良的、使用最为广泛的天然纤维。随着人们生活水平的提高,对天然纯棉织物的需求不断增加,同时对品质的要求也愈来愈高。因此,提高棉纤维产量和品质成为当前棉花遗传育种的重要目标,对棉纤维发育相关基因的克隆与功能研究是实现这一目标的重要基础。棉纤维发育由4个明显但又重叠的时期组成,包括纤维细胞的起始、伸长(初生壁合成)、次生壁合成和脱水成熟。起始是影响纤维细胞数量的重要时期,而纤维长度和强度的决定发生在次生壁合成期和脱水成熟期。棉纤维发育是一个复杂而有序的过程,由大量的基因参与调控。目前,已经有一些在棉纤维发育过程中发挥重要作用的基因被报道,包括各种转录因子、参与激素代谢基因、编码细胞壁蛋白和细胞骨架蛋白基因、活性氧代谢相关基因、以及参与糖和脂类代谢的基因等。文中对已报道的这些与棉花纤维发育相关基因的克隆和功能分析进行了系统总结,以期为棉花纤维发育及品质改良研究提供参考。     

草菇不同发育时期子实体菌柄细胞形态变化研究

测量了草菇子实体不同发育时期菌柄细胞的长与宽,同时通过对蛋形期菌柄标记,观察了菌柄不同部位的伸长速率,结果显示：菌柄细胞从蛋形期开始伸长,在伸长期的伸长速度显著高于其它发育时期,同时菌柄中上部的细胞伸长远多于菌柄底部细胞的伸长,并决定着整个菌柄的生长。     

Cyclic adenosine monophosphate stimulation of axonal elongation

Elevated concentrations of adenosine 3',5'-monophosphate induce a variety of cell movements. The role of adenosine 3',5'-monophosphate in promoting those movements associated with growth prompted our study of in vitro microtubule-dependent axonal elongation. Ganglia treated with adenosine 5'-monophosphate show no enhancement over controls; treatment with adenosine 3', 5'monophosphate or its dibutyryl derivative significantly enhances elongation, as measured by increases in both axonal numbers and length. Our study suggests that adenosine 3',5'-monophosphate promotes elongation by stimulation of microtubule assembly.     

MreB在鱼腥蓝细菌PCC 7120细胞分裂过程中的功能

为研究Anabaena sp. PCC 7120中的细胞形态决定蛋白MreB的亚细胞定位和功能,构建了由mreB自身启动子驱动的mreB-gfp融合载体,通过接合转移的方法将其转化到野生型Anabaena sp. PCC 7120中,获得绿色荧光蛋白标记的MreB的鱼腥蓝细菌菌株,同时用MreB的抑制剂A22处理菌株。结果显示,MreB的亚细胞定位随着细胞周期的进程而发生变化,A22处理可导致MreB无法正确聚合定位。     

BRITTLE CULM16(BRITTLE NODE) is required for the formation of secondary cell walls in rice nodes

Plant cell walls constitute the skeletal structures of plant bodies,and thus confer lodging resistance for grain crops.While the basic cell wall synthesis machinery is relatively well established now,our understanding of how the process is regulated remains limited and fragmented.In this study,we report the identification and characterization of the novel rice(Oryza sativa L.)brittle culm16(brittle node;bc16)mutant.The brittle node phenotype of the bc16 mutant appears exclusively at nodes,and resembles the previously reported bc5 mutant.Combined histochemical staining and electron microscopy assays revealed that in the bc16 mutant,the secondary cell wall formation and thickening of node sclerenchyma tissues are seriously affected after heading.Furthermore,cell wall composition assays revealed that the bc16 mutation led to a significant reduction in cellulose and lignin contents.Using a map-based cloning approach,the bc16 locus is mapped to an approximately 1.7-Mb region of chromosome 4.Together,our findings strengthen evidence for discretely spatial differences in the secondary cell wall formation within plant bodies.     

亚麻快速生长期细胞壁形成相关基因的表达分析

【目的】亚麻在快速生长期其韧皮纤维细胞发育在SP(the snap point)点上下端分别经历细胞伸长和次生细胞壁加厚2个不重叠时期。研究亚麻快速生长期不同组织、不同时期、不同器官中与细胞壁形成相关的β-半乳糖苷酶(Lu BGALs)、纤维素合酶(Lu CESAs)等家族基因的表达谱,探讨快速生长期亚麻韧皮纤维细胞细胞壁的发育模式,为改善亚麻纤维产量提供理论依据。【方法】以生长45 d的亚麻根、茎韧皮纤维、叶为材料,用透射电镜观察并测量茎韧皮纤维细胞细胞壁结构和厚度,采用实时荧光定量(q RT-PCR)方法,研究亚麻快速生长期Lu BGALs和Lu CESAs等细胞壁形成相关的基因在亚麻韧皮纤维不同阶段的表达特点。【结果】在SP点上部TOP端纤维细胞细胞壁薄,约110 nm;紧邻SP点茎中部的MID区(约500 nm)和茎下部的BOT区(约650 nm),细胞壁厚度明显增厚,细胞壁质地均一,没有明显的分层现象,说明SP点中部和下部韧皮纤维细胞细胞壁已经开始加厚但还未进入次生壁加厚阶段,与TOP明显不同。亚麻Lu BGAL1在TOP区的表达显著低于MID区和BOT区,表明其主要参与纤维细胞细胞壁加厚过程。而Lu BGAL3、Lu BGAL6、Lu BGAL9在TOP区表达最高,MID区次之,表明此类基因主要参与亚麻韧皮细胞伸长和细胞壁重建过程。Lu BGAL5在幼嫩的TOP区表达量高,在亚麻茎较为成熟的MID区较低,说明Lu BGAL5在细胞壁形成过程中起作用。其他BGALs基因的表达量均较低。在亚麻茎幼嫩的TOP区纤维细胞中,亚麻纤维素合酶基因Lu CESA1、Lu CESA3、Lu CESA7、Lu CESA8、Lu CESA9和Lu CESA10都检测出较高的表达量,且明显高于其在MID区和BOT区的表达。其中Lu CESA3和Lu CESA10在MID区的表达显著低于BOT区,其他几个CESAs基因在MID和BOT的表达并无明显差异。结合这些基因在亚麻快速生长期不同器官中的表达模式,结果说明,亚麻中6个CESA(Lu CESA1、Lu CESA3、Lu CESA7、Lu CESA8、Lu CESA9和Lu CESA10)主要促进亚麻韧皮纤维细胞的伸长。Lu Su Sy在幼茎韧皮纤维细胞中表达量高,表明亚麻茎伸长和加粗需要大量能量。Lu XTH4在亚麻细胞壁发育过程中发挥作用。【结论】快速生长期亚麻茎韧皮纤维细胞细胞壁没有次生加厚过程;Lu BGAL3、Lu BGAL5、Lu BGAL6、Lu BGAL9、Lu CESA1、Lu CESA3、Lu CESA9和Lu CESA10在亚麻细胞壁细胞伸长过程中起作用;Lu BGAL1主要促进亚麻细胞壁加厚过程;Lu Su Sy和Lu XTH4在亚麻细胞壁发育中发挥作用。     

Actin polymerization-driven molecular movement of mDia1 in living cells

mDia1, a Rho effector, belongs to the Formin family of proteins, which shares the conserved tandem FH1-FH2 unit structure. Formins including mDia1 accelerate actin nucleation while interacting with actin filament fast-growing ends. Here our single-molecule imaging revealed fast directional movement of mDia1 FH1-FH2 for tens of microns in living cells. The movement of mDia1 FH1-FH2 was blocked by actin-perturbing drugs, and the speed of mDia1 FH1-FH2 movement appeared to correlate with actin elongation rates. In vitro, mDia1 FH1-FH2 associated persistently with the growing actin barbed end. mDia1 probably moves processively along the growing end of actin filaments in cells, and Formins may be a molecular motility machinery that is independent from motor proteins.     

计算机辅助农业机械运动状态分析的多媒体系统

为了研究农业机械设计的最佳参数或参数范围,应用计算机的机构运动状态模拟分析方法和多媒体技术,开发了计算机辅助农业机械动态分析的多媒体系统,该系统将机构运动分析参数化,能更大地提高机构运动分析的效率和精确度,还将运动分析方法的全过程生动地展示出来.既可用于农业机械等机械设计中也可作为机构运动状态分析的辅助教学软件.     

Helical filaments produced by a Mycoplasma-like organism associated with corn stunt disease

Mycoplasma-like bodies with helical filaments were seen by phase contrast microscopy in juice expressed from tissues of plants infected with corn stunt agent. Each filament is bounded by a "unit membrane" and no cell wall, sheath, envelope, or second membrane has yet been discerned by electron microscopy. The association of these filaments with development of disease, their occurrence in phloem cells as seen by both freeze-etching and thin-section electron microscopy, the diagnosis of infection based on their presence in plants without symptoms, and their absence in noninfected corn are consistent with the hypothesis that these unusual filaments are formed by the mycoplasma-like organism presumed to be the corn stunt agent.     

Chemokine signaling controls endodermal migration during zebrafish gastrulation

Directed cell movements during gastrulation establish the germ layers of the vertebrate embryo and coordinate their contributions to different tissues and organs. Anterior migration of the mesoderm and endoderm has largely been interpreted to result from epiboly and convergent-extension movements that drive body elongation. We show that the chemokine Cxcl12b and its receptor Cxcr4a restrict anterior migration of the endoderm during zebrafish gastrulation, thereby coordinating its movements with those of the mesoderm. Depletion of either gene product causes disruption of integrin-dependent cell adhesion, resulting in separation of the endoderm from the mesoderm; the endoderm then migrates farther anteriorly than it normally would, resulting in bilateral duplication of endodermal organs. This process may have relevance to human gastrointestinal bifurcations and other organ defects.     

家蝇幼虫抗菌肽MDL-2干扰细菌功能的初步研究

研究了家蝇幼虫抗菌肽MDL-2对细菌细胞壁的溶解作用、细胞膜渗透性和代谢的影响.抗菌肽MDL-2在抗菌过程中首先与细菌的细胞壁相互作用,使其破裂.菌肽对革兰氏阴性细菌大肠杆菌的细胞壁的作用有浓度依赖性,而对革兰氏阳性细菌金黄色葡萄球菌,MDL-2在较低的浓度时即具有破坏细胞壁的作用;抗菌肽MDL-2与金黄色葡萄球菌的细胞膜作用曲线呈现典型的S型,有较强的协同效应,并随着作用时间的延长细胞的渗透性增强;抗菌肽对细菌的代谢功能有较强的干扰作用,阻碍菌体的生命活动,加速菌体死亡.     

Cortical flow in animal cells

A concerted flow of actin filaments associated with the inner face of the plasma membrane may provide the basis for many animal cell movements. The flow is driven by gradients of tension in the cell cortex, which pull cortical components from regions of relaxation to regions of contraction. In some cases cortical components return through the cytoplasm to establish a continuous cycle. This cortically located motor may drive cell locomotion, growth cone migration, the capping of antigens on a lymphocyte surface, and cytokinesis.     

Multienzyme systems of DNA replication

Replication is accomplished by multienzyme systems whose operations are usefully considered in respect to three stages of the process: initiation, elongation, anid termination. 1) Initiation entails synthesis of a short RNA fragment that serves as primer for the elongation step of DNA synthesis. This stage, probed by SS phage DNA templates, reveals three distinctive and highly specific systems in E. coli. The Ml3 DNA utilizes RNA polymerase in a manner that may reflect how plasmid elements are replicated in the cell. The ?X174 DNA does not rely on RNA-polymerase, but requires instead five distinctive proteins which may belong to an apparatus for initiating a host chromosome replication cycle at the origin. The G4 DNA, also independent of RNA polymerase, needs simply the dnaG protein for its distinctive initiation and may thus resemble the system that initiates the replication fragments at the nascent growing fork. In each case it is essential that in vitro the DNA-unwinding protein coat the viral DNA and influence its structure. 2) Elongation is achieved in every case by the multisubunit, holoenzyme form of DNA polymerase III. Copolymerase III, which is an enzyme subunit, and adenosine triphosphate are required to form a proper complex with the primer template but appear dispensable for the ensuing chain growth by DNA polymerase (33). 3) Termination requires excision of the RNA priming fragment, filling of gaps and sealing of interruptions to produce a covalently intact phosphodiester backbone. DNA polymerase I has the capacity for excision and gapfilling and DNA ligase is required for sealing. What once appeared to be a simple DNA polymerase-mediated conversion of a single-strand to a duplex circle (34) is now seen as a complex series of events in which diverse multienzyme systems function. Annoyance with the difficulties in resolving and reconstituting these systems is tempered by the conviction that these are the very systems used ,by the cell in replicating its chromosome and extrachromosomal elements. Thus, understanding of the regulation of replication events in the cell, their localization at membrane surfaces and integration with cell division, and their coordination with phage DNA maturation and particle assembly will all be advanced by knowledge of the components of the replicative machinery.     

Direct redox regulation of F-actin assembly and disassembly by Mical

Different types of cell behavior, including growth, motility, and navigation, require actin proteins to assemble into filaments. Here, we describe a biochemical process that was able to disassemble actin filaments and limit their reassembly. Actin was a specific substrate of the multidomain oxidation-reduction enzyme, Mical, a poorly understood actin disassembly factor that directly responds to Semaphorin/Plexin extracellular repulsive cues. Actin filament subunits were directly modified by Mical on their conserved pointed-end, which is critical for filament assembly. Mical posttranslationally oxidized the methionine 44 residue within the D-loop of actin, simultaneously severing filaments and decreasing polymerization. This mechanism underlying actin cytoskeletal collapse may have broad physiological and pathological ramifications.