Crystalline order to high resolution in the sheath of Methanospirillum hungatei: a cross-beta structure

Electron microscopy and electron diffraction indicate that the outer sheath of the cell wall of the archaebacterium Methanospirillum hungatei contains a two-dimensional crystalline lattice having, at least to low resolution, p2 symmetry in projection with a = 5.66 nm, b = 2.81 nm and gamma = 85.6 degrees. At a resolution of 2 nm, the unit cell contains two lobes, whereas high-angle electron diffraction shows the presence of a substantial quantity of beta structure, with the 0.47 nm spacing (between polypeptide chains within a sheet) oriented circumferentially. The sheath is unusual when compared to other regular surface arrays found on bacteria in that it is a compact structure with small subunits. It may have a structural role analogous to barrel hoops since it tends to fragment perpendicular to its axis to give rings or hoops.     

Crystalline fibril structure of type II collagen in lamprey notochord sheath

We report here the existence of a crystalline molecular packing of type II collagen in the fibrils of the lamprey notochord sheath. This is the first finding of a crystalline structure in any collagen other than type I.The lamprey notochord sheath has a composition similar to that of cartilage, with type II collagen, a minor collagen component with 1α, 2α and 3α chains, and cartilage-like proteoglycan. The high degree of orientation of fibrils in the notochord makes it possible to use X-ray diffraction to determine collagen fibril organization in this type II-containing tissue. The low angle equatorial scattering shows the fibrils are all about 17 nm in diameter and have an average center-to-center separation of 31 nm. These results are supported by electron microscope observations. A set of broad equatorial diffraction maxima at higher angles represents the sampling of the collagen molecular transform by a limited crystalline lattice, extending over a lateral dimension close to the diameter of one fibril. This indicates that each 17 nm fibril contains a crystalline array of molecules and, although a unit cell is difficult to determine because of the broad overlapping reflections, it is clear that the quasi-hexagonal triclinic unit cell of type I collagen in rat tail tendon is not consistent with the data. The meridional diffraction pattern showed 26 orders with the characteristic 67 nm periodicity found for tendon. However, the intensities of these reflections differ markedly from those found for tendon and cannot be explained by an unmodified gap/ overlap model within each 67 nm period. Both X-ray diffraction and electron microscope data indicate a low degree of contrast along the fibril axis and are consistent with a periodic binding of a non-collagenous component in such a way as to obscure the gap region.     

MICROCYLINDERS WITHIN MITOCHONDRIAL CRISTAE IN THE RAT PINEALOCYTE

A minute cylindrical structure with a dense core, designated as "microcylinder," has been observed within enlarged spaces of mitochondrial cristae in pinealocytes of some adult rats (osmium tetroxide fixed, methacrylate embedded). The microcylinders are 270 to 330 A in diameter and of indeterminate length. Their wall is found to be made up of slender filamentous subunits, probably 6 in number, and surrounds a central filament. The microcylinders are arranged parallel to one another, forming monolayered or, more frequently, multilayered aggregates. Their number within a crista varies considerably. Packets of microcylinders may be seen located in the outer mitochondrial chamber, but are never found in the mitochondrial matrix. They have been observed neither in other cell types of the pineal gland nor in neurons, ependymal, and glial cells of the nearby epithalamic tissue. The origin and nature of the microcylinders are unknown. Glycogen-like particles have been encountered, though very infrequently, accompanying bundles of microcylinders within mitochondrial cristae.     

Ultrastructure and partial characterization of a regular array in the cell wall of Lactobacillus brevis.

The cell wall of Lactobacillus brevis was revealed by electron microscopy to have an outer layer composed of a regular array. The morphological unit of the regular array appeared to consist of four spherical subunits, each about 2 nm in diameter, which were arranged in a tetragonal pattern about 4.5 by 7.0 nm in dimension. The regular array was composed of the tetragonal units in rows in two directions at an angle of about 75 degrees to each other. The average spacing between the rows was about 10 nm in one direction and about 7 nm in the other. The tetragonally arranged subunits were removed from the cell wall by treatment with guanidine hydrochloride, urea, or sodium dodecyl sulfate (SDS) but not by the action of ethylenediaminetetraacetate, nonionic detergents, or proteolytic enzymes except pepsin. The regular subunits were shown to be composed of a protein with a molecular weight of about 51,000 by SDS-polyacrylamide gel electrophoresis.     

Heliobacterium chlorum: cell organization and structure

The basic cellular organization of Heliobacterium chlorum is described using the freeze-etching technique. Internal cell membranes have not been observed in most cells, leading to the conclusion that the photosynthetic apparatus of these organisms must be localized in the cell membrane of the bacterium. The two fracture faces of the cell membrane are markedly different. The cytoplasmic (PF) face is covered with densely packed particles averaging 8 nm in diameter, while the exoplasmic (EF) face contains far fewer particles, averaging approximately 10 nm in diameter. Although a few differentiated regions were noted within these fracture faces, the overall appearance of the cell membrane was remarkably uniform. The Heliobacterium chlorum cell wall is a strikingly regular structure, composed of repeating subunits arranged in a rectangular pattern at a spacing of 11 nm in either direction. We have isolated cell wall fragments by brief sonication in distilled water, and visualized the cell wall structure by negative staining as well as deep-etching.Abbreviations PF protoplasmic fracture face - EF exoplasmic fracture face     

感染病毒的小麦全蚀菌的超微结构

将感染病毒的小麦全蚀菌山东烟台株培养 20天的菌体细胞,进行超微结构的研究。于电镜下观察到球状病毒颗粒,平均直径23—30nm,多是无规则松散的分布于胞质中;或紧密聚集于液泡、线粒体周围;或排列成线状;或7—8个颗粒排列成环状。病毒仅分布于细胞质中,细胞核、脂肪体内均未见病毒颗粒。病毒浓度在较老的菌体内有增加的趋势。全蚀菌的菌丝细胞壁有三层,外层电子致密内含纤维状物,内层电子较为透明,中层为一电子致密度很深的狭窄夹层。壁的厚度不均,外缘不规则;在菌丝体产生隔膜的早期阶段,于隔膜附近有1—3个外被膜结构的沃罗宁体 Woronin body,隔膜形成的后期,见电子致密物质沉积在核膜孔上,形成中的隔膜顶端为尖状突起向基部逐渐增宽略成金字塔形。     

The fine structure of the endogenous stages of Eimeria labbeana: 2. Mature macrogamonts and young oocysts.

The fine structure of the mature macrogamonts and intracellular oocysts of Eimeria labbeana from the ileal mucosa of experimentally infected Pigeons (Columbia livia) was investigated and described. The macrogamont reached a maximum size of 12.0 x 9.5 mum (average equals 10.8 x 8.8 mum), and was located within a narrow parasitophorus vacuole. Most of the macrogamonts were limited by two membranes. Intravacuolar tubules, 1.2 mum long and 58 nm in diameter, established direct connections between the parasite and the host cell. Each tabule was composed of 9 subunits arranged around the central lumen. Cytoplasmic canaliculi were composed of bundles of microtubule-like structures (8-10 nm wide). Type 1 wall-forming bodies reached a maximum size of 1.8 x 1.5 mum, and many had centric or eccentric electron transparent portions within them. They were frequently seen lodged within peripherally-located mitochondria. Type 2 wall-forming bodies averaged 1.5 mum in diameter. The role of the two types of wall-forming bodies in forming the outer and inner layers of the wall of the oocyst was similar to that in other species of Eimeria. The oocyst wall was 0.2 mum thick and composed of a limiting membrane (20 nm thick), an outer layer (75 nm thick), and an inner layer (100 nm thick).     

The structural features of beef heart mitochondrial creatine kinase.

Two forms of mitochondrial creatine kinase (Mi-CK) having Mr 320 kDa and 240 kDa as determined by gel-filtration on Sephacryl S-300 in 0.1 M Tris-HCl pH 7.4 were investigated. The sedimentation coefficient values for these two forms were found to be identical and equal to 12.3 S. When studied by electron microscopy the main type of images for the 320 kDa and 240 kDa Mi-CK appeared as annular particles, 12-14 nm in diameter, with a well-detected subunit structure and a central hollow, 3-4nm in diameter filled with the dye. The results of the averaging of the main type of individual Mi-CK images and particles of the two-dimensional crystal layer point to the overall geometry of the Mi-CK molecule structure as containing eight subunits arranged by a 4-fold symmetry around the central hollow. It may be that the eight identical subunits of crystalline Mi-CK are arranged with a P422 symmetry. However in both cases the averaged main images do not show a mirror symmetry. The multiplicity of the observed projections close to annular one provides additional evidence in favour of the great lability and structural mobility of the Mi-CK subunits. It allows to assume that two forms (320 kDa and 240 kDa) are not the different oligomers but they are two functionally distinct conformational states of octameric molecule of Mi-CK.     

Some observations on the fine structure of the giant nerve fibers of the earthworm,Eisenia foetida

Sectioned dorsal giant fibers of the earthworm Eisenia foetida have been studied with the electron microscope. The giant axon is surrounded by a Schwannian sheath in which the lamellae are arranged spirally. They can be traced from the outer surface of the Schwann cell to the axon-Schwann membranes. Irregularities in the spiral arrangement are frequently observed. Desmosome-like attachment areas occur on the giant fiber nerve sheath. These structures appear to be arranged bilaterally in columns which are oriented slightly obliquely to the long axis of the giant fiber and aligned linearly from the axon to the periphery of the sheath. At these sites they bind together apposing portions of Schwann cell membrane comprising the sheath. Longitudinal or oblique sections of the nerve sheath attachment areas are reminiscent of the Schmidt-Lantermann clefts of vertebrate peripheral nerve. Septa of the giant fibers have been examined. They are symmetrical or non-polarized and consist of the two plasma membranes of adjacent nerve units. Characteristic vesicular and tubular structures are associated with both cytoplasmic surfaces of these septa.     

Some Observations on the Fine Structure of the Giant Nerve Fibers of the Earthworm, Eisenia foetida

Sectioned dorsal giant fibers of the earthworm Eisenia foetida have been studied with the electron microscope. The giant axon is surrounded by a Schwannian sheath in which the lamellae are arranged spirally. They can be traced from the outer surface of the Schwann cell to the axon-Schwann membranes. Irregularities in the spiral arrangement are frequently observed. Desmosome-like attachment areas occur on the giant fiber nerve sheath. These structures appear to be arranged bilaterally in columns which are oriented slightly obliquely to the long axis of the giant fiber and aligned linearly from the axon to the periphery of the sheath. At these sites they bind together apposing portions of Schwann cell membrane comprising the sheath. Longitudinal or oblique sections of the nerve sheath attachment areas are reminiscent of the Schmidt-Lantermann clefts of vertebrate peripheral nerve. Septa of the giant fibers have been examined. They are symmetrical or non-polarized and consist of the two plasma membranes of adjacent nerve units. Characteristic vesicular and tubular structures are associated with both cytoplasmic surfaces of these septa.     

Three-dimensional structure of a cell wall glycoprotein

Electron microscopy and computer image analysis have been used to determine the three-dimensional structure of the crystalline glycoprotein cell wall layer of the alga Lobomonas piriformis. Images of negatively stained specimens, tilted through a range of angles up to 70 °, were combined to give a map of the molecular envelope to a resolution of 2.0 nm. The cell wall layer consists of crystalline plates the centres and edges of which display distinctly different but isomorphous structures. A comparison of three-dimensional reconstructions of the two areas shows the difference probably to be due to a conformational change of one of the glycoprotein subunits. The structure consists of two sets of dimers composed of rod-shaped subunits which lie with their long axes approximately in the plane of the crystal. The centre-edge transition may have significance in the pathway of accretion of new subunits during cell wall growth.     

ELECTRON MICROSCOPIC OBSERVATIONS ON TUBULAR STRUCTURES INVOLVED IN CRYSTAL FORMATION IN THE RED ALGA PLEONOSPORIUM SQUARRULOSUM (HARV.) ABBOTT1

Hexagonal or angular crystalline inclusions in Pleonosporium (Naeg.) Hauck vegetative cells were examined using electron microscopy. Ultrastructural analysis reveals that the inclusions initially contain tubular elements resembling microtubules but, with continued differentiation, are transformed into rod containing crystals. The tubular structures initially measure 25 nm in diameter. Scattered tubules become arranged in a parallel and alternate pattern and undergo subsequent enlargement to approximately 29 nm. Following enlargement, each tubule apparently disaggregates into rods that form a crystal having hexagonally arranged rod-like subunits. It is suggested that these tubules may represent microtubules and the resultant crystals are composed of tubulin.     

Dorsal giant fibre septum of earthworm. Fine structural details and further evidence for gap junctions

The fine structure of the dorsal giant fibre septa of earthworm (Lumbricus terrestris and Eisenia foetida) was studied by thin sectioning. E-PTA, BIUL and ZIO impregnation were carried out to characterize the various membrane specializations found. A septum, which is formed by two axon membranes only ca. 7 nm apart, is a heterogeneous structure showing a number of different specializations (intermediate junctions, dense projection-like humps, septate regions, vesicles quite often associated with a widened intercellular gap and membrane thickening). Septal areas referred to as septal complexes are of particular interest. They comprise up to 20% of septal cross-sections and are characterized by membrane appositions (diameter ca. 25 nm) which bridge the septal gap, protrude 16-25 nm into the respective axoplasms, and form a more or less hexagonal array with a centre-to-centre spacing of ca. 33 nm (ca. 29 nm after E-EPTA treatment) in en face sections. We interpret the septal complexes as gap junctions, i.e. sites of electronic coupling, and emphasize their unusually large dimension. The vesicles found at the septum could not be stained by ZIO treatment.     

Three-dimensional reconstruction of innermost chorion layer from Drosophila melanogaster

A low-resolution three-dimensional structure of the crystalline innermost chorion layer (ICL) has been calculated from electron microscope images of tilted negatively stained crystals. The isolated ICL is a single layer, about 12 nm thick and appears to be made up of two types of subunits, each approximately 3 nm in diameter, arranged regularly as groups of four heterodimers in space group C222. Linking density between these groups of subunits, maintaining the integrity of the layer, appears to be confined mainly to the outer surfaces of the ICL.     

Three-dimensional reconstruction of innermost chorion layer from Drosophila melanogaster

A low-resolution three-dimensional structure of the crystalline innermost chorion layer (ICL) has been calculated from electron microscope images of tilted negatively stained crystals. The isolated ICL is a single layer, about 12 nm thick and appears to be made up of two types of subunits, each approximately 3 nm in diameter, arranged regularly as groups of four heterodimers in space group C222. Linking density between these groups of subunits, maintaining the integrity of the layer, appears to be confined mainly to the outer surfaces of the ICL.     

On the egg investments and fertilization reaction inPomatoceros triqueter L.

Summary After a specific time of glutaraldehyde-acrolein fixation, microtubule walls appear to be composed of single 6.5–7.5 nm diameter osmiophilic subunits. Variations in the duration of glutaraldehyde-acrolein and also glutaraldehyde-osmium fixation reveal a two layered wall containing osmiophilic subunits, 4.0–4.5 nm in diameter, arranged radially, in tandem. The double-layered wall is demonstrated by microdensitometer traces. These observations are discussed in relation to previously proposed models of microtubule substructure.     

A new interpretation of plasmodesmatal ultrastructure

Summary It is shown that simple, unbranched, plasmodesmata between young xylem ray cells of willow have no direct intercellular continuity apart from the plasmalemma which limits the cytoplasm and lines the plasmodesmatal canal. Each plasmodesma is traversed by a 200 Å diameter tubule (the desmotubule) which has a wall with probably 11 subunits arranged around a central cavity through which runs a 40 Å diameter rod. This rod is connected to the inside of the tubule wall, by fine filaments. At the ends of each plasmodesma the plasmalemma and cell wall are closely appressed to the tubule, thus precluding direct continuity between the cytoplasm of adjacent cells. Through the central part of the plasmodesmata the tubule is separated from the plasmalemma by a 90–100 Å wide gap. Cytoplasmic microtubules in the same tissue have a diameter of approximately 250 Å and a wall probably composed of 13 subunits: both desmotubules and cytoplasmic microtubules therefore have a centre-to-centre subunit spacing of about 47 Å. It is suggested that the desmotubules are not microtubules but may be nuclear spindle fibres which become trapped in the wall during cell plate formation. The endoplasmic reticulum, while closely approaching the plasmodesmata, is not continuous across them. It is thought most unlikely that the endoplasmic reticulum traverses plasmodesmata, as the dimensions of the central tubule — found here as well as by other workers — are smaller than those which would be expected to allow a stable molecular configuration in a unit membrane. The plasmalemma, where it lines the plasmodesmatal canal, appears to have particulate subunits in the outer opaque layers and the presence of these subunits may be attributable to the need for stability in membranes arranged about so small a radius.     

Changes in the distribution of filament-containing septate junctions as coelenterate myoepithelial cells change shape

This paper describes the redistribution of septate junctions during an increase in diameter of myoepithelial cells from mesenteries of the sea anemone Metridium senile (L). Each septum was composed of a filament core, 9.5-10.2 nm in diameter, which had a double row of lateral projections from each side to the adjacent cell membrane. Septa were arranged in patches in which neighbouring septa lay parallel, 28-33 nm apart. When anaesthetized mesenteries were stretched, myoepithelial cell layers decreased from a mean of 32 to 8 micron thick; each cell shortened and its apical diameter increased. The integrity of the septate junctions was, however, maintained. The mean perimeter of septate junctions, corresponding to that of the cells, increased from 20 to 31 micron; mean depth decreased from 3.7 to 2.1 micron. There was no significant change in spacing between septa. Patches of septa, free to move in a fluid matrix of junction cell membranes, may form mobile attachment sites between cells, thus allowing those cells to change shape. Number and distribution density of microvilli decreased when cell diameter increased. This implies that the microvilli contribute membrane to the cell surface as its surface area increases. Gastrodermal cells are compared with epidermal cells that do not undergo dramatic changes in diameter.     

Electron microscopy of seed-storage globulins

The quaternary structures of a range of seed globulins, including examples of both the so-called 7 S and 11 S types, have been examined by electron microscopy. The legume 7 S proteins, phaseolin (bean), beta-conglycinin (soybean), and vicilin (pea), appear as flat discs of diameter ca. 8.5 nm and thickness ca. 3.5 nm formed by association of three subunit domains. Phaseolin converts to an 18 S tetramer at acid pH, and images recorded under these conditions suggest that four of the 7 S protomer discs associate to form the faces of a regular tetrahedron. The classical 11 S seed globulins, cucurbitin (pumpkin) and legumin (pea), are approximately spherical molecules of diameter ca. 8.8 nm composed of six subunits. In contrast, the hexameric 10 S storage protein from lupin seed, conglutin gamma, appears toroidal in shape with outer diameter ca. 10.3 nm and thickness ca. 2.2 nm. These results indicate that constraints imposed on seed proteins by their role in sustaining the germinating plant may have allowed a variety of different globulin structures to accumulate in the protein-storage bodies of seeds.     

Electron microscopy of nickel-containing methanogenic enzymes: methyl reductase and F420-reducing hydrogenase.

Methanogens catalyze the hydrogen-dependent eight-electron reduction of carbon dioxide to methane. Two of the key catalysts in the eight-electron reduction pathway are the nickel-containing enzymes F420-reducing hydrogenase and methyl reductase. In the present study, the structures of these archaebacterial enzymes from Methanobacterium thermoautotrophicum delta H have been determined by electron microscopy. By negative stain techniques, F420 hydrogenase was found to be a ring structure with a diameter of 15.7 nm and an inner channel 4 nm in diameter. Shadow-casting experiments demonstrated that the rings were 8.5 nm deep, indicating a holoenzyme molecular weight of 8.0 X 10(5). Methyl reductase appeared to be an oligomeric complex of dimensions 8.5 by 9 by 11 nm, with a central stain-penetrating region. The morphology and known subunit composition suggest a model in which the subunits are arranged as an eclipsed pair of open trimers. Methyl reductase was also found in the form of larger aggregates and in paracrystalline arrays derived from highly concentrated solutions. The extremely large size of F420 hydrogenase and the methyl reductase supramolecular assemblies may have relevance in vivo in the construction of multiprotein arrays that function in methane biogenesis.