Virus electron micrographs

Figure 16.2 The icosahedron (top) and dodecahedron (bottom) have identical symmetries but different shapes. Protein subunits of spherical viruses form a coat around the nucleic acid with the same symmetry arrangement as these geometrical objects. Electron micrographs of these viruses have shown that their shapes are often well represented by icosahedra. One each of the twofold, threefold, and fivefold symmetry axes is indicated by an ellipse, triangle, and pentagon, respectively.

Rossmann suggested that the canyons form the binding site for the rhi-novirus receptor on the surface of the host cells. The receptor for the major group of rhinoviruses is an adhesion protein known as lCAM-1. Cryoelectron microscopic studies have since shown that ICAM-1 indeed binds at the canyon site. Such electron micrographs of single virus particles have a low resolution and details are not visible. However, it is possible to model components, whose structure is known to high resolution, into the electron microscope pictures and in this way obtain rather detailed information, an approach pioneered in studies of muscle proteins as described in Chapter 14. 

Some virus particles have their protein subunits symmetrically packed in a helical array, forming hollow cylinders. The tobacco mosaic virus (TMV) is the classic example. X-ray diffraction data and electron micrographs have revealed that 16 subunits per turn of the helix project from a central axial hole that runs the length of the particle. The nucleic acid does not lie in this hole, but is embedded into ridges on the inside of each subunit and describes its own helix from one end of the particle to the other. 

Helical symmetry was thought at one time to exist only in plant viruses. It is now known, however, to occur in a number of animal virus particles. The influenza and mumps viruses, for example, which were first seen in early electron micrographs as roughly spherical particles, have now been observed as enveloped particles within the envelope, the capsids themselves are helically symmetrical and appear similar to the rods of TMV, except that they are more flexible and are wound like coils of rope in the centre of the particle. 

Figure 5.4 Structure and manner of assembly of a simple virus, tobacco mosaic virus, (a) Electron micrograph at high resolution of a portion of the virus particle, (b) Assembly of the tobacco mosaic virion. The RNA assumes a helical configuration surrounded by the protein capsomeres. The center of the particle is hollow.

Figure 5.29 Uptake of an enveloped virus particle by an animal cell, (a) The process by which the viral nucleocapsid is separated from its envelope, (b) Electron micrograph of adenovinis panicles entering a cell. Each panicle is about 70 nm in diameter.

Electron microscopic studies have suggested that the alphavirus particle has icosahedral symmetry (see below). The triangulation number is not certain, however (Murphy, 1980). Previous estimates for the molecular weight were compatible with 240 subunits per virus particle, and electron micrographs appear to show a T = 4 surface lattice (von Bons-dorff and Harrison, 1975). More information is now needed to determine the surface organization, since compositional data show fewer than 240 subunits. 

FIG. 1.12 Electron micrograph of two different types of particles that represent extreme variations from spherical particles (a) tobacco mosaic virus particles (Photograph courtesy of Carl Zeiss, Inc., New York) and (b) clay particles (sodium kaolinite) of mean diameter 0.2 fim (by matching circular fields). In both (a) and (b), contrast has been enhanced by shadow casting (see Section 1.6a.2a and Figure 1.21). (Adapted from M. D. Luh and R. A. Bader, J. Colloid Interface Sci. 33, 539(1970). 

Figure 4.12a shows plots of the intrinsic viscosity —in volume fraction units —as a function of axial ratio according to the Simha equation. Figure 4.12b shows some experimental results obtained for tobacco mosaic virus particles. These particles —an electron micrograph of which is shown in Figure 1.12a—can be approximated as prolate ellipsoids. Intrinsic viscosities are given by the slopes of Figure 4.12b, and the parameters on the curves are axial ratios determined by the Simha equation. Thus we see that particle asymmetry can also be quantified from intrinsic viscosity measurements for unsolvated particles. 

Figure 5-13 Electron micrograph of a DNA molecule (from a bacterial virus bacteriophage T7) undergoing replication. The viral DNA is a long ( 14 pm) duplex rod containing about 40,000 base pairs. In this view of a replicating molecule an internal "eye" in which DNA has been duplicated is present. The DNA synthesis was initiated at a special site (origin) about 17% of the total length from one end of the duplex. The DNA was stained with uranyl acetate and viewed by dark field electron microscopy. Micrograph courtesy J. Wolfson and D. Dressier.

Figure 7-8 (A) Electron micrograph of the rod-shaped particles of tobacco mosaic virus. Omikron, Photo Researchers. See also Butler and Klug.42 (B) A stereoscopic computer graphics image of a segment of the 300 nm long tobacco mosaic virus. The diameter of the rod is 18 nm, the pitch of the helix is 2.3 nm, and there are 16 1 3 subunits per turn. The coat is formed from 2140 identical 17.5-kDa subunits. The 6395-nucleotide genomic RNA is represented by the dark chain exposed at the top of the segment. The resolution is 0.4 nm. From Namba, Caspar, and Stubbs.47 (C) A MolScript ribbon drawing of two stacked subunits. From Wang and Stubbs.46...

Electron crystallography 131 Electron micrograph of bacteria 4 of cell junctions 27 of plant cell 13 of starch granules 172 of viruses 246... 

Figure 10.6 (a) Electron micrograph and (b) Schematic representation of the tobacco mosaic virus. 

Figure 1 Electron micrograph of an artificial viral envelope. In a unilamellar lipid bilayer of a diameter of approximately 200 nm are surface glycoproteins of the respiratory syncytial virus (RSV) inserted as described in detail by Chander Schreier [7] (electron micrographic image by G. Erdos, EM Core Laboratory, University of Florida, Gainesville, EL).

Figure 9.2 Transmission electron micrograph of Cowpea mosaic virus particles negatively stained with uranyl acetate. The scale bar is lOOnm.

Fig-1 Transmission electron micrographs of thin sections of infected cells of Phaeocystis pouchetii. (a) The virus-like particles (indicated by arrow) are found in the cytoplasm of the cells, (b) Detail of virus-like particles showing the hexagonal outline of the viruses... 

Fig. 12. Mass spectra of Rice Yellow Mottle Virus (RYMV) and Tobacco Mosaic Virus (TMV) particles analyzed with an electrospray ionization charge detection time-of-flight mass spectrometer. Inset, electron micrographs of the icosahedral RYMV (diameter 28.8 nm) and the cylindrical TMV (-300 nm long and 17 nm in diameter). The known molecular weight of RYMV and TMV are 6.5X106 and 40.5X106 Daltons,respectively...

Figure 21.8 Transmission electron micrograph of Ebola virus. CDC/C. Goldsmith...

Fig. 5. Imaging and image quality assessment by incoherent averaging of Fourier transforms, (a and b) A focal pair of 400-keV electron micrographs of ice-embedded rice dwarf virus (RDV) was recorded in a JEOL4000 electron cryomicroscope with a Gatan cryoholder operated at —170 C. Images were taken at x50,000 magnification with an electron dose less than 20 electrons/A. One corresponding RDV particle is...

Crowther, R. A. (1971). Procedures for three-dimensional reconstruction of spherical viruses by Fourier synthesis from electron micrographs. Philos. Trans. R. Soc. Land. B Biol. Sci. 261, 221-230. 

Figure 3.50. Complex Quaternary Structure. The coat of rhinovirus comprises 60 copies of each of four subunits. (A) A schematic view depicting the three types of subunits (shown in red, blue, and green) visible from outside the virus. (B) An electron micrograph showing rhinovirus particles. [Courtesy of Norm Olson, Dept, of Biological Sciences, Purdue University.]... 

FIGURE 2.1. An electron micrograph of the rhombic form of tobacco necrosis virus/ showing the regular arrangement of spherical particles (molecules) in the crystal and also the formation of crystal faces. In this micrograph the viral particles each have a diameter of approximately 250 A. (Micrograph courtesy R. W. G. Wyckoff.)... 

Figure 7-8 (A) Electron micrograph of the rod-shaped particles of tobacco mosaic virus. Omikron, Photo Researchers. See also Butler and Klug. ...

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