Isometric viruses

Twenty five years on from the first spherical virus structures, the complexity of the isometric virus... 

C. Structures of Membrane-Containing Isometric Viruses Fusion Machines... 60... 

Mature capsid structure, of bacteriophage T7 procapsid assembly, 310—315 Membrane-containing isometric viruses, structures of, 60-63 Membrane fusion models, of influenza hemagglutinin (HA), 340-343... 

To a first approximation, the sizes of isometric viruses can be estimated by comparing the experimental maxima and minima with the theoretical curves calculated for spheres and hollow spheres [492-494,504]. However, viruses are composed of protein shells and nucleic acid cores (with carbohydrate and lipid in more complex viral structures), so a full analysis requires the explicit consideration of non-uniform scattering densities. In addition, the principle of icosahedral symmetry in the assembly of the protein shell means that, at large Q, deviations from spherical symmetry will influence the scattering curve. The separation of the scattering curve... 

A related area of virus solution scattering is the study of the large bacteriophages, which have head-and-tail structures and so sure in principle anisometric. The volume of the tail is usually less than 0% of the phage volume, so the scattering curves are dominated by the phage head. For phages that possess isometric heads and sufficiently short tails, the analyses can be performed exactly as for isometric viruses above [514-518,523,527,528,529,531,532,534,536,563]. 

Other anisometric viruses have rod-like helical or cylindrical structures, such as tobacco mosaic virus [495,496,509,533] or alfalfa mosaic virus [551,561,562]. Thus cross-sectional parameters can be determined using / xs Q) Q q->o addition to Rq d I 0) data [537,550]. Stuhrmann plots of the / xs data lead to information on the cross-sectional distribution of protein and RNA. Shell models for the cross-section can likewise be made by analogy with the isometric viruses [550,561,562]. The radial scattering density of the cross-section can be calculated by applying the Hankel transformation to the scattering curve [509]. 

Volumes 5 and 6 represent the first in a series that focuses primarily on the structure and assembly of virus particles. Volume 5 is devoted to general structural principles involving the relationship and specificity of interaction of viral capsid proteins and their nucleic acids, or host nucleic acids. It deals primarily with helical and the simpler isometric viruses, as well as with the relationship of nucleic acid to protein shell in the T-even phages. Volume 6 is concerned with the structure of the picornaviruses, and with the reconstitution of plant and bacterial RNA viruses. 

Fig. 32. Typical experimental data ( + ) for an isometric protein-RNA plant virus, southern bean mosaic virus (SBMV) in its (a) compact and (b) swollen states [555]. For a given H20 buffer content, note the shift in the position of the subsidiary mimima to lower Q which accompanies the increase in the virus radius of about 10% in the swollen state. The continuous curve at each contrast is that calculated for a four-shell model of SBMV, and has been smeared to allow for neutron beam divergence and wavelength spread.

Crick and Watson were the first to suggest that small viruses were built up of small protein subunits packed together symmetrically to form a protective shell for the nucleic acid. They reasoned that formation of small identical molecules was an efficient use of the limited information contained in the virus nucleic acid. They also realized that, of the types of symmetry possible for a three-dimensional structure enclosing space, only the cubic point groups could lead to an isometric particle, which was the known symmetry of many viruses at the time. Three types of cubic symmetry exist namely,... 

The picornavirus particle is composed of a molecule of single-stranded RNA (50 by weight) enclosed in a capsid of protein (70%) There is no good evidence for the presence of carbohydrate or lipid in the virion (1). As viewed in the electron microscope by negative staining, the particle is isometric with a dry diameter of 27-28 nm. In solution it behaves as a spheroid (frictional ratio of 1.05-1 10) with a diameter of about JO nm, and containing some 0.25 g water per gram of dry virus (2). 

Structure. Virus particles display a wide range of different forms, but the particle size and shape of each species are usually constant and characteristic (Fig.l). The diameter of isometric species ranges between 15 and 300 nm, while rod-shaped and helical particles vary in length between 180 and 2,000 nm. 

Hexon 13, 6 Hybrid viruses adeno-SV40 10, 4 13, 1 plant isometric 6, 4 plant rod-shaped 6, 2 Hydroxymethyluracil-containing phages 7, 3 8, 1... 

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