FINE STRUCTURE OF REOVIRUS TYPE 2

Analysis of herpes simplex virus type I nuclear particles by flow cytometry

Flow cytometry has been instrumental to characterize cell populations and examine their inner molecules and processes. In most instances, whole cells are analyzed, and hence, particle size is not an issue. Viruses are 2-3 orders of magnitude smaller than cells so flow cytometry has typically been used to study viral markers within whole infected cells. However, the ability to separate and purify viral particles representing different maturation stages within a viral life cycle would be a useful tool to analyze them in details and characterize the host proteins they associate with. Herpes simplex virus Type 1 is a 250 nm enveloped DNA virus that replicates in the nucleus where it assembles new viral particles called capsids. These capsids eventually travel to the cell surface and are modified along the way, producing several intermediate particles. In the nucleus, three types of stable nonenveloped 125 nm nuclear capsids exist that differ in protein composition and genome content. This includes so-called nuclear C-capsids that are the precursors of mature extracellular virions. We report that we can apply flow cytometry to sort these nuclear C-capsid intermediates by labeling the viral genome with Syto 13, a fluorescent marker that binds to nucleic acids. This is the first time flow cytometry has been used not only to detect but also to purify an intracellular viral maturation intermediate. This opens new research avenues in virology to study capsid assembly, maturation and egress, analyze mutant phenotypes, and define host factors associated with specific viral intermediates.

Structure of infectious bursal disease virus

Infectious bursal disease virus of chickens was purified, and its structure was examined by the negative-staining technique in the electron microscope. The buoyant density of infectious bursal disease virus in CsCl was found to be 1.34 g/cm(3). The morphological details suggest that the capsid of the virion consists of a single layer of 32 capsomeres arranged in 5:3:2 symmetry. The virion measured about 55 nm in diameter and had no envelope.

Electron microscopical and biochemical characterization of infectious pancreatic necrosis virus

An electron microscopical and biochemical examination of the properties of infectious pancreatic necrosis virus (IPN) and of its ribonucleic acid (RNA) was made. The buoyant density of IPN in CsCl was found to be 1.33 g/cm(3). Electron microscopical examination of the banded virus revealed structures similar in size (74 nm) and shape to reoviruses but lacking a characteristic inner capsid structure. Polyacrylamide gel electrophoretic analysis of IPN-RNA revealed a single non-segmented component of molecular weight 3.2 x 10(6). Its susceptibility to ribonuclease, base composition, and resistance to thermal denaturation indicated a single-stranded RNA structure. However, its sedimentation behavior (16S) independent of ionic strength in sucrose gradients, partial solubility in 2 m LiCl, and ribonuclease resistance in the presence of Mg(2+) suggest an unusual secondary structure of unknown nature. The accumulated data indicate that IPN virus does not belong to either the picornavirus or reovirus groups and may represent a new group of viruses.

Spontaneous degradation of reovirus capsid into subunits

When a plaque-purified strain of reovirus type 3 (Dearing strain) degraded during storage, electron micrographs of the degraded virion were made which clearly demonstrated the substructural components of the outer capsid.

Purification, morphology and partial characterization of a reovirus-like agent associated with neonatal calf diarrhea

Studies have been conducted on a virus which has recently been implicated as an etiological agent in widely disseminated cases of calf diarrhea. The virus was isolated from feces and intestinal mucosa from experimentally infected calves and from cell cultures which had been inoculated with material from infected calves. The virus was purified by differential centrifugation, nuclease treatment, extraction with Genetron 113 and cesium chloride (CsC1) gradient. Purified and non-purified samples were studied by electron microscopy. Both types of preparations contained unenveloped particles approximately 64 nm in diameter with a hexagonal core region 36 nm in diameter. Subunits of the capsids appeared to be arranged in accordance with cubic synmetry. Essential lipids were not associated with the virus. The buoyant density of purified virus was 1.359. Viral nucleic acid was determined by chemical methods to be ribonucleic acid (RNA). The morphology and some chemical characteristics appear to be similar to those of the reovirus group. However, the exact classification of this virus and its relationship to other groups remains to be established.

Defective virions of reovirus

When purified preparations of stock reovirus, type 3, were digested with chymotrypsin, the virions were converted into two different types of particle. These new particles could be separated from each other by isopycnic centrifugation in cesium chloride gradients. One particle banded at a buoyant density of 1.43 g/cm(3), the other at a density of 1.415 g/cm(3). The former particle is termed the heavy (H) particle, the latter is the light (L) particle. The ratio of H/L particles varied between 0.5 and 0.25 in various purified preparations of virus. In electron micrographs, both H and L particles had the appearance and dimensions of viral cores. H particles were infectious for L cells. When plaques formed by stock virus, or by H particles, were picked and propagated in L cells, the majority of the clones gave rise only to H particles on chymotrypsin digestion. On continued serial passage of the clones, virions containing L particles again appeared in the progeny. The simplest explanation of these results was that stock virus was comprised of two populations of virions. One type of virion which contained H particles was infectious, whereas the other, which contained L particles, was not itself infectious and could replicate only in cells coinfected with an H particle virion. Added weight was given to this hypothesis by two observations. First, a small but definite separation of H and L virions could be achieved by isopycnic centrifugation in a gradient of cesium chloride. Second, L particles and virions containing L particles were both shown to lack the largest of the ten segments of double-stranded ribonucleic acid genome. Thus, L particle virions have defective genomes.

Structural proteins of reoviruses

Polyacrylamide gel electrophoresis of the solubilized proteins from the three serotypes of reovirus revealed that each contained three major and four minor components. Subviral particles were prepared by brief treatment of complete virions with urea. Electron microscopy, density-gradient centrifugation, and chemical analyses of these particles indicated that their outer capsid structure had been selectively removed. They contained only two proteins, but their ribonucleic acid composition was similar to that of complete virions. The subviral particles were not infectious.

Separation of ten reovirus genome segments by polyacrylamide gel electrophoresis

Double-stranded ribonucleic acid (RNA) extracted from purified reoviruses of all three serotypes and from type 3 virus-infected cells was analyzed by polyacrylamide gel electrophoresis. It was calculated that each RNA includes 10 segments: 3 large, 3 intermediate, and 4 small fragments corresponding to molecular weights of about 2.5, 1.4, and 0.8 x 10(6) daltons, respectively, or a total of 15 x 10(6) daltons.