Barley stripe mosaic virions assoicated with spindle microtubules

Genotoxicity of barley stripe mosaic virus in infected host plants

A comparative study of the effect of barley stripe mosaic virus (BSMV) and gamma irradiation on mitotic divisions in barley (Hordeum vulgare L.) roots was performed by evaluating the mitotic index (MI), micronucleus (MN) frequency and sister chromatid exchanges (SCE). Results indicate that, similarly to gamma irradiation at doses of 100, 150 and 250 Gy, BSMV reduces the mitotic activity, increases the micronucleus frequency and the rate of SCE and promotes the formation of C-metaphases. In root meristematic cells of the three barley cultivars studied (Galactic, Sonor and Unirea), the mitotic index of infected plants was found to be 52.5, 54.48 and 64.17%, respectively, lower than the uninfected control. An increase in frequency of sister chromatid exchanges was observed in all the experimental variants. In treatments involving viral infection alone or in combination with gamma irradiation chromosomes with three and more chromatid exchanges were observed, while their percentage in the control or in treatments with gamma irradiation alone was reduced. The results of the study indicate that in plants derived from irradiated seeds, BSMV produces an effect that is correlated nonlinearly with the radiation dose applied. Cytological analysis of mitotic divisions in barley roots revealed the genotoxicity of BSMV infection.

Interaction of frog virus-3 with the cytoskeleton. I. Altered organization of microtubules, intermediate filaments, and microfilaments

The progressive cytoskeletal alterations of frog virus 3-infected baby hamster kidney (BHK) and fathead minnow (FHM) cells were studied by immunofluorescence and electron microscopy. The virus assembly sites, which contain viral genomes and viral proteins, were detected in the cytoplasm at 4 h (FHM) or 6 h (BHK) and mature virions appeared 2 h later. When infected cells were treated with Triton X-100, the assembly sites were found in association with the cytoskeleton. In infected cells, the number of microtubules progressively decreased but a few microtubules traversing in the vicinity of the assembly sites remained intact. Early in infection, the intermediate filaments retracted from the cell periphery, delimited the forming assembly sites, and remained there throughout infection. We suggest that intermediate filaments are involved in the formation of assembly sites. In addition, the filaments either by themselves or in conjunction with microtubules may anchor the assembly sites near the nucleus. The microfilament bundles (stress fibers) disappeared with the formation of assembly sites, and late in infection many projections containing microfilaments and virus particles appeared at the cell surface. The observation suggests a role for microfilaments in virus release. Taken together, these results provide the first example of a virus-infected cell in which all three cytoskeletal filaments show profound organizational changes and suggest an active participation of the host cytoskeleton in viral functions.

Binding of adenovirus to microtubules. II. Depletion of high-molecular-weight microtubule-associated protein content reduces specificity of in vitro binding

A specific in vitro association between adenovirus and pruified rat brain microtubules has been previously demonstrated (R. B. Luftig and R. R. Weihing, 1975). When examined by negative-staining electron microscopy, approximately 90% of the virus associated with microtubules was edge bound, i.e., associated within +/-4 nm of the microtubule edge. Similar results are now found for the association of adenovirus with purified chick brain microtubules. When the content of the high-molecular-weight proteins (MAPs) normally present as projections on the surface of microtubules is depleted by fractionation of cold-depolymerized microtubules on agarose A-15M columns or by brief treatment of polymerized microtubules with trypsin, the percentage of edge-bound microtubule-associated viruses is reduced to a level close to that found for particles such as reovirus, coliphage f2, or polystyrene latex spheres, which randomly associate with microtubules (54 to 64% for column-fractionated microtubules; 45 to 68% for trypsin-treated microtubules). Counts of adenovirus particles specifically bound to microtubules, corrected for variations in microtubule and virus concentrations, gave values 2.5 to 3.5 times higher for unfractionated microtubules than for microtubule-associated protein-depleted microtubules. These results are consistent with the suggestion that the specific association between adenovirus and microtubules is mediated by microtubule-associated proteins.

High-voltage electron microscope study of the release of vaccinia virus from whole cells

High-voltage (1,000-kV) electron microscope examination of whole BSC-1 cells infected with vaccinia virus at different times after infection revealed the presence of increasing numbers of virions no longer confined to factories but situated along the cell periphery of monolayer cells. Stereoscopic images showed each virus enclosed within a membrane-like component of the host cell cytoplasm. Viruses within factories appeared to lack similar enclosures. Cytochalasin B, but not vinblastine, caused the enclosures to disrupt. Vaccinia viruses were observed to escape the host cell individually from the tips of microvillie and within packets of cytoplasm. Observations suggest that the intracellular movement and release of vaccinia virus utilize a host cell cytoplasmic network that involves microfilaments for stability.

Adenovirus binds to rat brain microtubules in vitro

We have found by negative staining electron microscopy that when similar concentrations of adenovirus and reovirus (viruses of about the same diameter, 75 to 80 nm, and density, 1.34 to 1.36 g/cm3) were incubated with a carbon support film containing microtubules, 72% of adenovirus on the grid, but only 32% (equivalent to random association) of reovirus, were associated with microtubules. Similar concentrations of both larger and smaller particles, such as polystyrene latex spheres and coliphage f2, also exhibited a low degree of interaction, viz., 17 to 37%, with microtubules. Moreover, 90% of microtubule-associated adenovirus binds to within +/- 4 nm of the edge of microtubules, but lower fractions (again equivalent to a random association) of the other particles bind to the edge of the microtubules. The mechanism behind this phenomenon, which we denote as "edge binding," is presently obscure; however, it provides us with a second, albeit empirical, method to distinguish between the microtubular association of adenovirus and other particles. We found that edge binding of adenovirus also occurred when adenovirus was initially placed on the carbon support film and then incubated with microtubules and when adenovirus and microtubules were mixed prior to placement on the support. In contrast, reovirus or the other particles prepared by similar techniques exhibited a random amount of edge binding. The binding of adenovirus appears to involve the hexon capsomers of the virion since (i) high resolution electron micrographs showed that the edge of the virus was in contact with the edge of the microtubules, and (ii) adenovirions briefly treated with formamide to remove pentons and fibers bind as efficiently as intact virions. Core structures, which were obtained by further formamide degradation of the virion, do not associate with microtubules. These observations support the hypothesis of Dales and Chardonnet (1973) that the transport of adenovirions within infected cells is mediated by interaction with microtubules.