Morphology of BHK-21 Cells Infected with Sindbis Virus Temperature-Sensitive Mutants in Complementation Groups D and E

BHK-21 cells infected with temperature-sensitive mutants of Sindbis virus in complementation groups D and E differed in their appearance under nonpermissive conditions. Cells infected at nonpermissive temperature with virus defective in complementation group E had nucleocapsids attached in large numbers to the inside surface of the host plasma membrane. Infection with a group D mutant produced nucleocapsids that did not attach to the plasma membrane but rather remained free in the cell cytoplasm.     

Rotavirus RNA replication: VP2, but not VP6, is necessary for viral replicase activity.

Temperature-sensitive mutants of simian rotavirus SA11 were previously developed and organized into 10 of a possible 11 recombination groups on the basis of genome reassortment studies. Two of these mutants, tsF and tsG, map to genes encoding VP2 (segment 2) and VP6 (segment 6), respectively. To gain insight into the role of these proteins in genome replication, MA104 cells were infected with tsF or tsG and then maintained at permissive temperature (31 degrees C) until 9 h postinfection, when some cells were shifted to nonpermissive temperature (39 degrees C). Subviral particles (SVPs) were recovered from the infected cells at 10.5 and 12 h postinfection and assayed for associated replicase activity in a cell-free system shown previously to support rotavirus genome replication in vitro. The results showed that the level of replicase activity associated with tsF SVPs from cells shifted to nonpermissive temperature was ca. 20-fold less than that associated with tsF SVPs from cells maintained at permissive temperature. In contrast, the level of replicase activity associated with tsG SVPs from cells maintained at nonpermissive temperature was only slightly less (twofold or less) than that associated with tsG SVPs from cells maintained at permissive temperature. Analysis of the structure of replicase particles from tsG-infected cells shifted to nonpermissive temperature showed that they were similar in size and density to virion-derived core particles and contained the major core protein VP2 but lacked the major inner shell protein VP6. Taken together, these data indicate that VP2, but not VP6, is an essential component of enzymatically active replicase particles.     

Electron microscope studies of temperature-sensitive mutants of herpes simplex virus type 2.

Nine temperature-sensitive mutants of herpes simplex virus type 2 representing eight complementation groups were assigned to two classes as a consequence of the virion forms and virus-specific cellular alterations observed in thin sections of mutant-infected human embryonic lung cells grown at the nonpermissive temperature. Mutants in class A, one DNA- and one DNA +, failed to synthesize detectable virus particles. Mutants in class B, 4DNA- and 3DNA+, produced moderate to large numbers of empty nucleocapsids. Dense-cored nucleocapsids were not observed in thin sections of cells infected with any of the nine mutants at this temperature. Virus-specific cellular alterations consisted primarily of margination of chromating and nulcear membrane thickening and duplication.     

Impaired processing of precursor polypeptides of temperature-sensitive mutants of Rauscher murine leukemia virus.

The synthesis and processing of virus-specific precursor polypeptides in NIH/3T3 cells infected at the permissive temperature (31 degrees C) with temperature-sensitive (ts) mutants of Rauscher murine leukemia virus was studied in pulse-chase experiments at the permissive and nonpermissive (39 degrees C) temperatures. The newly synthesized virus-specific polypeptides were analyzed by sodium dodecyl sulfate polyacrylamide gel electrophoresis after immunoprecipitation with polyvalent and monospecific antisera against Rauscher murine leukemia virus proteins. In cells infected with ts mutants defective in early replication steps (the early mutants ts17 and ts29), and ts mutants defective in postintegration steps (the late mutants ts25 and ts26), the processing of the primary gag gene product was impaired at the nonpermissive temperature. gag-pr75 of all four mutants was converted into gag-pr65; however, gag-pr65 accumulated at the nonpermissive temperature, and the main internal virion polypeptide p30 was not formed. Therefore, the proteolytic cleavage is blocked beyond gag-pr65. Concomitantly, the formation of the env gene-related polypeptide p12(E) of all four mutants was blocked at the restrictive temperature. In contrast, cells infected with the late mutant ts28, which produced noninfectious virions at 39 degrees C, showed a normal turnover of the gag and env precursor polypeptides.     

Action of temperature-sensitive mutants of myeloproliferative sarcoma virus suggests that fibroblast-transforming and hematopoietic transforming viral properties are related

The myeloproliferative sarcoma virus is molecularly related to the Moloney sarcoma virus (Pragnell et al., J. Virol. 38:952-957, 1981), but causes both fibroblast transformation in vitro and leukemic changes--including spleen focus formation--in adult mice. The fibroblast transforming properties of myeloproliferative sarcoma virus were used to select viral temperature-sensitive mutants at 39.5 degrees C, the nonpermissive temperature. These mutants are temperature sensitive in the maintenance of the transformed state. This was also shown by cytoskeletal changes of the infected cells at permissive and nonpermissive temperatures. Viruses released from cells maintained at both the permissive and nonpermissive temperature are temperature sensitive in fibroblast transformation functions. All temperature-sensitive mutants show only a low reversion rate to wild-type transforming function. The myeloproliferative sarcoma virus temperature-sensitive mutants are inefficient in causing leukemic transformation (spleen enlargement, focus formation) in mice at the normal temperature. A method to maintain a low body temperature (33 to 34 degrees C) in mice is described. One temperature-sensitive mutant was checked at low body temperature and did not induce leukemia. These data thus indicate that the same or related viral functions are responsible for hematopoietic and fibroblast transformation.     

Properties of mammalian cells transformed by temperature-sensitive mutants of avian sarcoma virus.

Fibroblasts from European field vole (Microtus agrestis) and from normal rat kidney (NRK) have been infected by avian sarcoma virus mutants which are temperature-sensitive for the maintenance of transformation. These cells are transformed at 33 degrees C, but show normal cell characteristics in morphology, colony formation in agar, saturation density, sugar uptake and membrane proteins at 39 degrees C and 40 degrees C, the nonpermissive temperatures. Ts mutant virus was rescued from most of the ts transformed cell lines. NRK cells infected by avian sarcoma virus ts mutants and kept at the nonpermissive temperature can be transformed by wild-type avian sarcoma virus. The susceptibility of the temperature-sensitive NRK lines to this transformation is higher than the susceptibility of uninfected NRK at either permissive or nonpermissive temperature.     

Ultrastructural characterization of an early, nonstructural polypeptide of herpes simplex virus type 1.

An immunoperoxidase procedure was employed to study the expression of a large-molecular-weight, virus-induced polypeptide (VP175; molecular weight, 175,000) at the light and electron microscopic levels in Vero cells infected with herpes simplex virus type 1 or with tsB2, a DNA-negative, temperature-sensitive mutant of herpes simplex virus type 1. In cells infected with herpes simplex virus type 1 and in cells infected with tsB2 at the permissive temperature (34 degrees C), VP175 was found within the nucleus. The protein was detected as early as 2 h postinfection and, by 3 h postinfection, was generally distributed in a marginated pattern contiguous with, and extending from, the inner lamella of the nuclear membrane. At 6 h postinfection, protein accumulations were dispersed throughout the nucleus, and, by 9 h postinfection, these accumulations tended to be localized in a marginated pattern near the nuclear membrane. It was also noted that, at 9 h postinfection, under permissive conditions, VP175 was not found in association with nucleocapsids or enveloped particles. In contrast, in cells infected with tsB2 at the nonpermissive temperature (39 degrees C) and harvested at 6 or 9 h postinfection, accumulations of VP175 were identified not only within the nucleus, but also within the cytoplasm in the form of annular or globular aggregates. These aggregates consisted of a granular matrix and were not bound by membranes.     

Characterization of herpes simplex virus 2 temperature-sensitive mutants whose lesions map in or near the coding sequences for the major DNA-binding protein.

By marker rescue with cloned herpes simplex virus 2 DNA fragments, we have mapped the temperature-sensitive mutations of a series of herpes simplex virus 2 mutants to a region of the herpes simplex virus 2 genome that lies within or near the coding sequences for the major DNA-binding protein, ICP8. In cells infected with certain of these mutants at the nonpermissive temperature, the association of the major DNA-binding protein with the cell nucleus was defective. In these cells, the DNA-binding protein accumulated in the cytoplasmic and the crude nuclear detergent wash fractions. At the permissive temperature, the maturation of the mutant ICP8 was similar to that of the wild-type viral protein. With the remainder of the mutants, the nuclear maturation of ICP8 was similar to that encoded by the wild-type virus at the nonpermissive and permissive temperatures as assayed by cell fractionation.     

Herpes Simplex Virus Glycoproteins: Isolation of Mutants Resistant to Immune Cytolysis

Immune cytolysis mediated by antibody and complement is directed against components of the major herpes simplex virus (HSV) glycoprotein complex (molecular weight, 115,000 to 130,000), comprised of gA, gB, and gC, and against glycoprotein gD-all present on the surfaces of infected cells. Tests with a temperature-sensitive (ts) mutant of HSV-1 (tsA1) defective in glycoprotein synthesis at the nonpermissive temperature (39 degrees C) demonstrated that over 90% of mutant-infected cells maintained at 39 degrees C and treated with antibody and complement were not lysed, presumably due to the absence of viral glycoproteins on the surface of infected cells at this temperature. Furthermore, a small number of tsA1-infected cells could be detected among a large excess of wild-type virus-infected cells by virtue of their failure to be lysed at 39 degrees C by antibody and complement. Making use of the involvement of viral glycoproteins in immune cytolysis and the ability of cells infected with glycoprotein-defective mutants to escape cytolysis, we sought mutants defective in the expression of individual viral glycoproteins. For this purpose, antisera directed against the VP123 complex and against the gC and combined gA and gB glycoprotein subcomponents of this complex were first tested for their ability to lyse wild-type virus-infected cells in the presence of complement. Wild-type virus-infected cells were lysed after treatment with each of the three antisera, demonstrating that the gC glycoprotein and the combined gA and gB glycoproteins can act as targets in the immune cytolysis reaction. Next, these antisera were used to select for mutants which were resistant to immune cytolysis. Cells infected with wild-type virus which had been mutagenized with 2-aminopurine and incubated at 39 degrees C were treated with one of the three types of antisera (anti-VP123 complex, anti-gC, or anti-gAgB) and lysed by the addition of complement. Cells which survived immune cytolysis were plated, and virus in the resulting plaques was isolated. Plaque isolates were tested for temperature sensitivity of growth and altered cytopathic effects in cell culture at 34 degrees C (the permissive temperature) and 39 degrees C. A total of 73 mutants was isolated in this manner. Selection with glycoprotein-specific antisera resulted in a 2- to 16-fold enrichment for mutants compared with "mock" -selected mutants using normal rabbit serum. Phenotypically, 24 mutants were temperature sensitive for growth, 27 were partially temperature sensitive, and 22 were not temperature sensitive but exhibited markedly altered cytopathic effects at both permissive and nonpermissive temperatures. Nine mutants of each phenotype (temperature sensitive, partially temperature sensitive, and non-temperature sensitive) were selected at random for confirmatory immune cytolysis tests with the antisera used in their selection. Cells infected with eight of the nine mutants were shown to be significantly more resistant to immune cytolysis at the nonpermissive temperature than were the mock-selected mutants or the wild-type virus from which they were derived.     

Replication of herpesvirus DNA. V. Maturation of concatemeric DNA of pseudorabies virus to genome length is related to capsid formation.

The maturation of pseudorabies virus DNA from the replicative concatemeric form to molecules of genome length was examined using nine DNA+ temperature-sensitive mutants of pseudorabies virus, each belonging to a different complementation group. At the nonpermissive temperature, cells infected with each of the mutants synthesized concatemeric DNA. Cleavage of the concatemeric DNA to genome-length viral DNA was defective in all the DNA+ ts mutants tested, indicating that several viral gene products are involved in the DNA maturation process. In none of the ts mutant-infected cells were capsids with electron-dense cores (containing DNA) formed. Empty capsids with electron-translucent cores were, however, formed in cells infected with six of the nine temperature-sensitive mutants; in cells infected with three of the mutants, no capsid assembly occurred. Because these three mutants are deficient both in maturation of DNA and in the assembly of viral capsids, we conclude that maturation of viral DNA is dependent upon the assembly of capsids. In cells infected with two of the mutants (tsN and tsIE13), normal maturation of viral DNA occurred after shiftdown of the cells to the permissive temperature in the presence of cycloheximide, indicating that the temperature-sensitive proteins involved in DNA maturation became functional after shiftdown. Furthermore, because cycloheximide reduces maturation of DNA in wild-type-infected cells but not in cells infected with these two mutants, we conclude that a protein(s) necessary for the maturation of concatemeric DNA, which is present in limiting amounts during the normal course of infection, accumulated in the mutant-infected cells at the nonpermissive temperature. Concomitant with cleavage of concatemeric DNA, full capsids with electron-dense cores appeared after shiftdown of tsN-infected cells to the permissive temperature, indicating that there may be a correlation between maturation of DNA and formation of full capsids. The number of empty and full capsids (containing electron-dense cores) present in tsN-infected cells incubated at the nonpermissive temperature, as well as after shiftdown to the permissive temperature in the presence of cycloheximide, was determined by electron microscopy and by sedimentation analysis in sucrose gradients. After shiftdown to the permissive temperature in the presence of cycloheximide, the number of empty capsids present in tsN-infected cells decreased with a concomitant accumulation of full capsids, indicating that empty capsids are precursors to full capsids.     

Viral DNA synthesis in cells infected with temperature-sensitive mutants of herpes simplex virus type 1.

Temperature-sensitive mutants of herpes simplex virus type 1 representing eight DNA-negative complementation groups were grouped into the following three categories based on the viral DNA synthesis patterns after shift-up from the permissive to the nonpermissive temperature and after shift-down from the nonpermissive to the permissive temperature in the presence and absence of inhibitors of RNA and protein synthesis. (i) Viral DNA synthesis was inhibited after shift-up in cells infected with tsB, tsH, and tsJ. After shift-down, tsB- and tsH-infected cells synthesized viral DNA in the absence of de novo RNA and protein synthesis whereas tsJ-infected cells synthesized no viral DNA in the absence of protein synthesis. The B, H, and J proteins appear to be continuously required for the synthesis of viral DNA. (ii) Viral DNA synthesis continued after shift-up in cells infected with tsD and tsK whereas no viral DNA was synthesized after shift-down in the absence of RNA and protein synthesis. Mutants tsD and tsK appear to be defective in early regulatory functions. (iii) Cells infected with tsL, tsS, and tsU synthesized viral DNA after shift-up and after shift-down in the absence of RNA and protein synthesis. The functions of the L, S, and U proteins cannot yet be determined.     

Method for Isolation and Selection of Temperature-Sensitive Mutants of Herpes Simplex Virus

A method for induction and selection of temperature-sensitive mutants of herpes simplex virus is presented. After the infected cells were treated with 5-bromodeoxyuridine, the virus was extracted by repeated freezing and thawing and cloned in microcultures which were then incubated at permissive temperature until viral plaques appeared. The microcultures were then replicated at nonpermissive temperature. Clones not forming plaques in these latter were further purified and examined for temperature-sensitive characteristics. Viral clones mutated in plaque-forming ability or in yield were obtained and preliminarily characterized.     

Assembly of vesicular stomatitis virus: distribution of the glycoprotein on the surface of infected cells.

This study demonstrates that the glycoprotein of vesicular stomatitis virus clusters in the plasma membrane of infected Chinese hamster lung cells during morphogenesis and suggests that viral nucleocapsids are required for this clustering. A mutant virus (ts E-1) which is temperature sensitive for the synthesis of viral nucleocapsids but not viral membrane proteins was used. The surface distribution of the viral glycoprotein in cells infected by this virus was determined by a specific indirect immunoferritin stain. Early in infection at permissive temperatures, the glycoprotein was randomly distributed on membrane ghosts. Later, clusters of ferritin the size and shape of virus particles were seen. In contrast, ghosts prepared from virus-infected cells maintained at a restrictive temperature always had a random distribution of viral glycoprotein.     

Characterization of ts 16, a temperature-sensitive mutant of vaccinia virus.

We have characterized a temperature-sensitive mutant of vaccinia virus, ts16, originally isolated by Condit et al. (Virology 128:429-443, 1983), at the permissive and nonpermissive temperatures. In a previous study by Kane and Shuman (J. Virol 67:2689-2698, 1993), the mutation of ts16 was mapped to the I7 gene, encoding a 47-kDa protein that shows partial homology to the type II topoisomerase of Saccharomyces cerevisiae. The present study extends previous electron microscopy analysis, showing that in BSC40 cells infected with ts16 at the restrictive temperature (40 degrees C), the assembly was arrested at a stage between the spherical immature virus and the intracellular mature virus (IMV). In thawed cryosections, a number of the major proteins normally found in the IMV were subsequently localized to these mutant particles. By using sucrose density gradients, the ts16 particles were purified from cells infected at the permissive and nonpermissive temperatures. These were analyzed by immunogold labelling and negative-staining electron microscopy, and their protein composition was determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. While the ts16 virus particles made at the permissive temperature appeared to have a protein pattern identical to that of wild-type IMV, in the mutant particles the three core proteins, p4a, p4b, and 28K, were not proteolytically processed. Consistent with previous data the sucrose-purified particles could be labelled with [3H]thymidine. In addition, anti-DNA labelling on thawed cryosections suggested that most of the mutant particles had taken up DNA. On thawed cryosections of cells infected at the permissive temperature, antibodies to I7 labelled the virus factories, the immature viruses, and the IMVs, while under restrictive conditions these structures were labelled much less, if at all. Surprisingly, however, by Western blotting (immunoblotting) the I7 protein was present in similar amounts in the defective particles and in the IMVs isolated at the permissive temperature. Finally, our data suggest that at the nonpermissive temperature the assembly of ts16 is irreversibly arrested in a stage at which the DNA is in the process of entering but before the particle has completely sealed, as monitored by protease experiments.     

Studies on the budding process of a temperature-sensitive mutant of murine leukemia virus with a scanning electron microscope.

The scanning electron microscope was used to study the budding process of the wild-type Moloney murine leukemia virus and one of its temperature-sensitive mutants, designated ts 3. A considerably larger number of budding particles was observed on TB cells infected with ts 3 at the nonpermissive temperature (39 C) than at the permissive temperature (34 C). No apparent difference was noted between the number of particles on ts 3-infected cells at (34 C) and wild-type-infected cells at 34 or 39 C. Virions were detected at the cell membrane of ts 3-infected cells at 39 C as early as 8 h postinfection. Virion density increased progressively up to 48 h after which no increase was observed. An average of 1,600 virus particles was observed at the cell surface at the peak of virus production. The distribution of these on the cell membrane appeared to be random. The maximum proportion of the cell surface occupied by the viral particles did not exceed 10%. After temperature shift from 39 to 34 C, approximately 90% of the particles had dissociated from the cell membrane within 1 h.     

Cells transformed by temperature-sensitive mutants of avian sarcoma virus cause tumors in vivo at permissive and nonpermissive temperatures.

Chick embryo (CE) fibroblasts and normal rat kidney (NRK) cells transformed by temperature-sensitive (ts) mutants of avian sarcoma virus (NY68, LA23, LA24, LA25, LA29, LA31, GI201, GI202, GI251, GI253 induce tumors on the chorioallantoic membrane (CAM) of chick eggs at temperatures that correspond to the permissive and nonpermissive temperatures used to induce conditional expression of the "transformed" phenotype in these cells when cultured in vitro. Chick embryo cells infected with transformation-defective mutants of ASV (td101, td108) or RAV-50 were nontumorigenic under the same conditions, as were nontransformed CE and NRK cells. This indicates that the CAM is not an unusually susceptible substrate for cell growth and that the ability of tsASV-transformed cells to form tumors at nonpermissive temperatures reflects their true tumorigenicity. In contrast, a ts mutant chemically transformed rat liver cell line, ts-223, only formed tumors on the CAM under permissive conditions. The wild-type parent cells (W-8) of this mutant produced tumors at both permissive and nonpermissive temperatures. Direct implantation of microprobe thermometers into tumors caused by ts-ASV-transformed cells at nonpermissive temperatures confirmed that tumor formation occurred in a stable temperature environment and was not due to temperature fluctuations which might have created semi-permissive conditions for tumor growth. Cells isolated from tumors formed at nonpermissive temperatures and recultured in vitro displayed temperature-dependent hexose transport and colony formation in agar similar to the orginal parent cell inoculum. Similarly, virus recovered from tumors at nonpermissive temperatures retained the ts mutation.     

Mutants of vesicular stomatitis virus blocked at different stages in maturation of the viral glycoprotein

Maturation of the vesicular stomatitis virus (VSV) glycoprotein (G) to the cell surface is blocked at the nonpermissive temperature in cells infected with temperature-sensitive mutants in the structural gene encoding for G. We show here that these mutants fall into two discrete classes with respect to the stage of post-translational processing at which the block occurs. In all cases the mutant glycoproteins are inserted normally into the endoplasmic reticulum membrane, receive the two-high-mannose oligosaccharides, and apparently lose the NH2-terminal signal sequence of 16 amino acids. In cells infected with one class of mutants, no further processing of the glycoprotein occurs, and we conclude that the mutant protein is blocked at a pre-Golgi stage. In cells infected with ts L511(V), however, addition of the terminal sugars galactose and sialic acid occurs normally. Thus the maturation of G proceeds through several Golgi functions but is blocked before its appearance on the cell surface. The oligosaccharide chain of ts L511(V) G, accumulated at either the permissive (where surface maturation occurs) or the nonpermissive temperature, lacks one saccharide residue, probably fucose. In addition, no fatty acid residues are added to the ts L511(V) G protein at the nonpermissive temperature, although addition does occur under permissive conditions.