Analysis of structural proteins of purified murine cytomegalovirus

Animal cytomegaloviruses

Cytomegaloviruses are agents that infect a variety of animals. Human cytomegalovirus is associated with infections that may be inapparent or may result in severe body malformation. More recently, human cytomegalovirus infections have been recognized as causing severe complications in immunosuppressed individuals. In other animals, cytomegaloviruses are often associated with infections having relatively mild sequelae. Many of these sequelae parallel symptoms associated with human cytomegalovirus infections. Recent advances in biotechnology have permitted the study of many of the animal cytomegaloviruses in vitro. Consequently, animal cytomegaloviruses can be used as model systems for studying the pathogenesis, immunobiology, and molecular biology of cytomegalovirus-host and cytomegalovirus-cell interactions.

Synthesis and processing of a 22-26K murine cytomegalovirus glycoprotein recognized by a neutralizing monoclonal antibody

A 22-26K glycoprotein (gp24) of the murine cytomegalovirus (MCMV) virion was immunoprecipitated by a monoclonal antibody (MAb) 6A1.21A that neutralized MCMV infectivity only in the presence of complement. Pulse-chase experiments demonstrated that gp24, which contained only N-linked, complex-type oligosaccharides, was processed from an 18.4K high-mannose precursor (gp18.4). Analyses by two-dimensional (nonreducing/reducing) gel electrophoresis have shown that both gp18.4 and gp24 are present as disulfide-linked complexes, and rapid oligomerization of the 18.4K precursor is an early step in the processing pathway of gp24. Finally, we demonstrated that gp24 belongs to the "late" class of MCMV proteins.

Characterization of a major virion envelope glycoprotein complex of murine cytomegalovirus and its immunological cross-reactivity with human cytomegalovirus

Three glycoproteins on the murine cytomegalovirus (MCMV) virion with apparent molecular weights of 150K (gp 150), 105K (gp 105), and 52K (gp52) were immunoprecipitated by two monoclonal antibodies (MAbs) 8G5.12A and 2E.12A. However, only 8G5.12A was able to neutralize MCMV infectivity in the presence of complement. The accessibility of these three glycoproteins to radiolabeling by surface-iodination reactions suggested that they were exposed on the surface of the virion. Western blot analysis of the three glycoproteins showed that gp150 shared antigenic determinants with gp105 and gp52. Briefly, the MAb 8G5.12A reacted with gp150 and gp105, whereas the MAb 2E8.12A reacted with gp150 and gp52. A third MAb 3H2.12A was also found to be reactive with gp150 and gp105 in Western blots, but was unable to immunoprecipitate these glycoproteins. Data from pluse-chase experiments suggested that all three virion glycoproteins were synthesized from a common 128K precursor, providing a partial explanation of their antigenic relatedness. Furthermore, we have demonstrated the presence of high-molecular-weight complexes formed by disulfide bonding between gp150, gp105, and gp52. Lastly, the MAb 8G5.12A was able to immunoprecipitate 84K and 99-110K glycoproteins from human CMV-infected WI-38 cells, demonstrating that conserved determinants exist between murine and human CMV envelope glycoproteins.

A neutralizing monoclonal antibody recognizes an 87K envelope glycoprotein on the murine cytomegalovirus virion

An 87K glycoprotein (gp87) on the murine cytomegalovirus (MCMV) virion was immunoprecipitated by the neutralizing monoclonal antibody (MAb) 8D1.11A. The 87K glycoprotein is also radiolabeled in a surface iodination reaction, suggesting that it is exposed on the surface of the virion. Using a nondenaturing system of polyacrylamide gel electrophoresis in combination with Western blotting, we have shown that the epitope recognized by the MAb 8D1.11A resides on gp87. The failure of 8D1.11A to react with gp87 in a reduced and denatured form suggests that the epitope is recognized only when disulfide linkages are preserved. Our data also indicated that gp87 is present in the MCMV virion both in a monomeric form and as a component of disulfide-linked complexes. Using a two-dimensional gel electrophoresis system, we have demonstrated the presence of disulfide linkages between gp87 and virion polypeptides with apparent molecular weights of 138K, 46K, and 20K. Finally, the difference in migration rates of gp87 in SDS-polyacrylamide gels under reducing and nonreducing conditions suggests the existence of intramolecular disulfide bonds.

Detection of antibody to murine cytomegalovirus by enzyme-linked immunosorbent and indirect immunofluorescence assays

We have compared murine cytomegalovirus (MCMV) antibody determination by enzyme-linked immunosorbent assay (ELISA) and indirect immunofluorescence assay. A comparison of antibody detection with 146 serum samples at a 1:20 dilution showed 100% agreement (60 negatives and 86 positives) between the assays. There was close agreement of endpoint determinations of sera by both methods. After experimental MCMV infection, antibody to MCMV was detected by both assays as early as day 7, and high titers persisted as late as 6 months. In contrast to immunocompetent littermates, athymic nude mice did not develop antibody after infection. Mice lacking antibody detectable by ELISA were susceptible to lethal MCMV challenge. In a survey of animals from five commercial sources, MCMV antibody was not detected unless mice were experimentally infected. MCMV antibody determination by ELISA is a convenient method, comparable to the indirect immunofluorescence assay in sensitivity and specificity.

Late-phase expression of a murine cytomegalovirus immediate-early antigen recognized by cytolytic T lymphocytes

The cloned murine cytolytic T-lymphocyte line IE1-IL and several sublines detect a murine cytomegalovirus immediate-early (IE) membrane determinant in conjunction with Ld class I major histocompatibility glycoprotein. The lines retained cytolytic activity, strict antigen specificity, and self-restriction even when adapted to long-term, antigen-independent growth in the presence of interleukin-2 only (M. J. Reddehase, H.-J. Bühring, and U. H. Koszinowski, J. Virol. 57:408-412). These attributes allowed us to use IE1-IL as a stable, monospecific probe for tracing the expression of the IE membrane antigen throughout the viral replication cycle. Presentation of the antigen at the cell membrane proved to be most effective when expression of IE genes in infected mouse embryo fibroblasts was selectively enhanced by consecutive cycloheximide-actinomycin D treatment, whereas without enhancement high numbers of IE1-IL cytolytic T lymphocytes were required to demonstrate the antigen in the IE phase. In the early phase of infection when IE genes were no longer transcribed, cytolysis was not observed, although IE proteins were detectable in the nuclei of the infected cells. Without application of inhibitors IE membrane antigen expression was most prominent during the late phase of infection. Reinitiation of transcription from the genomic region encoding the major IE protein (pp89) and de novo synthesis of pp89 correlated with this reexpression of the IE membrane antigen.

The cytolytic T lymphocyte response to the murine cytomegalovirus. II. Detection of virus replication stage-specific antigens by separate populations of in vivo active cytolytic T lymphocyte precursors

During the acute cytolytic T lymphocyte (CTL) response of mice to infection with the murine cytomegalovirus two independent populations of activated interleukin-receptive CTL precursors can be demonstrated. One population is specific for cell membrane-incorporated viral structural antigens, whereas the second population detects an antigen, whose appearance is correlated with the synthesis of viral immediate early proteins. Since this new type of antigen is only defined by lymphocyte recognition, it is referred to as the lymphocyte-detected immediate early antigen (LYDIEA). Expression of immediate early antigen precedes the production of viral progeny and, therefore, it is possible that LYDIEA-specific CTL could serve as indicator cells for the very first activities of the viral genome, even during nonproductive infection.

Herpesvirus sylvilagus I. Polypeptides of virions and nucleocapsids

Herpesvirus sylvilagus was propagated in juvenile cotton tail rabbit kidney cells and purified from the cytoplasmic fraction of the infected cells. The purification procedure included zonal centrifugation through a 5 to 30% dextran t-10 gradient, followed by equilibrium centrifugation in a 5 to 50% potassium tartrate gradient. H. sylvilagus formed one band after centrifugation through the tartrate gradient at a density of 1.22 g/cm3. Contamination of the purified virus preparation by cellular proteins was less than 0.2% as determined by the removal of radioactivity from an artificially mixed sample containing [35S]methionine-labeled control cells and nonlabeled infected cells. H. sylvilagus nucleocapsids were isolated from infected cell nuclei and purified by sedimentation through a 36% sucrose cushion, followed by equilibrium centrifugation in 5 to 50% tartrate gradient. Forty-four polypeptides ranging in molecular weight from 18,000 to 230,00 were resolved when [35S]methionine-labeled enveloped H. sylvilagus was analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Seventeen polypeptides found within the enveloped virus were also identified with the nucleocapsid. Six additional nucleocapsid polypeptides han no counterparts within the enveloped virus. The major polypeptide within both the virus and the nucleocapsid had a molecular weight of 150,000.

Proteins Specified by bovine herpesvirus 1 (infectious bovine rhinotracheitis virus)

An electrophoretic analysis of radioactively labeled, purified, "empty" and DNA-containing infectious bovine rhinotracheitis virions revealed the presence of 25 to 33 structural (virion) polypeptides. A total of 11 of these polypeptides could be labeled with [3H]glucosamine and were identified as glycoproteins. In addition to the 25 structural polypeptides, infectious bovine rhinotracheitis virus infected cells also contained at least 15 nonstructural (nonvirion) polypeptides that were not present in purified virions. Expression of the viral polypeptides in infected cells was controlled temporally. Thus, most viral polypeptides could be categorized as "alpha" (immediate early), "beta" (early), or "gamma" (late) on the basis of their order of appearance in infected cells and whether their syntheses were dependent upon prior viral protein or DNA synthesis. None of the glycoproteins belongs to the alpha class, although at least one (GVP11) was synthesized in the absence of viral DNA synthesis. Serum from a cow in which infectious bovine rhinotracheitis virus lesions were reactivated by dexamethasone precipitated both structural and nonstructural polypeptides.

Analysis of structural polypeptides of purified human cytomegalovirus

Human cytomegalovirus strain C87 was purified by the following procedures. (i) Extracellular virus was concentrated by centrifugation at 100,000 X g for 90 min and passed through a Bio-Rad Bio-Gel A-15m column. Most of the virus was recovered in the void volume. (ii) After two consecutive isopycnic potassium tartrate gradient centrifugations (20 to 50%), coinciding peaks of plaque titer, protein, and radioactivity were found at a density of from 1.20 to 1.21 g/cm3. To characterize the structural polypeptides of human cytomegalovirus and to establish relative purification criteria, virus was purified from two mixtures: (i) [35S]methionine-labeled extracellular virus mixed with an equal volume of unlabeled normal culture fluid; (ii) unlabeled extracellular virus mixed with an equal volume of [357a1methionine-labeled normal culture fluid. The extent of purification, as judged by the ratio of cellular to viral radioactivity, was 39-fold; i.e. about 2.5% of the protein in the purified virus preparation could be accounted for by host protein contamination. Electrophoresis of purified [35S]methionine-labeled virus on a polyacrylamide gel slab showed that there were at least 33 viral structural polypeptides (VPs), and their molecular weights ranged from 11,000 to 290,000. Autoradiograms obtained from electropherograms of purified [14C]glucosamine labeled virus showed six bands. Four of these were so broad that several VPs corresponded to each of the glycosylated bands. When heavy (two fractions close to 1.21 g/cm3) and light (two fractions close to 1.20 g/cm3) fractions of the PFU peak from the second potassium tartrate gradient were analyzed separately, the number of polypeptides observed was the same, but the relative amounts of some polypeptides differed. The major polypeptide, VP17, was found in greater amounts in the heavy fraction (35%) than in the light fraction (22%). The amount of DNA as a percentage of the weight of protein was 2% for the light fraction and 1% for the heavy fraction.