Analysis of structural polypeptides of purified human cytomegalovirus

Mass Spectrometry-Based Characterization of the Virion Proteome, Phosphoproteome, and Associated Kinase Activity of Human Cytomegalovirus

The assembly of human cytomegalovirus (HCMV) virions is an orchestrated process that requires, as an essential prerequisite, the complex crosstalk between viral structural proteins. Currently, however, the mechanisms governing the successive steps in the constitution of virion protein complexes remain elusive. Protein phosphorylation is a key regulator determining the sequential changes in the conformation, binding, dynamics, and stability of proteins in the course of multiprotein assembly. In this review, we present a comprehensive map of the HCMV virion proteome, including a refined view on the virion phosphoproteome, based on previous publications supplemented by new results. Thus, a novel dataset of viral and cellular proteins contained in HCMV virions is generated, providing a basis for future analyses of individual phosphorylation steps and sites involved in the orchestrated assembly of HCMV virion-specific multiprotein complexes. Finally, we present the current knowledge on the activity of pUL97, the HCMV-encoded and virion-associated kinase, in phosphorylating viral and host proteins.

Global aspects of viral glycosylation

Enveloped viruses encompass some of the most common human pathogens causing infections of different severity, ranging from no or very few symptoms to lethal disease as seen with the viral hemorrhagic fevers. All enveloped viruses possess an envelope membrane derived from the host cell, modified with often heavily glycosylated virally encoded glycoproteins important for infectivity, viral particle formation and immune evasion. While N-linked glycosylation of viral envelope proteins is well characterized with respect to location, structure and site occupancy, information on mucin-type O-glycosylation of these proteins is less comprehensive. Studies on viral glycosylation are often limited to analysis of recombinant proteins that in most cases are produced in cell lines with a glycosylation capacity different from the capacity of the host cells. The glycosylation pattern of the produced recombinant glycoproteins might therefore be different from the pattern on native viral proteins. In this review, we provide a historical perspective on analysis of viral glycosylation, and summarize known roles of glycans in the biology of enveloped human viruses. In addition, we describe how to overcome the analytical limitations by using a global approach based on mass spectrometry to identify viral O-glycosylation in virus-infected cell lysates using the complex enveloped virus herpes simplex virus type 1 as a model. We underscore that glycans often pay important contributions to overall protein structure, function and immune recognition, and that glycans represent a crucial determinant for vaccine design. High throughput analysis of glycosylation on relevant glycoprotein formulations, as well as data compilation and sharing is therefore important to identify consensus glycosylation patterns for translational applications.

Analysis of baculovirus genomes with restriction endonucleases

The viral DNAs from nine wild-type insect baculoviruses have been isolated and the EcoR-1 restriction endonuclease fragment patterns compared. Genomic heterogeneity could be detected in the DNA restriction patterns of four of these wild-type baculoviruses. Three infectious virus forms (two that are occluded in the nucleus and an extracellular virus that has budded from the plasma membrane of infected cells) of a nuclear polyhedrosis virus with multiple nucleocapsids per envelope (MNPV) from the lepidopteran insect, Autographa californica are shown to be phenotypically distinct by comparison of viral structural polypeptides by polyacrylamide gel electrophoresis and autoradiography of L[35S]methio-nine-labeled virus proteins. The three phenotypic forms were cloned by successive plaque purification and eight distinct variants were identified from 11 plaque-purified viruses by genotypec analysis with EcoR-1 and HindIII restriction endonuclease. Isolation of variants from the three phenotypic forms has shown that each of the infectious forms is heterogeneous and that no segregation of genotypes among the three forms was evident. The characteristic restriction fragment patterns of several variants were maintained upon multiple passage in cell culture.

The human cytomegalovirus UL35 gene encodes two proteins with different functions

The human cytomegalovirus (HCMV) virion is a complex structure that contains at least 30 proteins, many of which have been identified. We determined that the HCMV UL35 gene encodes two proteins, including a previously unidentified virion protein. A 22-kDa phosphoprotein (ppUL35(A)) was translated from a 1.2-kb UL35 transcript by 4 h postinfection; a second phosphoprotein of 75 kDa (ppUL35) was translated from a 2.2-kb transcript predominantly late in infection. The 22-kDa protein localized to the nucleus, while the 75-kDa protein localized to the juxtanuclear compartment and was packaged into virion particles. The 22-kDa protein was identical to the COOH-terminal end of the 75-kDa protein but was not found in virions, thus defining the NH(2)-terminal portion of the 75-kDa protein as essential for packaging. Expression of the 22-kDa protein inhibited activation of the major immediate-early promoter by ppUL82 (pp71), suggesting that the UL35 22-kDa protein may modulate expression of the major immediate-early gene.

Wild isolates of murine cytomegalovirus induce myocarditis and antibodies that cross-react with virus and cardiac myosin

The laboratory-adapted K181 strain of murine cytomegalovirus (MCMV) induces both acute and chronic myocarditis, associated with autoantibodies to cardiac myosin, in susceptible BALB/c mice. However, the K181 MCMV strain has been maintained in the laboratory for many years and may not resemble naturally occurring strains of MCMV in its ability to induce myocarditis. Accordingly, six different isolates of MCMV from wild Mus domesticus were compared with K181 MCMV for their ability to induce myocarditis and autoantibodies to cardiac myosin in BALB/c mice. These isolates were shown to induce acute myocarditis similar to K181 MCMV, with associated focal and diffuse myocardial inflammation. However, the levels of myocarditis induced by the wild isolates during the chronic phase of the disease (days 32-56 post-infection) were low in contrast to the K181 strain. Interestingly, 30% of wild-trapped mice showed histological evidence of myocarditis and all were sero-positive to MCMV. Sera from BALB/c mice infected with wild MCMV isolates and from wild-trapped mice contained antibodies that cross-reacted with MCMV and cardiac myosin (S2 region). The cross-reactive region of MCMV was found to be a 50,000-55,000 MW viral polypeptide. These findings suggest that molecular mimicry may be involved in the pathogenesis of autoimmune myocarditis following infection with both laboratory and wild MCMV strains.

Proteins associated with purified human cytomegalovirus particles

Virion-associated proteins isolated from purified human cytomegalovirus particles (strain AD169) were used as substrates for chemical sequence analysis. Extracellular virions, noninfectious enveloped particles, and dense bodies were purified by negative viscosity-positive density gradient centrifugation, and their component proteins were separated by denaturing polyacrylamide gel electrophoresis. The deduced amino acid sequence of individual protein bands was used to identify six corresponding viral genes whose products have not previously been identified as virion constituents: UL47, UL25, UL88, UL85, UL26, and UL48.5. In addition, a 45-kDa cellular protein was identified, and the protein fragments sequenced have a high degree of amino acid identity with actin. However, antiactin monoclonal and polyclonal antibodies did not react with a specific protein in the virus preparations, suggesting that this 45-kDa protein is an immunologically distinct isoform of actin. The newly identified viral and cellular proteins were resistant to protease treatment of purified virions, suggesting that they are unlikely to be contaminants of the viral preparations.

A murine cytomegalovirus-neutralizing monoclonal antibody exhibits autoreactivity and induces tissue damage in vivo

The autoreactivity of murine cytomegalovirus (MCMV)-neutralizing monoclonal antibody (mAb) AC1 was examined in vitro and in vivo. Both mAb AC1 and a human antiserum reactive with U1-small nuclear ribonucleoprotein (U1-snRNP) stained uninfected mouse embryo fibroblasts (MEF) in a speckled nuclear pattern and reacted with 70,000 molecular weight (MW) MEF nuclear antigens by immunoblotting, suggesting that mAb AC1 cross-reacted with the 70,000 MW component of U1-snRNP. However, only mAb AC1 cross-reacted with an additional epithelial cytoplasmic autoantigen present in cultured HEp2 cells. On tissue sections from uninfected mice, mAb AC1 predominantly reacted with a component of central and peripheral nervous systems, although cross-reactivity with the stratum spinosum of the skin and the outer sheath of hair follicles was also observed. Immunoblotting revealed that mAb AC1 reacted with phosphorylated epitopes present on a 98,000 MW MCMV structural protein and the 200,000 MW mouse neurofilament protein (NFP). Treatment of uninfected mice with mAb AC1 resulted in a severe interstitial pneumonia with greatly thickened and congested alveolar septa. Severe oedema of the hypodermis and a mild mesangial proliferative glomerulonephritis were also observed. These results demonstrate that a mAb reacting with a MCMV structural phosphoprotein which can protect mice against the dissemination of MCMV, can also promote the development of autoimmune disease. Therefore, the production of such cross-reactive antibodies may be an important mechanism in the development of autoimmunity following viral infection.

Human cytomegalovirus binding to fibroblasts is receptor mediated

The binding of radiolabeled human cytomegalovirus (HCMV) strain AD169 to human lymphocytes, lymphoblastoid cell lines, monocytes, and fibroblasts varied over a 20-fold range. Since maximum binding was observed with human foreskin fibroblasts (HFF), interactions of radiolabeled HCMV with this cell type were analyzed quantitatively. Binding of HCMV to HFF at 4 degrees C was specific and saturable; at low viral inputs specific binding averaged 16.4% of input and nonspecific binding was less than 1% of input. Binding curves yielded single-component linear Scatchard plots indicating an average Kd of 1.1 nM and 5,262 available virus-binding sites per cell. A two-component Scatchard curve was obtained at 37 degrees C and reflected viral internalization, since it could be converted to a single-component curve by the use of paraformaldehyde-fixed cells. HCMV strain Towne was found to bind to the receptor used by HCMV strain AD169 with similar affinity. HCMV failed to bind to protease-treated HFF or to HFF grown in the presence of inhibitors of glycosylation. Sialic acid residues, however, were not found to be important in binding. These data indicate that a single type of molecule, likely a glycoprotein, on the surface of HFF serves as a specific receptor for the virus.

Pattern of anti-cytomegalovirus IgM antibodies determined by immunoblotting. A study of kidney graft recipients developing a primary or recurrent CMV infection

In order to improve the knowledge of the humoral immune response to CMV infection, we developed an immunoblotting technique which allowed a better analysis of the changes in the pattern of anti CMV-polypeptides IgM. We examined 234 sera belonging to 27 renal allograft recipients developing a primary or recurrent CMV infection and 12 non infected recipients. Thus we found that 11 main anti CMV-polypeptides IgM antibodies were present in over 25% of the infected patients. They reacted with proteins whose molecular weights ranged from 32K to 205K. We showed that anti-p 45-47 IgM antibodies were present in 100% of CMV infected recipients and never in the non-infected population. They appeared very early in the course of the infection (5.43 weeks post-graft for primary infection and 5.00 weeks for recurrent ones) and, therefore, constitute a good marker of active infection. Two other CMV-specific IgM antibodies (anti-p 60-64 and anti-p 100) were found exclusively in the course of primary infections. Anti-p 60-64 IgM was observed at a high frequency (57.1%) and with a mean delay of 6.57 weeks post-graft. Therefore, the anti-p 60-64 IgM detection could be helpful for the diagnosis of primary infection. In almost 100% of both primary and recurrent infections, we observed anti-p 140 and anti-p 38 IgM antibodies. Only about 50% of non-infected patients had low levels of anti-p 140 and anti-p 38 IgM. The follow-up of recurrent infections showed that the anti CMV-polypeptides IgM antibodies appeared earlier than in primary infection. When we compared anti-p 45-47 IgM detection by immunoblotting and anti-CMV IgM detected by ELISA we observed that immunoblotting permitted the diagnosis 2.5 weeks earlier for primary infection, and 1 week earlier for recurrent infection, than ELISA. In addition, the detection of anti-p 45-47 IgM antibodies also occurred earlier than virus excretion.

The human cytomegalovirus strain Towne glycoprotein H gene encodes glycoprotein p86

The gene encoding the glycoprotein H (gH) homologue of CMV strain Towne was cloned, sequenced, and expressed. The predicted 742 amino acid gH protein had characteristics typical of a membrane glycoprotein including hydrophobic signal and transmembrane domains and six possible N-linked glycosylation sites. The CMV (Towne) gH gene had a 95% nucleotide identity and a 96.6% amino acid identity with the CMV (AD169) gH gene, as described by M. P. Cranage, G. L. Smith, S. E. Bell, H. Hart, C. Brown, A. T. Bankier, P. Tomlinson, B. G. Barrell, and T. C. Minson (1988, J. Virol. 62, 1416-1422). Transcriptional analysis of the gH gene revealed that the 2.9-kilobase (kb) gH transcript was not detected until late after CMV infection, indicating that the kinetics of gH expression were typical of the late class of CMV genes. The gH gene was expressed in COS cells using a vector in which transcription was driven by the SV40 early promoter. The expression of gH was detected by immunofluorescence using the virus neutralizing murine monoclonal antibody 1G6, which is specific for an 86-kilodalton (kDa) CMV virion membrane protein (p86). Amino acid sequence analysis of p86 tryptic peptides revealed sequence identity with peptides from the deduced gH amino acid sequence, confirming that the gH gene encodes p86. These results indicate that CMV gH can induce virus neutralizing antibodies and establishes gH as a candidate antigen for a subunit vaccine against CMV.

Identification of the major capsid protein gene of human cytomegalovirus

The coding region for the major capsid protein (MCP) of human cytomegalovirus (HCMV) was identified by comparing the protein sequence with the respective sequences of herpes simplex virus (HSV), Epstein-Barr virus, and varicella-zoster virus. The predicted length of the HCMV MCP was 1,370 amino acids. Comparison of the MCP sequences of the different human herpesviruses showed a homology of 25% to the MCP of HSV type 1, a homology of 29% to the MCP of Epstein-Barr virus, and a homology of 23% to the MCP of varicella-zoster virus. A subfragment of the HSV type 1 KpnI i fragment encoding the MCP VP5 cross-hybridized with the HCMV HindIII U fragment containing part of the MCP gene. Northern (RNA) blot analyses with subclones out of the coding region for the HCMV MCP detected one large transcript of about 8 kilobases. A portion of the open reading frame was expressed in Escherichia coli plasmid pBD2 IC2OH as a beta-galactosidase fusion protein and was used to generate polyclonal antibodies in New Zealand White rabbits. The obtained antisera reacted in Western immunoblots with the MCP of purified HCMV virions. A monoclonal antibody against the human MCP and a monospecific rabbit antiserum against strain Colburn of simian cytomegalovirus detected the fusion protein as well as the MCP of purified virions in immunoblots.

Genomic locus for a 140 kDa structural protein (pp150) of human cytomegalovirus in strains Towne and AD169

An HCMV specific clone was isolated from a genomic library of human cytomegalovirus (HCMV) DNA cloned into the expression vector lambda gt11. This clone (lambda 111-1) expressed an HCMV/beta-galactosidase fusion protein which was reactive with rabbit antibody prepared against purified HCMV virions and dense bodies as well as human HCMV immune serum. By probing Western blots of HCMV virion proteins or HCMV-infected cells with antibody prepared against the fusion protein, the authentic gene product of clone lambda 111-1 was identified as a high molecular weight polypeptide of 140. Probing the restriction digests of HCMV DNA with insert DNA from the immunoreactive lambda gt11 clone permitted us to localize the coding sequence for the 140 kDa polypeptide to the long unique region (map coordinates of 0.16-0.18) on HCMV Towne and AD169 genomes.

Genomic localization of the gene encoding a 32-kDa capsid protein of human cytomegalovirus

We have determined the map position of a viral gene encoding a 32-kDa late structural protein of human cytomegalovirus (HCMV) using a murine monoclonal antibody. This monoclonal antibody was reactive with two protein bands of 32 and 27 kDa in HCMV-infected cell lysates and with a single 32-kDa protein band in HCMV virions as detected by immunoblot analysis. When purified HCMV envelope preparation was used for immunoblotting, the monoclonal antibody did not display a detectable band. We used this monoclonal antibody to screen a cDNA library that was constructed from poly(A)+ RNA of late HCMV-infected cells and cloned into the expression vector lambda gt11. A cDNA clone that expressed an immunoreactive epitope of the late HCMV protein fused to beta-galactosidase was identified. Probing the restriction digests of HCMV (Towne and AD169) DNA with insert DNA from the immunoreactive lambda gt11 clone permitted us to localize the coding sequence within the long unique region between map coordinates of 0.62 and 0.64 of HCMV Towne and AD169 genomes. Using the same probe, a single transcript of 1.4 kb was detected in total RNA from HCMV-infected cells at late times after infection.

Isolation and characterization of a human cytomegalovirus glycoprotein containing a high content of O-linked oligosaccharides

Several disulfide linked glycoprotein complexes were extracted from human cytomegalovirus with a non-ionic detergent and separated by anion exchange high performance liquid chromatography (HPLC). One complex had a molecular weight of 93,000 and was classified as gCII-93. Another complex had a molecular weight greater than 200,000 and was classified as gCII-200. Both complexes immunoprecipitated with a monoclonal antibody (9E10). A third set of complexes (classified as gC-I) immunoprecipitated with another monoclonal antibody (41C2). Isolated complexes were reduced, alkylated, and individual glycoproteins separated by gel-filtration HPLC. Glycoproteins with molecular weights of 50-52,000 from gCII-93 and gCII-200 appeared to be the same glycoprotein since they could be immunoprecipitated by 9 E 10 and had identical peptide maps. The amino sugar content of these glycoproteins was compared to that of higher molecular weight glycoproteins obtained from gCII-200 and to a glycoprotein with a molecular weight of 93,000 (gp93 (I] from gCI. Glycoproteins with molecular weights of 50-52,000 from gCII-93 and gCII-200 contained similar amounts of galactosamine (GalN), glucosamine (GlcN) and sialic acid. However, they contained 2-3 times more GalN than any other glycoprotein from gCII-200 and 10 times more GalN than was detected in gp93 (I). All glycoproteins from gCII-93 or gCII-200 also contained more sialic acid when compared to gp93 (I). GalN in these glycoproteins was present in O-linked oligosaccharides. This was demonstrated by release of low molecular weight oligosaccharides from high molecular weight glycopeptides by mild base hydrolysis and the conversion of GalN to galactosaminitol. Thus, gp52(II) appears to have a unique phenotype marked by a high amount of O-linked oligosaccharides.

Characterization of two different human cytomegalovirus glycoproteins which are targets for virus neutralizing antibody

In previous studies we have identified two viral polypeptides detected by murine monoclonal antibodies which neutralize the infectivity of human cytomegalovirus (CMV) AD169. One is an 86,000-Da polypeptide (p86) and the second is a complex of two major coimmunoprecipitating polypeptides of 130,000 and 55,000 Da (p130/55). In this study we have shown that the two viral polypeptides are immunologically unrelated and have distinct peptide cleavage patterns. We have characterized these polypeptides as glycoproteins and studied their biosynthesis in human embryonic lung cells. The oligosaccharides found on both the p86 and the p130/55 were characterized by endoglycosidase digestion as N-linked high-mannose carbohydrates. Inhibitors of glycosylation were used to further characterize the oligosaccharides. Tunicamycin, which inhibits the biosynthesis of N-linked oligosaccharides on the endoplasmic reticulum, inhibited both the infectivity and biosynthesis of the p86 and p130/55. The underglycosylated forms in tunicamycin-treated cultures could be detected only under conditions of pulse-labeling with L-[35S]methionine. Monensin, which inhibits the modification of glycoproteins from simple to complex forms in the Golgi, reduced viral infectivity at concentrations which had no effect on viral protein synthesis, but did not alter the apparent molecular weight of either the p86 or the p130/55. The oligosaccharides were critical for the in vitro immunologic reactivity of the p86 in immunoblots. However, endoglycosidase F-treated p86 was comparable to the native form in inducing virus neutralizing antibody in guinea pigs. Endoglycosidase F-treated p130/55 retained its ability to bind antibody in Western blots.

Virion basic phosphoprotein from human cytomegalovirus contains O-linked N-acetylglucosamine

A 149-kDa virion protein of human strains of cytomegalovirus is the principal acceptor for galactose added in vitro by bovine milk galactosyltransferase. Peptide comparisons with other biochemical characteristics of the galactosylated protein identified it as the virus-encoded basic phosphoprotein. This protein is an abundant constituent of the virion and is located in the tegument region, between the capsid and the envelope, rather than in the envelope layer with the recognized virion glycoproteins. The galactosylated carbohydrate was resistant to a commercial preparation of endoglycosidase F but was sensitive to removal by alkali-induced beta-elimination, indicating an O-linkage to the protein. Chromatographic and electrophoretic determinations identified the beta-eliminated material as the alditol of Gal beta 1-4GlcNAc, establishing that the human cytomegalovirus virion basic phosphoprotein contains single O-linked residues of N-acetylglucosamine.

Comparative study with monospecific and monoclonal antibodies against a 65 K human cytomegalovirus protein

Immunofluorescence assay using monospecific and monoclonal antibodies to the 65 K major protein of human cytomegalovirus (HCMV) was carried out to monitor the expression of this protein in infected cells. Regardless of differences in the reactivity of the monoclonal antibodies, as determined by immunoblotting and immunofluorescent staining, all stained cytoplasmic inclusion bodies localized to the site of the HCMV-induced receptor for the Fc portion of IgG, suggesting that most of the 65 K major protein of HCMV colocalizes with the HCMV-induced FcR.

Human cytomegalovirus structural components: intracellular and intraviral localization of p28 and p65-69 by immunoelectron microscopy

Using specific monoclonal antibodies, we have localized two structural proteins (p65-69 and p28) of human cytomegalovirus in infected cells and in virions and/or virus-related particles by immunoelectron microscopy using protein A-gold. Protein p65-69 is present in some roundish structures in the nuclei, often in contact with the viroplasm, and in the cytoplasm, exclusively within the dense body matrix. Protein p28 is present only in the outline of cytoplasmic capsids, and reaches the highest density in the large aggregates of virions and dense bodies which are particularly numerous during the late phases of the viral replication cycle.

Production and characterisation of a human monoclonal antibody to cytomegalovirus and its use in an early nuclear fluorescence assay

A human monoclonal antibody to cytomegalovirus (CMV) was produced by transforming peripheral blood mononuclear cells of a patient with recent CMV infection. It is directed against a late antigen located in the nucleus of CMV infected fibroblasts at 24-72 hours postinfection and immuneprecipitates 65K and 48K proteins from 35S-labelled CMV infected cells. Results of its use in an early nuclear fluorescence assay for rapid diagnosis are presented.

Analysis by immunoblotting of human cytomegalovirus antibody in sera of renal transplant recipient

Human cytomegalovirus-infected cell polypeptides were immunoreacted by sera of renal transplant recipients and compared with those reactive with sera of healthy adult donors by means of the Western immunoblotting technique. At least 15 polypeptides with molecular weights of 155K, 123K, 102K, 89K, 79K, 71K, 65K, 60K, 55K, 50K, 46K, 42K, 38K, 33K, and 28K were immunoreacted. Sera obtained serially from renal transplant recipients reacted with most of these polypeptides and reacted more frequently and intensely with the smaller polypeptide species such as 38K, 33K, and 28K, compared with sera of healthy seropositive adults. The implications of these findings are discussed.

Molecular biology and immunology of cytomegalovirus

The application of modern biochemical techniques has led to a rapid improvement in our knowledge of the molecular biology of CMV. Several coding regions of the DNA genome have been identified with certainty and major virus-coded proteins have been given provisional names. The cascade expression of the CMV genome has been shown to be controlled by mechanisms similar to those found in other herpes viruses, together with novel post-transcriptional controls which remain to be defined. The control of CMV replication by the host involves both non-specific and specific defence mechanisms. The induction of natural killer cells and interferon early after CMV infection appears to be the most important aspects of the non-specific host defence against the virus. The cell-mediated immune response, in particular the generation of Tc cells against CMV early antigens, is probably the most important facet of the specific immune defence against CMV. When intact these defence mechanisms appear to be efficient in restricting viral replication; however, when such immunity is compromised, the balance rapidly swings in favour of the virus. As our understanding of the interaction between the host and the virus increases, it may be possible to redress the balance in such cases in favour of the host.

Prospects for the clinical management of human cytomegalovirus infections

Infection of susceptible populations by human cytomegalovirus (HCMV) is a significant public health problem in Western societies. Vaccination with live attenuated strains of HCMV has demonstrated some degree of clinical benefit but objections based on the possibility of these viruses becoming latent and their potential oncogenicity must be considered. Our knowledge of the biology and immunology of HCMV, although advancing rapidly, is still a long way short of being able to predict candidate subunit vaccines based on virus encoded proteins or glycoproteins. Treatment of the disease by injection of antibodies awaits a breakthrough and chemicals effective in the control of other human herpes viruses are disappointingly ineffective against HCMV. Clearly, prophylaxis is preferable to therapy and it is in the design of new effective vaccines that endeavours must be channelled so that we can control complications associated with severe clinical infection with this virus.

Confirmation of human cytomegalovirus by reverse passive hemagglutination with monoclonal antibodies reactive to the major glycosylated peptide (GP-66)

Sheep erythrocytes coated with three monoclonal antibodies, each reactive to a different epitope of the 66-kilodalton cytomegalovirus (CMV) matrix protein, were used in a reverse passive hemagglutination test with CMV-infected cell lysate to identify and confirm the CMV. The test is specific only for CMV, since 5 laboratory strains of CMV (AD169, Davis, Espilat, C-87, and Towne) and 10 clinical isolates reacted well, but uninfected MRC-5 cell lysate, lysates of herpes simplex virus types 1 and 2, varicella-zoster virus, and adenoviruses did not react. The reactive CMV lysate was confirmed by the pretreatment of CMV lysate with the three monoclonal antibodies followed by the addition of antibody-coated erythrocytes. The reverse passive hemagglutination test and the confirmatory blocking test are performed at the same time, requiring 2 h to complete. Since V-bottom microtiter 96-well plates and a 25-microliter pipette can be used to perform the test, it is ideal for CMV confirmation, especially when the equipment to read the fluorescent-antibody test or enzyme-linked immunosorbent assay is not available.

Primate cytomegalovirus glycoproteins: lectin-binding properties and sensitivities to glycosidases

The lectin-binding properties and glycosidase sensitivities of the virion glycoproteins of primate cytomegaloviruses (CMVs) were examined. Three simian CMV (SCMV) strains, including Colburn, and four human CMV (HCMV) strains were compared. Their proteins were separated in denaturing polyacrylamide gels and electrotransferred onto nitrocellulose, and the glycosylated species were visualized with iodinated concanavalin A or wheat germ agglutinin (WGA). Virions of both HCMV and SCMV strains contained six principal and several minor lectin-reactive bands. Neuraminidase treatment abolished WGA binding and reduced the charge and charge heterogeneity of the SCMV (i.e., Colburn) virion glycoproteins and had a similar, although less dramatic, effect on those of HCMV. The specificities of concanavalin A and WGA in these assays were evaluated with endo-beta-N-acetylglucosaminidase H and endo-beta-N-acetylglucosaminidase F, and a combination of lectins and glycosidases was used to demonstrate that many of the primate CMV glycoproteins contain both high-mannose and complex, N-linked oligosaccharides. Results suggest that the HCMV virion glycoproteins are more extensively glycosylated or have more completely processed carbohydrate side chains, or both, than their SCMV counterparts.

Synthesis and processing of the envelope gp55-116 complex of human cytomegalovirus

The envelope of human cytomegalovirus has been reported to contain between three and eight glycoproteins. Major constituents of the envelope include two abundant glycoproteins with estimated molecular weights of 55,000 (gp55) and 116,000 (gp116). These two glycoproteins have been shown to exist as a disulfide-linked complex (gp55-116) within the envelope of mature virions. Utilizing a panel of monoclonal antibodies reactive with the gp55-116 complex, we characterized the synthesis and processing of these two virion proteins. Infected cells were shown to contain two glycosylated proteins of 160,000 and 150,000 daltons as well as the mature gp55 and gp116. Pulse-chase analysis indicated that gp150 was a precursor protein of gp160. The mature gp55 and gp116 were generated, in turn, by cleavage of gp160. Antigenic and structural analysis revealed that gp55 and gp116 shared little structural homology and no detectable antigenic cross-reactivity. The results of this study are discussed in relation to the synthesis of envelope proteins of other herpesviruses.

Human immune response to cytomegalovirus structural polypeptides studied by immunoblotting

To define better the human immune response to individual structural proteins of human cytomegalovirus (HCMV), 55 human sera with different IgG and IgM titres were studied for their reactivity with HCMV structural polypeptides separated by SDS-PAGE and electrotransferred to nitrocellulose paper. The results obtained showed that antibody titres detected by immunoassay correlate with the intensity and the number of polypeptides reacting by immunoblotting (IB). The IB profiles of HCMV polypeptides reacting with different sera having the same antibody titres show considerable variation. Sera with high levels of IgG antibody and that are IgM-positive frequently react with 155, 149, 82.5, 74.5, 67, 57, 55, 38.5, and 28 kD polypeptides; all these sera react with 155, 67, 57, 55, 38.5 kD polypeptides. Sera with high levels of IgG antibody but that are IgM negative frequently react with all these polypeptides, with the exception of 149 and 74.5. Only 155 and 28 kD polypeptides were recognized by all sera of this group. The sera with moderate levels of IgG antibody preferentially recognize 155, 110, 82.5, 62, 55, 38.5 and 28 kD polypeptides. The sera with low levels of antibody reacted especially with 155 and 62 kD polypeptides. IgM antibody seems to recognize preferentially 155, 110, 67, 57, 55, 38.5 kD polypeptides.

Anti-complement immunofluorescence establishes nuclear localization of human cytomegalovirus matrix protein

A monospecific, polyclonal antiserum to the 69-kDa matrix protein of human cytomegalovirus (HCMV) was prepared in a guinea pig and used to determine the intracellular distribution of this viral antigen. The resulting antiserum was specific for infected cells as tested by immunofluorescence, and specific for the HCMV matrix protein as determined by "nitrocellulose immunoassay" of electrophoretically separated, infected-cell proteins. Antibodies were reacted with fixed, infected human fibroblasts, and visualized by the anti-complement immunofluorescence procedure to avoid complications arising from the strong IgG Fc binding activity of the infected-cell-specific cytoplasmic inclusion. Results establish that the matrix protein is located in the nucleus, and indicate that it is concentrated in the nucleoplasm rather than within the intranuclear inclusions.

Distinction of viral and host-derived glycopolypeptides induced by "early" functions of human cytomegalovirus

Human cytomegalovirus is shown to induce in phosphonoacetic acid-treated human fibroblasts glycosylation of five polypeptides with approximate molecular weights of 200-250, 150, 135, 130 and 100 kilodaltons (kd). Except for the 130 kd product, these glycopolypeptides (gp) separate with the cytoplasmic fraction, only one (200-250 kd) with the chromatin fraction as well. The gp of 135 and 100 kd were found to be virus-specified as determined by immunoblotting and immunoprecipitation. The gp of 200-250 kd exhibited an immunological relatedness to fibronectin and are therefore considered host-specific products. Both subsets of gp participate in virus-induced surface membrane alterations as documented by living cell immunofluorescence.

Human cytomegalovirus polypeptides stimulate neutralizing antibody in vivo

At least three human cytomegalovirus polypeptides are targets for virus neutralizing antibody; a single protein of 86,000 molecular weight (p86) and two coimmunoprecipitating proteins of 130,000 and 55,000 molecular weight (p130/55). These polypeptides have been isolated by immunoaffinity chromatography and tested for immunogenicity in guinea pigs. Neutralizing antibody was detected after immunization with both p86 and p130/55. Hyperimmune sera to p130/55, but not p86, were dependent upon guinea pig complement for virus neutralization.

Cloning and physical mapping of a gene fragment coding for a 64-kilodalton major late antigen of human cytomegalovirus

We have isolated a clone containing a gene fragment coding for a 64-kilodalton glycoprotein that is the major late antigen of human cytomegalovirus (HCMV). Based upon the amino acid sequence of a tryptic peptide of this glycoprotein (HCMVgp64), two sets of mixed-sequence probes, one consisting of a mixture of 16 heptadecadeoxyribonucleotides and the other a mixture of 32 icosadeoxyribonucleotides, were synthesized. A subgenomic library of HCMV (Towne strain) DNA was constructed in plasmid pBR327 and transformants were screened with 32P-labeled aliquots of these synthetic oligodeoxyribonucleotide probes. Two clones among 15,000 gave strong positive signals. Plasmid DNA was isolated from the positive clones and characterized by restriction mapping and Southern blot analysis using both probes. The plasmid DNA contained a 2.3-kilobase insert, which yielded an 800-base-pair and a 1500-base-pair fragment after Sau3A digestion. Only the 800-base-pair fragment hybridized to the mixed probes, and DNA sequence analysis revealed that it contains nucleotide sequences compatible with amino acid sequences of tryptic peptides of HCMVgp64. Restriction mapping studies of HCMV DNA using this 32P-labeled 800-base-pair cloned DNA have allowed us to locate this gene fragment in the long unique region of HCMV (Towne strain) genome at approximately equal to 0.5-0.51 map unit.

Equine cytomegalovirus: structural proteins of virions and nucleocapsids

Enveloped virions and nucleocapsids of equine cytomegalovirus (ECMV; equine herpesvirus type 2) have been purified from the supernatants and the nuclear extracts of infected rabbit kidney (RK) cells, respectively, and their structural protein compositions have been analyzed. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis revealed that ECMV nucleocapsids were composed of nine proteins (average molecular weights = 148K, 52K, 49.5K, 46K, 43.5K, 38.5K, 27K, 20K, and 18K), which together constituted 89% of the total nucleocapsid protein on the basis of incorporated 3H-labeled amino acids. The 148K protein comprised 47.3% of the total protein and thus appeared to be similar in molecular weight and proportional composition to the major capsid proteins of other herpesviruses. Purified virions were composed of 37 proteins whose average molecular weights ranged from 14K to greater than 200K. Three intense glycoprotein bands (83K, 78K, and 73.5K) as well as four less intensely labeled glycoproteins were detected in [3H]glucosamine-labeled virion preparations. At least 14 structural proteins were readily detected in extracts of infected cells which had been [35S]methionine labeled late in infection, and 11 of these were immunoprecipitated by rabbit antiserum against purified virions. The protein composition of ECMV differs substantially from those of equine herpesvirus type 1 and type 3 as well as from those of other herpesviruses.

Murine monoclonal antibody to a single protein neutralizes the infectivity of human cytomegalovirus

Murine monoclonal antibodies to human cytomegalovirus (CMV) strain AD169 were selected that neutralized virus infectivity. One monoclonal antibody-producing hybridoma, 1G6, was used to produce ascites fluid from which immunoglobulin was isolated. This antibody efficiently neutralized CMV AD169, other laboratory strains (Towne, Davis), and clinical isolates of CMV in early tissue culture passage (less than 10) in the absence of complement. The antibody immunoprecipitated a single 86,000-dalton protein from both laboratory and clinical strains. This viral protein was demonstrated by indirect immunofluorescence to be localized in the cytoplasm of CMV-infected cells.

Characterization of monoclonal antibodies and polyclonal immune sera directed against human cytomegalovirus virion proteins

Four classes of monoclonal antibody-producing cell lines have been obtained that detect human cytomegalovirus virion structural proteins. These antibodies react with (1) a major outer membrane virion glycoprotein(s) gp58-gp130, whose molecular weight varies between strains of cytomegalovirus, (2) a phosphoprotein, pp71, localized inside the virion membrane, (3) a phosphorylated nucleocapsid protein, pp155, and (4) a virion-associated phosphoprotein, pp29. Polyclonal immune human sera react with a large number of virion proteins including those detected by these monoclonal antibodies. These monoclonal antibodies were employed in a radioimmune assay to detect low levels (6 X 10(3) PFU/ml) of human cytomegalovirus in solution and human urine. These antibodies were also employed in a fluorescent antibody format to identify cytomegalovirus-infected cells obtained from human urine and nasopharyngeal aspirates. These reagents provide useful tools for studying the molecular biology of virus replication, for diagnosing cytomegalovirus infections, and for studying virus latency and activation.

Isolation and partial chemical characterization of a 64,000-dalton glycoprotein of human cytomegalovirus

A guanidinium chloride extract of [3H]glucosamine- and [35S]methionine-labeled virions plus dense bodies of human cytomegalovirus (Towne) was separated by reverse-phase high-pressure liquid chromatography. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the eluate revealed the major peak to be a glycoprotein with a relative mass of 64,000. This glycoprotein (HCMVgp64) was characterized by amino acid analysis and a high-pressure liquid chromatographic map of its tryptic peptides.

Isolation and characterization of a noninfectious virion-like particle released from cells infected with human strains of cytomegalovirus

Three types of virus particles have been recovered from the culture medium of human foreskin fibroblasts infected with human strains of cytomegalovirus (HCMV). Two of these, virions and dense bodies, are routinely observed and have been described by others. The third, produced in lesser amounts, has not been previously characterized. This particle, separable from virions by rate-velocity sedimentation, is morphologically distinguished from them only by core structure. Radiolabeling and biological assays have established that these particles, like dense bodies, lack DNA and are not infectious. Based on these properties, we have designated this virion-like structure as a noninfectious enveloped particle (NIEP). Comparisons of the protein constituents of these three particles has shown that dense bodies have the simplest composition. Approximately 95% of their protein mass is represented by a 69,000 Da (69K) matrix-like protein. While dense bodies appear to have a normal complement of virion glycoproteins, they completely lack other predominant virion species. The protein compositions of virions and NIEPs are more complex than that of dense bodies, and are distinguished from one another by the presence in NIEPs of a 35,000 Da (35K) protein absent from the two other particles. Biosynthetic radiolabeling and cell fractionation experiments have demonstrated that this 35K protein is produced only in infected cells, is phosphorylated and partitions with the nuclear fraction. These and other results suggest that this protein is the HCMV counterpart of the previously described B-capsid proteins VP22a of herpes simplex and 37K of CMV (strain Colburn). NIEPs are produced by all HCMV strains examined and have not been observed in preparations of herpes simplex virus- or Old World monkey CMV-infected cells. Although this particle is generally present in much lower amounts than virions, strain AD169 overproduces NIEPs by approximately 10-fold. We have also found that the additional NIEP protein of AD169 has an apparently larger size (i.e., 36K) than the corresponding protein of other strains. The correlation between AD169 NIEP overproduction and its altered protein suggests that the two may be causally related.

Production and characterization of monoclonal antibodies specific for a glycosylated polypeptide of human cytomegalovirus

Nine hybrid cell lines producing antibodies specific for cytomegalovirus (CMV) antigen were obtained after fusion of P3/X63-Ag8 myeloma cells with spleen cells from BALB/c mice immunized with CMV complement-fixing antigen. By the immunoblot technique, five of nine antibodies (4D11, 7B4, 7D2, 8E3, and 8E10) were identified as being reactive to a CMV glycosylated polypeptide with molecular weight of 66,000 (GP66). Four other antibodies (1B8, 8E9, 4D2, and 7E2) appeared to be reactive with CMV antigen(s) only if the antigen was not denatured by sodium dodecyl sulfate. These remain unassigned until further studies are done. With the enzyme-linked immunosorbent assay (ELISA), competitive bindings were performed with a constant amount of horseradish peroxidase-conjugated antibody and various concentrations of unconjugated homologous and heterologous antibodies on CMV antigen-coated ELISA wells, and the antigenic determinant specific for each antibody was determined. The nine antibodies could be classified into six different groups, each group reacting with a different epitope or a different region with two or more antigenic determinants which are so close to each other that they cause binding inhibition. They are groups A (4D11), B (7B4, 8E10), C (7D2), D (4D2, 7E2, 8E9), E (8E3), and F (1B8). The extent of competition among antibodies within each group was the same. By using the two antibodies that reacted with different epitopes on GP66, a double-antibody sandwich ELISA method was developed. The method was sensitive enough to detect as little as 50% of the antigen present in one infected cell or 0.000245 U of CMV complement-fixing antigen per test well. Other strains of CMV (David, Kerr, Espilat, C-87, and five clinical isolates) gave positive results, whereas herpes simplex virus types 1 and 2, varicella-zoster virus and Epstein-Barr virus nuclear antigen preparations did not. By the indirect immunofluorescence assay, antibodies 4D11 and 8E3 were able to detect GP66 in the nucleus of CMV-infected F-5000 human embryonic fibroblasts as early as 2 h postinfection and were superior in this respect to the remaining seven antibodies tested. By the double-antibody sandwich ELISA, the presence of GP66 in CMV-infected cells was detected as early as 2 h postinfection.

Phosphorylation, maturational processing, and relatedness of strain Colburn matrix proteins

The intracellular 66,000 D (66K) and 69,000 D (69K) matrix-like phosphoproteins of strain Colburn cytomegalovirus (CMV) have been compared with each other and with their electrophoretic counterparts in the virion. Three lines of evidence indicate that the 66K and 69K proteins are products of separate genes, and that the intracellular and virion species are closely related. First, "pulse-chase" radiolabeling experiments showed that these proteins have separate precursors; that modification of each to the mature form correlated with phosphorylation; and that phosphorylation of the 69K precursor occurred more slowly than that of the 66K precursor, and resulted in a more dramatic slowing of its electrophoretic mobility. Second, comparisons of the 66K and 69K proteins based on partial proteolysis of [35S]methionine-labeled proteins, using V8 protease, and complete proteolysis of [32P]orthophosphate-labeled proteins, using trypsin or Pronase, provided no evidence of sequence relatedness. These analyses also suggested that the distribution of phosphorylated residues differs in the two proteins--clustered for the 69K and more disperse for the 66K. Phosphoamino acid analyses showed only phosphoserine in the 66K protein. The 69K protein contained, in addition to phosphoserine, an electrophoretically faster moving, unidentified spot. And third, immunological comparisons showed these proteins to exhibit little or no antigenic cross-reactivity. They did, however, demonstrate that the nuclear proteins are immunologically cross-reactive with their respective virion counterparts. Additional comparisons of these nuclear and virion proteins established that the virion 69K protein (v69) differs in electrophoretic mobility and net charge from the nuclear 69K protein but that it, as well as the virion 66K protein, has a two-dimensional phosphopeptide pattern similar to its nuclear counterpart.

Protein counterparts of human and simian cytomegaloviruses

Cytomegalovirus (CMV) proteins from isolates of both human (HCMV) and simian (SCMV) origin have been compared. Three classes were analyzed: the immediate-early (IE) proteins, other infected-cell-specific proteins not present in virus particles, and the proteins that constitute the mature extracellular virion. Comparisons were based on one- and two-dimensional (charge-size) separations in denaturing polyacrylamide gels, and on the selectivity of biosynthetic radiolabeling with [32P]orthophosphate and [3H]glucosamine. Results indicate that most, if not all, of the HCMV and SCMV proteins recognized, have counterparts in strain Colburn. As a group, the simian strains exhibit protein similarities that distinguish them from the human strains. Among the most diagnostic of these are the 205K and 145K virion proteins, each of which is about 7K smaller than its HCMV counterpart, and the predominant IE proteins, which are 10K to 20K (depending upon the strain) larger than their HCMV counterparts. The proteins of strain Colburn are shown to be more like those of the simian isolates than the human, and more like those of a vervet strain than rhesus. Leads provided by experiments using strain Colburn have aided in the identification of a previously unrecognized, abundant virion protein that is a principal phosphate acceptor, both in vivo and in vitro. Three additional phosphorylated proteins are identified in HCMV virions, as well as three glycoproteins. Only two HCMV strain-specific protein differences were detected by comparisons based on separation in SDS-containing polyacrylamide gels--one in the IE protein of strains Towne and Davis; the other in a virus capsid protein of strain AD169.

Genome structure and virion polypeptides of the primate herpesviruses Herpesvirus aotus types 1 and 3: comparison with human cytomegalovirus

Two serologically distinguishable primate herpesviruses, Herpesvirus aotus type 1 and type 3, were examined with regard to their genomes and structural polypeptides. The duplex DNA genomes of these two viruses were found to be essentially identical in molecular weight (Mr approximately equal to 145 X 10(6)) and guanine plus cytosine composition (55%). Both contained unique and inverted repeat nucleotide sequences of the same size and arrangement, which, as judged by DNA-DNA hybridization and restriction enzyme analyses, were at least 95% homologous. In addition, no differences were observed in electrophoretic profiles of virion polypeptides. Because of their great similarity with respect to these criteria, the two viruses ought to be considered independent isolates (or strains) of a single virus, which should be designated H. aotus type 1. The elevated molecular weight and presence of two sets of inverted repeat sequences closely resemble the structure of the human cytomegalovirus genome. However, no sequence homology (less than 5%) nor similarity in virion polypeptides was detected between H. aotus type 1 and human cytomegalovirus.

Electrophoretic analysis of polypeptides immune precipitated from cytomegalovirus-infected cell extracts by human sera

Serodiagnosis of cytomegalovirus (CMV) infection by complement fixation tests depends on showing a fourfold rise in antibody titer from acute- to convalescent-phase sera. Freeze-thaw and glycine-extracted, infected cell culture antigens used for these tests give markedly different titers in reactions with the same sera. In this study, we characterized the CMV-infected cell polypeptides contained in freeze-thaw and glycine-extracted antigens and identified the proteins precipitated by 23 pairs of human acute and convalescent sera. Our results were as follows. First, freeze-thaw and glycine-extracted antigens prepared from infected cells radiolabeled with [35S]methionine and subjected to electrophoresis in sodium dodecyl sulfate-polyacrylamide gels yielded similar patterns, and the bulk of the label was contained in late structural proteins and glycoproteins. Glycine-extracted preparations contained a greater proportion of soluble 66,000- and 50,000-molecular-weight proteins than did freeze-thaw antigens. Second, convalescent sera precipitated proteins migrating with apparent molecular weights of 150,000, 130,000, 110,000, 96,000, 74,000, 66,000, 50,000, 34,000, 32,000, and 25,000. Of these the 130,000-, 110,000-, 96,000-, 66,000-, 50,000-, and 25,000-molecular-weight proteins comigrated with glucosamine-labeled polypeptides. Both immunoglobulin G and M antibodies in human sera precipitated these proteins from CMV-infected cell preparations. Implications of the results for serodiagnosis of CMV infections are discussed.

Monoclonal antibodies to human cytomegalovirus: three surface membrane proteins with unique immunological and electrophoretic properties specify cross-reactive determinants

Seventy-seven clones of hybridomas selected for reactivity by immunofluorescence with human cytomegalovirus (CMV)-infected cells were produced by fusing mouse myeloma cells with the spleen cells of mice immunized with CMV strain AD169. The clones were classified into seven groups on the basis of the electrophoretic properties of the polypeptides immune precipitated from extracts of CMV-infected cells. Studies on the three groups of monoclonal antibodies directed against CMV surface membrane antigens showed the following. Clones in each group were differentiated by immunoglobulin subclass, neutralizing activity, and reactivity with the antigenic domains of proteins exposed on the surface membranes of intact CMV-infected cells. Monoclonal antibodies in each group precipitated one slowly migrating protein and multiple faster migrating forms which shared antigenic determinants. The first group of monoclonal antibodies precipitated four glycosylated polypeptides with apparent molecular weights of 130,000, 110,000, 100,000, and 60,000. Monoclonal antibody CH51 of this group neutralized infectious virus but failed to react with antigenic domains on the surfaces of infected cells. The second group of monoclonal antibodies precipitated four polypeptides with apparent molecular weights of approximately 66,000, 55,000, 50,000, and 46,000. Monoclonal antibodies CH65 and CH134 in this group had neutralizing activity and reacted with antigenic domains of proteins exposed on the surface of CMV-infected cells. The third group of monoclonal antibodies precipitated four polypeptides with apparent molecular weights of 49,000, 48,000, 34,000, and 25,000. Serological analysis of 15 naturally occurring CMV strains with a panel of monoclonal antibodies to surface membrane proteins showed that the antigenic determinants reactive with the antibodies tested were conserved in all of the strains. Monoclonal antibodies to surface membrane proteins on CMV-infected cells may prove to be valuable reagents for identification of virus isolates.

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.

Polypeptides and major antigens of four new isolates of cytomegalovirus

Two strains of cytomegalovirus (CMV) from patients with renal transplants and two other strains of CMV from young children were successfully propagated in human diploid cells, MRC5. Analysis by SDS-PAGE of these viruses passaged only 4 to 6 times revealed 21 to 29 polypeptides. The freshly isolated strains differed from reference strains, Ad 169 and Davis, in the number of bands, in the intensity of labeling of each band and also in molecular weight of the polypeptides. Some bands were common to all six strains. Some bands of molecular weight above 105 K were present in some strains and not in others. This was also the case for some bands between 64 and 76 K and two to five bands at low molecular weight between 17 and 22 K were detected. The band at 68 K was the most intensely labeled. Several polypeptides were precipitated by immune sera containing antibodies to CMV (one positive human serum and one guinea pig antiserum to Davis strain). Different bands were observed after precipitation, depending on the antiserum used and the strain of virus. Interstrain differences were found to be associated with antigenic heterogeneity. However, these cannot be correlated as yet with primary infection or reactivation.

Unimpaired histone synthesis in human fibroblasts infected by human cytomegalovirus

Serum-starved human foreskin fibroblasts were infected by human cytomegalovirus (Towne strain) that is thought to induce DNA replication in host cells during lytic infection. At various times postinfection, the cultures were pulse labeled with either 3H-thymidine or 14C-thymidine and 3H-lysine to examine DNA synthesis and histone synthesis, respectively. Isopycnic centrifugation of labeled DNA in CsCl revealed that precursor incorporation into host-cell DNA was enhanced over the control around 24 h postinfection and decreased after onset of viral DNA synthesis which reached a peak around 72 h postinfection. For analysis of histones 3H-lysine-labeled proteins of lysates of unfractionated cells and of chromatin preparations were subjected to polyacrylamide gel electrophoresis in presence of sodium dodecyl sulfate and subsequent fluorography. Comparison of the fluorograms from the various pulses postinfection suggested that 3H-lysine incorporation into histones exhibited no major variations concurrent with the changes of host-cell DNA synthesis. In contrast, herpes simplex virus type 1 was found progressively to extinguish histone synthesis in the course of the cellular infection. Furthermore, histone synthesis in phosphonoacetic acid-treated cytomegalovirus-infected cultures was not enhanced over that in mock-infected controls. These observations do not support the view that human cytomegalovirus induces host-cell DNA replication under the conditions used.

Synthesis of proteins and glycoproteins in cells infected with human cytomegalovirus

In cytomegalovirus-infected cells, the rate of protein synthesis was detected as two peaks. One occurred during the early phase of infection, 0 to 36 h postinfection, and the other occurred during the late phase, after the initiation of viral DNA synthesis. Double-isotopic-label difference analysis demonstrated that host and viral proteins were synthesized simultaneously during both phases. In the early phase, approximately 70 to 90% of the total proteins synthesized were host proteins, whereas approximately 10 to 30% were viral, even at a multiplicity of infection of 20 PFU/cell. Virus-related proteins or glycoproteins were referred to as infected-cell specific (ICS). Two ICS glycoproteins (gp145 and 100) were clearly detectable and were synthesized preferentially in the early phase of infection. Their synthesis was concomitant with stimulation of the protein synthesis rate. In the late phase of infection, approximately 50 to 60% of the total protein synthesis was viral and approximately 40 to 50% was host. The ICS proteins and glycoproteins detected during the late phase of infection were viral structural proteins. Infectious virus was not detectable until 48 to 72 h postinfection. An inhibitor of viral DNA synthesis, phosphonoacetic acid, prevented the appearance of the late-phase ICS proteins and glycoproteins, but there was little or no effect on early ICS glycoprotein synthesis. Radiolabeled ICS proteins and glycoproteins were identified by their relative rates of synthesis, by their different electrophoretic mobilities compared with those of host proteins and host glycoproteins, and by their similar electrophoretic mobilities compared to those of proteins and glycoproteins associated with virions and dense bodies of cytomegalovirus. Structural viral antigens in the infected-cell extracts were removed by immunoprecipitation, using F(ab')(2) fragments of cytomegalovirus-specific antibodies, and identified as described above. The last two criteria were used to identify viral structural ICS proteins and glycoproteins. Although approximately 35 structural proteins were found to be associated with purified virions and dense bodies, the continued synthesis of host cell proteins complicated their identification in infected cells. Nevertheless, seven of the nine structural glycoproteins were identified as ICS glycoproteins.