Cellular synthesis and modification of murine hepatitis virus polypeptides

Seroprevalence of turkey coronavirus in North American turkeys determined by a newly developed enzyme-linked immunosorbent assay based on recombinant antigen

Turkey coronavirus (TCoV) causes diarrhea in young turkey poults, but little is known about its prevalence in the field. To address this, the complete nucleocapsid gene was cloned and expressed in Escherichia coli. Expressed nucleocapsid gene produced two distinct proteins (52 and 43 kDa); their specificity was confirmed by Western blotting using two different monoclonal antibodies. Recombinant N protein was purified and used as an antigen to develop an enzyme-linked immunosorbent assay (ELISA) for the serological detection of TCoV that was then validated using experimentally derived turkey serum. The N-based ELISA showed (97%) sensitivity and (93%) specificity for TCoV, which was significantly higher than an infectious bronchitis coronavirus-based commercial test for TCoV. To assess the utility of this recombinant ELISA, 360 serum samples from turkey farms in Ontario, Canada, and 81 serum samples from farms in Arkansas were tested for TCoV-specific antibodies. A high seroprevalence of TCoV was found in turkeys from the Ontario farms with 73.9% of breeders and 60.0% of meat turkeys testing seropositive using the N-based ELISA. Similarly, a high field prevalence was found in the turkeys from Arkansas, for which 64.2% of the serum samples tested seropositive.

The virion N protein of infectious bronchitis virus is more phosphorylated than the N protein from infected cell lysates

Because phosphorylation of the infectious bronchitis virus (IBV) nucleocapsid protein (N) may regulate its multiple roles in viral replication, the dynamics of N phosphorylation were examined. ³²P-orthophosphate labeling and Western blot analyses confirmed that N was the only viral protein that was phosphorylated. Pulse labeling with ³²P-orthophosphate indicated that the IBV N protein was phosphorylated in the virion, as well as at all times during infection in either chicken embryo kidney cells or Vero cells. Pulse-chase analyses followed by immunoprecipitation of IBV N proteins using rabbit anti-IBV N polyclonal antibody demonstrated that the phosphate on the N protein was stable for at least 1 h. Simultaneous labeling with ³²P-orthophosphate and ³H-leucine identified a 3.5-fold increase in the ³²P:³H counts per minute (cpm) ratio of N in the virion as compared to the ³²P:³H cpm ratio of N in the cell lysates from chicken embryo kidney cells, whereas in Vero cells the ³²P:³H cpm ratio of N from the virion was 10.5-fold greater than the ³²P:³H cpm ratio of N from the cell lysates. These studies are consistent with the phosphorylation of the IBV N playing a role in assembly or maturation of the viral particle.

Bovine coronavirus nucleocapsid protein processing and assembly

The coronavirus nucleocapsid protein (N) encapsidates the genomic RNA to form a helical nucleocapsid. The requirements for coronavirus nucleocapsid assembly are being studied. Two forms (approximately 50 kDa and 55 kDa) of the bovine coronavirus (BCV) N protein were detected in infected cells. However, only one form, a 50 kDa species, was detected in extracellular virions. After treatment with calf intestinal alkaline phosphatase (CIAP), the 55 kDa intracellular form increased in mobility to comigrate with the 50 kDa form; whereas, the 50 kDa intracellular species and N from extracellular virions was not sensitive to CIAP treatment. The data indicate that specificity exists with regard to assembly of N into the mature virion. The data suggests that processing of N may take place during assembly of either nucleocapsids or virions and that the processing may be a dephosphorylation event.

Characterization of coronavirus JHM variants isolated from Wistar Furth rats with a viral-induced demyelinating disease

Murine hepatitis virus (MHV) can cause neurological disease when inoculated intracerebrally (ic) into mice and rats. Specifically the JHM strain of MHV (MHV-JHM) generally causes an acute encephalitis when inoculated ic into 2-day-old Wistar Furth rats. In contrast, JHM generally produces a chronic demyelinating disease with resulting posterior paralysis when inoculated ic into 10-day-old Wistar Furth rats. In addition, while JHM readily produces a productive infection in a mouse fibroblast cell line (L-2), it does not form syncytia or replicate well in a tissue cell line of glial origin (G26-24). We have isolated and characterized three MHV-JHM viral variants from the central nervous system of two Wistar Furth rats with a MHV-JHM-induced demyelinating disease. The pattern of viral-specific mRNA for all three of these variants differed from what was observed for the wild-type parental MHV-JHM that had been passaged only in tissue culture. One of these variants, ATllf cord virus, which induced a chronic demyelinating disease in 2- or 10-day-old intracerebrally inoculated Wistar Furth rats, had a deletion in the coding region of the peplomer glycoprotein mRNA. In addition, this variant formed massive syncytia and replicated well in G26-24 cells. We have not detected this deletion in the other two JHM variants, ATllf brain virus and ATlle brain virus. ATllf brain virus and ATlle brain virus primarily produced an acute encephalitis when reinoculated into 2- or 10-day-old Wistar Furth rats. In addition, these two variants did not form syncytia and had a reduced ability to replicate in G26-24 cells.

In vivo and in vitro models of demyelinating disease: activation of the adenylate cyclase system influences JHM virus expression in explanted rat oligodendrocytes

The specificity of JHM virus (JHMV) tropism for rat oligodendrocytes, as one of the primary host cells in the central nervous system, is maintained after explanation (S. Beushausen and S. Dales, Virology 141:89-101, 1985). The temporal correlation between onset of resistance to JHMV infection in vivo, completion of myelination, and maturation of the central nervous system can be simulated in vitro by inducers of oligodendrocyte differentiation (Beushausen and Dales, Virology, 1985). Stimulation of differentiation through the elevation of intracellular cyclic AMP (cAMP) levels suggests a possible connection between activation of the adenylate cyclase system and coronavirus expression. Chromatographic analysis of cAMP-dependent protein kinase activity in cytosol extracts prepared from astrocytes or oligodendrocytes revealed that both glial cell types were deficient in protein kinase I, indicating that expression of coronavirus in differentiated cells was not contingent upon the presence of protein kinase I. However, treatment with N6,2'-O-dibutyryladenosine-3',5'-cyclic monophosphate (dbcAMP) resulted in a severalfold enhancement of the free regulatory subunit (RI) in oligodendrocytes but not in astrocytes. The RII subunit in both neural cell types was relatively unaffected. Rapid increase in RI due to dbcAMP treatment was correlated with inhibition of JHMV expression. Other differentiation inducers, including 8-Br cAMP and forskolin which, by contrast, caused a decrease in detectable RI, also blocked JHMV expression. This apparent anomaly can be attributed to an increased turnover of RI due to destabilization of the molecule which occurs upon site-specific binding of the cyclic nucleotides. On the basis of these observations, we conclude that the state of oligodendrocyte differentiation manifested with the modulation of RI regulates JHMV expression. The differentiation process did not affect either virus adsorption or sequestration but appeared to inhibit the expression of viral RNA and proteins, implying that replication was inhibited at some step between penetration and initiation of genomic functions, perhaps at the stage of uncoating. We therefore examined the possibility that protein kinases and phosphatases, which influence cellular regulation during cAMP-induced differentiation, may be responsible for the phenomenon of coronavirus suppression in oligodendrocytes. Evidence was obtained indicating that normal processing of the phosphorylated nucleocapsid protein is inhibited in differentiated oligodendrocytes, consistent with the notion that JHMV replication might be arrested during uncoating.

The role of protease-dependent cell membrane fusion in persistent and lytic infections of murine hepatitis virus

We have defined three categories of cultured cell lines on the basis of their permissiveness (susceptibility to initial infection) to mouse hepatitis virus (MHV). Fully permissive L-2 cells gave rise to 100-1000-fold higher numbers of infectious centers than did semi-permissive LM, LM-K or C-1300 cells, whereas non-permissive Vero or C-6 cells were refractory to MHV infection. On an infected cell basis, there was no deficiency on the part of semi-permissive cell lines to replicate total viral RNA, viral polypeptides or progeny virions. Two of the semi-permissive cell lines (LM and LM-K) supported persistent MHV infection, while a third (C-1300) succumbed to lytic infection. LM and LM-K cells, but not C-1300 cells showed resistance to MHV-induced membrane fusion, even when placed in contact with fusion-active MHV-infected L-2 cells. The ability of a given cell to undergo fusion did not correlate with membrane lipid characteristics (unsaturated fatty acid and sterol content) which contribute to membrane "fluidity". In order to more closely study the parameters of MHV-induced cell fusion, membranes were prepared from MHV-infected L-2 cells and monitored for their fusogenic potential with permissive L-2 cells, semi-permissive LM cells and non-permissive vero cells. Fusion was only observed with the permissive L-2 cells, and only when exogenous protease (trypsin or chymotrypsin) was added. When the membranes were prepared from 35S-methionine-labeled MHV-infected L-2 cells and subjected to protease treatment, the radiolabeled 180,000 dalton form of the E2-glycoprotein underwent proteolytic cleavage to yield a major product of approximately 90,000 daltons. Both trypsin and chymotrypsin were effective in this proteolytic cleavage and in activating membrane fusion. In a normally permissive, fusogenic infection of MHV in L-2 cells, the protease inhibitors TPCK and ZPCK, but not TLCK, were found to inhibit cell fusion. In MHV-infected L-2 cells, E2 was found almost exclusively as the 180,000 dalton form but turned over rapidly as shown by pulse-chase studies. TPCK and ZPCK but not TLCK inhibited turnover. The results suggest that L-2 cells contain a protease which cleaves at aromatic amino acids such as phenylalanine, and that this protease cleaves the 180,000 dalton form of the E2 to peptide fragments, one or more of which may activate cell fusion.

RNA-binding proteins of coronavirus MHV: detection of monomeric and multimeric N protein with an RNA overlay-protein blot assay

RNA-binding proteins of coronavirus MHV-A59 were identified using an RNA overlay-protein blot assay (ROPBA). The major viral RNA-binding protein in virions and infected cells was the phosphorylated nucleocapsid protein N (50K). A new 140K virus structural protein was identified as a minor RNA-binding protein both in virions and in infected cells. The 140K protein was antigenically related to N, and upon reduction, yielded only 50K N. Thus, the 140K protein is probably a trimer of N subunits linked by intermolecular disulfide bonds. Several cellular RNA-binding proteins were also detected. RNA-binding of N was not nucleotide sequence specific. Single-stranded RNA of MHV, VSV, or cellular origin, a DNA probe of the MHV leader sequence, and double-stranded bovine rotavirus RNA could all bind to N. Binding of MHV RNA was optimal between pH 7 and 8, and the RNA could be eluted in 0.1 M NaCl. The ROPBA is a useful method for the initial identification of RNA-binding proteins, such as N and the 140K protein of murine coronavirus.

In vivo and in vitro models of demyelinating diseases. XII. Persistence and expression of corona JHM virus functions in RN2-2 Schwannoma cells during latency

The coronavirus JHMV persistently infects rat Schwannoma cells RN2-2 at 32.5 degrees C and enters a host-imposed reversible, latent state at 39.5 degrees C. JHMV can remain up to 20 days in the latent state and about 14 days before the cultures lose the capacity to resume virus production upon return to 32.5 degrees C. Although persistently and latently infected RN2-2 cells display resistance to superinfection by a heterologous agent VSV, these cells do not release detectable soluble mediators (e.g., interferon) of the antiviral state. Nevertheless, RN2-2 cells are competent to synthesize and release interferon when treated with the appropriate inducers. These observations suggest that interferon does not play any role or may not be the major factor in the control of latency in the Schwannoma cell. Hybridization with virus-specific cDNAs shows that all viral mRNAs are present during latency and that viral mRNAs are present in the polysomes of infected cells at 39.5 degrees C. Western immunoblotting with hybridoma antibodies demonstrates that viral specific proteins are produced at the restrictive temperature. These results suggest that despite the absence of production of infectious virus at 39.5 degrees C, there is active transcription and translation into virus-specified products.

Attenuation of murine coronavirus infection by ammonium chloride

Ammonium chloride at a concentration of 20 mM delayed by 4-5 hr the production of virus progeny in mouse L-2 cells infected at high multiplicity with mouse hepatitis virus (MHV). This delay was seen in the production of both intracellular and extracellular virus. However, the final titers were similar to those produced by MHV-infected cells maintained in normal medium. The manifestation of virus-induced cell fusion was similarly found to be delayed, but not otherwise decreased in severity, when ammonium chloride was present in the culture medium. Ammonium chloride caused similar delays in production of virus-specific, positive-sense RNAs and of viral polypeptides. The relative proportions and apparent molecular weights of viral RNAs and polypeptides were similar to those found in MHV-infected cells cultured in normal medium. In vitro translation of endogenously produced viral RNAs in cell extracts, prepared from MHV-infected cells, was not inhibited by ammonium chloride. Thus, ammonium chloride has no specific, inhibitory effect on viral protein synthesis. Ammonium chloride did not reduce the number of virus-infected cells in culture, as monitored by infectious center assay. Analysis of early events in MHV infection showed that ammonium chloride did not affect adsorption or internalization of MHV by L-2 cells. However, the subsequent eclipse phase, as monitored by decline in infectivity of internalized virus inoculum proceeded less efficiently in the presence of ammonium chloride. On the basis of the known inhibitory effects of ammonium chloride on lysosomal/endosomal functions, the results suggest an endosomal mechanism of MHV uncoating. Thus the primary effect of ammonium chloride on MHV infection of L-2 cells is to attenuate virus uncoating, thereby chronologically displacing all subsequent virus-encoded functions.

Coronavirus multiplication: locations of genes for virion proteins on the avian infectious bronchitis virus genome

Six overlapping viral RNAs are synthesized in cells infected with the avian coronavirus infectious bronchitis virus (IBV). These RNAs contain a 3'-coterminal nested sequence set and were assumed to be viral mRNAs. The seven major IBV virion proteins are all produced by processing of three polypeptides of ca. 23, 51, and 115 kilodaltons. These are the core polypeptides of the small membrane proteins, the nucleocapsid protein, and the 155-kilodalton precursor to the large membrane proteins GP90 and GP84, respectively. To determine which mRNAs specify these polypeptides, we isolated RNA from infected cells and translated it in a messenger-dependent rabbit reticulocyte lysate. Proteins of 23, 51, and 110 kilodaltons were produced. Two-dimensional tryptic peptide mapping demonstrated that these proteins were closely related to the major virion proteins. Fractionation of the RNA before cell-free translation permitted the correlation of messenger activities for synthesis of the proteins with the presence of specific mRNAs. We found that the smallest RNA, RNA A, directs the synthesis of P51, the nucleocapsid protein. RNA C, which contains the sequences of RNA A, directs the synthesis of the small membrane protein P23. RNA E directs the synthesis of the large virion glycoproteins. These results supported a model in which only the unique 5'-terminal domain of each IBV mRNA is active in translation and enabled us to localize genes for virion proteins on the IBV genome.

Protein synthesis in cells infected by murine hepatitis viruses JHM and A59: tryptic peptide analysis

The structural and intracellular proteins of the murine hepatitis viruses MHV-JHM and MHV-A59 were studied by tryptic peptide mapping. The results demonstrated that the virions contained three distinct proteins: the two related chains of the E2 complex, the nucleocapsid protein and the heterogeneous E1 complex. Five distinct virus-specific proteins were synthesized by infected cells. Three of the five intracellular proteins contained tryptic-peptides with properties similar to the three structural proteins. Models describing the evolution of the proteins are proposed. Although the pathogenic properties of MHV-JHM and MHV-A59 differ greatly, the tryptic peptide maps of the corresponding proteins of these MHV strains were remarkably similar.

Synthesis and subcellular localization of the murine coronavirus nucleocapsid protein

The synthesis and processing of the nucleocapsid protein (pp60) of the JHM strain of murine coronaviruses were examined. Pulse-chase experiments showed that pp60 was synthesized initially as a protein of approximately 57,000 in molecular weight (p57). Immunoprecipitation using mouse anti-JHMV antiserum indicated that p57 was virus specific. Immunoprecipitation with monoclonal antibodies specific for pp60 showed that p57 was antigenically related to pp60 and was not phosphorylated, while the intracellular protein that comigrated with the virion nucleocapsid protein, pp60, was phosphorylated. The p57 was found exclusively in the cytosol while the majority of pp60 was associated with the membrane fraction but pp60 was not an integral membrane protein.

Fusion resistance and decreased infectability as major host cell determinants of coronavirus persistence

Mouse hepatitis virus persists in cultures of a subline (designated LM-K) of mouse LM cells but produces a lytic infection in L-2 cells. Persistence in the LM-K cells was not accompanied by production of ts mutants or of soluble anti-MHV factors. Infectious center assay demonstrated an approximately 500-fold lower level of infectibility by MHV of the LM-K cells as compared to L-2 cells. On an infected cell basis, production levels of infectious progeny and viral RNA were comparable between the two cell lines. The extent of virus-induced cell-cell fusion, however, was markedly reduced in the LM-K cells. Cell-mixing experiments showed that both infected L-2 and LM-K cells have the capacity of fusing with neighboring uninfected L-2 cells but not with uninfected LM-K cells. This suggests that the decreased level of fusion observed in the LM-K infection is due not to absence of viral fusion protein at the cell surface, but rather to an inherent resistance of the LM-K cell membrane to MHV-induced fusion. It is believed that such fusion resistance in LM-K cells moderates virus dissemination throughout the culture, thereby contributing to a state of virus persistence.

The molecular biology of coronaviruses

Coronaviruses have recently emerged as an important group of animal and human pathogens that share a distinctive replicative cycle. Some of the unique characteristics in the replication of coronaviruses include generation of a 3' coterminal-nested set of five or six subgenomic mRNAs, each of which appears to direct the synthesis of one protein. Two virus-specific RNA polymerase activities have been identified. Many of the distinctive features of coronavirus infection and coronavirus-induced diseases may result from the properties of the two coronavirus glycoproteins. The intracellular budding site, which may be important in the establishment and maintenance of persistent infections, appears to be due to the restricted intracytoplasmic migration of the E1 glycoprotein, which acts as a matrix-like transmembrane glycoprotein. E1 also exhibits distinctive behavior by self-aggregating on heating at 100°C in sodium dodecyl sulfate (SDS) and by its interaction with RNA in the viral nucleocapsid. The E1 of mouse hepatitis virus (MHV) is an O -linked glycoprotein, unlike most other viral glycoproteins. Thus, the coronavirus system may be a useful model for the study of synthesis, glycosylation, and transport of O -linked cellular glycoproteins.

Coronavirus proteins: biogenesis of avian infectious bronchitis virus virion proteins

We examined the synthesis of viral structural proteins in cultured cells infected with the avian coronavirus infectious bronchitis virus. Tryptic peptide mapping was used to determine the structural relationships of the intracellular proteins to the virion polypeptides. Pulse-chase experiments were performed to identify precursors to the virus-specific proteins. We found that the nucleocapsid protein, P51, and the small viral membrane proteins GP31, GP28, and P23 do not undergo post-translational proteolytic processing. In contrast, GP90 and GP84, the two large virion membrane proteins, were found to be produced by cleavage of a single precursor, GP155. This demonstrated that at least one coronavirus mRNA specifies two virion proteins.

Coronavirus proteins: structure and function of the oligosaccharides of the avian infectious bronchitis virus glycoproteins

The recent finding that the E1 glycoproteins of murine coronaviruses contain only O-linked oligosaccharides suggested that this unusual modification might be a distinguishing feature of coronaviruses and might play an essential role in the life cycle of this family of viruses. To examine these possibilities, we analyzed the oligosaccharide moieties of the membrane proteins of the avian coronavirus infectious bronchitis virus. In addition, we determined the effect of inhibiting the glycosylation of these proteins on viral maturation and infectivity. Infectious bronchitis virus virions contain nine proteins. Four of these proteins, GP36, GP31, GP28, and P23, are closely related structurally and appear to be homologous to the E1 proteins of murine coronaviruses. We found that the oligosaccharides of GP31 and GP28 could be removed with endoglycosidase H and that neither of these glycoproteins was detectable in tunicamycin-treated cells. These two results indicated that GP31 and GP28 contain N-linked oligosaccharides. Therefore, O-linked oligosaccharides are not a universal feature of the small coronavirus membrane glycoproteins. Tunicamycin inhibited glycosylation of all of the viral glycoproteins but did not inhibit production of virions by infectious bronchitis virus-infected cells. The virions released by these cells contained only the three non-glycosylated viral proteins P51, P23, and P14. These particles were not infectious. Therefore, it appears that glycosylated infectious bronchitis virus polypeptides are not required for particle formation. However, the viral glycoproteins are apparently indispensible for viral infectivity.

Structural analysis of virion proteins of the avian coronavirus infectious bronchitis virus

We have found six major polypeptides in virions of the avian coronavirus infectious bronchitis virus grown in tissue culture: four glycoproteins, GP84, GP36, GP31, and GP28, and two non-glycosylated proteins, P51 and P23. In addition, we detected three minor species: two glycoproteins, GP90 and GP59, and one non-glycosylated protein, P14. Two-dimensional tryptic peptide mapping showed that GP36, GP31, GP28, and P23 comprise a group of closely related proteins which we have designated the "P23 family," but that the other proteins are distinct. Analysis by partial proteolytic digestion of P23 family, but that the other proteins are distinct. Analysis by partial proteolytic digestion of the P23 family labeled biosynthetically with [35S] methionine, and P23, labeled with [35S] formyl-methionine by in vitro translation of RNA from infected cells, revealed that the proteins of the P23 family differ in their amino-terminal domains. Similar analysis of GP31 and Gp36 labeled with [3H] mannose showed that the partial proteolytic fragments unique to these proteins were glycosylated. This suggests that differences in glycosylation in the amino-terminal domains contributes to the marked polymorphism os the P23 family. The results are discussed with respect to possible models for synthesis of the virion proteins.

RNA and polypeptide homology among murine coronaviruses

Using a 32P complementary DNA (cDNA) prepared against the A59 nucleocapsid protein messenger RNA, we have investigated the extent of homology between A59 and four other strains of murine hepatitis virus (MHV). Analysis by hybridization kinetics of the annealing between A59 [32P]cDNA and infected cell RNA from the other four MHV strains demonstrated 70–80% homology. By gel transfer analysis, the A59 [32P]cDNA was able to detect subgenomic-size virus-specific RNAs in cells infected with all of the five MHV strains. A similar pattern of seven viral RNAs was detected in cells infected with A59, MHV-1, MHV-3, and JHM. In contrast, cells infected with MHV-S contained seven virus-specific RNAs, of which only the two smallest species comigrated with RNAs from the other four strains. The results suggest that as previously shown with A59 (S. Cheley, R. Anderson, M. J. Cupples, E. C. M. Lee Chan, and V. L. Morris (1981) Virology , 112, 596–604), all MHV strains tested encode a nested set of subgenomic RNAs. Analysis of [35S]methionine-labeled viral proteins by SDS-polyacrylamide gel electrophoresis revealed that each strain of MHV specified four major viral polypeptides with apparent molecular weights very similar to those previously reported for the E2, N, El, and PEI polypeptides of A59. The strong degree of interstrain homology among the five MHV strains investigated was confirmed by comparative chymotryptic peptide mapping of the viral N proteins. A majority of the chymotryptic peptides from each of the [35Sknethionine-labeled N proteins was found to coelute by high-performance liquid chromotography. Moreover, this technique of peptide mapping indicated a particularly strong relatedness between MHV-1 and MHV-S and among MHV-3, JHM, and A59.

Intracellular murine hepatitis virus-specific RNAs contain common sequences

A major polyadenylated viral RNA of approximately 0.8 × 10⁶ daltons was isolated from murine hepatitis virus (A59)-infected cells by preparative polyacrylamide gel electrophoresis in formamide. This RNA was shown to encode the viral nucleocapsid protein by direct in vitro translation in a cell-free, reticulocyte-derived system. Single stranded ³²P-labeled complementary DNA was prepared from this RNA and was demonstrated to be virus specific. Using this complementary DNA in a Northern blotting procedure, we were able to identify six major virus-specific intracellular RNA species with estimated molecular weights of 0.8, 1.1, 1.4, 1.6, 3, and 4 × 10⁶ daltons. All of these RNA species were polyadenylated. Our results support the idea that coronavirus-infected cells contain multiple intracellular polyadenylated RNAs which share common sequences.