Functional analysis of the coronavirus MHV-JHM surface glycoproteins in vaccinia virus recombinants

Molecular interactions in the assembly of coronaviruses

This chapter describes the interactions between the different structural components of the viruses and discusses their relevance for the process of virion formation. Two key factors determine the efficiency of the assembly process: intracellular transport and molecular interactions. Many viruses have evolved elaborate strategies to ensure the swift and accurate delivery of the virion components to the cellular compartment(s) where they must meet and form (sub) structures. Assembly of viruses starts in the nucleus by the encapsidation of viral DNA, using cytoplasmically synthesized capsid proteins; nucleocapsids then migrate to the cytosol, by budding at the inner nuclear membrane followed by deenvelopment, to pick up the tegument proteins.

A 12-amino acid stretch in the hypervariable region of the spike protein S1 subunit is critical for cell fusion activity of mouse hepatitis virus

The spike (S) glycoprotein of mouse hepatitis virus (MHV) plays a major role in the viral pathogenesis. It is often processed into the N-terminal S1 and the C-terminal S2 subunits that were evidently important for binding to cell receptor and inducing cell-cell fusion, respectively. As a consequence of cell-cell fusion, most of the naturally occurring infections of MHV are associated with syncytia formation. So far, only MHV-2 was identified to be fusion-negative. In this study, the S gene of MHV-2 was molecularly cloned, and the nucleotide sequence was determined. The MHV-2 S protein lacks a 12-amino acid stretch in the S1 hypervariable region from amino acid residue 446 to 457 when compared with the fusion-positive strain MHV-JHM. In addition, there are three amino acid substitutions in the S2 subunit, Tyr-1144 to Asp, Glu-1165 to Asp, and Arg-1209 to Lys. The cloned MHV-2 S protein exhibited the fusion-negative property in DBT cells as the intrinsic viral protein. Furthermore, similar to the fusion-positive MHV-JHM strain, proteolytic cleavage activity was detected both in DBT cells infected with the fusion-negative MHV-2 and in the transfected cells that expressed the cloned MHV-2 S protein. Domain swapping experiments demonstrated that the 12-amino acid stretch missing in the MHV-2 S1 subunit, but not the proteolytic cleavage site, was critical for the cell-fusion activity of MHV.

The molecular biology of coronaviruses

This chapter discusses the manipulation of clones of coronavirus and of complementary DNAs (cDNAs) of defective-interfering (DI) RNAs to study coronavirus RNA replication, transcription, recombination, processing and transport of proteins, virion assembly, identification of cell receptors for coronaviruses, and processing of the polymerase. The nature of the coronavirus genome is nonsegmented, single-stranded, and positive-sense RNA. Its size ranges from 27 to 32 kb, which is significantly larger when compared with other RNA viruses. The gene encoding the large surface glycoprotein is up to 4.4 kb, encoding an imposing trimeric, highly glycosylated protein. This soars some 20 nm above the virion envelope, giving the virus the appearance-with a little imagination-of a crown or coronet. Coronavirus research has contributed to the understanding of many aspects of molecular biology in general, such as the mechanism of RNA synthesis, translational control, and protein transport and processing. It remains a treasure capable of generating unexpected insights.

Interaction of mouse hepatitis virus (MHV) spike glycoprotein with receptor glycoprotein MHVR is required for infection with an MHV strain that expresses the hemagglutinin-esterase glycoprotein

In addition to the spike (S) glycoprotein that binds to carcinoembryonic antigen-related receptors on the host cell membrane, some strains of mouse coronavirus (mouse hepatitis virus [MHV]) express a hemagglutinin esterase (HE) glycoprotein with hemagglutinating and acetylesterase activity. Virions of strains that do not express HE, such as MHV-A59, can infect mouse fibroblasts in vitro, showing that the HE glycoprotein is not required for infection of these cells. The present work was done to study whether interaction of the HE glycoprotein with carbohydrate moieties could lead to virus entry and infection in the absence of interaction of the S glycoprotein with its receptor glycoprotein, MHVR. The DVIM strain of MHV expresses large amounts of HE glycoprotein, as shown by hemadsorption, acetylesterase activity, and immunoreactivity with antibodies directed against the HE glycoprotein of bovine coronavirus. A monoclonal anti-MHVR antibody, MAb-CC1, blocks binding of virus S glycoprotein to MHVR and blocks infection of MHV strains that do not express HE. MAb-CC1 also prevented MHV-DVIM infection of mouse DBT cells and primary mouse glial cell cultures. Although MDCK-I cells express O-acetylated sialic acid residues on their plasma membranes, these canine cells were resistant to infection with MHV-A59 and MHV-DVIM. Transfection of MDCK-I cells with MHVR cDNA made them susceptible to infection with MHV-A59 and MHV-DVIM. Thus, the HE glycoprotein of an MHV strain did not lead to infection of cultured murine neural cells or of nonmurine cells that express the carbohydrate ligand of the HE glycoprotein. Therefore, interaction of the spike glycoprotein of MHV with its carcinoembryonic antigen-related receptor glycoprotein is required for infectivity of MHV strains whether or not they express the HE glycoprotein.

Cell receptor-independent infection by a neurotropic murine coronavirus

The cellular receptors for a coronavirus, mouse hepatitis virus (MHV), have been recently identified as one or more members of the carcinoembryonic antigen (CEA) family. The neurotropic JHM strain of MHV (MHV-JHM) possesses a highly fusogenic surface (S) glycoprotein. This protein is now shown to promote the spread of MHV into cells lacking the specific CEA-related MHV receptor. Resistant cells are recruited into MHV-induced syncytium with consequent production of progeny virus. Cell-to-cell spread of virus via membrane fusion without the requirement for specific cell surface receptor offers a novel way for virus to spread within infected hosts.

Coronavirus IBV-induced membrane fusion occurs at near-neutral pH

The lysosomotropic agent NH4Cl caused a reduction of 80-95% in the number of chick kidney (CK) cells and Vero cells infected by infectious bronchitis virus (IBV) strain Beaudette, as determined by immunofluorescence at the end of the first replication cycle. Inhibition only occurred when NH4Cl was present during the first 2 h after infection. Syncytium formation was studied during replication of IBV-Beaudette in Vero cells. Some cell-cell fusion occurred at pH 7.0 and pH 6.5 but it was optimal at pH 6.7. IBV strain UK/123/82 did not replicate in Vero cells and was studied in CK cells in which it grew well but without forming syncytia. In contrast to IBV-Beaudette, NH4Cl had virtually no effect on the replication of UK/123/82. The results show that the IBV spike glycoprotein induces membrane fusion at near neutral pH although some IBV strains may require a mildly acidic environment for the efficient uncoating of the virion RNA.

Isolated HE-protein from hemagglutinating encephalomyelitis virus and bovine coronavirus has receptor-destroying and receptor-binding activity

Bovine coronavirus (BCV) and hemagglutinating encephalomyelitis virus (HEV) from swine were found to grow to high titers in MDCK I cells, a subline of Madin Darby canine kidney cells. Virus grown in these cells was used to isolate and purify the HE-protein. This protein has been shown recently to have acetylesterase activity and to function as the receptor-destroying enzyme of BCV. Here we show that HEV contains this enzyme, too. The glycoproteins were solubilized by treatment of virions with octylglucoside. Following centrifugation through a sucrose gradient the surface proteins S and HE (hemagglutinin-esterase) were obtained in purified form. After removal of the detergent by dialysis, HE formed rosettes as shown by electron microscopy. The purified HE protein retained acetylesterase activity and was able to function as a receptor-destroying enzyme rendering red blood cells resistant against agglutination by both coronaviruses. HE protein released from the viral membrane failed to agglutinate red blood cells. However, it was found to recognize glycoconjugates containing N-acetyl-9-O-acetylneuraminic acid as indicated by a binding assay with rat serum proteins blotted to nitrocellulose and by its ability to inhibit the hemagglutinating activity of BCV, HEV, and influenza C virus. The purified enzyme provides a useful tool for analyzing the cellular receptors for coronaviruses.