CORONA VIRIDAE-Properties of Coronaviridae

 

The coronaviruses were so named because the unusually large club-shaped peplomers projecting from the envelope give the particle the appearance of a solar corona .

 The virion is pleomorphic, being roughly spherical in the case of the genus Coronavirus but often disk-, kidney-, or rod-shaped in the case of the genus Tnrovirus, and can range in size from 60 to 220 nm. 

The tubular nucleo capsid, difficult to discern in electron micrographs, is composed of a phosphorylated nucleoprolein IN) and seems to be connected directly to an unusual transmembrane protein, M, which spans the viral envelope three times and performs the role normally fillsed by matrix protein in other enveloped viruses.

 A very large (200K), heavily glycosylated envelope glycoprotein S (for spike) forms the bulky peplomcrs and carries cell binding, membrane fusion, and hemagglutinating activities.

 The human coronavirus  0C43 also possesses a third glycoprotein, HE (for hemagglutininesterase) which closely resembles a similar cell-binding receptor-destroying enzyme found in influenza C virus.

The genome consists of a single linear molecule of ssRNA of positive polarity, about 30 kh in size, which is 5' capped and 3' polyadenylated, and is infectious .

Since the second report of the Coronavirus Study Group in 1978 , considerable data, especially on the structure and replication of coronaviruses, have been published, and we feel a new report is justified. 

The Coronaviridae are a monogeneric family of pleomorphic, ether-labile, enveloped viruses. The virions have a diameter ranging from 60 to 220 nm and an average density in sucrose of 1.18 g/nil. They characteristically bear clubshaped surface projections about 20 nm in length from which the group derives its name (Latin corona, crown) [1], The genomic RNA is an infectious single-stranded molecule which is capped and polyadenylated. The molecular weight is between 5x 106 and 7* 10°, corresponding to about 15,000-20,000 nucleotides. 

There is no extensive sequence reiteration in the coronavirus genome. Coronavirions characteristically have three types of protein: a phosphorylated nucleocapsid protein [mol.wt. (50-60) x 103], complexed with the genome as a helical ribonucleoprotein (RNP); an jV-glycosylated surface peplomer protein, associated with glycopolypeptides of (90-180) x 103 molecular weight, which isacylated and is responsible for virus attachment and cell-to-cell fusion (this protein may be removed by protease treatment); and a transmembrane matrix protein, associated with polypeptides of molecular weight (20-35) x 103 which have variable degrees of glycosylation. 

In the case of murine and bovine coronaviruses this polypeptide bears O-glycosidically linked oligosaccharides, and in the case of avian infectious bronchitis virus it bears Nglycosidically linked oligosaccharides. Most coronaviruses replicate in tissue culture within 12 h at 37°. Infection is often accompanied by cytopathic changes. There are conflicting reports as to whether a nuclear function is required for coronavirus replication. There are few data about the early events (adsorption, penetration, uncoating, etc.) involved in coronavirus replication. It is assumed that upon entering the cell the positivestranded genome encodes protein(s) whose function is to replicate the genomic RNA and produce subgenomic mRNA. 

Recently, there have been reports of virus-specific RNA polymerases in coronavirus-infected cells, but the components of the enzyme have not been identified. Characteristic of coronavirus infection is the production of 3' coterminal subgenomic RNAs which form a nested set extending in a 5' direction. These RNAs are capped and polyadenylated. The replicative structures from which they are produced have not been characterized, but it has been demonstrated that the negative-stranded template from which murine hepatitis virus mRNAs are copied is of genome length. UV inactivation studies indicate that coronavirus mRNAs are not produced by the processing of a larger RNA, although extensive sequence homologies have been detected at the 5' ends of ail murine hepatitis virus-specific subgenomic RNAs. For murine hepatitis virus, the mRNA function of each of the subgenomic viral RNAs has been demonstrated in vitro, and the mRNAs encoding each of the virion proteins, or its precursors, have been identified . 

Comparing the size of each mRNA with its translation product suggests that the expressed information lies within the 5' sequences of each RNA which are not found in the next smallest RNA. For murine hepatitis virus, glycosylation of the peplomer protein is initiated cotranslationally in the rough endoplasmic reticulum, whereas the transmembrane protein is glycosylated posttranslationally in the Golgi apparatus. The infectious bronchitis virus matrix protein is glycosylated on the nascent polypeptide. After synthesis, genomic RNA and virion proteins are assembled at the rough endoplasmic reticulum and virions bud into cisternae, acquiring their lipid membranes from the cell. The virions are subsequently transported to and accumulate in Golgi and smooth-walled vesicles. There is an absence of budding from the plasmaiemma. The mechanism of virus release has not been characterized.