STRUCTURE:
Corona viruses are pleomorphic, ether-labile, enveloped viruses.
Diameter ranges from 60 to 220 nm.
The average density in Sucrose is 1.18 g/ml.
The particles have characteristic club-shaped surface projections of approximately 20 nm in length.
The genomes of Corona viruses are single molecules of single-stranded RNA of molecular weight between 5.8x10^6 and 6.5x10^6. The RNA is polyadenylated at the 3' terminus and all the coronaviruses studied so far are capped at the 5' terminus.
Three main structural proteins have been identified, comprising:
1. The Nucleocapsid protein,
2. The Surface projection or Peplomer protein and
3. The Transmembrane or Matrix protein
The nucleocapsid protein is phosphorylated, has a molecular weight of between 50K and 60K and forms a complex with the RNA genome.
The surface projection protein is a high molecular weight glycoprotein of about 180,000. It is also possible that the 180,000 glycoprotein is an association of two non-identical subunits of about 90,000 as seen with IBV.
The transmembrane protein is glycosylated and has a molecular weight of between 20K and 35K depending upon its degree of glycosylation. A small hydrophilic region extends outside the viral envelope and contains all the carbohydrate of the molecule, whilst the hydrophobic region contains disulphide bonds and resides within the lipid bilayer. A small part of the molecule resides on the inner surface of the envelope and maybe associated with the nucleocapsid.
BIOLOGICAL PROPERTIES:
Some HCV strains, namely 229E and serologically related strains, are readily propagated in human tissue culture cell lines.
Othe HCV strains, namely OC43 and serologically related strains are difficult to propagate in tissue cultures and most isolations of OC43- related strains have been done in organ cultures. OC43 strain has been successfully adopted to the brains of suckling mice.
Because human corona virus do not grow well in cell cultures, details of Viral Replication have come from the studies with Mouse Hepatitis Virus (MHV), which is closely related to human strain - OC43.
ANTIGENIC STRUCTURE:
Three distinct antigenic molecules are found in the virions. They are:
1. Nucleocapsid,
2. Surface projection and
3. Transmembrane proteins.
The nucleocapsid protein antigens maybe common to all Corona viruses.
The main antigenic determinants of individual viruses reside on the surface projections.
Surface projection antigens are used foe serological grouping of Corona viruses.
One avain and two mammalian serological groups have been established.
HCV strains fall into each of the mammalian serological groups and are named the OC43 and 229E serological groups, respectively.
The tests most frequenly used for the serological grouping of Corona viruses are:
1. Enzyme linked immunosorbent assay (ELISA) and
2. Immunofluorescence (IF).
Also the following tests are used less frequently:
1. Radio immuno assay (RIA)
2. Immuno electron microscopy (IEM)
3. Neutralisation (N) and
4. Haemagglutination Inhibition (HI).
