Systematic Veterinary Virology

MORPHOLOGY

• The virions are enveloped and slightly pleomorphic or spherical in shape with 60-220 nm in diameter. The surface of the virus has surface projections, which are distinct and club-shaped. These projections are spaced widely apart and dispersed evenly over all over the surface providing a crown like appearance and hence these viruses are known as corona viruses.
• The nucleocapsid is filamentous with helical symmetry. The virions contain one molecule of linear positive-sense single stranded RNA, which also act as mRNA.
• The total genome length is 20000-33000 nt. The 5' end of the genome has a cap and 3' end has a poly (A) tail. The virus has five structural proteins – S, M, N, HE and E.
• Surface glycoprotein (or spike, S), which S protein is responsible for attachment to cells, hemagglutination and membrane fusion.
• Integral membrane protein (M) which spans the virus envelope three times
• Nucleocapsid protein (N)
• Hemagglutinine-esterase protein (HE), which forms short surface projections, and have receptor binding, hemagglutination and receptor destroying activities
• Envelope protein (E), small, envelope-associated protein
• Unlike (-)sense RNA viruses polymerase enzymes are not present in the nucleocapsid