Picornaviruses

Picornaviruses

	Picornaviruses - Picornaviruses ('pico' = small, rna = RNA) are small (26-30 nm) icosahedral, ssRNA animal viruses, There are four subgroups which are structurally related, and have essentially identical protein compositions. 1. Human enterovirus-found in the alimentary canal. Poliovirus - causes poliomyelitis. Coxsackie virus - causes a variety of symptoms including myositis. ECHO (Enteric Cytopathogenic Human Orphan) virus - causes paralysis, diarrhoea and aseptic meningitis.
2. Cardioviruses of rodents, e.g. encephalomyocarditis virus, mouse Elberfield virus and Mengo virus 3. Rhinoviruses - cause human respiratory infections like the common cold, bronchitis and bronchopneumonia. 4. Foot and mouth disease viruses of cattle. The icosahedral shell consists of 60 identical crystallographic subunits or protomers. Each polypeptide of the virus occurs about 60 times, i.e. once for each subunit.
The virion RNA (358-378) is a single-stranded positive (infectious) molecule which acts as mRNA in infected cells. It has a molecular weight of about 2.6*10 corresponding to a length of 7,500 bases; with a coding capacity of 2,500 amino acids The 3 terminus of poliovirus mRNA has about 75 nucleotides of adenosine (poly (A) sequence). Removal of poly(A) renders the poliovirus' RNA noninfectious. Virion RNA has a protein linked to pUp at its 5' end, while intracellular RNA (mRNA) has a free terminus of pUp.
The whole viral RNA appears to function as mRNA. The entire genome is translated into a large precursor polypeptide ('polyprotein') (MW 200,000), which is then cleaved to form, all the viral proteins. The togaviruses are the only other viruses to have this strategy of protein synthesis. According to the hypothesis of Jacobson and Baltimore (1968), a single mRNA can produce only a single polypeptide. One mRNA has only one polypeptide initiation site. Picornaviruses follow this, albeit the polypeptide is later cleaved. Translation of proteins takes place on large polysomes consisting of 30-40 host cell ribosomes.
	There are 4 capsid proteins, VP1, VP2, VP3 and VP4",and several noncapsid virus-specific proteins, (NCVPI-NCPV10,Summers el. al, 1965). The polyprotein (MW 200,000) undergoes. Nascent cleavage to produce three protein, NCVP1a (MW 110,000), NCVPX (MW 34,000) and NCVP1b (MW 93, 000). The last protein is also called NCVP1½or 1.5. NCVP1a is the precursor the four capsid proteins VP1-VP4. NCVP1b is cleaved, to NCVP2, which is further cleaved to NCVP4 (enzymatic protein). The other types of cleavages are die secondary or cytoplasmic cleavages, which take place after the protein is released from the ribosomes, and the morphogenetic or maturation cleavages (VPO → VP4 + VP2)

Assembly starts with the formation of a 5s particle the 'immature protomer' containing one molecule each of VPO, VP1 and VP3, There is a step wise formation of progressively larger structures. Five S8 particles are assembled to form the 14S structure.

Twelve 14S particles associate to produce a 74S procapsid. Viral RNA (35S) then enters the procapsid to produce a 125S provirion structure. The protein VPO undergoes morphogenetic cleavage resulting in the formation of 150S mature virions.

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Herpesviruses Herpesviruses And Their Infections Poxviruses Reoviruses Reovirus RNAs and Proteins Picornaviruses Strucures of Picornavirus During Morphogenesis Togaviruses (Arboviruses) Rhabdoviruses Proteins of the Vesicular Stomatitis Orthomyxoviruses Multiplication in RNA Viruses Rous Sarcoma Virus (RSV) Interferon

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