Picornaviruses. Plus Strand RNA Virus Families. PICORNAVIRIDAE • More than 200 viruses prevalent world-wide. • cause many serious diseases of animals and man. • Foot and mouth virus first animal virus described (1898). • Poliovirus is an important model:
• More than 200 viruses prevalent world-wide.
• cause many serious diseases of animals and man.
• Foot and mouth virus first animal virus described (1898).
• Poliovirus is an important model:
- first virus purified and crystallized.
- first inactivated vaccine used (Salk 1950’s).
- first picornavirus to be sequenced.
- first infectious cDNA clone of an animal virus.
- first picornavirus structure to be solved.
Members of the Picornaviridae Cause Many Serious Diseases of Man and Animals
- Poliovirus type member, 3 major types cause paralysis.
- cause respiratory tract infections, acid labile, cause
colds in humans (110 types) and pigs.
- Hepatitis A, contagious liver infections.
- EMC group, cause heart and brain inflammation
acid labile, source is a rodent reservoir.
- Foot and mouth disease, most destructive in Africa.
Capsid is a pseudo T=3 icosahedron consisting of 60 identical asymmetric protomers arranged as 12 pentamers.
Each protomer is composed of a single copy of each of the four capsid proteins, VP1, VP2, VP3 and VP4.
VP4 is located on the inner surface of the protein shell formed by VP1, VP2 and VP3.
All three proteins contain a central b–barrel jelly roll.
The jelly roll is a wedge-shaped structure that consists of two antiparallel b-sheets.
IG Like - Members of the immunoglobulin family of receptors found on cells of the immune system.
Scr-short consensus repeat
LDL-low density lipoprotein
A single type of receptor mediates virus Human Rhinovirus binding and entry
•Poliovirus genome contains a single ORF, which encodes a 247 kD polyprotein. It has VPg at its 5’ end and a poly(A) tail at its 3’ end
•Processing occurs in 3 steps. The first is to cleave the P1 capsid protein precursor, which is catalyzed by 2Apro
• The second step is to process the noncapsid and the capsid precursors catalyzed by 3Cpro and 3CDpro
• The third step is the processing of VP0 into VP4 and VP2
During translation, the 2A proteinase cleaves at its amino terminus immediately after it is translated.
Polyproteins are Proteolytic Processed During Translation
Protein Synthesis Is Bimodal in Infected Cells
Poliovirus 2A protein Dephosphorylates eIF4Ebp
and Cleaves the eIF4G subunit.
A highly based paired structure formed from the 108 nucleotides at the 5’ end of the (+) strand RNA of poliovirus, which forms a cloverleaf like structure.
The viral protein 3CD and a PCBP a host Cellular protein, (PolyrC Binding Protein) interact with different loops of the cloverleaf. This interaction brings the 5’ end close to the 3’ end. At this point VPg priming can occur.
Upon completion of the minus strand a similar clover leaf is formed at the 5’ end of the minus strand. Formation of this ribonucleoprotein complex is required for synthesis of both plus and minus strand RNA.
Steps in the Assembly of Poliovirus
Assembly steps are not reversible
The capsid binds to host receptors and releases Viral RNA into the cell.
Translation of the released RNA produces a polyprotein that is processed by the 2A and 3CD proteinases to form the viral gene products.
RNA synthesis is initiated by the 3AB proteins and elongated by the 3D protein.
As proteins viral capsid assembly begins in the cytoplasm from P1 products.
In 1952 there were 57,000 cases of polio in USA mostly in children between five to nine years old.
By 1961 less than 100 cases were reported & by 1970 less than 10 cases occurred per year.
Live Vaccine induced cases of polio have occurred. Now the eradication programs are switching back to the killed virus vaccine.
C - Nucleocapsid protein that forms the enveloped icosahedron.
prM - Structural glycoprotein. Cleaved to pr and Membrane protein by furin.
E - Envelope Protein
NS1 - Nonstructural glycoprotein required for RNA replication.
NS2A - Hydrophobic protein that anchors replication machinery in the Endoplasmic Reticulum.
NS2B - Hydrophobic protein. Serves as cofactor for the NS3 protease.
NS3 - Serine protease; Helicase; component of capping enzyme.
NS4A & B - Hydrophobic proteins that may anchor replication machinery in the Endoplasmic Reticulum.
NS5 - RNA polymerase contains capping enzyme activity.
The immature virion moves to the Golgi where the prM and envelope proteins are glycosylated and the envelope protein and a nonstructural protein (NSI) are trimmed by Host Signalase enzymes.