## *Mehtap Erdinç's Lecture Note*
# Picornaviruses (*Picornaviridae*)

Payne, S. (2017) Virüsler , Picornaviruses Jawetz Melnick & Adelbergs Medical

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## Picornaviridae
- Members of the family Picornaviridae are small (pico) RNA viruses. Family Picornaviridae is one of five named families in the order Picornavirales, a large group of unenveloped positive-strand RNA viruses of plants, insects, and animals. Virions are unenveloped with T53 icosahedral capsids.

**Virion structure of a picornavirus**

-Virions are unenveloped with T53 icosahedral symmetry. Virions are about 30 nm in diameter. Virions are assembled from 180 capsid proteins: 60 copies each of VP1, VP2, VP3 assemble to form the outer shell (picornavirus capsids are sometimes referred to as pseudo T53 because they are not assembled from 180 copies of a single capsid protein).

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**Structure of a typical picornavirus**
- Exploded diagram showing internal location of the RNA genome surrounded by capsid composed of pentamers of proteins VP1, VP2, VP3, and VP4. Note the “canyon” depression surrounding the vertex of the pentamer

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- Binding of cellular receptor to the floor of the canyon. The major rhinovirus receptor (intercellular adhesion molecule-1 [ICAM-1]) has a diameter roughly half that of an immunoglobulin G (IgG) antibody molecule.

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> **TAXONOMY**


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Location of a drug-binding site in VP1 of a rhinovirus. The antiviral drug shown, WIN 52084, prevents viral attachment by deforming part of the canyon floor.


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**Negative stain electron micrograph of PV**


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Schematic diagram of a picornaviral genome showing a single long ORF flanked by 50 and 30 UTRs. The 50 end of the genomic RNA is covalently linked to a small viral protein, VPg. A highly base-paired structure in the 50 UTR, called the internal ribosome entry site or IRES, is the site of ribosome assembly.

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**Organization and expression of the picornavirus genome**

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The picornavirus genome is an mRNA and the full complement of viral proteins is synthesized from the poly(A)-containing genome length mRNA.

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- In order for the PV genome to be replicated, it must be free of ribosomes. Both virally encoded and host proteins are required for genome replication, which occurs in the cytoplasm and is associated with cell membranes.

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- Picornaviral RdRp (3Dpol) uses a protein primer to initiate RNA synthesis. VPg is a short (22-24 amino acids) polypeptide found covalently linked to the 50 ends of newly synthesized viral RNAs.
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**Overview of the picornavirus replication cycle**


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**Genome Replication**


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**Assembly**
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-The process of capsid assembly begins when local concentrations of proteins are sufficiently high.

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-VP1, VP3, and VP0 (VP21VP4) form protomers and five protomers assemble to form a pentamer.

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-Twelve pentamers assemble to form an empty capsid. RNA is then inserted into the empty capsid.-

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**Maturation**
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PV virions are released upon cell death. After release VP0 is cleaved to produce VP21VP4. VP4 is located inside the capsid, associated with RNA. Now the virion is ready to infect a new cell.