| Picornavirus | |
|---|---|
| Virus classification | |
| Group: | Group IV ((+)ssRNA) |
| Order: | Picornavirales |
| Family: | Picornaviridae |
| Genera | |
|
Aphthovirus |
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A picornavirus is a virus belonging to the family Picornaviridae. Picornaviruses are non-enveloped, positive-stranded RNA viruses with an icosahedral capsid. The genome RNA is unusual because it has a protein on the 5' end that is used as a primer for transcription by RNA polymerase. The name is derived from pico, meaning small, and RNA, referring to the ribonucleic acid genome, so "picornavirus" literally means small RNA virus.
Picornaviruses are separated into a number of genera and include many important pathogens of humans and animals.[1] The diseases they cause are varied, ranging from acute "common-cold"-like illnesses, to poliomyelitis, to chronic infections in livestock. Additional species not belonging to any of the recognised genera continue to be described.[2]
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Picornaviruses are separated into a number of genera. Contained within the picornavirus family are many organisms of importance as vertebrate and human pathogens, shown in the table below.
Enteroviruses infect the enteric tract, which is reflected in their name. On the other hand, rhinoviruses infect primarily the nose and the throat. Enteroviruses replicate at 37°C, whereas rhinoviruses grow better at 33°C, as this is the lower temperature of the nose. Enteroviruses are stable under acid conditions and thus they are able to survive exposure to gastric acid. In contrast, rhinoviruses are acid-labile (inactivated or destroyed by low pH conditions) and that is the reason why rhinovirus infections are restricted to the nose and throat.
| Genus | Species (* signifies type species) | Serotypes |
|---|---|---|
| Enterovirus | Bovine enterovirus | 2 types: bovine enterovirus (BEV) 1-2 |
| Human enterovirus A | 21 types including some coxsackie A viruses and enteroviruses | |
| Human enterovirus B | 59 types including enteroviruses, coxsackie B viruses, echoviruses, and swine vesicular disease virus | |
| Human enterovirus C * | 19 types including poliovirus (PV) 1-3, some coxsackie A viruses and enteroviruses | |
| Human enterovirus D | 3 types: EV-68, EV-70, EV-94 | |
| Porcine enterovirus B | 2 types: porcine enterovirus (PEV) 9-10 | |
| Simian enterovirus A | 1 type: simian enterovirus (SEV) A1 | |
| Human rhinovirus A * | 75 types | |
| Human rhinovirus B | 25 types | |
| Human rhinovirus C | 7+ types | |
| Cardiovirus | Encephalomyocarditis virus * | 1 type: encephalomyocarditis virus (EMCV). Note: Columbia SK virus, Maus Elberfeld virus and Mengovirus are strains of EMCV. |
| Theilovirus | 12 types: Theiler's murine encephalomyelitis virus (TMEV), Vilyuisk human encephalomyelitis virus (VHEV), Thera virus (TRV), Saffold virus (SAFV) 1-9 | |
| Aphthovirus | Foot-and-mouth disease virus[3] * | 7 types: O, A, C, Southern African Territories (SAT) 1, SAT 2, SAT 3 and Asia 1 |
| Equine rhinitis A virus | 1 type: equine rhinitis A virus (ERAV) | |
| Bovine rhinitis B virus | 1 type: bovine rhinitis B virus (BRBV) | |
| Hepatovirus | Hepatitis A virus * | 1 type: Hepatitis A virus (HAV) |
| Parechovirus | Human parechovirus * | 14 types: Human parechovirus (HPeV) 1-14 |
| Ljungan virus | 4 types: Ljungan virus (LV) 1-4 | |
| Erbovirus | Equine rhinitis B virus * | 3 types: equine rhinitis B virus (ERBV) 1-3 |
| Kobuvirus | Aichi virus * | 1 type: Aichi virus (AiV) |
| Bovine kobuvirus | 1 type: bovine kobuvirus (BKV) | |
| Teschovirus | Porcine teschovirus * | 11 serotypes: porcine teschovirus (PTV) 1 to 11 |
| Sapelovirus | Porcine sapelovirus * (formerly Porcine enterovirus A) | 1 type: porcine sapelovirus (PSV) (formerly PEV-8) |
| Simian sapelovirus | 3 types: simian sapleovirus (SSV) 1-3 | |
| Avian sapelovirus | 1 type: avian sapelovirus (ASV) | |
| Senecavirus | Seneca Valley virus * | 1 type: Seneca Valley virus (SVV) |
| Tremovirus | Avian encephalomyelitis virus | 1 type: avian encephalomyelitis virus (AEV) |
| Avihepatovirus | Duck hepatitis A virus | 3 types: duck hepatitis A virus (DHAV) 1-3 |
The plant picornaviruses have a number of properties that are distinct from the animal viruses. They have been classified into the family Secoviridae containing the subfamily Comovirinae (genera Comovirus, Fabavirus and Nepovirus), and genera Sequivirus, Waikavirus, Cheravirus, Sadwavirus and Torradovirus (type species Tomato torrado virus)).
A number of picorna like viruses have been described infecting insects. These include Perina nuda picorna-like virus of the tussock moth, infectious flacherie virus of the silkworm and Sacbrood virus of the honeybee,[4] Plautia stali intestine virus[5] kelp fly virus,[6] Ectropis obliqua picorna-like virus,[7] deformed wing virus,[8] acute bee paralysis virus, Drosophila C virus, Rhopalosiphum padi virus, and Himetobi P virus.[9] Several have been placed in a separate family - the Dicistroviridae. Others have been placed into a new family Iflaviridae. This family includes Infectious flacherie virus and SeIV-1 virus.[10]
Picornaviruses are classed under Baltimore's viral classification system as group IV viruses as they contain a single stranded, positive sense RNA genome of between 7.2 and 9.0 kb (kilobases) in length. Like most positive sense RNA genomes, the genetic material alone is infectious; although substantially less virulent than if contained within the viral particle, the RNA can have increased infectivity when transfected into cells.
The capsid is an arrangement of 60 protomers in a tightly packed Icosahedral structure. Each protometer consists of 4 polypeptides known as VP (viral protein)1, 2, 3 and 4. VP2 and VP4 polypeptides originate from one protomer known as VP0 that is cleaved to give the different capsid components. The Icosahedral is said to have a triangulation number of 3, this means that in the icosahedral structure each of the 60 triangles that make up the capsid are split into 3 little triangles with a subunit on the corner. Depending on the type and degree of dehydration the viral particle is around 27-30 nm in diameter. The viral genome is around 2500 nm in length so we can therefore conclude that it must be tightly packaged within the capsid along with substances such as sodium ions in order to cancel out the negative charges on the RNA caused by the phosphate groups.
The genome itself is the same sense as mammalian mRNA, being read 5' to 3'. Unlike mammalian mRNA picornaviruses do not have a 5' cap but a virally encoded protein known as VPg. However, like mammalian mRNA, the genome does have a poly(A) tail at the 3' end. There is an un-translated region (UTR) at both ends of the picornavirus genome. The 5' UTR is longer, being around 600-1200 nucleotides (nt) in length, compared to that of the 3' UTR, which is around 50-100 nt. It is thought that the 5' UTR is important in translation and the 3' in negative strand synthesis; however the 5' end may also have a role to play in virulence of the virus. The rest of the genome encodes structural proteins at the 5' end and non-structural proteins at the 3' end in a single polyprotein.
The polyprotein is organised as follows: L-1ABCD-2ABC-3ABCD with each letter representing a protein.
The 3B protein is the VPg protein. The 3D protein is the RNA polymerase.
The viral particle binds to cell surface receptors. This causes a conformational change in the viral capsid proteins, and myristic acid are released. These acids form a pore in the cell membrane through which RNA is injected[2]. Once inside the cell, the RNA un-coats and the (+) strand RNA genome is replicated through a double-stranded RNA intermediate that is formed using viral RDRP (RNA-Dependent RNA polymerase). Translation by host cell ribosomes is not initiated by a 5' G cap as usual, but rather is initiated by an IRES (Internal Ribosome Entry Site). The viral lifecycle is very rapid with the whole process of replication being completed on average within 8 hours. However as little as 30 minutes after initial infection, cell protein synthesis declines to almost zero output – essentially the macromolecular synthesis of cell proteins is “shut off”. Over the next 1–2 hours there is a loss of margination of chromatin and homogeneity in the nucleus, before the viral proteins start to be synthesized and a vacuole appears in the cytoplasm close to the nucleus that gradually starts to spread as the time after infection reaches around 3 hours. After this time the cell plasma membrane becomes permeable, at 4–6 hours the virus particles assemble, and can sometimes be seen in the cytoplasm. At around 8 hours the cell is effectively dead and lyses to release the viral particles.
Experimental data from single step growth-curve-like experiments have allowed scientists to look at the replication of the picornaviruses in great detail. The whole of replication occurs within the host cell cytoplasm and infection can even happen in cells that do not contain a nucleus (known as enucleated cells) and those treated with actinomycin D (this antibiotic would inhibit viral replication if this occurred in the nucleus.)
In 1897, foot-and-mouth disease virus (FMDV), the first animal virus, was discovered. FMDV is the prototypic member of the Aphthovirus genus in the Picornaviridae family.[3] The plaque assay was developed using poliovirus. Both RNA dependent RNA polymerase and polyprotein synthesis were discovered by studying poliovirus infected cells.
Additional material
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