Origin of the name and classification
The family name Picornaviridae is a compound word that perfectly reflects the main physicochemical properties of its representatives. The first part of the word comes from the Spanish pico, which translates as "small" and indicates the extremely small size of the virions. The second part — rna — indicates the type of nucleic acid that forms the genome (ribonucleic acid).
The taxonomic hierarchy of the family is built on a classical principle: genera consist of species, and species, in turn, are detailed into serotypes. To date, the family includes over 230 representatives.
Among the main genera are:
- Enterovirus: the most numerous genus, including 260 serotypes.
- Aphthovirus: includes 7 serotypes.
- Hepatovirus: includes 2 serotypes (one infects humans, the second infects monkeys).
- Cardiovirus: represented by 2 serotypes.
In addition, new genera have been introduced into modern classification: Parechovirus, Erbovirus, Kobuvirus, and Teschovirus.
Morphology and molecular structure
Picornaviruses belong to the category of simply organized viruses — they lack an additional lipid envelope (supercapsid). The diameter of the viral particle is only about 30 nm.
The capsid (protein shell) has a strict icosahedral, or cubic, type of symmetry. Its architecture follows a clear mathematical model:
- The shell is built from 12 vertices shaped like pentagons. These structures are called pentamers.
- Each of the 12 pentamers consists of 5 protein subunits, which are called protomers.
- At the molecular level, each protomer is formed by a complex of four different viral polypeptides: VP1, VP2, VP3, and VP4.
Inside the securely protected capsid lies the viral genome. It is represented by a single-stranded plus-sense RNA ((+)RNA), which is itself infectious and capable of directly functioning as messenger (informational) RNA in the cell. An important feature is that a specific genomic viral protein — VPg (Viral Protein genome-linked) — is covalently linked to this RNA molecule.
Features of the reproductive cycle
The entire cycle of picornavirus reproduction takes place strictly in the cytoplasm of the infected host cell. This process is always accompanied by a pronounced cytopathic effect, meaning it inevitably leads to cell death. When viruses are cultured in laboratory conditions (on a cell culture under an agar overlay), this property manifests as the formation of so-called "plaques" — visible zones of cell lysis.
The virus life cycle can be divided into six successive stages:
- Penetration: The virion enters the host cell either via endocytosis or by direct injection of its RNA genome directly through the cell membrane.
- Uncoating: The capsid is destroyed, and the genomic (+)RNA is released into the cytoplasm. At the same time, the attached VPg protein remains preserved at the 5'-end of the RNA molecule.
- Translation: The released genomic (+)RNA immediately begins to function as messenger RNA (mRNA). Translation occurs on cellular ribosomes, resulting in the synthesis of a single giant precursor protein — the polyprotein.
- Processing: The synthesized polyprotein undergoes cleavage (slicing) into separate functional units. As a result, structural capsid proteins (VP1–VP4) and enzymatic proteins are formed, including the key enzyme — RNA-dependent RNA polymerase.
- Genome replication: The newly formed viral polymerase begins the synthesis of the complementary minus strand ((-)RNA), using the genomic (+)RNA as a template. The resulting (-)RNA becomes the replicative intermediate: based on it, the enzyme synthesizes multiple copies of new genomic (+)RNAs. The VPg protein is immediately attached to each new RNA copy.
- Assembly and release: New nucleocapsids are formed from the synthesized structural proteins and fresh copies of RNA. Fully formed virions leave the cell at the moment of its lysis (destruction).
Briefly: answers to topical questions
What human diseases are caused by members of the genus Enterovirus?
Members of the genus Enterovirus cause a wide range of clinically diverse diseases, including acute respiratory viral infections (ARVI) and acute bronchitis. Depending on the pathogen group (e.g., Coxsackie viruses), they can cause the following pathologies:
- Herpangina — vesicular eruptions on the posterior pharyngeal wall, dysphagia, and fever.
- Hand, foot, and mouth disease — "hand-foot-mouth" syndrome.
- Pleurodynia — Bornholm disease, accompanied by painful chest attacks and pleurisy.
- Myocarditis — visceral pathology with heart damage.
Enteroviruses are also capable of causing poliomyelitis-like diseases, encephalitis, exanthema, and diarrhea in children.
Which human virus belongs to the genus Hepatovirus?
The genus Hepatovirus includes the hepatitis A virus (HAV), which is the type species of this taxon. This RNA-containing virus causes infectious hepatitis (Botkin's disease) in humans. The disease is characterized by an acute course, is transmitted by the fecal-oral route, and does not lead to liver cirrhosis. In total, two serotypes are distinguished in this genus: one of them is pathogenic for humans, and the second for monkeys.
Into which functional proteins is the picornavirus polyprotein cleaved?
During the reproduction process, the giant picornavirus polyprotein is cleaved into separate functional proteins. These include:
- Structural proteins (VP1, VP2, VP3, VP4) — capsid polypeptides from which the protomers of the virion protein shell are built.
- Enzymes required for viral replication, including RNA-dependent RNA polymerase.
You can remember the structure of the picornavirus capsid by the formula "12-5-4": 12 vertices (pentamers), each with 5 protomers, and each protomer glued together from 4 proteins (VP1, VP2, VP3, VP4).
Class and exam questions
Why is the picornavirus genome called infectious?
Because it is represented by plus-strand RNA, which immediately acts as messenger RNA (mRNA) upon entering the cell cytoplasm. Ribosomes can immediately begin reading information from it without prior transcription.
What is the role of the VPg protein?
The VPg protein (Viral Protein genome-linked) is covalently linked to the 5'-end of the viral RNA. It protects the genome from degradation and plays a critical role in initiating the replication of the viral genome.
What is the cytopathic effect of picornaviruses?
The reproduction process of these viruses ends with the mass assembly of virions, which exit into the external environment via lysis (shell rupture and complete destruction) of the infected cell.
What else to study on this topic
- Diversity of serotypes in the genus Enterovirus (260 variants)
- Features of human and monkey Hepatovirus
- Mechanisms of endocytosis and genome translocation across the membrane
- Function of picornavirus RNA-dependent RNA polymerase
- Cytopathic effect and methods of plaque counting under agar
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