Name Origin and Classification
The family name Picornaviridae is a portmanteau reflecting the primary physicochemical properties of its members. The first part comes from the Spanish pico, meaning "small," which highlights the extremely small size of the virions. The second part, rna, indicates the type of nucleic acid forming the genome (ribonucleic acid).
The taxonomic hierarchy follows a classical scheme: genera consist of species, and species are further subdivided into serotypes. Today, the family includes more than 230 members.
Major genera include:
- Enterovirus: the largest genus, including 260 serotypes.
- Aphthovirus: contains 7 serotypes.
- Hepatovirus: includes 2 serotypes (one infects humans, the other infects monkeys).
- Cardiovirus: represented by 2 serotypes.
Additionally, modern classification includes newer genera: Parechovirus, Erbovirus, Kobuvirus, and Teschovirus.
Morphology and Molecular Structure
Picornaviruses belong to simply organized viruses lacking an outer lipid envelope (supercapsid). The viral particle diameter is only about 30 nm.
The capsid (protein coat) possesses strict icosahedral, or cubic, symmetry. Its architecture follows a precise mathematical model:
- The shell is built from 12 pentagonal vertices called pentamers.
- Each of the 12 pentamers consists of 5 protein subunits called protomers.
- At the molecular level, each protomer is formed by a complex of four distinct viral polypeptides: VP1, VP2, VP3, and VP4.
Inside the protected capsid lies the viral genome. It is represented by a single-stranded positive-sense RNA ((+)RNA) molecule, which is infectious on its own and can directly function as messenger RNA (mRNA) inside the host cell. A key feature is that a specific viral protein, VPg (Viral Protein genome-linked), is covalently attached to this RNA molecule.
Reproductive Cycle
The entire replication cycle of picornaviruses takes place strictly in the cytoplasm of the host cell. This process is always accompanied by a pronounced cytopathic effect, inevitably leading to cell death. When cultured in a laboratory (in cell culture under an agar overlay), this property manifests as the formation of "plaques"—visible zones of cell lysis.
The viral life cycle can be divided into six sequential stages:
- Penetration: The virion enters the host cell either via endocytosis or by direct injection of its RNA genome across the cell membrane.
- Uncoating: The capsid disintegrates, releasing the genomic (+)RNA into the cytoplasm. The VPg protein remains attached to the 5' end of the RNA molecule.
- Translation: The released genomic (+)RNA immediately functions as mRNA. Cellular ribosomes translate it into a single giant precursor protein called a polyprotein.
- Processing: The synthesized polyprotein is cleaved into individual functional units. This yields structural capsid proteins (VP1–VP4) and enzymatic proteins, including the key enzyme—RNA-dependent RNA polymerase.
- Genome Replication: The newly formed viral polymerase synthesizes a complementary negative strand ((-)RNA) using the genomic (+)RNA as a template. The resulting (-)RNA acts as a replicative intermediate from which the enzyme synthesizes multiple copies of new genomic (+)RNA. The VPg protein is promptly attached to each new RNA copy.
- Assembly and Exit: New nucleocapsids are formed from the synthesized structural proteins and fresh RNA copies. Fully assembled virions leave the cell upon lysis (destruction) of the host cell.