Basic virion structure
The main component of any virus is the nucleocapsid, which is a combination of genetic material (nucleic acid) and a protective protein coat called the capsid. The capsid itself consists of many identical subunits called capsomeres, which in turn are formed by polypeptide chains.
The capsid performs critical functions:
- Protection: shields the viral genome from degradation by enzymes and adverse environmental factors.
- Receptor function (in non-enveloped viruses): ensures initial attachment (adsorption) to a susceptible cell.
Non-enveloped and enveloped viruses
Based on structural features, viruses are divided into two large groups:
- Non-enveloped viruses: consist solely of nucleic acid and a capsid. After replication, they typically leave the host cell by destroying it (lysis). Capable of forming crystals (e.g., foot-and-mouth disease virus).
- Enveloped viruses: possess an additional lipoprotein membrane over the nucleocapsid — the supercapsid (or peplos).
The supercapsid is formed when the virion buds off from the host cell, acquiring a portion of its membranes (plasma, nuclear, or endoplasmic reticulum). Special glycoprotein spikes called peplomers are frequently located on this envelope. In some viruses, an additional layer — the matrix protein (M protein) — lies beneath the supercapsid. Notably, enveloped viruses are sensitive to lipid solvents (e.g., ether): destruction of the lipid envelope leads to virus inactivation.
Types of symmetry
The shape of the virion largely depends on how the nucleocapsid is arranged. Several types of symmetry are distinguished:
- Helical: the nucleocapsid is wound in a screw-like manner (characteristic of coronaviruses and influenza viruses).
- Icosahedral (cubic): the capsid forms an isometric hollow body inside which the nucleic acid is enclosed (e.g., herpes virus).
- Complex: a combination of various structural types.
Proteins and internal structure
The contents inside the capsid are called the core. Its composition varies among different viruses: for example, in adenoviruses, DNA is bound to histone-like proteins, while reoviruses contain inner capsid proteins.
All viral proteins are divided into:
- Structural: form the "body" of the virion.
- Non-structural: function as enzymes and factors required for viral replication inside the cell.
Most proteins are encoded by the viral genome itself, although during assembly, the virion may also incorporate certain host cell proteins. Enveloped viruses also contain carbohydrates (polysaccharides) and lipids in their envelopes.
Briefly: answers to topical questions
What functions do peplomers perform on the surface of enveloped viruses?
Peplomers (glycoprotein "spikes" of the supercapsid) perform receptor and enzymatic functions, facilitate viral entry into the cell, and act as strong immunogens.
- Hemagglutinin and attachment proteins (HN, H, G, E2) ensure adsorption of the virus to specific target cell receptors.
- Fusion protein (F) mediates the fusion of the viral lipid envelope with the cell membrane and causes syncytium formation.
- Neuraminidase (N) disrupts hemagglutinin bonds with the epithelium, facilitating virion penetration and the release of new viral particles.
- Envelope glycoproteins (E1) induce the formation of virus-neutralizing antibodies.
Which virus families have helical symmetry?
Helical nucleocapsid symmetry (screw-like structure) is characteristic of a number of enveloped RNA virus families.
- Orthomyxoviruses (Orthomyxoviridae) — the causative agents of influenza.
- Paramyxoviruses (Paramyxoviridae) — include parainfluenza, measles, mumps, and respiratory syncytial virus.
- Coronaviruses (Coronaviridae).
- Filoviruses (Filoviridae) — the causative agents of Marburg and Ebola fevers.
Tobacco mosaic virus also has a helical capsid arrangement.
Which virus families have icosahedral (cubic) symmetry?
Icosahedral (cubic) capsid symmetry is found in both non-enveloped and enveloped viruses across various families.
- Herpesviruses (Herpesviridae).
- Adenoviruses (Adenoviridae).
- Reoviruses (Reoviridae).
- Picornaviruses (Picornaviridae) — include enteroviruses and rhinoviruses.
- Flaviviruses (Flaviviridae) — include yellow fever, tick-borne encephalitis, and hepatitis C viruses.
Additionally, an icosahedral capsid is characteristic of hepatitis E virus.
What function does the matrix protein (M protein) perform in the virion structure?
The matrix protein (M protein) primarily serves binding and structural roles, forming the internal framework of the virion. It lines the inner surface of the outer lipoprotein envelope (supercapsid) and structurally links it to the nucleocapsid. In some viruses, the M protein performs additional regulatory tasks. For example, in orthomyxoviruses, high accumulation of the M1 protein induces the export of viral genome segments from the cell nucleus using a nuclear export protein.
Which specific enzymes belong to non-structural viral proteins?
Non-structural proteins include virus-induced enzymes that are not part of the virion itself, but are synthesized in the infected cell to support viral reproduction.
- Replication and transcription enzymes — viral RNA polymerases and DNA polymerases.
- Modifying enzymes — proteinases (e.g., papain-like protease) and protein kinases that modify viral proteins.
Non-structural enzymes also include specific proteins required for the replication of particular families, such as the NS2–NS5 enzyme complex in hepatitis C virus.
Class and exam questions
What are peplomers and where are they located?
Peplomers are glycoprotein projections ("spikes") located on the surface of the supercapsid (outer envelope) of enveloped viruses.
What types of symmetry do viruses exhibit?
Helical, icosahedral (cubic), and complex types of symmetry are distinguished, depending on how capsomeres are arranged around the nucleic acid.
Why are enveloped viruses sensitive to ether?
The supercapsid is a lipoprotein membrane. Ether and other lipid solvents destroy the envelope lipids, leading to virus inactivation.
What else to study on this topic
- Mechanisms of viral adsorption to the host cell
- The process of budding and supercapsid formation
- Specificity of viral receptors
- Role of structural and non-structural proteins in reproduction
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