capsid (rus. капсид) — protein shell of a virus formed through self-assembly of one or more proteins into a geometrically ordered structure.

Description

A capsid is composed of individual protein subunits (capsomeres), organised in one or two layers; its symmetry can be cubic or helical. The main functions of the capsid include protection of the viral genome, providing virion (structurally mature infectious particle) adsorption to the cell, and its penetration into the cell through interaction with cellular receptors. The symmetry favours a highly compact capsid structure containing a large number of capsomeres required for genome packaging. The formation of a capsid resembles the crystallisation process and is based on the principle of self-assembly. The number of capsomeres is strictly specific for each species and depends on the size and morphology of virions. The self-assembly procedure allows native virus particles, particles consisting of a viral capsid and a heterologous nucleic acid, to be obtained, as well as virus-like particles that do not contain nucleic acids.

Symmetrically arranged viral particles or self-assembled (repolymerised) proteins of the viral protein shell can be used as scaffolds for creating various bio-inorganic materials: nanotubes, nanoelectrodes , nanocontainers. Some metals (gold, platinum) or metal-containing compounds (chlorides of silver, gold, platinum, iron oxide, cadmium and sulphur sulphides) can bind to the viral shell proteins on the surface or in the inner cavity of a capsid. Capsids metallised with gold or platinum are being used in development of nanodevices (nanowires, nanoelectrodes, nano-optical probes, etc.).

The ability of viruses to form crystals is also used to obtain three-dimensional long-living nanostructures. In the crystalline state viruses form strictly symmetrical porous structures of a particular architecture. These cavities and channels can be used to produce three-dimensional nanocomposites of noble metals.

Capsid protein modification or replacement by structural components of other viruses, bacteria, or specialised ligands allows target-specific recombinant viral nanoparticles (RVN) to be created. Chemical modification of the RVN protein shell, for example, protein modification with polymers, reduces the immune response to RVN shell proteins and thus helps to increase RVN protection from immune cells of the host. Thus, the RVN circulation in the host can be extended, the dose of RVNs can be lowered reducing toxic effects. Such RVNs can be used for the targeted delivery of drugs, therapeutic nanoparticles, and components of gene therapy into the target cells.

Illustrations

 Exohedral capsid of an edenovirus.
 Exohedral capsid of an edenovirus.

Authors

  • Naroditsky Boris S.
  • Shirinsky Vladimir P.
  • Nesterenko Lyudmila N.

Sources

  1. Alberts B., Johnson A., Lewis J. et al. Molecular Biology of the Cell. 4th ed. — N.Y.: Garland Publishing, 2002. — 265 p.
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  7. Falkner J. C., Turner M. E., Bosworth J. K., Trenter T. J. et al. Virus crystals as nanocomposite scaffolds // J. Am. Chem. Soc. 2005. V. 127. P. 5274–5275.

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