Virophage Concept, The

Abstract

The existence of small viruses that depend on the coinfection of their host cells by another virus has been known for a long time. These viruses are considered to be satellites of their helper viruses. The discovery of Sputnik, Mavirus and the Organic Lake virophage shook this vision of subviral agents. Indeed, these new viruses are not only dependent on coinfection but are also noxious for their so‚Äźcalled host viruses, leading to sick particles. This surprising capability established the concept of a virophage, which is a virus infecting a virus. Studies on the morphology, life cycle and genomes of virophages have enriched this concept by highlighting unexpected features. Thus, the question of the classification of the virophages among the satellite viruses within the virosphere has been brought to light.

Key Concepts:

  • Virophages are small viruses discovered to be associated with the largest viruses of the virosphere.

  • As satellite viruses, virophages are dependent on coinfection of their host cells with their host viruses.

  • This coinfection results in the lysis of the cell, diminution of the production and infectivity of the host virus and an increase in the production of abnormal particles of the giant virus.

  • The virophages may hijack the replication mechanisms of their host viruses within their host cells.

  • Virophage replication occurs in the viral factory, a dense cytoplasmic region that appears a few hours after infection by the host virus with or without its virophage.

Keywords: virophage; satellite virus; Mimiviridae; nucleocytoplasmic large DNA viruses; virosphere

Figure 1.

Sputnik particles observed by TEM (a, scale bar 200 nm; b, scale bar 500 nm).

Figure 2.

Diagram summarising the general structure of the Sputnik particle.

Figure 3.

The intracellular life cycle of Sputnik: (a) Scheme summarising the life cycle of Sputnik, with the different steps of coinfection of an amoeba together with the host virus. In (1), the giant virus and the virophage enter the amoeba, and then are internalised in vacuoles (2). The virion factory (VF) appears in (3), followed by the production of virophage's particles (4) and then by the particles of the giant virus (5). Finally, the amoeba is lysed and the two viruses are liberated (6). ‘N’ refers to the nucleus. (b) TEM of steps 4 and 5, (c) and (d) IFF (performed using mouse anti‐Sputnik and anti‐Mimivirus antibodies and DAPI staining) of steps 4 and 5 with Mimivirus (c) and Courdo11 (d), another Mimiviridae strain.

Figure 4.

TEM of a normal particle of Mamavirus (a, scale bar 200 nm) compared with abnormal particles. A Mamavirus particle with an accumulation of several capsid layers (b, scale bar 200 nm) and an empty particle of Mamavirus (c, scale bar 500 nm).

Figure 5.

Illustration of the genomes of Sputnik2, Mavirus and OLV.

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Further Reading

Pearson H (2008) ‘Virophage’ suggests viruses are alive. Nature 454: 677.

Web Links

Fleming N (2008) http://www.newscientist.com/article/dn14480‐first‐virophage‐could‐take‐the‐fight‐to‐viruses.html

Yong E (2012) http://www.the‐scientist.com/?articles.view/articleNo/32840/title/A‐Parasite‐s‐Parasites/

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Gaia, Morgan, Colson, Philippe, Desnues, Christelle, and La Scola, Bernard(May 2013) Virophage Concept, The. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0024410]