Sputnik virophage

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Classification

Domain; Phylum; Class; Order; family [Others may be used. Use NCBI link to find]

Viruses; unclassified virophages

Species

NCBI: Taxonomy

Genus species

Description and Significance

Describe the appearance, habitat, etc. of the organism, and why you think it is important.

Genome Structure

Describe the size and content of the genome. How many chromosomes? Circular or linear? Other interesting features? What is known about its sequence?

Cell Structure, Metabolism and Life Cycle

The Sputnik Virophage is an icosahedral, un-enveloped capsid with a diameter of 50 nm (740 Å). The triangulation number of the capsid is T=27 (1) with a total of 1620 capsid proteins (2). There is a lipid membrane located beneath the capsid proteins. Studies have shown that the most abundant protein of Sputnik is the V20 protein which is hypothesized to self-assemble and compose the icosahedral capsid (3). Studies of the capsid proteins of Sputnik have identified the major capsid protein of the virophage, protein V20, to have two consecutive jelly roll domains. The double jelly roll domains (two eight-stranded antiparallel β-barrels) are organized into capsomers and then arranged in hexagonal arrays. The capsomers of Sputnik are made up of three monomeric double jelly roll domains that create a pseudohexagonal shape. Sputnik also has fiber-like structures on its surface that are hypothesized to help stabilize the individual capsomers or aid in host recognition and attachment (1).

Sputnik is a virophage of the Acanthamoeba polyphaga mimivirus (APMV) family and is able to infect the common host Acanthamoeba without association with its mimivirus host. However, viral lysis was not observed within seven days post-infection without the mimivirus host(4). The genome is composed of dsDNA, structural (capsid) protein and non-structural capsid proteins (DNA polymerase, genome packaging ATPase), however it is unable to produce its own viral factory within the Acanthamoeba hose and instead utilizes the giant viral machiney that is produced by APMV (5). Recent studies of Mimivirus have shown that Sputnik infection is not present when the Mimivirus is bald, so it thought that the fibers on the Mimivirus capsid are involved with adhesion of the virophage and host virus.(6)

Sputnik particles have been identified within the amoeba cell as soon as 30 minutes post-infection. Between 6 and 8 hours the Sputnik virions are observed in the amoeba, however there is a lag phase between Sputnik and Mimivirus production. Mimivirus is not observable within the cell until 8 hours post-infection. Co-infection of Mimivirus with the Sputnik Virophage is deleterious to Mimivirus production and results in production of abnormal Mimivirus particles. Side effects of co-infection that have been observed are an 11% increase in Mimivirus capsid thickness, multiple Mimivirus capsid layers and asymmetric accumulation of capsid fibrils. Co-infection with Sputnik decreases the yield of infectious mimivirus particles by 70% (4) and decreases cell lysis at 24 hours by 13% (6).

Ecology and Pathogenesis

Habitat; symbiosis; biogeochemical significance; contributions to environment.
If relevant, how does this organism cause disease? Human, animal, plant hosts? Virulence factors, as well as patient symptoms.

References

[Sample reference] Takai, K., Sugai, A., Itoh, T., and Horikoshi, K. "Palaeococcus ferrophilus gen. nov., sp. nov., a barophilic, hyperthermophilic archaeon from a deep-sea hydrothermal vent chimney". International Journal of Systematic and Evolutionary Microbiology. 2000. Volume 50. p. 489-500.

1 – Sun, S., La Scola, B., Bowman, V. et al. “Structural Studies of the Sputnik Virophage”. Journal of Virology. 2010 January; 84(2): 894–897. Published online 2009 November 4.

2 - Gasteiger E., Gattiker A., Hoogland C., Ivanyi I., Appel R.D., Bairoch A. T=27 icosahedral capsid. ExPASy: the proteomics server for in-depth protein knowledge and analysis Nucleic Acids Res. Retrieved from http://viralzone.expasy.org/all_by_protein/1556.html.

3 - The UniProt Consortium. B4YNG0 (CAPSD_SPTNK). Universal Protein Knowledge Base. September 2011. Retrieved from http://www.uniprot.org/uniprot/B4YNG0.

4 – La Scola, B., Desnues, C., Pagnier, I. et al. “The virophage as a unique parasite of the giant mimivirus”. Nature. 2008 August; 455: 100-104.

5 - Desnues, C., Raoult, D. “Virophages question the existence of satellites”. Nature Reviews Microbiology. March 2012; 10: 234.

6 – Desnues, C., Boyer, M., Raoult, D. “Sputnik, a Virophage Infecting the Viral Domain of Life”. Advances in Virus Research. 2012; 82: 63-89.

Author

Page authored by _____, student of Prof. Jay Lennon at Michigan State University.

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