Cypovirus

Cypovirus, short for cytoplasmic polyhedrosis virus, is a genus of double-stranded RNA viruses in the family Reoviridae and subfamily Spinareovirinae. Cypoviruses have only been isolated from insects. Diseases associated with this genus include: larvae chronic disease. There are currently 16 species in this genus including the type species Cypovirus 1.[1][2]

Cypovirus
Cryo-EM capsid and asymetric unit cell protein structure of cypovirus
TEM of occlusion body and virions of dendrolimus punctatus cypovirus
Virus classification
(unranked): Virus
Realm: Riboviria
Kingdom: Orthornavirae
Phylum: Duplornaviricota
Class: Resentoviricetes
Order: Reovirales
Family: Reoviridae
Subfamily: Spinareovirinae
Genus: Cypovirus
Type species
Cypovirus 1

Cypoviruses structurally have a lot in common with the more widely studied nucleopolyhedroviruses, a genus of arthropod viruses in the family Baculovirus. However, cypoviruses have an RNA genome and replicate in the cytoplasm of the infected cells while nucleopolyhedroviruses have a DNA genome and replicate in the nucleus.

Structure

Viruses in the genus Cypovirus are non-enveloped, with icosahedral geometries, and T=2 symmetry.[1] Cypoviruses have only a single capsid shell, which is similar to the orthoreovirus inner core. They exhibit striking capsid stability which is fully capable of endogenous RNA transcription and processing.[3] The capsid diameter is around 65 nm.[1]

Genome

Genomes are linear and segmented. The genome codes for 10 to 12 proteins.[1] Classification of cypoviruses is based on the electrophoretic migration profiles of their genome segments. The overall folds of cypovirus proteins are similar to those of other reoviruses, however, they have insertional domains and unique structures that contribute to their extensive intermolecular interactions. The cypovirus turret protein contains two methylase domains with a highly conserved helix-pair/β-sheet/helix-pair sandwich fold but lacks the β-barrel flap present in orthoreovirus λ2. The stacking of turret protein functional domains and the presence of constrictions and A spikes along the mRNA release pathway indicate a mechanism that uses pores and channels to regulate the highly coordinated steps of RNA transcription, processing, and release.[3]

Life cycle
Diagram of a cypovirus polyhedra

Viral replication is cytoplasmic. Entry into the host cell is achieved by attachment to host receptors, which mediates endocytosis. Replication follows the double-stranded RNA virus replication model. Double-stranded RNA virus transcription is the method of transcription. Translation takes place by ribosomal skipping. The virus exits the host cell by monopartite non-tubule guided viral movement and exists in occlusion bodies after cell death and remaining infectious until finding another host.

Insect serve as the natural host. Transmission routes are fecal-oral, parental, and egg transmission.[1]

Infection occurs when a susceptible insect consumes the polyhedra, usually as a contaminant on the insect’s food (in most cases, foliage of a plant). The polyhedra dissolve in the digestive tract of the insect, releasing the virus particles that penetrate the gut epithelial cells. Replication of the virus is often confined to these cells and the progeny virus, in the form of new polyhedra are excreted in the insect feces, thus contaminating more foliage resulting in the spread of the disease to additional insects. The progression of the disease can be rather slow, but the virus infection is normally fatal.

Taxonomy

The genus Cypovirus has sixteen species:

  • Cypovirus 1
  • Cypovirus 2
  • Cypovirus 3
  • Cypovirus 4
  • Cypovirus 5
  • Cypovirus 6
  • Cypovirus 7
  • Cypovirus 8
  • Cypovirus 9
  • Cypovirus 10
  • Cypovirus 11
  • Cypovirus 12
  • Cypovirus 13
  • Cypovirus 14
  • Cypovirus 15
  • Cypovirus 16

See also

References
  1. "Viral Zone". ExPASy. Retrieved 15 June 2015.
  2. ICTV. "Virus Taxonomy: 2014 Release". Retrieved 15 June 2015.
  3. Zhou ZH (2008). "Cypovirus". Segmented Double-stranded RNA Viruses: Structure and Molecular Biology. Caister Academic Press. ISBN 978-1-904455-21-9.
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