Togaviridae

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Togaviridae
Semliki-Forest-Virus.jpg

Semliki Forest Virus

Systematics
Classification : Viruses
Area : Riboviria
Empire : Orthornavirae
Phylum : Kitrinoviricota
Class : Alsuviricetes
Order : Martellivirales
Family : Togaviridae
Taxonomic characteristics
Genome : (+) ssRNA linear
Baltimore : Group 4
Symmetry : icosahedral
Cover : available
Scientific name
Togaviridae
Left
NCBI  Taxonomy: 11018
ViralZone ( ExPASy , SIB): 3
ICTVdB Virus Code : 00.073

The Togaviridae family ( Togaviruses ) currently comprises two genera of enveloped viruses (Latin toga : coat) with a single-stranded RNA with positive polarity . The Togaviridae belong to the genus Alphavirus, important virus species that transmit diseases to humans and animals through insects .

morphology

The spherical virus particles ( virions ) of the Togaviridae are about 70 nm in diameter and surrounded by a lipid membrane (virus envelope). Heterodimers of the two viral envelope proteins E1 and E2 (45-58  kDa ) are embedded in the envelope as distinct spikes . Within the genus Alphavirus , the heterodimers combine to form 80 trimers and form an icosahedral envelope structure (otherwise only found in capsids ). With some alphaviruses, a third envelope protein E3 (10 kDa) can also appear.
A capsid with a diameter of about 40 nm and made up of 240 molecules of the core protein CP (30-33 kDa) with an icosahedral symmetry (T = 4) is found very close to the shell . The fixed ratio of envelope proteins to capsid proteins (1: 1) is particularly important when assembling the capsids of the alphaviruses.

The capsid of the Rubella virus (genus Rubivirus ) consists of 180 homodimers of the core protein in an icosahedral symmetry (T = 3) and does not have such a close association with the virus envelope as in the case of alphaviruses; the rubella virus is therefore often variable in shape. This genus was moved to a separate family Matonaviridae by the ICTV 2018/2019 with the Master Species List # 34 .

The genome of the Togaviridae consists of a single-stranded linear RNA of positive polarity ((+) ssRNA) and is from 9.7 to 11.8 kb in size. At its 5 'end there is a 5' cap structure , followed by the sequences for the mostly four non-structural proteins (nsP1-4), the structural proteins (CP, E1, E2) and a poly at the 3 'end (A) tail . The genome can thus serve as messenger RNA (mRNA) for translation .

Replication

Togaviruses attach to the cell surface via specific receptors and are taken up by an endosome vesicle that forms. At a neutral pH outside the cell, the E2 coat protein covers the E1 protein. Inside the endosome, at acidic pH, the outer domains of the E1 protein are exposed, which contain a fusion sequence and induce the fusion of the endosome membrane and the virus envelope. This releases the capsid into the cytosol, disintegrates and releases the genome.
The (+) ssRNA initially only serves to translate the non-structural proteins, which are synthesized as a large polyprotein and cleaved from it by cellular signal peptidases . The sequences for the structural proteins, on the other hand, are first multiplied by the viral RNA polymerase (replicase) via a complementary (-) ssRNA as a template and translated as a separate short mRNA. This short (subgenomic) mRNA is also packaged in the virion in the rubella virus .
After the synthesis of the new genomic RNA and the viral structural proteins, the glycosylated envelope proteins begin to be transported from the endoplasmic reticulum (ER) via the Golgi apparatus to the cell membrane. Here, finally, the assembly of shell and capsid and budding (find budding ) of new virions instead. In the case of alphaviruses, the capsid with the RNA is formed beforehand; in the case of the rubella virus , the capsid formation only begins when it is primarily attached to the membrane of the ER, from which the rubella virus also buds .

Systematics

Internal system

The following structure of the Togaviridae follows the specifications of the International Committee on Taxonomy of Viruses (ICTV) as of February 2019, the groups of the rank of a subgenus have a proposed character and have not yet been confirmed by the ICTV, as are the species listed in quotation marks.

  • Togaviridae family
  • Group 'Barmah Forest Virus Complex' (BFV complex or BFV group, possibly to the SFV group, see below)
  • Eastern equine encephalitis complex group (EEEV group)
  • 'Middelburg virus complex' group (MIDV complex or MIDV group, possibly to the SFV group, see below)
  • 'Ndumu virus complex' group (NDUV complex or NDUV group)
  • Semliki Forest Virus Complex group (SFV complex or SFV group)
  • Venezuelan equine encephalitis complex group (VEEV group)
  • Western equine encephalitis complex group (WEEV group)
  • without suggested group:

The genus Rubivirus (with the species rubella virus , en. Rubella virus , RuV) has been prepared by ICTV 2018/2019 into its own family Matonaviridae since March 2020 order Hepelivirales placed in the same class.

External system

In 2015, Koonin et al assigned the Togaviridae taxonomically (due to their relationship) to the supergroup 'Alphavirus-like superfamily' they postulated. Sister group is thereafter the family Hepeviridae . The members of this proposed supergroup belong to different groups of the Baltimore classification , usually they are single-stranded RNA viruses of positive polarity ((+) ssRNA, Baltimore group 4), but there are also double-stranded representatives (marked with dsRNA, Find Baltimore Group 3).

This suggestion has now been replaced by the ICTV Master Species List # 35 of March 2020. A comparison of the cladograms can be found in Tymovirales §ICTV Master Species List # 35 .

Individual evidence

  1. ↑ a b c d ICTV Master Species List 2018b v1 MSL # 34, Feb. 2019
  2. ↑ a b c d e ICTV: ICTV Master Species List 2019.v1 , New MSL including all taxa updates since the 2018b release, March 2020 (MSL # 35)
  3. ↑ ICTV : Master Species List 2018a v1 , MSL (# 33) including all taxa updates since the 2017 release. Fall 2018
  4. ↑ Google Patents: RU2350622C2 "Conjugate of angiotensin peptide component with carrier, vaccine composition, method of animal immunization and method of treatment or prevention of physical disorder related to angiotensin system activated by renin"
  5. ↑ Since this group (referred to by the authors as English superfamily ) contains an order with the Tymovirales , their rank must be higher than this and should not be understood as a superfamily . Ranks higher than order were not given by the ICTV at the time of the work.
  6. ^ Eugene V. Koonin, Valerian V. Dolja, Mart Krupovic: Origins and evolution of viruses of eukaryotes: The ultimate modularity , in: Virology from May 2015; 479-480. 2-25, Epub March 12, 2015, PMC 5898234 (free full text), PMID 25771806

literature

  • S. Schlesinger and MJ Schlesinger: Togaviridae: The viruses and their replication . In: David M. Knipe, Peter M. Howley et al. (Ed.): Fields' Virology, 4th Edition, Philadelphia 2001
  • CM Fauquet, MA Mayo, et al. : Eighth Report of the International Committee on Taxonomy of Viruses , London, San Diego, 2004

Web links