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ICTV TAXONOMY PROFILES Chen et al., Journal of General Virology 2018;99:761–762 DOI 10.1099/jgv.0.001072 ICTV

ICTV Virus Taxonomy Profile: Togaviridae

Rubing Chen,1,* Suchetana Mukhopadhyay,2 Andres Merits,3 Bethany Bolling,4 Farooq Nasar,5 Lark L. Coffey,6 Ann Powers,7 Scott C. Weaver1 and ICTV Report Consortium

Abstract The Togaviridae is a family of small, enveloped with single-stranded, positive-sense RNA of 10–12 kb. Within the family, the includes a large number of diverse species, while the genus Rubivirus includes the single species virus. Most are mosquito-borne and are pathogenic in their hosts. Many are important human and veterinary (e.g. virus and eastern equine virus). is transmitted by respiratory routes among humans. This is a summary of the International Committee on Taxonomy of Viruses (ICTV) Report on the taxonomy of the Togaviridae, which is available at www.ictv.global/report/togaviridae.

Table 1. Characteristics of the family Togaviridae

Typical member: (J02363), species Sindbis virus, genus Alphavirus

Virion Enveloped, 65–70 nm spherical virions for alphaviruses or 50–90 nm pleomorphic virions for rubella virus, with a single and three or two envelope glycoproteins, respectively 10–12 kb of positive-sense, unsegmented RNA Replication Cytoplasmic, in vesicles derived from the plasma membrane/endosomal compartment. Assembled virions bud from plasma membrane (alphaviruses) or into the lumen of (rubella virus) Translation Non-structural are translated from genomic RNA, and structural proteins from subgenomic RNA, both as polyprotein precursors Host range Humans and nonhuman , equids, birds, amphibians, reptiles, , pigs, sea mammals, salmonids, mosquitoes and some other ; most members of genus Alphavirus are mosquito-borne Taxonomy Two genera (Alphavirus and Rubivirus) including more than 30 species

VIRION structural polyproteins are encoded by separate ORFs that Togavirus particles consist of a nucleocapsid core, a lipid are flanked by 5¢- and 3¢-terminal non-coding regions and bilayer and surface glycoproteins. However, there are some separated by an internal non-coding region (Fig. 2). The striking differences between alphaviruses and rubella viri- subgenomic promoter overlaps with the 3¢-end of the non- ons. Alphaviruses are spherical and about 70 nm in diame- structural and an internal non-coding region [3]. ter. Particles contain a distinct icosahedral core and an icosahedral glycoprotein layer [1]. In contrast, rubella virus REPLICATION particles are pleomorphic, often tube-like and do not have Replication complexes (spherules) are formed on the plasma icosahedral symmetry. Rubella virus capsid proteins form membrane. Non-structural polyproteins are cleaved in an homodimers in a grid-like pattern and the glycoproteins are ordered manner by a viral cysteine to form (1) a arranged in rows on the virion surface [2] (Table 1, Fig. 1). short-lived replication complex for negative-sense RNA GENOME synthesis and later (2) a stable replication complex for syn- thesis of positive-sense RNA genomes and subgenomic The virus genome is unsegmented RNA of 9.7–12 kb (alpha- mRNAs. Replication enzymes also include a methyl- viruses) or 9.8–10 kb (rubella virus). Non-structural and guanylyl transferase, an RNA and an RNA-

Received 18 April 2018; Accepted 20 April 2018 Author affiliations: 1Department of Microbiology and Immunology, University of Texas Medical Branch, Galveston, TX 77555, USA; 2Department of Biology, Indiana University, Bloomington, IN 47405, USA; 3Institute of Technology, University of Tartu, Nooruse 1, 50411 Tartu, Estonia; 4Arbovirus Laboratory, Texas Department of State Health Services, 1100 West 49th Street, Austin, TX 78714, USA; 5Virology Division, United States Army Medical Research Institute of Infectious Diseases, 1425 Porter Street, Frederick, MD 21702, USA; 6Department of Pathology, Microbiology and Immunology, School of Veterinary Medicine, University of California, Davis, CA 95616, USA; 7Division of Vector-Borne Diseases, Centers for Disease Control and Prevention, Fort Collins, CO 80521, USA. *Correspondence: Rubing Chen, [email protected] Keywords: Togaviridae: taxonomy; ICTV Report; chikungunya virus; eastern equine encephalitis virus; rubella virus.

001072 This is an open access article under the terms of the http://creativecommons.org/licenses/by/4.0/, which permits unrestricted use, distribution and reproduction in any medium, provided the original author and source are credited. 761 Chen et al., Journal of General Virology 2018;99:761–762

Fig. 2. Alphavirus and rubivirus genome organisation. Black lines – non-coding regions; open boxes – ORFs (NS-ORF, non structural Fig. 1. Structure of togavirus particles. (a) Three-dimensional cryo- protein ORF; S-ORF, structural protein ORF) with processed products electron reconstruction of chikungunya virus at 10.2 Å resolution indicated beneath. An asterisk indicates the stop codon present in (courtesy of J. C. Y. Wang). The triangle outlines one icosahedral unit some alphaviruses that must be translationally read through to pro- with the symmetry axes labelled. (b–d) Representation of three differ- duce a precursor containing nsP4. Mtr, methyl transferase; Pro. prote- ent rubella virions, each determined using cryo-electron tomography ase; Hel, helicase; X, unknown function; RdRP, RNA-dependent RNA and no averaging procedures. The resolution of the reconstructions is polymerase (courtesy of T. K. Frey). not absolute, but is estimated to be better than 50 Å. All scale bars represent 10 nm. Modified from [2]. genome sequences are not monophyletic with respect to those of other positive-sense ssRNA viruses. dependent RNA polymerase. The orders of these conserved functional motifs in non-structural polyproteins are differ- RESOURCES ent between the two genera [4]. Full ICTV Online (10th) Report: Structural polyproteins are processed by cellular enzymes www.ictv.global/report/togaviridae. and, for alphaviruses, also by autoprotease activity of the capsid protein. The capsid protein assembles with the viral genomic RNA to form viral nucleocapsids. The glycopro- Funding information Production of this summary, the online chapter and associated resour- teins are co-translationally inserted into the endoplasmic ces was funded by a grant from the Wellcome Trust (WT108418AIA). reticulum and translocated to the plasma membrane (alpha- viruses) or to membranes of the Golgi apparatus (rubella Acknowledgements virus). From these respective sites, alpha- and rubella Members of the ICTV Report Consortium are Elliot J. Lefkowitz, Andrew J. Davison, Stuart G. Siddell, Peter Simmonds, Sead Sabanad- viruses bud to form virions with lipid envelopes [5]. zovic, Donald B. Smith, Richard J. Orton and Nick J. Knowles.

TAXONOMY Conflicts of interest The authors declare that there are no conflicts of interest. Alphavirus References This genus includes >30 species. Most alphaviruses are 1. Li L, Jose J, Xiang Y, Kuhn RJ, Rossmann MG. Structural changes mosquito-borne, transmitting alternatively between mos- of envelope proteins during alphavirus fusion. Nature 2010;468: quito vectors and vertebrate hosts including humans, non- 705–708. human primates, equids, birds, amphibians, reptiles, rodents 2. Mangala Prasad V, Klose T, Rossmann MG. Assembly, maturation and pigs. Chikungunya virus is an important human and three-dimensional helical structure of the teratogenic rubella virus. PLoS Pathog 2017;13:e1006377. ; most other alphaviruses cannot develop sufficient 3. Kuhn RJ. Togaviridae. In: Knipe DM and Howley PM (editors). Fields’ viraemia in humans to infect mosquitoes. [6], Virology. Philadelphia, USA: Lippincott Williams & Wilkins; 2013. pp. and probably Taï Forest alphavirus [7], are insect-specific. 629–650. The aquatic alphaviruses, southern elephant seal virus and 4. Rupp JC, Sokoloski KJ, Gebhart NN, Hardy RW. Alphavirus RNA salmon pancreas disease virus, appear to be transmitted synthesis and non-structural protein functions. J Gen Virol 2015;96: horizontally. 2483–2500. 5. Frey TK. Molecular biology of rubella virus. Adv Virus Res 1994;44: Rubivirus 69–160. Members of the single species in this genus, Rubella virus, 6. Nasar F, Palacios G, Gorchakov RV, Guzman H, da Rosa AP et al. Eilat virus, a unique alphavirus with host range restricted to insects infect only humans and are transmitted via respiratory by RNA replication. Proc Natl Acad Sci USA 2012;109:14622–14627. routes. The taxonomic relationship of Rubivirus to Alphavi- 7. Hermanns K, Zirkel F, Kopp A, Marklewitz M, Rwego IB et al. Dis- rus is under active assessment because of significant differ- covery of a novel alphavirus related to Eilat virus. J Gen Virol 2017; ences in their members’ virion structures and because their 98:43–49.

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