Properties of Togaviridae

Togavirus virions are spherical, 70 nm in diameter, and consist of an ico-sahedral capsid surrounded by a tightly adherent lipid envelope covered with glycoprotein peplomers (Fig. 25-1). The genome is a single linear 11-12 kb

Specific Proteins Rubella Virus
Fig. 25-1 Togavmdac, genus Alphavirus. Negatively stained virions of Semliki Forest virus. Bar, 100 nm. (Courtesy Di C -H von Bonsdorff.)

molecule of ssRNA of positive polarity, which is 5' capped and 3' polyadeny-lated, and is infectious. Togaviruses are not very stable in the environment and are easily inactivated by disinfectants. Other properties of the togaviruses are summarized in Table 25-1.

Classification

All alphaviruses share common antigenic determinants, notably on the nucleo-capsid protein, C. The envelope glycoproteins carry some determinants that are common to certain species only, providing the.basis for partition of the genus into six antigenic "complexes." Individual alphaviruses can be differentiated on the basis of species-specific epitopes on the envelope glycoproteins. Rubella virus, the only member of the genus Rubtvirus, displays no antigenic cross-reactivity with any alphavirus.

Table 25-1

Properties of Togavindae

Two genera Alphavirus and Rubtvirus

Spherical virion, enveloped, with peplomers, diameter 70 nm Icosahedral capsid, diameter 40 nm

Linear, plus sense ssRNA genome, 11-12 kb (Alplmi'irits), 10 kb (Rubmius), 5' capped, 3' poly-ad en y la ted, infectious

Genes for nonstructural proteins located al 5' end of genome, those for structural proteins at the

Two envelope glycoproteins, EI and E2, containing virus-specific neutralizing epitopes and alphavirus serogroup and subgroup specificities, nucleocnpsid prolein, C, with broadly cross-reactive alphavirus group specificity Full-length and subgenomic RNA transcripts; posttranslationnl cleavage of polyproterns Cytoplasmic replication; budding from plasma membrane (alphaviruses) or intracytoplasmic membranes (rubella virus) of vertebrate cells, or from inlracytoplasmic membranes of invertebrate cells, in which alphaviruses are noncytocidal

Viral Replication

Alphavirus virions are taken up into coaled vesicles via receptor-mediated endocytosis (see Fig. 3-3); when these coated vesicles mature into phagolysosomes the low interior pH causes virions to release the genome into the cytosol, where the whole of the replication cycle takes place.

The 5' two-thirds of the capped viral RNA genome (Fig. 25-2) is translated into a large polyprotein, which then cleaves itself into four nonstructural proteins. The viral protease, as well as a helicase involved in viral RNA replication, is contained within nonstructural protein 2 (NSP2); NSP4 is thought to represent the RNA polymerase; NSP3 converts RNA replicase to a plus strand replicase; NSP1 is involved in methylation and capping of viral RNAs and in initiation of minus strand RNA synthesis. Thus all four nonstructural proteins are required for synthesis of a full-length complementary copy of the viral genome. From this minus strand template two types of RNA are produced: full-length progeny genomic RNA and a subgenomic mRNA corresponding to the 3' one-third of the genomic RNA which encodes all the

Nonstructural proteins

Polyprolein

Translation

Virion proteins ]—Afn) Stop codon i NSP1 I NSP2 I

Transcription

Minus strand complementary RNA

Transcription

Cap —f Subgenomic mñÑA~~]— A(n) [ Translation I Polyprolein

Cotranslalional processing of < virion proteins lc I

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