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
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.
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.
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
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
Virion proteins ]—Afn) Stop codon i NSP1 I NSP2 I
Minus strand complementary RNA
Cap —f Subgenomic mñÑA~~]— A(n) [ Translation I Polyprolein
Cotranslalional processing of < virion proteins lc I
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