In this article we will discuss about the structure and replication of togaviruses.
Structure of Togaviruses:
The virus is enveloped and forms spherical particle of 65-70 nm diameter. It includes icosahedral capsid, which is made up of 240 monomers (Fig. 17.26). It has a triangulation number of (74). Envelope consists of 80 trimer spikes; dimension of each spike is 3xE1/E2 heterodimers. The spikes consist of glycoprotein that acts as attachment proteins to receptor of host cell membrane.
Capsid consists of a mono-partite, single-stranded, (+) sense, non-segmented RNA of about 11.7 kb long (consisting of about 10,000-12,000 nucleotides). It account for 4-8% total weight of particle. The 5′-terminus carries a methylated nucleotide cap (5’cap) and the 3′-terminus has a polyadenylated tail (3′ poly-A) or a genome-linked protein (VPg); therefore, its genome resembles to cellular mRNA, (Fig. 17.27).
Replication of Togaviruses:
Virus attaches to host receptors though E glycoprotein. Thereafter, fusion of virus membrane with the vesicle membrane occurs. Consequently, virus is endocytosed into vesicles in the host cell. RNA genome is released into the cytoplasm. After entry into the cell, gene expression and replication takes place within the cytoplasm (Fig. 17.28).
The virion RNA is infectious and serves as both genome and viral messenger RNA (Fig. 17.29). The whole genome is translated in a non-structural (NS) polyprotein which is processed by host and viral proteases. Structural polyprotein is expressed through a sub-genomic mRNA (Fig. 17.29).
Characteristically, there are two rounds of translation: (+) sense genomic RNA (’49S’ = 11.7kb) acts directly as mRNA and is partially translated (5′ end) to produce NS proteins. These proteins are responsible for replication, forming a complementary (-) sense strand (cRNA) as the template for further (+) sense strand synthesis.
Consequently, two species of (+) RNA are synthesized, full length genomic RNA and sub-genomic mRNA (’26S’ = 4.1kb). Translation of the newly synthesized sub-genomic RNA results in the production of structural proteins from 3′ end of genome (Fig. 17.29).
Replication takes place in cytoplasm of the host which occurs at the surface of endoplasmic reticulum. By using the genomic RNA as a template, a negative-sense single-stranded complementary RNA (cRNA) is synthesized. Both new genomic RNA as well as sub-genomic RNA are synthesized by using the negative-sense RNA as a template.
Sub-genomic RNA is translated in structural proteins. Virus assembly occurs at the endoplasmic reticulum. The virion buds at the endoplasmic reticulum is transported to the Golgi apparatus. Assembly occurs al the cell surface, and the envelope is acquired as the virus buds from the cell. Release and maturation almost simultaneous and then bud from the cell membrane (Fig. 17.30).