Like coronaviruses and flaviviruses, togaviruses have the following properties:
- + strand ssRNA genomes
- Use insects as secondary hosts
- Wide range and severity of diseases
There are two classes of togaviruses:
|Alphaviruses||The prototype is Sindbis, which forms very clean plaques; result in disease with an enormous range of symptoms.|
|Rubiviruses||The prototype is rubella; result in disease with very mild symptoms.|
The most important feature of togavirus replication is that is has a dicistronic genome. Two mRNAs are used, one to produce non-structural proteins and another to produce structural proteins. This is important because it is very efficient. Picornaviruses produce one polypeptide, which means that equal amounts of each protein are produced even though unequal amounts are needed. As a result, picornavirus replication is inefficient. Togaviruses, however, regulate synthesis of non-structural and structural proteins by using two mRNAs. In fact, there is 4 times as much subgenomic RNA as genomic RNA in cells infected with togavirus. Two classes of proteins are synthesiszed from different mRNAs, which allows temporal regulation and qualitiative replication. Genomic RNA can do two things: make non-structural proteins (such as replicase) by synthesizing the minus strand, or make structural proteins through synthesis of subgenomic RNA and synthesize positive strand RNA.
A minus strand is made that is complementary to the genomic plus RNA. This minus strand has two initiation sites for replicase: one for non-structural proteins and one for structural proteins. The site for non-structural proteins is at the 3' end, and if replicase initiates there it will produce the plus RNA encoding non-structural proteins. The site for structural protiens is about mid-way through, and if replicase initiates there then a subgenomic (smaller than genomic) plus RNA will be synthesized. This will encode structural proteins.
Togavirus subgenomic RNA was discovered via the following experiment:
- Cells are infected with togavirus and incubated in labeled uridine
- This labeled uridine is incorporated into all RNA
- Actinomycin D shuts off synthesis of cellular RNA
- As a result, only viral RNAs are labelled
- This RNA is run on a gel, and 3 kinds of togavirus RNA are detected:
- Genome (plus strand)
- Negative strand
- Further experiments revealed how the 2 mRNAs could be synthesized at different levels, and that each had only one ORF.
There are five structural proteins: capsid, E1 (composed of E2 and E3), and p62. The capsid protein autolytically cleaves itself from the other structural proteins. The capsid protein assembles with the genomic RNA into a nucleocapsid. E1 and p62 proteins contain a sequence which directs them into the lumen of the endoplasmic reticulum. They move to the Golgi and the trans-Golgi as if they were host cell proteins. They are also modified just like host cell proteins via glycosylation and further cleavage to p62, E2, and E3. Glycoproteins E1, E2, and E3 form a trimer in the cell membrane, which becomes the viral envelope. Incorporation of this trimer into the membrane is very important for the viral infectivity. Cytoplasmic tails of the viral glycoproteins are also required to bind to the nucleocapsid and ensure the membrane forms aorund the new viruses.