Flavivirus structure

Cleavages of flavivirus polyprotein by host and viral protease   Dengue virus. Transmission EM. CDC

  Rift valley fever virus. Note the virus budding through internal membranes. Transmission EM   CDC

West Nile Virus. Colorized negative stain EM   CDC

• Virion size: 50nm
• RNA: single positive strand. Not polyadenylated. 5' methyl nucleotide cap. RNA is infectious as it serves both as genome and viral mRNA
• Envelope: Yes (from host cell endoplasmic reticulum and Golgi membrane)
• Replication site: Entirely cytoplasmic (as is the case with most RNA viruses)
• Symmetry: Icosahedral
• Genome size: 10kB
• Cell receptor: G protein coupled receptor kinase 2
• Name: Comes from yellow fever virus (flavus: Latin: Yellow)

Flaviviruses are positive sense RNA viruses and, like most RNA viruses replicate wholly in the cytoplasm. Although the RNA is positive sense (like mRNA) there is no polyA tail but, like mRNAs, there is a methyl guanosine cap. The RNA is translated into a single large polyprotein which is cleaved by virally-encoded and host cell proteases to structural proteins:

• Capsid
• Matrix
• Envelope (Two: M and E)

and eight non-structural proteins that encode enzymes including an RNA-dependent RNA polymerase and those that make the mRNA cap

• RNA triphosphatase (N3 protein)
• guanyl transferase (NS5 protein)
• N7-methyl transferase (NS5 protein)
• 2'-O methyl transferase(NS5 protein)

The polyprotein has an autocatalytic activity that cuts away the first viral protein that then acts as a protease to cleave the other proteins. Among the products is the polymerase which transcribes the positive strand to a negative intermediate which is then transcribed to genomic RNA (which acts also as mRNA).

Because the virus encodes a single polypeptide that must be cleaved within the cell to give, among others, a surface glycoprotein, it must bud through the endoplasmic reticulum membrane where the first cleavage is by host cell signal peptidase and followed by other Golgi proteases (see electron micrograph above) . The remaining proteins assemble on the cytoplasmic surface of the endoplasmic reticulum. The virus is then released from the Golgi by cell lysis.