Question

Decribe Life Cycle of West Nile Virus (WNV)

In your description of the life (replication) cycle. You must name the stage and specify which of the events occur in it.

The 5 stages are

1(attachment),

2(entry),

3(uncoating),

4 (biosynthesis-replication and viral protein synthesis),

5(maturation and release)

Answer 1

Answer:

The life cycle of West Nile Virus (WNV) within cells is similar to other RNA viruses that replicate cytoplasmically.

1. Attachment:

WNV enters cells via receptor-mediated endocytosis, and is transported into endosomes. The WNV receptor is unknown. Also, the receptor(s) required for WNV binding and entry may vary by cell type.

2. Entry:

After binding to the cell's membrane, the virus is taken up via clathrin-mediated endocytosis.

3. Uncoating: (The entry and uncoating events overlap)

In the acidified endosome the E protein undergoes conformational changes resulting in fusion between the viral and cellular membranes and release of the viral nucleocapsid into the cytoplasm.

Besides the mildly acidic endosomal pH, it is the composition of the target membrane that plays an important role in the membrane fusion process of flaviviruses. For example, in vitro studies have found that addition of cholesterol to target membranes has strong promoting effect on the membrane fusion capacity with TBEV and WNV.

4. Biosynthesis:

Translation:

The viral RNA serves as mRNA and translated into a single polyprotein, which is proteolytically processed to yield three structural proteins (the envelope protein E; the membrane precursor protein prM; and the capsid protein C) and seven non-structural (NS) proteins (NS1, NS2a, NS2b, NS3, NS4a, NS4b, and NS5).

The cleavages at the junctions C-prM, prM-E, E-NS1, NS4A-NS4B, and likely also NS1- NS2A, are performed by the host signal peptidase located within the lumen of the ER. The remaining peptide bonds are cleaved by the virus encoded NS3 protease.

Replication:

Genome replication is carried out in structures termed replication complexes (RC) that are established by the viral proteins and surrounded host membranes. Replication requires the viral protein NS5, which is an RNA-dependent RNA polymerase. An "antisense" negative strand RNA is produced by this enzyme, which then serves as a template for the synthesis of many new copies of the infectious positive strand of RNA genome.

5. Maturation and release:

WNV assembles on virus-induced membranes derived from the endoplasmic reticulum and buds into the lumen as immature virions on which E and prM proteins form 60 heterotrimeric spikes. Transit of the immature virion through the mildly acidic compartments of the trans-Golgi network triggers a rearrangement of E proteins on the immature virion; the lower pH induces a structural transition such that E proteins lie flat as antiparallel dimers on the surface of the virion.

Under acidic conditions, prM remains associated with the virion and protrudes from the surface of an otherwise smooth virus particle. This pH-dependent conformational change increases the susceptibility of prM for a furin-like serine protease. Cleavage and release of prM completes the virion maturation process, and is a required step in the virus lifecycle. Mature virions exit cells into extracellular milieu by exocytosis and/or budding.