Viral Structure

The West Nile Virus is a member of the Flavivirus genus. Like all flaviviruses, it has a spherical envelope, and has an icosahedral nucleocapsid which contains a single-stranded RNA genome (2).

The West Nile Virus genome is about 10,000 bases long. The genome 3’ terminus end forms a loop structure, and the 5’ end has a methylated nucleotide cap which allows for translation, or a genome-linked protein (VPg) (7).

Figure 1.1: Map of a flavivifus genome (2).

The capsid has an icosahedral shape, and is about 25 nm in diameter (4).

Figure 1.2: The red depicts the envelope, cut away to show the capsid structure (8).

Viral envelopes typically help the virus infect the host and are composed of phospholipids and membrane proteins derived from host cell, and proteins and glycoproteins of viral origin. The West Nile Virus envelope is composed of proteins, lipids, trace metals, and carbohydrates, and is about 50 nm in diameter (4). The surface structure varies between immature and mature viruses (1). The immature viral envelope consists of 60 trimetric glycoprotein spikes in an icosahedral symmetric pattern. The fusion loops of each glycoprotein are pointed inward, and pre-membrane proteins (prM) are attached to the spikes, preventing the virus membrane from fusing with the cell membrane. When the virus encounters an environment of low pH, the prMs are released, allowing maturation to begin. During maturation, a conformational change occurs, rearranging the trimetric spikes to produce a smooth surface. (1)

Figure 1.3: The envelope structure of the immature virus (left) in contrast to the mature virus (right) (1).

The mature viral envelope also has an icosahedral shape, but its sets of three glycoproteins are arranged in a herringbone pattern. It contains two virus-encoded membrane proteins, the M protein and the E dimer, and180 copies of its envelope glycoprotein. The glycoproteins contain three domains (DI, DII, DIII), each having a specific role during cell entry. DII contains the fusion loop, which is responsible for the fusion of the viral envelope to the cell membrane (1).

Figure 1.4: The divisions of a single glycoprotein (1).