During infection, the virus population contains both mature and immature virus particles containing a varying number of immature prM protein molecules on the surface ( 57, 239).Įntry of WNV is through receptor-mediated endocytosis after virus attachment to the cell surface. The prM protein is also known to embed in the lipid bilayer and is thought to protect E from undergoing premature fusion upon virus exocytosis to the cell surface. The envelope protein is responsible for binding the receptor on the cell surface for viral entry ( 134). During virus assembly, the envelope protein embeds in the lipid bilayer of the virus and is exposed to the virion surface. Although the nuclear functions of capsid are not fully understood, recent evidence suggests a role in gene regulation through binding with histone proteins ( 41). A high proportion of capsid protein localizes to the nucleus, while viral assembly takes place in the cytoplasm, with budding in the endoplasmic reticulum (ER) ( 17, 41, 183). The West Nile virus virion is an icosahedral particle with the capsid protein associating with the RNA genome to form the nucleocapsid, which is surrounded by a lipid bilayer. Several of the nonstructural proteins, including NS2A, NS2B, NS4A, and NS4B, have been shown to inhibit one or more components of the innate immune response against viral infection ( 116, 121, 122, 139). The NS5 protein serves as the viral polymerase and encodes a methyltransferase, and it is necessary for viral replication ( 117, 174). NS3 is the viral protease responsible for cleaving other nonstructural proteins from the viral polyprotein and encodes enzyme activities, and these functions have been widely characterized ( 118). NS1 has both a “cellular” form and a secreted form and is highly immunogenic but has no described role in virion assembly, though it has been suggested to play a role in replication ( 234). The nonstructural proteins have many diverse functions, which is understandable as the virus has a very limited number of proteins and they must each serve multiple purposes during infection. The 5′ end of the genome encodes the structural proteins, which are necessary for virus entry and fusion as well as encapsidation of the viral genome during assembly ( 118). The viral RNA is translated as a single polyprotein that is post- and cotranslationally cleaved by both host and viral proteases, resulting in three structural (capsid, envelope, and premembrane) and seven nonstructural (NS1, NS2A, NS2B, NS3, NS4A, NS4B, and NS5) proteins ( 174). Both the 5′ and 3′ noncoding regions of the genome form stem-loop structures that aid in replication, transcription, translation, and packaging ( 63, 92, 196). The genome consists of a single open reading frame of approximately 11 kb with no polyadenylation tail at the 3′ end. WNV is an enveloped virion containing a single-stranded, positive-sense RNA genome.
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