Mosquito Surveillance for Prevention and Control of Emerging Mosquito-Borne Diseases in Portugal — 2008–2014

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. 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). Although the nuclear functions of capsid are not fully understood, recent evidence suggests a role in gene regulation through binding with histone proteins

The virus-containing endosome matures during internalization from the cell surface, with the pH dropping from neutral to slightly acidic in the early endosome and becoming more acidic during maturation to the late endosome. Within the late endosome, the envelope protein will undergo a conformational change resulting in fusion of the viral lipid membrane with the endocytic membrane and the release of the viral RNA genome into the cell cytoplasm (134)

In situ hybridization of DNA probes specific for viral RNA demonstrated the trafficking of uncoated viral RNA genomes to the endoplasmic reticulum.

The E protein of flaviviruses has been demonstrated to play crucial role in mediating virus-host cellular receptors interaction

Cleavage of prM by cellular furin or related proteases during virus maturation pathway would then release matured virus particles containing M proteins (37).

Flavivirus RNA genome encodes for three structural proteins (capsid [C], precursor of membrane [prM], and envelope [E]), and seven nonstructural proteins (NS1, NS2A, NS2B, NS3, NS4A, NS4B, and NS5) that are essential for intracellular replication of the virus (12, 13, 49, 58). The RNA genome is packaged within a spherical nucleocapsid that composed of multiple copies of the capsid proteins (11, 59). The nucleocapsid is further enwrapped by a modified lipid bilayer derived from host cellular membranes through the insertion of virus structural proteins, envelope, and precursor membrane.

he family Flaviviridae are positive-sense, single-stranded RNA viruses that replicate in the cytoplasm of infected cells.

Heterodimers of prM and E protein become embedded in the lipid bilayer of the virus during assembly and are exposed on the virion surface [32]. The prM protein is thought to protect the immature virion from undergoing premature fusion prior to viral budding from the cell surface by blocking the fusion loop of E and is cleaved off during the viral maturation process [32–36]. During infection, mature, immature, and partially mature virus particles are produced, containing a varying number of immature prM protein molecules on the surface [37]. The E protein mediates both binding of the receptor on the cell surface for viral entry and fusion with the membrane of the host cell [38–40].