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The article reviews the structure of BTV and the role of capsid proteins in the replication process. The BTV is an icosahedral virus with a genome of approximately 19 200 base pairs and composed of ten linear segments of double-stranded RNA (dsRNA). These RNA segments are packaged in a triple layered protein capsid (88 nm in diameter). The outer shell of capsid is composed of two structural proteins: 60 trimers of VP2, which acting as a hemagglutinin (HA) is responsible for glycoprotein receptor binding, and 120 trimers of VP5 that mediate the release of viral particles from endosomal compartments into cytoplasm and undergo pH dependent conformational changes that allow membrane fusion and syncytium formation. The intermediate layer consists of VP7 - protein the major immunodominant of BTV. This layer surrounds the subcore (54 nm) which consist of 12 decamers of the VP3 protein (role in the structural integrity of the virus core) and three minor structural proteins: VP1 (replicase), VP4 (mRNA capping enzyme) and VP6 (RNA-dependent ATPase and helicase). Moreover, the viral subcore consists of nonstructural proteins: NS1, NS2, NS3 and NS3A, which participate in the control of BTV replication, maturation and export from infected cell.
The paper reviews molecular markers and determinants associated with virulence, host adaptation and drug resistance in avian influenza viruses (AIV). The virulence of AIV is mostly dependant on the presence of multiple amino acids (mainly arginine and lysine) at the cleavage site of the haemagglutinin (HA) protein. The major factors contributing to host adaptation are also harbored within the HA protein: amino acids at positions 226 and 228 determine virus binding affinity to receptors present in cell membranes of birds or humans. It has been shown that pathogenicity and host adaptation are also dependant on the amino acid sequences of the polymerase complex (PB2-PB1-PA) and the most significant mutation (E627K in PB2) is related to the increased replication of the virus in mammalian cells. Molecular markers associated with an increased resistance to antiviral drugs are localized in neuraminidase (NA) and matrix (M) proteins. For example, a histidine to tyrosine substitution at position 274 of NA (H274Y) decreases viral susceptibility to neuraminidase inhibitors (e.g. oseltamivir), the most frequently used drugs in flu treatment. Monitoring of the molecular changes in the viral genome of AIV is very important from an epidemiological point of view and can be a valuable part of an early warning system.
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