Summary
The factors that determine the virulence and pathogenesis characteristics of bluetongue virus (BTV), African horse sickness virus (AHSV) and other orbiviruses are not well known. With respect to the viral proteins that are expected to play a role it may be assumed that proteins, such as the outer capsid proteins VP2 and VP5, that are involved in the attachment of virus particles to target cells and influence replication efficiency are particularly important. Equally important are viral proteins such as non-structural protein NS3 that influence the release of virus particles from a target host or vector cell. The authors compare the amino acid sequence variation, structural motifs and some phenotypic characteristics of proteins VP2, VP5 and NS3 of different orbiviruses, such as AHSV, BTV and equine encephalosis virus (EEV). The most variable protein is VP2 and a pair-wise alignment of VP2 sequences of different serotypes of both BTV and AHSV indicated variation of between 48% to 64% and 46% to 52% for most isolates, respectively. Several regions of high variability can be identified. VP5 of BTV is much less variable than VP2 but still more so than the cognate AHSV VP5. In contrast, the NS3 protein of AHSV is much more variable than its BTV or EEV counterpart with maximum levels of NS3 variation up to 36% as compared to 10% for BTV. The AHSV NS3 variation is clustered into three discreet phylogenetic groups. All orbivirus NS3/NS3A proteins share a number of highly conserved structural features that include two hydrophobic domains (HD1 and HD2) that are involved in the interaction with the membrane. Most of the NS3 variation is located in HD1 and the adjacent variable region between HD1 and HD2. In the case of AHSV this region only has 13% identity compared to 64% in the case of BTV. NS3 of AHSV is also a highly toxic protein and mutation analysis has indicated that the toxicity is associated with the two hydrophobic domains. Expression of NS3 deletion mutants in bacterial cells has shown that both HD1 and HD2 are necessary for cytotoxicity and that removal of the adjacent N-terminal domains increases cytotoxicity. Preliminary results with different AHSV strains and the corresponding NS3 equivalent have indicated that the membrane permeabilisation effect of the individual NS3 proteins correlate with the permeabilisation effect of the corresponding viruses. These results would suggest that characterisation of the NS3 protein by itself might predict some phenotypic characteristics and potential membrane destabilisation effect of the corresponding virus.

Keywords
African horse sickness virus, Bluetongue virus, Cytotoxic protein, Hydrophobic domains, Membrane permeability, Non-structural proteins, Orbiviruses, Pair-wise alignment, Pathogenesis, Viroporin, Virulence.