Determination of Critical Requirements for Classical Swine Fever Virus NS2-3-Independent Virion Formation

For Flaviviridae members, the nonstructural proteins are essential for virion formation and thus exert a dual role in RNA replication and virion morphogenesis. However, it remains unclear how these proteins are functionalized for either process. In wild-type pestiviruses, the NS3/4A complex is selectively active in RNA replication, while NS2-3/4A is essential for virion formation. Mutations recently identified in BVDV-1 rendered NS3/4A capable of supporting NS2-3-independent virion morphogenesis. A comparison of NS3/4A complexes incapable/capable of supporting virion morphogenesis revealed that changes in NS3/NS4A surface interactions are decisive for the gain of function. However, so far, the role of the NS2 mutations as well as the accessory mutations additionally required in the NS2-IRES-NS3 virus variant has not been clarified. To unravel the course of genome packaging, the additional sets of mutations obtained for a second pestivirus species (CSFV) are of significant importance to develop mechanistic models for this complex process.