A positively charged surface patch on the pestivirus NS3 protease module plays an important role in modulating NS3 helicase activity and virus production

Pestivirus nonstructural protein 3 (NS3) is a multifunctional protein with protease and helicase activities that are essential for the virus replication. In this study, we used a combination of biochemical and genetic approaches to investigate the relationship between the positively charged patch on protease module and NS3 function. The surface patch was composed of four basic residues R50, K74 and K94 in NS3 protease domain and H24 in the structurally integrated cofactor NS4A PCS . Single residue or simultaneous four-residue substitution in the patch to alanine or aspartic acid hardly affect ATPase activity. However, the single R50, K94 or H24 residue or simultaneous four-residue substitution resulted in the apparent changes of the helicase activity and RNA-binding ability of NS3. When these mutations were introduced into a classical swine fever virus (CSFV) cDNA clone, the single K94 residue or simultaneous four-residue substitution (Qua_A or Qua_D) impaired the infectious virus production. Furthermore, the replication efficiency of the CSFV variants was partially correlated to the helicase activity of NS3 in vitro . Our results suggest that the conserved positively charged patch on the NS3 plays an important role in modulating the NS3 helicase activity in vitro and CSFV production.