The nsp1α and nsp1β papain-like autoproteinases are essential for porcine reproductive and respiratory syndrome virus RNA synthesis

The two N-terminal cleavage products, nsp1alpha and nsp1beta, of the replicase polyproteins of porcine reproductive and respiratory syndrome virus (PRRSV) each contain a papain-like autoproteinase domain, which have been named PCPalpha and PCPbeta, respectively. To assess their role in the PRRSV life cycle, substitutions and deletions of the presumed catalytic cysteine and histidine residues of PCPalpha and PCPbeta were introduced into a PRRSV infectious cDNA clone. Mutations that inactivated PCPalpha activity completely blocked subgenomic mRNA synthesis, but did not affect genome replication. In contrast, mutants in which PCPbeta activity was blocked proved to be non-viable and no sign of viral RNA synthesis could be detected, indicating that the correct processing of the nsp1beta/nsp2 cleavage site is essential for PRRSV genome replication. In conclusion, the data presented here show that a productive PRRSV life cycle depends on the correct processing of both the nsp1alpha/nsp1beta and nsp1beta/nsp2 junctions.