The nsp1 alpha and nsp1 beta papain-like autoproteinases are essential for porcine reproductive and respiratory syndrome virus RNA synthesis

The two N-terminal cleavage products, nsp1 alpha and nsp1 beta, of the replicase polyproteins of porcine reproductive and respiratory syndrome virus (PRRSV) each contain a papain-like autoproteinase domain, which have been named PCP alpha and PCP beta, respectively. To assess their role in the PRRSV life cycle, substitutions and deletions of the presumed catalytic cysteine and histidine residues of PCPa and PCP beta were introduced into a PRRSV infectious cDNA clone. Mutations that inactivated PCPa activity completely blocked subgenomic mRNA synthesis, but did not affect genome replication. In contrast, mutants in which PCPP 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 nsp1 beta/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 nsp1 alpha/nsp1 beta and nsp1 beta/nsp2 junctions.