Positive-strand RNA genomes function as mRNA for viral protein synthesis which is fully reliant on host cell translation machinery. Competing with cellular protein translation apparatus needs to ensure the production of viral proteins, but this also stifles host innate defense. In the present study, we showed that porcine reproductive and respiratory syndrome virus (PRRSV), whose replication takes place in the cytoplasm, imprisoned host cell mRNA in the nucleus, which suggests a novel mechanism to enhance translation of PRRSV genome. PRRSV nonstructural protein (nsp) 1 beta was identified as the nuclear protein playing the role for host mRNA nuclear retention and subversion of host protein synthesis. A SAP (SAF-A/B, Acinus, and PIAS) motif was identified in nsp1 beta with the consensus sequence of (126)-LQxxLxxxGL-(135). In situ hybridization unveiled that SAP mutants were unable to cause nuclear retention of host cell mRNAS and did not suppress host protein synthesis. In addition, these SAP mutants reverted PRRSV-nsp1 beta-mediated suppression of interferon (IFN) production, IFN signaling, and TNF-alpha production pathway. Using reverse genetics, a series of SAP mutant PRRS viruses, vK124A, vL126A, vG134A, and vL135A were generated. No mRNA nuclear retention was observed during vL126A and vL135A infections. Importantly, vL126A and vL135A did not suppress IFN production. For other arteriviruses, mRNA nuclear accumulation was also observed for LDV-nsp1 beta and SHFV-nsp1 beta. EAV-nspl was exceptional and did not block the host mRNA nuclear export.