Porcine reproductive and respiratory syndrome virus (PRRSV) infection of swine results in substantial economic losses to the swine industry worldwide. Identification of cellular factors involved in PRRSV life cycle not only will enable a better understanding of virus biology but also has the potential for the development of antiviral therapeutics. The PRRSV nonstructural protein 1 (nsp1) has been shown to be involved in at least two important functions in the infected hosts: (i) mediation of viral subgenomic (sg) mRNA transcription and (ii) suppression of the host's innate immune response mechanisms. To further our understanding of the role of the viral nsp1 in these processes, using nsp1β, a proteolytically processed functional product of nsp1 as bait, we have identified the cellular poly(C)-binding proteins 1 and 2 (PCBP1 and PCBP2) as two of its interaction partners. The interactions of PCBP1 and PCBP2 with nsp1β were confirmed both by coimmunoprecipitation in infected cells and/or in plasmid-transfected cells and also by in vitro binding assays. During PRRSV infection of MARC-145 cells, the cytoplasmic PCBP1 and PCBP2 partially colocalize to the viral replication-transcription complexes. Furthermore, recombinant purified PCBP1 and PCBP2 were found to bind the viral 5' untranslated region (5'UTR). Small interfering RNA (siRNA)-mediated silencing of PCBP1 and PCBP2 in cells resulted in significantly reduced PRRSV genome replication and transcription without adverse effect on initial polyprotein synthesis. Overall, the results presented here point toward an important role for PCBP1 and PCBP2 in regulating PRRSV RNA synthesis.