Identification of porcine reproductive and respiratory syndrome virus ORF1a-encoded non-structural proteins in virus-infected cells

The porcine reproductive and respiratory syndrome virus (PRRSV) replicase gene consists of two large ORFs, ORF1a and ORF1b, the latter of which is expressed by ribosomal frameshifting. The ORF1a-encoded part of the resulting replicase polyproteins (pp1a and pp1ab) is predicted to be processed proteolytically into ten non-structural proteins (nsps), known as nsp1-8, with both the nsp1 and nsp7 regions being cleaved internally (yielding nsp1α and nsp1β, and nsp7α and nsp7β, respectively). The experimental verification of these predictions depends strongly on the ability to identify individual cleavage products with specific antibodies. In this study, a panel of monoclonal and polyclonal antibodies was generated, which together were able to recognize eight ORF1a-encoded PRRSV nsps. Using these reagents, replicase cleavage products were detected in PRRSV-infected MARC-145 cells using a variety of immunoassays. By immunofluorescence microscopy, most nsps could be detected by 6 h post-infection. During the early stages of infection, nsp1β, nsp2, nsp4, nsp7α, nsp7β and nsp8 co-localized in distinct punctate foci in the perinuclear region of the cell, which were determined to be the site of viral RNA synthesis by in situ labelling. Western blot and immunoprecipitation analysis identified most individual nsps and several long-lived processing intermediates (nsp3-4, nsp5-7, nsp5-8 and nsp3-8). The identification and subcellular localization of PRRSV nsps in virus-infected cells documented here provides a basis for the further structure-function studies. Thus, this PRRSV antibody panel will be an important tool for future studies on the replication and pathogenesis of this major swine pathogen.