Chimeric arteriviruses generated by swapping of the M protein ectodomain rule out a role of this domain in viral targeting.

Arteriviruses are enveloped, positive-strand RNA viruses for which the two major envelope proteins GP(5) and M occur as disulfide-linked heterodimers. These were assumed to serve the viral targeting functions, but recent ectodomain swapping studies with equine arteritis virus (EAV) indicate that the GP(5) protein does not determine arteriviral tropism. Here, we focused on the short, 13- to 18-residue ectodomain of the M protein. Using an infectious cDNA clone of the Lelystad virus isolate of porcine reproductive and respiratory syndrome virus (PRRSV), we substituted the genomic sequence encoding the M ectodomain by that of murine lactate dehydrogenase-elevating virus, EAV, and the US PRRSV-isolate, VR2332. Viable viruses with a chimeric M protein were obtained in all three cases, but for the latter two only after removal of the genomic overlap between the M and GP(5) genes. Characterization of the chimeric viruses revealed that they could be distinguished immunologically from wild-type virus, that they were genetically stable in vitro, but that they were impaired in their growth, reaching lower titers than the parental virus. The latter appeared to be due to an increased particle-to-infectivity ratio of the chimeric virus particles. Interestingly, the chimeric viruses had retained their ability to infect porcine cells and had not acquired tropism for cells susceptible to the viruses from which the foreign ectodomains were derived. We conclude that the surface structures composed by the arterivirus M and GP(5) ectodomains do not determine viral tropism.