The role of porcine reproductive and respiratory syndrome (PRRS) virus structural and non-structural proteins in virus pathogenesis.

Porcine reproductive and respiratory syndrome (PRRS) is an economically devastating viral disease affecting the swine industry worldwide. The etiological agent, PRRS virus (PRRSV), possesses a RNA viral genome with nine open reading frames (ORFs). The ORF1a and ORF1b replicase-associated genes encode the polyproteins pp1a and pp1ab, respectively. The pp1a is processed in nine non-structural proteins (nsps): nsp1a, nsp1b, and nsp2 to nsp8. Proteolytic cleavage of pp1ab generates products nsp9 to nsp12. The proteolytic pp1a cleavage products process and cleave pp1a and pp1ab into nsp products. The nsp9 to nsp12 are involved in virus genome transcription and replication. The 30 end of the viral genome encodes four minor and three major structural proteins. The GP2a, GP3 and GP4 (encoded by ORF2a, 3 and 4), are glycosylated membrane associated minor structural proteins. The fourth minor structural protein, the E protein (encoded by ORF2b), is an unglycosylated membrane associated protein. The viral envelope contains two major structural proteins: a glycosylated major envelope protein GP5 (encoded by ORF5) and an unglycosylated membrane M protein (encoded by ORF6). The third major structural protein is the nucleocapsid N protein (encoded by ORF7). All PRRSV non-structural and structural proteins are essential for virus replication, and PRRSV infectivity is relatively intolerant to subtle changes within the structural proteins. PRRSV virulence is multigenic and resides in both the non-structural and structural viral proteins. This review discusses the molecular characteristics, biological and immunological functions of the PRRSV structural and nsps and their involvement in the virus pathogenesis.

Amino acid substitutions in σ1 and μ1 outer capsid proteins are selected during mammalian reovirus adaptation to Vero cells

Establishment of viral persistence in cell culture has previously led to the selection of mammalian reovirus mutants, although very few of those have been characterized in details. In the present study, reovirus was adapted to Vero cells that, in contrast to classically-used L929 cells, are inefficient in supporting the early steps of reovirus uncoating and are also unable to produce interferon as an antiviral response once infection occurs. The Vero cell-adapted reovirus exhibits amino acids substitutions in both the σ1 and μ1 proteins. This contrasts with uncoating mutants from persistently-infected L929 cells, and various other cell types, that generally harbor amino acids substitutions in the σ3 outer capsid protein. The Vero cell-adapted virus remained sensitive to an inhibitor of lysosomal proteases; furthermore, in the absence of selective pressure for its maintenance, t he virus has partially lost its ability to resist interferon. The positions of the amino acids substitutions on the known protein structures suggest an effect on binding of the viral σ1 protein to the cell surface and on μ1 disassembly from the outer capsid.

Amino acids substitutions in σ1 and μ1 outer capsid proteins of a Vero cell-adapted mammalian orthoreovirus are required for optimal virus binding and disassembly

In a recent study, the serotype 3 Dearing strain of mammalian orthoreovirus was adapted to Vero cells; cells that exhibit a limited ability to support the early steps of reovirus uncoating and are unable to produce interferon as an antiviral response upon infection. The Vero cell-adapted virus (VeroAV) exhibits amino acids substitutions in both the σ1 and μ1 outer capsid proteins but no changes in the σ3 protein. Accordingly, the virus was shown not to behave as a classical uncoating mutant. In the present study, an increased ability of the virus to bind at the Vero cell surface was observed and is likely associated with an increased ability to bind onto cell-surface sialic acid residues. In addition, the kinetics of μ1 disassembly from the virions appears to be altered. The plasmid-based reverse genetics approach confirmed the importance of σ1 amino acids substitutions in VeroAV's ability to efficiently infect Vero cells, although μ1 co-adaptation appears necessary to optimize viral infection. This approach of combining in vitro selection of reoviruses with reverse genetics to identify pertinent amino acids substitutions appears promising in the context of eventual reovirus modification to increase its potential as an oncolytic virus.