1.A.116.  The Pore-forming Porcine Reproductive and Respiratory Syndrome Virus Viroporin (PRRSV) Family

The small envelope (E) protein of porcine reproductive and respiratory syndrome virus (PRRSV) is a hydrophobic 73 amino acid protein encoded in the internal open reading frame (ORF) of the bicistronic mRNA2. E gene expression was blocked in a full-length infectious clone by mutating the ATG translational initiation codon to GTG, such that the full-length mutant genomic clone was unable to synthesize the E protein (Lee and Yoo 2006). An E gene knocked-out genomic clone showed the absence of virus infectivity. The E protein is essential for PRRSV infection but dispensable for virion assembly. Electron microscopy suggested that the P129-DeltaE virions assembled in the absence of E had a similar appearance to the wild-type particles. E protein-negative, non-infectious P129-DeltaE virus particles were able to enter cells, but further steps of replication were interrupted. The entry of PRRSV had been suggested to be via receptor-mediated endocytosis, and lysomotropic basic compounds and known ion-channel blocking agents both inhibited PRRSV replication effectively during the uncoating process. The expression of E protein in E. coli arrested growth and increased membrane permeability. Cross-linking experiments in cells infected with PRRSV or transfected with E gene showed that the E protein was able to form homo-oligomers. Thus, the PRRSV E protein is likely an ion-channel protein embedded in the viral envelope that facilitates uncoating of the virus and releases of the genome into the cell cytoplasm (Lee and Yoo 2006).