Theiler’s Murine encephalomyelitis virus (TMEV) belongs to the Theilovirus species within the Cardiovirus genus. The leader (L) protein encoded at the N terminus of viral polyprotein is multifunctional and was shown to antagonize the innate immune response. We reported that wild type but not mutant L protein can inhibit stress granules (SG) assembly in infected cells.
Recent results suggested that this inhibition was consequent to L-mediated inhibition of the double-stranded RNA (dsRNA)-activated protein kinase R (PKR). Indeed, wild type but not L-mutant viruses inhibited PKR phosphorylation. However, we found no evidence for interaction between L protein and PKR or viral dsRNA suggesting that PKR inhibition by L might be indirect. Recently, we identified MAP kinases of the RSK family as binding partners of the L protein. To analyze the involvement of RSK kinases on SG assembly, eIF3 (SG marker) immunolabeling was performed on cells where RSK1 and/or RSK2 expression was knocked down using shRNAs or knocked out by CRISPR-Cas9. Interestingly, wild type virus failed to inhibit PKR phosphorylation and stress granule assembly in cells that lacked RSK1/2 expression, showing the involvement of RSK kinases in PKR inhibition.
In conclusion, RSK kinases are required for TMEV L-mediated inhibition of PKR activity.