A Conserved Domain in the Leader Proteinase of Foot-and-Mouth Disease Virus Is Required for Proper Subcellular Localization and Function

The leader proteinase (L-pro) of foot-and-mouth disease virus (FMDV) is involved in antagonizing the innate immune response by blocking the expression of interferon (IFN) and by reducing the immediate-early induction of IFN-beta mRNA and IFN-stimulated genes. In addition to its role in shutting off cap-dependent host mRNA translation, L-pro is associated with the degradation of the p65/RelA subunit of nuclear factor kappa B (NF-kappa B). Bioinformatics analysis suggests that L-pro contains a SAP (for SAF-A/B, Acinus, and PIAS) domain, a protein structure associated in some cases with the nuclear retention of molecules involved in transcriptional control. We have introduced a single or a double mutation in conserved amino acid residues contained within this domain of L-pro. Although three stable mutant viruses were obtained, only the double mutant displayed an attenuated phenotype in cell culture. Indirect immunofluorescence analysis showed that L-pro subcellular distribution is altered in cells infected with the double mutant virus. Interestingly, nuclear p65/RelA staining disappeared from wild-type (WT) FMDV-infected cells but not from double mutant virus-infected cells. Consistent with these results, NF-kappa B-dependent transcription was not inhibited in cells infected with double mutant virus in contrast to cells infected with WT virus. However, degradation of the translation initiation factor eIF-4G was very similar for both the WT and the double mutant viruses. Since L-pro catalytic activity was demonstrated to be a requirement for p65/RelA degradation, our results indicate that mutation of the SAP domain reveals a novel separation-of-function activity for FMDV L-pro.