We next tested whether RVFV NSs downregulated PKR by promoting PKR degradation through a ubiquitin-proteasome pathway. 293 cells were infected with rMP12-rLuc or MP-12, and were immediately treated with proteasome inhibitor MG132 or lactacystin. Cells were harvested at 8 h.p.i. and analyzed in Western blotting. As expected, rMP12-rLuc replication did not induce PKR downregulation ( Figure 7F, lane 1) , while MP-12 replication induced the reduction of PKR abundance ( Figure 7F,  lanes 4) . It was evident that the treatment of MP-12-infected cells with those proteasome inhibitors suppressed NSs-induced PKR downregulation ( Figure 7F, lanes 5 and 6) , suggesting that NSs promoted PKR downregulation by the degradation through the proteasome pathway.