Inactivation of the RNase activity of glycoprotein E-rns of classical swine fever virus results in a cytopathogenic virus

Envelope glycoprotein E-rns of classical swine fever virus (CSFV) has been shown to contain RNase activity and is involved in virus infection. Two short regions of amino acids in the sequence of E-rns are responsible for RNase activity. In both regions, histidine residues appear to be essential for catalysis. They were replaced by lysine residues to inactivate the RNase activity. The mutated sequence of E-rns was inserted into the p10 locus of a baculovirus vector and expressed in insect cells. Compared to intact E-rns, the mutated proteins had lost their RNase activity. The mutated proteins reacted with E-rns-specific neutralizing monoclonal and polyclonal antibodies and were still able to inhibit infection of swine kidney cells (SK6) with CSFV, but at a concentration higher than that measured for intact E-rns. This result indicated that the conformation of the mutated proteins was not severely affected by the inactivation. To study the effect of these mutations on virus infection and replication, a CSFV mutant with an inactivated E-rns (FLc13) was generated with an infectious DNA copy of CSFV strain C. The mutant virus showed the same growth kinetics as the parent virus in cell culture. However, in contrast to the parent virus, the RNase-negative virus induced a cytopathic effect in swine kidney cells. This effect could be neutralized by rescue of the inactivated E-rns gene and by neutralizing polyclonal antibodies directed against E-rns, indicating that this effect was an inherent property of the RNase-negative virus. Analyses of cellular DNA of swine kidney cells showed that the RNase-negative CSFV induced apoptosis. We conclude that the RNase activity of envelope protein E-rns plays an important role in the replication of pestiviruses and speculate that this RNase activity might be responsible for the persistence of these viruses in their natural host.