Characterization of interaction of classical swine fever virus NS3 helicase with 3′ untranslated region

The classical swine fever virus (CSFV) full-length NS3 protein (NS3F) and the truncated NS3 protein (NS3H) with postulated helicase domain were expressed and demonstrated to have helicase activity. Further, the electrophoretic mobility shift assays containing NS3H and the viral 3′ terminal sequences showed that NS3H specifically bound to the plus- and minus-strand 3′UTR. The minus-strand 3′UTR had higher binding activity. The 21-nt fragments at the 3′-most terminal sequences of both 3′UTRs were essential to NS3H binding. A 12-nt insertion, CUUUUUUCUUUU, present in the 3′UTR of a CSFV live attenuated vaccine strain, was also found to be deleterious to helicase binding. Intact secondary structure of 3′ terminal sequence of 3′UTR might be important in helicase binding. Our results show that interaction between the helicase and the viral 3′UTR is similar to that between the replicase and the 3′UTR, suggesting that NS3 helicase is important for CSFV genomic replication.