Potyviral NIa proteinase, a proteinase with novel deoxyribonuclease activity

The NIa proteinase from pepper vein banding virus (PVBV) is a sequence-specific proteinase required for processing of viral polyprotein in the cytoplasm. It accumulates in the nucleus of the infected plant cell and forms inclusion bodies. The function of this protein in the nucleus is not clear. The purified recombinant NIa proteinase was active, and the mutation of the catalytic residues His-46, Asp-81, and Cys-151 resulted in complete loss of activity. Most interesting, the PVBV NIa proteinase exhibited previously unidentified activity, namely nonspecific double-stranded DNA degradation. This DNase activity of the NIa proteinase showed an absolute requirement for Mg2+. Site-specific mutational analysis showed that of the three catalytic residues, Asp-81 was the crucial residue for DNase activity. Mutation of His-46 and Cys-151 had no effect on the DNase activity, whereas mutant D81N was partially active, and D81G was completely inactive. Based on kinetic analysis and molecular modeling, a metal ion-dependent catalysis similar to that observed in other nonspecific DNases is proposed. Similar results were obtained with glutathione S-transferase-fused PVBV NIa proteinase and tobacco etch virus NIa proteinase, confirming that the DNase function is an intrinsic property of potyviral NIa proteinase. The NIa protein present in the infected plant nuclear extract also showed the proteinase and the DNase activities, suggesting that the PVBV NIa protein that accumulates in the nucleus late in the infection cycle might serve to degrade the host DNA. Thus the dual function of the NIa proteinase could play an important role in the life cycle of the virus.