The N-terminal protein of the polyprotein encoded by the potyvirus tobacco vein mottling virus is an RNA-binding protein

The first predicted polypeptide encoded by the potyvirus tobacco vein mottling virus (TVMV) is a highly positively charged protein of predicted M(r) 29K that functions as a protease to perform the first predicted cleavage in the potyvirus polyprotein. We expressed this protein (P 1(pro)) fused with glutathione S-transferase (GST) and purified the fusion protein from engineered Escherichia coli. We found that the intact fusion protein, as well as samples in which the P1(pro) portion was liberated from GST by pretreatment with thrombin, was able to bind RNA. Binding activity was optimal at relatively high KCl concentrations, suggesting an interaction dependent on a specific protein structure and not just on the binding of the negatively charged phosphate backbone by the positively charged P1(pro) polypeptide. The TVMV P1(pro) preferred ssRNA over DNA or dsRNA, and showed a possible preference for sequences containing oligo(G) tracts. Like other potyvirus-encoded proteins, the TVMV P1(pro) therefore possesses more than one demonstrable biochemical activity and probably plays multiple roles in the TVMV life cycle.