Several mutations were introduced into the putative serine protease domain of the tick-borne encephalitis virus NS3 protein and into a possible internal cleavage site within the protein. The influence of these mutations on proteolytic activity of NS3 protein and NS3' protein formation was tested in vitro. It was found that NS3' formation was not dependent on the activity of the NS3 N-terminal serine protease. Mutations affecting the Ser-138 residue of the NS3 protein prohibited cleavage between NS2B and NS3 proteins when the NS2B-NS3 part of the viral genome was expressed in vitro, suggesting the key role of Ser-138 in viral serine protease functioning.
