Characterization of the norovirus 3C-like protease

The recombinant 3C-like protease of Chiba virus, a Norovirus, expressed in Escherichia coli cells was purified and characterized as to effects of pH, temperature, salt contents, and SH reagents on its proteolytic activity. The optimal pH and temperature of the 3C-like protease for the proteolytic activity were 8.6 and 37 °C, respectively. Increased concentration (∼100 mM) of monovalent cations such as Na+ and K+ was inhibitory to the activity. Hg2+ and Zn2+ remarkably inhibited the protease activity, while Mg2+ and Ca2+ had no virtual effect. Several sulfhydryl reagents such as p-chloromercuribenzoic acid, methyl methanethiosulfonate, N-ethylmaleimide and N-phenylmaleimide also blocked the activity, confirming the previous result that cysteine residue(s) were responsible for the proteolysis.