Inhibition studies on the nuclear inclusion protein a protease of turnip mosaic potyvirus C5

The nuclear inclusion protein a (NIa) protease of turnip mosaic potyvirus is responsible for processing the viral precursor polyprotein into mature proteins. The NIa protease was found to be inhibited by several metal ions at micromolar concentrations, especially copper, zinc, and cadmium ions. This implies that the NIa protease may contain cysteine or histidine residues essential for the catalytic activity. Substitution of His-46 or Cys-151 to Tyr or Ser, respectively, abolished the catalytic activity almost completely, supporting the hypothesis that cysteine and histidine are involved in the catalysis. Nα-p-tosyl-L-phenylalanine chloromethylketone (TPCK) and Nα-p-tosyl-L-lysine chloromethylketone (TLCK) exhibited significant inhibitory effects on the catalytic activity of the NIa protease with IC₅₀ values of 50 μM and 200 μM, respectively. This suggests chloromethylketone-conjugated peptides could work as potent inhibitors against NIa protease. Iodoacetamide, iodoacetate, and N-ethylmaleimide, which are known to modify cysteine or histidine, showed moderate inhibitory effects. The protease was inhibited negligibly by other serine or cysteine protease inhibitors such as leupeptin, antipain, aprotinin, phenylmethylsulfonyl fluoride, elastatinal, L-trans-epoxysuccinyl-leucylamido(4-guanidino)butane (E-64), and cystatin. These results suggest that although the active site of the NIa protease is structurally similar to that of the chymotrypsin-like serine protease, it has a unique active site specificity distinct from those of other serine proteases.