Protease inhibitors as potential antiviral agents for the treatment of picornaviral infections

Inhibitory effects of cystatins on proteolytic activities of the Plum pox potyvirus cysteine proteinases

In an effort to develop new antiviral strategies effective against potyviruses, several cystatins were evaluated for their ability to inhibit the cysteine proteinases of Plum pox potyvirus (PPV) using in vitro proteolytic assays. The following cystatins were purified as GST fusion proteins and shown to be active against papain:oryzacystatins I and II (OCI and OCII), corn cystatin II (CCII), human stefin A (HSA), the domain 8 of tomato multicystatin (TMC-8) and a large 24kDa tomato cystatin (LTCyst). These cystatins did not inhibit the activity of purified recombinant PPV NIa proteinase, a serine-like cysteine proteinases related to the 3C proteinases of picornaviruses and to chymotrypsin. The cystatins were shown to inhibit slightly the activity of the PPV HC-Pro proteinase with CCII being the best inhibitor. However a large excess of the cystatins was required to observe any inhibition. Based on these results and on the documented pleiotropic effects of cystatins on the metabolism of plants, we conclude that they are not the best candidates for antiviral strategies targeted to viral cysteine proteinases. The availability of soluble active recombinant PPV NIa proteinase will be instrumental for the selection of other proteinase inhibitors with increased affinity and specificity for this proteinase.

Structural basis for the substrate specificity of tobacco etch virus protease

Because of its stringent sequence specificity, the 3C-type protease from tobacco etch virus (TEV) is frequently used to remove affinity tags from recombinant proteins. It is unclear, however, exactly how TEV protease recognizes its substrates with such high selectivity. The crystal structures of two TEV protease mutants, inactive C151A and autolysis-resistant S219D, have now been solved at 2.2- and 1.8-A resolution as complexes with a substrate and product peptide, respectively. The enzyme does not appear to have been perturbed by the mutations in either structure, and the modes of binding of the product and substrate are virtually identical. Analysis of the protein-ligand interactions helps to delineate the structural determinants of substrate specificity and provides guidance for reengineering the enzyme to further improve its utility for biotechnological applications.