The NS3 protein of hepatitis C virus is unusual because it encodes two unrelated enzymatic activities in linked protease and helicase domains. It has also been intensively studied because inhibitors targeting its protease domain have potential to significantly improve treatment options for those infected with this virus. Many enzymological studies and inhibitor discovery programs have been carried out using the isolated protease domain in complex with a peptide derived from NS4A which stimulates activity. However, some recent publications have suggested that the NS3 helicase domain may influence inhibitor binding and thus suggest work should focus on the full-length NS3-NS4A protein. Here we present the characterization of a single-chain protease in which the NS4A peptide activator is linked to the N-terminus of the NS3 protease domain. This protein behaves well in solution, and its protease activity is very similar to that of full-length NS3-NS4A. We find that this fusion protein, as well as the noncovalent complex of the NS4A peptide with NS3, gives similar Ki values, spanning 3 orders of magnitude, for a set of 25 structurally diverse inhibitors. We also show that simultaneous mutation of three residues on the surface of the helicase domain which has been hypothesized to interact with the protease does not significantly affect enzymatic activity or inhibitor binding. Thus, the protease domain with the NS4A peptide, in a covalent or noncovalent complex, is a good model for the protease activity of native NS3-NS4A.
[Show abstract][Hide abstract] ABSTRACT: The flavivirus genome comprises a single strand of positive-sense RNA, which is translated into a polyprotein and cleaved by a combination of viral and host proteases to yield functional proteins. One of these, nonstructural protein 3 (NS3), is an enzyme with both serine protease and NTPase/helicase activities. NS3 plays a central role in the flavivirus life cycle: the NS3 N-terminal serine protease together with its essential cofactor NS2B is involved in the processing of the polyprotein, whereas the NS3 C-terminal NTPase/helicase is responsible for ATP-dependent RNA strand separation during replication. An unresolved question remains regarding why NS3 appears to encode two apparently disconnected functionalities within one protein. Here we report the 2.75-A-resolution crystal structure of full-length Murray Valley encephalitis virus NS3 fused with the protease activation peptide of NS2B. The biochemical characterization of this construct suggests that the protease has little influence on the helicase activity and vice versa. This finding is in agreement with the structural data, revealing a single protein with two essentially segregated globular domains. Comparison of the structure with that of dengue virus type 4 NS2B-NS3 reveals a relative orientation of the two domains that is radically different between the two structures. Our analysis suggests that the relative domain-domain orientation in NS3 is highly variable and dictated by a flexible interdomain linker. The possible implications of this conformational flexibility for the function of NS3 are discussed.
Journal of Virology 09/2009; 83(24):12895-906. DOI:10.1128/JVI.00942-09 · 4.44 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The nonstructural protein 3 (NS3) of hepatitis C virus (HCV) is a bifunctional enzyme with a protease and a helicase functionality located in each of the two domains of the single peptide chain. There is little experimental evidence for a functional role of this unexpected arrangement since artificial single domain forms of both enzymes are catalytically competent. We have observed that low concentrations of certain protease inhibitors activate the protease of full-length NS3 from HCV genotype 1a with up to 100%, depending on the preincubation time and the inhibitor used. The activation was reduced, but not eliminated, by increased ionic strength, lowered glycerol concentration, or lowered pH. In all cases, it was at the expense of a significant loss of activity. Activation was not seen with the artificial protease domain of genotype 1b NS3 fused with a fragment of the NS4A cofactor. This truncated and covalently modified enzyme form was much less active and exhibited fundamentally different catalytic properties to the full-length NS3 protease without the fused cofactor. The most plausible explanation for the activation was found to involve a slow transition between two enzyme conformations, which differed in their catalytic ability and affinity for inhibitors. Equations derived based on this assumption resulted in better fits to the experimental data than the equation for simple competitive inhibition. The mechanism may involve an inhibitor-induced stabilization of the helicase domain in a conformation that enhances the protease activity, or an improved alignment of the catalytic triad in the protease. The proposed mnemonic mechanism and derived equations are viable for both these explanations and can serve as a basic framework for future studies of enzymes activated by inhibitors or other ligands.
[Show abstract][Hide abstract] ABSTRACT: The bifunctional NS3 protease–helicase of hepatitis C virus (HCV), together with its cofactor protein NS4A, is an important target for antiviral drugs which can cure HCV infections. HCV strains are divided into six major genotypes based on sequence diversity, and the great majority of reports on NS3 have focused exclusively on genotype 1 proteins. Here we report the cloning, expression, and preliminary characterization of NS3–NS4A gene products from HCV genotypes 4, 5, and 6. This work complements our earlier characterization of genotype 2 and 3 proteins . We compare NS3–NS4A protease and helicase activities of genotypes 4a, 5a, and 6a to those of common reference strains Con1 (genotype 1b) and JFH1 (genotype 2a). The specific activities of the proteases of the newly isolated proteins were similar to those of the reference proteins. Furthermore, the reference inhibitor BILN 2061 had similar activity against all of the proteins except for that of JFH1, which had an apparent Ki that was 11-fold higher relative to Con1. RNA and DNA unwinding activities were also similar for genotypes 1, 4, 5, and 6 proteins, but significantly higher for genotype 2 JFH1. With the availability of these proteins, inhibitors developed based on their activity against genotype 1 can be tested against all the other major genotypes, providing a path to improved treatment for all HCV patients.
Biochemical and Biophysical Research Communications 01/2010; 391(1-391):692-697. DOI:10.1016/j.bbrc.2009.11.122 · 2.30 Impact Factor
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