Dengue protease activity: the structural integrity and interaction of NS2B with NS3 protease and its potential as a drug target.

Flaviviral NS3 serine proteases require the NS2B cofactor region (cNS2B) to be active. Recent crystal structures of WNV (West Nile virus) protease in complex with inhibitors revealed that cNS2B participates in the formation of the protease active site. No crystal structures of ternary complexes are currently available for DENV (dengue virus) to validate the role of cNS2B in active site formation. In the present study, a GST (glutathione transferase) fusion protein of DENV-2 cNS2B49-95 was used as a bait to pull down DENV-2 protease domain (NS3pro). The affinity of NS3pro for cNS2B was strong (equilibrium-binding constant <200 nM) and the heterodimeric complex displayed a catalytic efficiency similar to that of single-chain DENV-2 cNS2B/NS3pro. Various truncations and mutations in the cNS2B sequence showed that conformational integrity of the entire 47 amino acids is critical for protease activity. Furthermore, DENV-2 NS3 protease can be pulled down and transactivated by cNS2B cofactors from DENV-1, -3, -4 and WNV, suggesting that mechanisms for activation are conserved across the flavivirus genus. To validate NS2B as a potential target in allosteric inhibitor development, a cNS2B-specific human monoclonal antibody (3F10) was utilized. 3F10 disrupted the interaction between cNS2B and NS3 in vitro and reduced DENV viral replication in HEK (human embryonic kidney)-293 cells. This provides proof-of-concept for developing assays to find inhibitors that block the interaction between NS2B and NS3 during viral translation.

NMR analysis of the dynamic exchange of the NS2B cofactor between open and closed conformations of the West Nile virus NS2B-NS3 protease.

BACKGROUND: The two-component NS2B-NS3 proteases of West Nile and dengue viruses are essential for viral replication and established targets for drug development. In all crystal structures of the proteases to date, the NS2B cofactor is located far from the substrate binding site (open conformation) in the absence of inhibitor and lining the substrate binding site (closed conformation) in the presence of an inhibitor. METHODS: In this work, nuclear magnetic resonance (NMR) spectroscopy of isotope and spin-labeled samples of the West Nile virus protease was used to investigate the occurrence of equilibria between open and closed conformations in solution. FINDINGS: In solution, the closed form of the West Nile virus protease is the predominant conformation irrespective of the presence or absence of inhibitors. Nonetheless, dissociation of the C-terminal part of the NS2B cofactor from the NS3 protease (open conformation) occurs in both the presence and the absence of inhibitors. Low-molecular-weight inhibitors can shift the conformational exchange equilibria so that over 90% of the West Nile virus protease molecules assume the closed conformation. The West Nile virus protease differs from the dengue virus protease, where the open conformation is the predominant form in the absence of inhibitors. CONCLUSION: Partial dissociation of NS2B from NS3 has implications for the way in which the NS3 protease can be positioned with respect to the host cell membrane when NS2B is membrane associated via N- and C-terminal segments present in the polyprotein. In the case of the West Nile virus protease, discovery of low-molecular-weight inhibitors that act by breaking the association of the NS2B cofactor with the NS3 protease is impeded by the natural affinity of the cofactor to the NS3 protease. The same strategy can be more successful in the case of the dengue virus NS2B-NS3 protease.

NMR study of complexes between low molecular mass inhibitors and the West Nile virus NS2B-NS3 protease.

The two-component NS2B-NS3 protease of West Nile virus is essential for its replication and presents an attractive target for drug development. Here, we describe protocols for the high-yield expression of stable isotope-labelled samples in vivo and in vitro. We also describe the use of NMR spectroscopy to determine the binding mode of new low molecular mass inhibitors of the West Nile virus NS2B-NS3 protease which were discovered using high-throughput in vitro screening. Binding to the substrate-binding sites S1 and S3 is confirmed by intermolecular NOEs and comparison with the binding mode of a previously identified low molecular mass inhibitor. Our results show that all these inhibitors act by occupying the substrate-binding site of the protease rather than by an allosteric mechanism. In addition, the NS2B polypeptide chain was found to be positioned near the substrate-binding site, as observed previously in crystal structures of the protease in complex with peptide inhibitors or bovine pancreatic trypsin inhibitor. This indicates that the new low molecular mass compounds, although inhibiting the protease, also promote the proteolytically active conformation of NS2B, which is very different from the crystal structure of the protein without inhibitor.