Flexibility between the protease and helicase domains of the dengue virus NS3 protein conferred by the linker region and its functional implications

Novel ATP-independent RNA annealing activity of the dengue virus NS3 helicase

The flavivirus nonstructural protein 3 (NS3) bears multiple enzymatic activities and represents an attractive target for antiviral intervention. NS3 contains the viral serine protease at the N-terminus and ATPase, RTPase, and helicase activities at the C-terminus. These activities are essential for viral replication; however, the biological role of RNA remodeling by NS3 helicase during the viral life cycle is still unclear. Secondary and tertiary RNA structures present in the viral genome are crucial for viral replication. Here, we used the NS3 protein from dengue virus to investigate functions of NS3 associated to changes in RNA structures. Using different NS3 variants, we characterized a domain spanning residues 171 to 618 that displays ATPase and RNA unwinding activities similar to those observed for the full-length protein. Interestingly, we found that, besides the RNA unwinding activity, dengue virus NS3 greatly accelerates annealing of complementary RNA strands with viral or non-viral sequences. This new activity was found to be ATP-independent. It was determined that a mutated NS3 lacking ATPase activity retained full-RNA annealing activity. Using an ATP regeneration system and different ATP concentrations, we observed that NS3 establishes an ATP-dependent steady state between RNA unwinding and annealing, allowing modulation of the two opposing activities of this enzyme through ATP concentration. In addition, we observed that NS3 enhanced RNA-RNA interactions between molecules representing the ends of the viral genome that are known to be necessary for viral RNA synthesis. We propose that, according to the ATP availability, NS3 could function regulating the folding or unfolding of viral RNA structures.

A macrocyclic HCV NS3/4A protease inhibitor interacts with protease and helicase residues in the complex with its full-length target

Hepatitis C virus (HCV) infection is a global health burden with over 170 million people infected worldwide. In a significant portion of patients chronic hepatitis C infection leads to serious liver diseases, including fibrosis, cirrhosis, and hepatocellular carcinoma. The HCV NS3 protein is essential for viral polyprotein processing and RNA replication and hence viral replication. It is composed of an N-terminal serine protease domain and a C-terminal helicase/NTPase domain. For full activity, the protease requires the NS4A protein as a cofactor. HCV NS3/4A protease is a prime target for developing direct-acting antiviral agents. First-generation NS3/4A protease inhibitors have recently been introduced into clinical practice, markedly changing HCV treatment options. To date, crystal structures of HCV NS3/4A protease inhibitors have only been reported in complex with the protease domain alone. Here, we present a unique structure of an inhibitor bound to the full-length, bifunctional protease-helicase NS3/4A and show that parts of the P4 capping and P2 moieties of the inhibitor interact with both protease and helicase residues. The structure sheds light on inhibitor binding to the more physiologically relevant form of the enzyme and supports exploring inhibitor-helicase interactions in the design of the next generation of HCV NS3/4A protease inhibitors. In addition, small angle X-ray scattering confirmed the observed protease-helicase domain assembly in solution.

Conventional and unconventional mechanisms for capping viral mRNA

mRNAs are protected at their 5′ ends by a cap structure consisting of an N7-methylated GTP molecule linked to the first transcribed nucleotide by a 5′–5′ triphosphate bond.

The cap structure is essential for RNA splicing, export and stability, and allows the ribosomal complex to recognize mRNAs and ensure their efficient translation.

Uncapped RNA molecules are degraded in cytoplasmic granular compartments called processing bodies and may be detected as 'non-self' by the host cell, triggering antiviral innate immune responses through the production of interferons.

Conventional RNA capping (that is, of mRNAs from the host cell and from DNA viruses) requires hydrolysis of the 5′ γ-phosphate of RNA by an RNA triphosphatase, transfer of a GMP molecule onto the 5′-end of RNA by a guanylyltransferase, and methylation of this guanosine by an (guanine-N7)-methyltransferase. Subsequent methylations on the first and second transcribed nucleotides by (nucleoside-2′-O)-methyltransferases form cap-1 and cap-2 structures.

Viruses have evolved highly diverse capping mechanisms to acquire cap structures using their own or cellular capping machineries, or by stealing cap structures from cellular mRNAs.

Virally encoded RNA-capping machineries are diverse in terms of their genetic components, protein domain organization, enzyme structures, and reaction mechanisms and pathways, making viral RNA capping an attractive target for antiviral-drug design.

Capping the 5′ end of eukaryotic mRNAs with a 7-methylguanosine moiety enables efficient splicing, nuclear export and translation of mRNAs, and also limits their degradation by cellular exonucleases. Here, Canard and colleagues describe how viruses synthesize their own mRNA cap structures or steal them from host mRNAs, allowing efficient synthesis of viral proteins and avoidance of host innate immune responses.

In the eukaryotic cell, capping of mRNA 5′ ends is an essential structural modification that allows efficient mRNA translation, directs pre-mRNA splicing and mRNA export from the nucleus, limits mRNA degradation by cellular 5′–3′ exonucleases and allows recognition of foreign RNAs (including viral transcripts) as 'non-self'. However, viruses have evolved mechanisms to protect their RNA 5′ ends with either a covalently attached peptide or a cap moiety (7-methyl-Gppp, in which p is a phosphate group) that is indistinguishable from cellular mRNA cap structures. Viral RNA caps can be stolen from cellular mRNAs or synthesized using either a host- or virus-encoded capping apparatus, and these capping assemblies exhibit a wide diversity in organization, structure and mechanism. Here, we review the strategies used by viruses of eukaryotic cells to produce functional mRNA 5′-caps and escape innate immunity.

Monoclonal antibodies against dengue NS2B and NS3 proteins for the study of protein interactions in the flaviviral replication complex

The replication of dengue virus (DENV) RNA requires at least two viral non-structural (NS) proteins, NS3 and NS5. To facilitate the study of the DENV replication complex, human monoclonal IgG that are specific for NS proteins have been generated and characterised. The anti-NS3 IgG, 3F8, binds a conserved epitope (aa526-531) in the NS3 helicase domain, and cross-reacts with NS3 from all four DENV serotypes and the related yellow fever virus. The anti-NS2B IgG, 3F10, binds aa49-66 of NS2B (CF18), which forms part of the 47 aa hydrophilic cofactor region required for NS3 protease activity. The specificity of the IgG for their respective non-structural proteins has been demonstrated by immunofluorescence of cells infected with DENV and Western blotting. 3F8 is able to co-immunoprecipitate NS3 and NS5 from BHK-21 cells infected with DENV2, and 3F10 is able to detect an interaction between recombinant NS2B(CF18)NS3 full-length protein and the NS5 RNA-dependent RNA polymerase (RdRp) domain in an ELISA-based binding assay. The assay is specific and highly reproducible, with a clear binding curve seen when RdRp is incubated with increasing amounts of full-length NS3, but not the NS3 protease domain. The NS3 helicase domain competes with NS3 full-length for NS5 RdRp binding, with a K(d.) of 2.5μM. Since NS3 and NS5 are required for DENV replication, this fascile assay could be used to screen for non-nucleoside, allosteric inhibitors that disrupt the interaction between the two proteins.

RNA helicases and remodeling proteins

It is becoming increasingly clear that RNA molecules play a major role in all aspects of metabolism. The conformational state and stability of RNA are controlled by RNA remodeling proteins, which are ubiquitous motor proteins in the cell. Here, we review advances in our understanding of the structure and function of three major structural families of RNA remodeling proteins, the hexameric ring proteins, the processive monomeric RNA translocase/helicases, and the functionally diverse DEAD-box remodeling proteins. New studies have revealed molecular mechanisms for coupling between ATP hydrolysis and unwinding, the physical basis for regulatory control by cofactors, and novel functions for RNA remodeling proteins.

Small molecule pan-dengue and West Nile virus NS3 protease inhibitors

BACKGROUND

Dengue fever, dengue haemorrhagic fever, and dengue shock syndrome are caused by infections with any of the four serotypes of the dengue virus (DENV), and are an increasing global health risk. The related West Nile virus (WNV) causes significant morbidity and mortality as well, and continues to be a threat in endemic areas. Currently no FDA-approved vaccines or therapeutics are available to prevent or treat any of these infections. Like the other members of Flaviviridae, DENV and WNV encode a protease (NS3) which is essential for viral replication and therefore is a promising target for developing therapies to treat dengue and West Nile infections.

METHODS

Flaviviral protease inhibitors were identified and biologically characterized for mechanism of inhibition and DENV antiviral activity.

RESULTS

A guanidinylated 2,5-dideoxystreptamine class of compounds was identified that competitively inhibited the NS3 protease from DENV(1-4) and WNV with 50% inhibitory concentration values in the 1-70 μM range. Cytotoxicity was low; however, antiviral activity versus DENV-2 on VERO cells was not detectable.

CONCLUSIONS

This class of compounds is the first to demonstrate competitive pan-dengue and WNV NS3 protease inhibition and, given the sequence conservation among flavivirus NS3 proteins, suggests that developing a pan-dengue or possibly pan-flavivirus therapeutic is feasible.

Unmasking the active helicase conformation of nonstructural protein 3 from hepatitis C virus

The nonstructural protein 3 (NS3) helicase/protease is an important component of the hepatitis C virus (HCV) replication complex. We hypothesized that a specific β-strand tethers the C terminus of the helicase domain to the protease domain, thereby maintaining HCV NS3 in a compact conformation that differs from the extended conformations observed for other Flaviviridae NS3 enzymes. To test this hypothesis, we removed the β-strand and explored the structural and functional attributes of the truncated NS3 protein (NS3ΔC7). Limited proteolysis, hydrodynamic, and kinetic measurements indicate that NS3ΔC7 adopts an extended conformation that contrasts with the compact form of the wild-type (WT) protein. The extended conformation of NS3ΔC7 allows the protein to quickly form functional complexes with RNA unwinding substrates. We also show that the unwinding activity of NS3ΔC7 is independent of the substrate 3'-overhang length, implying that a monomeric form of the protein promotes efficient unwinding. Our findings indicate that an open, extended conformation of NS3 is required for helicase activity and represents the biologically relevant conformation of the protein during viral replication.

High affinity human antibody fragments to dengue virus non-structural protein 3

Background

The enzyme activities catalysed by flavivirus non-structural protein 3 (NS3) are essential for virus replication. They are distributed between the N-terminal protease domain in the first one-third and the C-terminal ATPase/helicase and nucleoside 5′ triphosphatase domain which forms the remainder of the 618-aa long protein.

Methodology/Principal Findings

In this study, dengue full-length NS3 protein with residues 49 to 66 of NS2B covalently attached via a flexible linker, was used as bait in biopanning with a naïve human Fab phage-display library. Using a range of truncated constructs spanning the NS2B cofactor region and the full-length NS3, 10 unique Fab were identified and characterized. Of these, monoclonal Fab 3F8 was shown to bind α3″ (residues 526 through 531) within subdomain III of the helicase domain. The antibody inhibits the ATPase and helicase activites of NS3 in biochemical assays and reduces DENV replication in HEK293 cells that were previously transfected with Fab 3F8 compared with mock transfected cells.

Conclusions/Significance

Antibodies such as 3F8 are valuable tools for studying the molecular mechanisms of flaviviral replication and for the monospecific detection of replicating dengue virus in vivo.

Dengue virus is the most prevalent mosquito transmitted infectious disease in humans and is responsible for febrile disease such as dengue fever, dengue hemorrhagic fever and dengue shock syndrome. Dengue non-structural protein 3 (NS3) is an essential, multifunctional, viral enzyme with two distinct domains; a protease domain required for processing of the viral polyprotein, and a helicase domain required for replication of the viral genome. In this study ten unique human antibody fragments (Fab) that specifically bind dengue NS3 were isolated from a diverse library of Fab clones using phage display technology. The binding site of one of these antibodies, Fab 3F8, has been precisely mapped to the third α-helix within subdomain III of the helicase domain (amino acids 526–531). The antibody inhibits the helicase activity of NS3 in biochemical assays and reduces DENV replication in human embryonic kidney cells. The antibody is a valuable tool for studying dengue replication mechanisms.