Crystal structure analysis of Flavivirus methyltransferases uncovered a flavivirus-conserved cavity located next to the binding site for its cofactor, S-adenosyl-methionine (SAM). Chemical derivatization of S-adenosyl-homocysteine (SAH), the product inhibitor of the methylation reaction, with substituents that extend into the identified cavity, generated inhibitors that showed improved and selective activity against dengue virus methyltransferase (MTase), but not related human enzymes. Crystal structure of dengue virus MTase with a bound SAH derivative revealed that its N6-substituent bound in this cavity and induced conformation changes in residues lining the pocket. These findings demonstrate that one of the major hurdles for the development of methyltransferase-based therapeutics, namely selectivity for disease-related methyltransferases, can be overcome.Methyltransferases (MTases) 3 play key roles in normal physiology and human diseases through methylating DNA, RNA, and proteins. Almost all MTases use S-adenosyl-L-methionine (SAM) as a methyl donor and generate S-adenosyl-Lhomocysteine (SAH) as a by-product. Pharmacological modulation of MTases by small molecules represents a novel approach to therapeutic intervention in cancer and other diseases (1). However, because the core domains of various MTases are conserved, designing inhibitors that specifically block the disease-related MTase without affecting other MTases, has been challenging. The ability to rationally design and generate selective inhibitors would have profound implications for development of new medicines for many methyltransferase-mediated diseases.Dengue virus (DENV), from genus Flavivirus in the family Flaviviridae, is the most prevalent mosquito-borne viral pathogen that infects humans. The four serotypes of DENV (DENV-1 to -4) pose a public health threat to 2.5 billion people worldwide, and cause 50 -100 million human infections each year. Neither vaccine nor antiviral therapy is currently available for DENV. The flavivirus MTase methylates the guanine N7 and ribose 2Ј-O positions of the viral RNA cap in a sequential manner (i.e. GpppA-RNA 3 m7GpppA-RNA 3 m7GpppAm-RNA) (2, 3). Recent studies have shown that flavivirus MTase is critical for viral replication and, therefore, represents a valid target for antiviral therapeutics (4 -6). We therefore examined the feasibility to design inhibitors that specifically modulate flavivirus MTase. EXPERIMENTAL PROCEDURESPreparation of DENV-3 MTases-The DNA fragment representing the MTase domain of DENV-3 was cloned into expression vector pGEX4T1 (Amersham Biosciences). Ala-substitution mutant MTases were prepared using a standard overlapping PCR procedure. Recombinant MTases, containing an N-terminal GST, were expressed in Escherichia coli. BL21 cells and purified through a GSTPrep TM FF 16/10 column (GE Healthcare). The GST tag was then cleaved by thrombin and removed from the MTases using the GST column. The MTases were further purified through gel filtration to ensure protein purity was Ͼ95%. The p...

We report that dengue virus (DENV) methyltransferase sequentially methylates the guanine N-7 and ribose 2'-O positions of viral RNA cap (GpppA-->m(7)GpppA-->m(7)GpppAm). The order of two methylations is determined by the preference of 2'-O methylation for substrate m(7)GpppA-RNA to GpppA-RNA, and the 2'-O methylation is not absolutely dependent on the prior N-7 methylation. A mutation that completely abolished the 2'-O methylation attenuated DENV replication in cell culture, whereas another mutation that abolished both methylations was lethal for viral replication, suggesting that N-7 methylation is more important than 2'-O methylation in viral replication. The latter mutant with lethal replication could be rescued by trans complementation using a wild-type DENV replicon. Furthermore, we found that chimeric DENVs containing the West Nile virus methyltransferase, polymerase, or full-length NS5 were nonreplicative, but the replication defect could also be rescued through trans complementation using the wild-type DENV replicon.

Dengue virus (DENV) causes several hundred million human infections and more than 20,000 deaths annually. Neither an efficacious vaccine conferring immunity against all four circulating serotypes nor specific drugs are currently available to treat this emerging global disease. Capping of the DENV RNA genome is an essential structural modification that protects the RNA from degradation by 5′ exoribonucleases, ensures efficient expression of viral proteins, and allows escape from the host innate immune response. The large flavivirus nonstructural protein 5 (NS5) (105 kDa) has RNA methyltransferase activities at its N-terminal region, which is responsible for capping the virus RNA genome. The methyl transfer reactions are thought to occur sequentially using the strictly conserved flavivirus 5′ RNA sequence as substrate (G ppp AG-RNA), leading to the formation of the 5′ RNA cap:To elucidate how viral RNA is specifically recognized and methylated, we determined the crystal structure of a ternary complex between the full-length NS5 protein from dengue virus, an octameric cap-0 viral RNA substrate bearing the authentic DENV genomic sequence (5′-m7 G 0ppp A 1 G 2 U 3 U 4 G 5 U 6 U 7 -3′), and S-adenosyl-L-homocysteine (SAH), the by-product of the methylation reaction. The structure provides for the first time, to our knowledge, a molecular basis for specific adenosine 2′-O-methylation, rationalizes mutagenesis studies targeting the K61-D146-K180-E216 enzymatic tetrad as well as residues lining the RNA binding groove, and offers previously unidentified mechanistic and evolutionary insights into cap-1 formation by NS5, which underlies innate immunity evasion by flaviviruses.dengue virus | nonstructural protein 5 methyltransferase-polymerase | 2′-O-ribose methyltransferase | cap-0 RNA | innate immunity evasion S everal members of the Flavivirus genus from the Flaviviridae family are major human pathogens, such as the four serotypes of dengue virus (DENV1-4), West Nile virus (WNV), Japanese encephalitis virus (JEV), and yellow fever virus (YFV). Recent largescale DENV vaccine trials using a tetravalent formulation and three dose injections have shown only limited protection against the four DENV serotypes, and no specific antiviral drug has reached the market so far (1-3). The flavivirus genome consists of a (+)-sense singlestranded RNA of ∼11 kb with a type 1 cap structure, followed by the strictly conserved dinucleotide sequence "AG": 5′-m7 G ppp A m2′-O-G-3′ (4, 5). Addition of the cap moiety to the 5′ end of the viral genome is crucial for viral replication, because this structure ensures efficient production of viral polyproteins by the host translation machinery and protection against degradation by 5′-3′ exoribonucleases, and also because it conceals the triphosphorylated (or diphosphorylated) end from recognition by host cell innate immune sensors (6-9). Following (+)-strand RNA synthesis by the C-terminal RNA-dependent RNA polymerase (RdRp) domain of nonstructural protein 5 (NS5), cap formation in flaviviruses resu...

Methyltransferases (MTases) from the genus Flavivirus encode both N-7 and 2'-O activities needed for type 1 (m(7)GpppNm) cap structure formation. We performed kinetic studies to understand the mechanisms of its progressive N-7 and 2'-O methylations. Sequential N-7 to 2'-O methylation occurred via a random bi bi and processive mechanism that does not involve enzyme-RNA dissociation. Analyses of steady state kinetic parameters showed that N-7 precedes 2'-O methylation as it turnovers RNA faster (k(cat)) resulting in 2.4-fold higher catalytic efficiency. Michaelis constants for S-adenosyl-methionine (AdoMet) in both reactions were about 10-fold lower than for their respective RNA substrates, suggesting that the rate-limiting steps in methylase reactions were associated with RNA templates. In the context of long viral RNA sequences, and compared to S-adenosyl-homocysteine, sinefungin was about 60- and 12-folds more potent against dengue N-7 and 2'-O MTase activity, exhibiting IC(50) values of 30 and 41nM, respectively.

Recent studies have shown that live attenuated influenza vaccines (LAIVs) expressing avian influenza virus hemagglutinins (HAs) prime for strong protective antibody responses to an inactivated influenza vaccine (IIV) containing the HA. To better understand this priming effect, we compared H7 HA head and stalk domain-specific B-cell responses in H7N7 LAIV-primed subjects and non-H7-primed controls after a single dose of H7N7 IIV. As previously reported, H7N7 LAIV-primed subjects but not control subjects generated strong hemagglutination-inhibiting and neutralizing antibody responses to the H7N7 IIV. Here, we found that the quantity, epitope diversity, and affinity of H7 head-specific antibodies increased rapidly in only H7N7 LAIV-primed subjects after receipt of the IIV. However, all cohorts generated a vigorous, high-affinity, stalk-specific antibody response. Consistent increases in circulating memory B-cell frequencies after receipt of the IIV reflected the specificity of high-affinity antibody production. Our findings emphasize the value of LAIVs as a vehicle for prepandemic vaccination.

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The N-terminal domain of the flavivirus NS5 protein functions as a methyltransferase (MTase). It sequentially methylates the N7 and 2′-O positions of the viral RNA cap structure (GpppA→7meGpppA→7meGpppA2′-O-me). The same NS5 domain could also have a guanylyltransferase activity (GTP+ppA-RNA→GpppA). The mechanism by which this protein domain catalyzes these three distinct functions is currently unknown. Here we report the crystallographic structure of DENV-3 MTase in complex with a 5′-capped RNA octamer (GpppAGAACCUG) at a resolution of 2.9 Å. Two RNA octamers arranged as kissing loops are encircled by four MTase monomers around a 2-fold non-crystallography symmetry axis. Only two of the four monomers make direct contact with the 5′ end of RNA. The RNA structure is stabilised by the formation of several intra and intermolecular base stacking and non-canonical base pairs. The structure may represent the product of guanylylation of the viral genome prior to the subsequent methylation events that require repositioning of the RNA substrate to reach to the methyl-donor sites. The crystal structure provides a structural explanation for the observed trans-complementation of MTases with different methylation defects.

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