Mickaël Bouvet

Coronavirus Nonstructural Protein 16 Is a Cap-0 Binding Enzyme Possessing (Nucleoside-2′O)-Methyltransferase Activity

ABSTRACT The coronavirus family of positive-strand RNA viruses includes important pathogens of livestock, companion animals, and humans, including the severe acute respiratory syndrome coronavirus that was responsible for a worldwide outbreak in 2003. The unusually complex coronavirus replicase/transcriptase is comprised of 15 or 16 virus-specific subunits that are autoproteolytically derived from two large polyproteins. In line with bioinformatics predictions, we now show that feline coronavirus (FCoV) nonstructural protein 16 (nsp16) possesses an S-adenosyl-l-methionine (AdoMet)-dependent RNA (nucleoside-2′O)-methyltransferase (2′O-MTase) activity that is capable of cap-1 formation. Purified recombinant FCoV nsp16 selectively binds to short capped RNAs. Remarkably, an N7-methyl guanosine cap (7MeGpppAC3-6) is a prerequisite for binding. High-performance liquid chromatography analysis demonstrated that nsp16 mediates methyl transfer from AdoMet to the 2′O position of the first transcribed nucleotide, thus converting 7MeGpppAC3-6 into 7MeGpppA2′OMeC3-6. The characterization of 11 nsp16 mutants supported the previous identification of residues K45, D129, K169, and E202 as the putative K-D-K-E catalytic tetrad of the enzyme. Furthermore, residues Y29 and F173 of FCoV nsp16, which may be the functional counterparts of aromatic residues involved in substrate recognition by the vaccinia virus MTase VP39, were found to be essential for both substrate binding and 2′O-MTase activity. Finally, the weak inhibition profile of different AdoMet analogues indicates that nsp16 has evolved an atypical AdoMet binding site. Our results suggest that coronavirus mRNA carries a cap-1, onto which 2′O methylation follows an order of events in which 2′O-methyl transfer must be preceded by guanine N7 methylation, with the latter step being performed by a yet-unknown N7-specific MTase.

In vitro reconstitution of SARS-coronavirus mRNA cap methylation.

SARS-coronavirus (SARS-CoV) genome expression depends on the synthesis of a set of mRNAs, which presumably are capped at their 5′ end and direct the synthesis of all viral proteins in the infected cell. Sixteen viral non-structural proteins (nsp1 to nsp16) constitute an unusually large replicase complex, which includes two methyltransferases putatively involved in viral mRNA cap formation. The S-adenosyl-L-methionine (AdoMet)-dependent (guanine-N7)-methyltransferase (N7-MTase) activity was recently attributed to nsp14, whereas nsp16 has been predicted to be the AdoMet-dependent (nucleoside-2′O)-methyltransferase. Here, we have reconstituted complete SARS-CoV mRNA cap methylation in vitro. We show that mRNA cap methylation requires a third viral protein, nsp10, which acts as an essential trigger to complete RNA cap-1 formation. The obligate sequence of methylation events is initiated by nsp14, which first methylates capped RNA transcripts to generate cap-0 7MeGpppA-RNAs. The latter are then selectively 2′O-methylated by the 2′O-MTase nsp16 in complex with its activator nsp10 to give rise to cap-1 7MeGpppA2′OMe-RNAs. Furthermore, sensitive in vitro inhibition assays of both activities show that aurintricarboxylic acid, active in SARS-CoV infected cells, targets both MTases with IC50 values in the micromolar range, providing a validated basis for anti-coronavirus drug design.

Crystal Structure and Functional Analysis of the SARS-Coronavirus RNA Cap 2′-O-Methyltransferase nsp10/nsp16 Complex

Cellular and viral S-adenosylmethionine-dependent methyltransferases are involved in many regulated processes such as metabolism, detoxification, signal transduction, chromatin remodeling, nucleic acid processing, and mRNA capping. The Severe Acute Respiratory Syndrome coronavirus nsp16 protein is a S-adenosylmethionine-dependent (nucleoside-2′-O)-methyltransferase only active in the presence of its activating partner nsp10. We report the nsp10/nsp16 complex structure at 2.0 Å resolution, which shows nsp10 bound to nsp16 through a ∼930 Å2 surface area in nsp10. Functional assays identify key residues involved in nsp10/nsp16 association, and in RNA binding or catalysis, the latter likely through a SN2-like mechanism. We present two other crystal structures, the inhibitor Sinefungin bound in the S-adenosylmethionine binding pocket and the tighter complex nsp10(Y96F)/nsp16, providing the first structural insight into the regulation of RNA capping enzymes in (+)RNA viruses.

Flaviviral methyltransferase/RNA interaction: Structural basis for enzyme inhibition

Abstract Flaviviruses are the causative agents of severe diseases such as Dengue or Yellow fever. The replicative machinery used by the virus is based on few enzymes including a methyltransferase, located in the N-terminal domain of the NS5 protein. Flaviviral methyltransferases are involved in the last two steps of the mRNA capping process, transferring a methyl group from S-adenosyl-l-methionine onto the N7 position of the cap guanine (guanine-N7 methyltransferase) and the ribose 2′O position of the first nucleotide following the cap guanine (nucleoside-2′O methyltransferase). The RNA capping process is crucial for mRNA stability, protein synthesis and virus replication. Such an essential function makes methyltransferases attractive targets for the design of antiviral drugs. In this context, starting from the crystal structure of Wesselsbron flavivirus methyltransferase, we elaborated a mechanistic model describing protein/RNA interaction during N7 methyl transfer. Next we used an in silico docking procedure to identify commercially available compounds that would display high affinity for the methyltransferase active site. The best candidates selected were tested in vitro to assay their effective inhibition on 2′O and N7 methyltransferase activities on Wesselsbron and Dengue virus (Dv) methyltransferases. The results of such combined computational and experimental screening approach led to the identification of a high-potency inhibitor.

RNA 3'-end mismatch excision by the severe acute respiratory syndrome coronavirus nonstructural protein nsp10/nsp14 exoribonuclease complex.

The replication/transcription complex of severe acute respiratory syndrome coronavirus is composed of at least 16 nonstructural proteins (nsp1–16) encoded by the ORF-1a/1b. This complex includes replication enzymes commonly found in positive-strand RNA viruses, but also a set of RNA-processing activities unique to some nidoviruses. The nsp14 protein carries both exoribonuclease (ExoN) and (guanine-N7)-methyltransferase (N7-MTase) activities. The nsp14 ExoN activity ensures a yet-uncharacterized function in the virus life cycle and must be regulated to avoid nonspecific RNA degradation. In this work, we show that the association of nsp10 with nsp14 stimulates >35-fold the ExoN activity of the latter while playing no effect on N7-MTase activity. Nsp10 mutants unable to interact with nsp14 are not proficient for ExoN activation. The nsp10/nsp14 complex hydrolyzes double-stranded RNA in a 3′ to 5′ direction as well as a single mismatched nucleotide at the 3′-end mimicking an erroneous replication product. In contrast, di-, tri-, and longer unpaired ribonucleotide stretches, as well as 3′-modified RNAs, resist nsp10/nsp14-mediated excision. In addition to the activation of nsp16-mediated 2′-O-MTase activity, nsp10 also activates nsp14 in an RNA processing function potentially connected to a replicative mismatch repair mechanism.

Molecular Mapping of the RNA Cap 2′-O-Methyltransferase Activation Interface between Severe Acute Respiratory Syndrome Coronavirus nsp10 and nsp16

Several protein-protein interactions within the SARS-CoV proteome have been identified, one of them being between non-structural proteins nsp10 and nsp16. In this work, we have mapped key residues on the nsp10 surface involved in this interaction. Alanine-scanning mutagenesis, bioinformatics, and molecular modeling were used to identify several “hot spots,” such as Val42, Met44, Ala71, Lys93, Gly94, and Tyr96, forming a continuous protein-protein surface of about 830 Å2, bearing very conserved amino acids among coronaviruses. Because nsp16 carries RNA cap 2′-O-methyltransferase (2′O-MTase) activity only in the presence of its interacting partner nsp10 (Bouvet, M., Debarnot, C., Imbert, I., Selisko, B., Snijder, E. J., Canard, B., and Decroly, E. (2010) PLoS Pathog. 6, e1000863), functional consequences of mutations on this surface were evaluated biochemically. Most changes that disrupted the nsp10-nsp16 interaction without structural perturbations were shown to abrogate stimulation of nsp16 RNA cap 2′O-MTase activity. More strikingly, the Y96A mutation abrogates stimulation of nsp16 2′O-MTase activity, whereas Y96F overstimulates it. Thus, the nsp10-nsp16 interface may represent an attractive target for antivirals against human and animal pathogenic coronaviruses.

Coronavirus Nsp10, a Critical Co-factor for Activation of Multiple Replicative Enzymes

Background: SARS-CoV nsp10 binds and stimulates both the nsp14 and nsp16 activities. Results: We mapped the nsp10 surface interacting with nsp14 and demonstrated that this surface plays a critical role in SARS-CoV replication. Conclusion: The core interaction domain of nsp10 is essential for SARS-CoV replication. Significance: This nsp10 core represents an attractive target for antiviral drug development against various pathogenic coronaviruses. The RNA-synthesizing machinery of the severe acute respiratory syndrome Coronavirus (SARS-CoV) is composed of 16 non-structural proteins (nsp1–16) encoded by ORF1a/1b. The 148-amino acid nsp10 subunit contains two zinc fingers and is known to interact with both nsp14 and nsp16, stimulating their respective 3′-5′ exoribonuclease and 2′-O-methyltransferase activities. Using alanine-scanning mutagenesis, in cellulo bioluminescence resonance energy transfer experiments, and in vitro pulldown assays, we have now identified the key residues on the nsp10 surface that interact with nsp14. The functional consequences of mutations introduced at these positions were first evaluated biochemically by monitoring nsp14 exoribonuclease activity. Disruption of the nsp10-nsp14 interaction abrogated the nsp10-driven activation of the nsp14 exoribonuclease. We further showed that the nsp10 surface interacting with nsp14 overlaps with the surface involved in the nsp10-mediated activation of nsp16 2′-O-methyltransferase activity, suggesting that nsp10 is a major regulator of SARS-CoV replicase function. In line with this notion, reverse genetics experiments supported an essential role of the nsp10 surface that interacts with nsp14 in SARS-CoV replication, as several mutations that abolished the interaction in vitro yielded a replication-negative viral phenotype. In contrast, mutants in which the nsp10-nsp16 interaction was disturbed proved to be crippled but viable. These experiments imply that the nsp10 surface that interacts with nsp14 and nsp16 and possibly other subunits of the viral replication complex may be a target for the development of antiviral compounds against pathogenic coronaviruses.

Crystallization and diffraction analysis of the SARS coronavirus nsp10-nsp16 complex.

The expression, purification and crystallization of the SARS coronavirus nsp16 RNA-cap AdoMet-dependent (nucleoside-2′O)-methyltransferase in complex with its activating factor nsp10 are reported.