Structural basis for sequence-nonspecific recognition of 5'-capped mRNA by a cap-modifying enzyme

Structural and functional insights into the 2'-O-methyltransferase of SARS-CoV-2

A unique feature of coronaviruses is their utilization of self-encoded nonstructural protein 16 (nsp16), 2'-O-methyltransferase (2'-O-MTase), to cap their RNAs through ribose 2'-O-methylation modification. This process is crucial for maintaining viral genome stability, facilitating efficient translation, and enabling immune escape. Despite considerable advances in the ultrastructure of SARS-CoV-2 nsp16/nsp10, insights into its molecular mechanism have so far been limited. In this study, we systematically characterized the 2'-O-MTase activity of nsp16 in SARS-CoV-2, focusing on its dependence on nsp10 stimulation. We observed cross-reactivity between nsp16 and nsp10 in various coronaviruses due to a conserved interaction interface. However, a single residue substitution (K58T) in SARS-CoV-2 nsp10 restricted the functional activation of MERS-CoV nsp16. Furthermore, the cofactor nsp10 effectively enhanced the binding of nsp16 to the substrate RNA and the methyl donor S-adenosyl-L-methionine (SAM). Mechanistically, His-80, Lys-93, and Gly-94 of nsp10 interacted with Asp-102, Ser-105, and Asp-106 of nsp16, respectively, thereby effectively stabilizing the SAM binding pocket. Lys-43 of nsp10 interacted with Lys-38 and Gly-39 of nsp16 to dynamically regulate the RNA binding pocket and facilitate precise binding of RNA to the nsp16/nsp10 complex. By assessing the conformational epitopes of nsp16/nsp10 complex, we further determined the critical residues involved in 2'-O-MTase activity. Additionally, we utilize an in vitro biochemical platform to screen potential inhibitors targeting 2'-O-MTase activity. Overall, our results significantly enhance the understanding of viral 2'-O methylation process and mechanism, providing valuable targets for antiviral drug development.

Adenine Methylation Enhances the Conformational Flexibility of an RNA Hairpin Tetraloop

The N6-methyladenosine modification is one of the most abundant post-transcriptional modifications in ribonucleic acid (RNA) molecules. Using molecular dynamics simulations and alchemical free-energy calculations, we studied the structural and energetic implications of incorporating this modification in an adenine mononucleotide and an RNA hairpin structure. At the mononucleotide level, we found that the syn configuration is more favorable than the anti configuration by 2.05 ± 0.15 kcal/mol. The unfavorable effect of methylation was due to the steric overlap between the methyl group and a nitrogen atom in the purine ring. We then probed the effect of methylation in an RNA hairpin structure containing an AUCG tetraloop, which is recognized by a "reader" protein (YTHDC1) to promote transcriptional silencing of long noncoding RNAs. While methylation had no significant conformational effect on the hairpin stem, the methylated tetraloop showed enhanced conformational flexibility compared to the unmethylated tetraloop. The increased flexibility was associated with the outward flipping of two bases (A6 and U7) which formed stacking interactions with each other and with the C8 and G9 bases in the tetraloop, leading to a conformation similar to that in the RNA/reader protein complex. Therefore, methylation-induced conformational flexibility likely facilitates RNA recognition by the reader protein.

5'-cap RNA/SAM mimetic conjugates as bisubstrate inhibitors of viral RNA cap 2'-O-methyltransferases

Viral RNA cap 2'-O-methyltransferases are considered promising therapeutic targets for antiviral treatments, as they play a key role in the formation of viral RNA cap-1 structures to escape the host immune system. A better understanding of how they interact with their natural substrates (RNA and the methyl donor SAM) would enable the rational development of potent inhibitors. However, as few structures of 2'-O-MTases in complex with RNA have been described, little is known about substrate recognition by these MTases. For this, chemical tools mimicking the state in which the cap RNA substrate and SAM cofactor are bound in the enzyme's catalytic pocket may prove useful. In this work, we designed and synthesized over 30 RNA conjugates that contain a short oligoribonucleotide (ORN with 4 or 6 nucleotides) with the first nucleotide 2'-O-attached to an adenosine by linkers of different lengths and containing S or N-heteroatoms, or a 1,2,3-triazole ring. These ORN conjugates bearing or not a cap structure at 5'-extremity mimic the methylation transition state with RNA substrate/SAM complex as bisubstrates of 2'-O-MTases. The ORN conjugates were synthesized either by the incorporation of a dinucleoside phosphoramidite during RNA elongation or by click chemistry performed on solid-phase post-RNA elongation. Their ability to inhibit the activity of the nsp16/nsp10 complex of SARS-CoV-2 and the NS5 protein of dengue and Zika viruses was assessed. Significant submicromolar IC50 values and Kd values in the µM range were found, suggesting a possible interaction of some ORN conjugates with these viral 2'-O-MTases.

Limited high-throughput screening compatibility of the phenuivirus cap-binding domain

Bunyaviruses constitute a large and diverse group of viruses encompassing many emerging pathogens, such as Rift Valley fever virus (family Phenuiviridae), with public and veterinary health relevance but with very limited medical countermeasures are available. For the development of antiviral strategies, the identification and validation of virus-specific targets would be of high value. The cap-snatching mechanism is an essential process in the life cycle of bunyaviruses to produce capped mRNAs, which are then recognized and translated into viral proteins by the host cell translation machinery. Cap-snatching involves cap-binding as well as endonuclease functions and both activities have been demonstrated to be druggable in related influenza viruses. Here, we explore the suitability of the phenuivirus cap-binding function as a target in medium- and high-throughput drug discovery approaches. We developed a range of in vitro assays aiming to detect the interaction between the cap-binding domain (CBD) and the analogue of its natural cap-ligand m7GTP. However, constricted by its shallow binding pocket and low affinity for m7GTP, we conclude that the CBD has limited small molecule targeting potential using classical in vitro drug discovery approaches.

Structural basis for RNA-cap recognition and methylation by the mpox methyltransferase VP39

Mpox is a zoonotic disease caused by the mpox virus (MPXV), which has gained attention due to its rapid and widespread transmission, with reports from more than 100 countries. The virus belongs to the Orthopoxvirus genus, which also includes variola virus and vaccinia virus. In poxviruses, the RNA cap is crucial for the translation and stability of viral mRNAs and also for immune evasion. This study presents the crystal structure of the mpox 2'-O-methyltransfarase VP39 in complex with a short cap-0 RNA. The RNA substrate binds to the protein without causing any significant changes to its overall fold and is held in place by a combination of electrostatic interactions, π-π stacking and hydrogen bonding. The structure also explains the mpox VP39 preference for a guanine base at the first position; it reveals that guanine forms a hydrogen bond that an adenine would not be able to form.

Discovery and structural characterization of monkeypox virus methyltransferase VP39 inhibitors reveal similarities to SARS-CoV-2 nsp14 methyltransferase

Monkeypox is a disease with pandemic potential. It is caused by the monkeypox virus (MPXV), a double-stranded DNA virus from the Poxviridae family, that replicates in the cytoplasm and must encode for its own RNA processing machinery including the capping machinery. Here, we present crystal structures of its 2'-O-RNA methyltransferase (MTase) VP39 in complex with the pan-MTase inhibitor sinefungin and a series of inhibitors that were discovered based on it. A comparison of this 2'-O-RNA MTase with enzymes from unrelated single-stranded RNA viruses (SARS-CoV-2 and Zika) reveals a conserved sinefungin binding mode, implicating that a single inhibitor could be used against unrelated viral families. Indeed, several of our inhibitors such as TO507 also inhibit the coronaviral nsp14 MTase.

Interplay of RNA 2'-O-methylations with viral replication

Viral RNAs (vRNAs) are decorated by post-transcriptional modifications, including methylation of nucleotides. Methylations regulate biological functions linked to the sequence, structure, and protein interactome of RNA. Several RNA viruses were found to harbor 2'-O-methylations, affecting the ribose moiety of RNA. This mark was initially shown to target the first and second nucleotides of the 5'-end cap structure of mRNA. More recently, nucleotides within vRNA were also reported to carry 2'-O-methylations. The consequences of such methylations are still puzzling since they were associated with both proviral and antiviral effects. Here, we focus on the mechanisms governing vRNA 2'-O-methylation and we explore the possible roles of this epitranscriptomic modification for viral replication.

CMTr mediated 2'-O-ribose methylation status of cap-adjacent nucleotides across animals

Cap methyltransferases (CMTrs) O methylate the 2' position of the ribose (cOMe) of cap-adjacent nucleotides of animal, protist, and viral mRNAs. Animals generally have two CMTrs, whereas trypanosomes have three, and many viruses encode one in their genome. In the splice leader of mRNAs in trypanosomes, the first four nucleotides contain cOMe, but little is known about the status of cOMe in animals. Here, we show that cOMe is prominently present on the first two cap-adjacent nucleotides with species- and tissue-specific variations in Caenorhabditis elegans, honeybees, zebrafish, mouse, and human cell lines. In contrast, Drosophila contains cOMe primarily on the first cap-adjacent nucleotide. De novo RoseTTA modeling of CMTrs reveals close similarities of the overall structure and near identity for the catalytic tetrad, and for cap and cofactor binding for human, Drosophila and C. elegans CMTrs. Although viral CMTrs maintain the overall structure and catalytic tetrad, they have diverged in cap and cofactor binding. Consistent with the structural similarity, both CMTrs from Drosophila and humans methylate the first cap-adjacent nucleotide of an AGU consensus start. Because the second nucleotide is also methylated upon heat stress in Drosophila, these findings argue for regulated cOMe important for gene expression regulation.

Overview of Methods for Large-Scale RNA Synthesis

In recent years, it has become clear that RNA molecules are involved in almost all vital cellular processes and pathogenesis of human disorders. The functional diversity of RNA comes from its structural richness. Although composed of only four nucleotides, RNA molecules present a plethora of secondary and tertiary structures critical for intra and intermolecular contacts with other RNAs and ligands (proteins, small metabolites, etc.). In order to fully understand RNA function it is necessary to define its spatial structure. Crystallography, nuclear magnetic resonance and cryogenic electron microscopy have demonstrated considerable success in determining the structures of biologically important RNA molecules. However, these powerful methods require large amounts of sample. Despite their limitations, chemical synthesis and in vitro transcription are usually employed to obtain milligram quantities of RNA for structural studies, delivering simple and effective methods for large-scale production of homogenous samples. The aim of this paper is to provide an overview of methods for large-scale RNA synthesis with emphasis on chemical synthesis and in vitro transcription. We also present our own results of testing the efficiency of these approaches in order to adapt the material acquisition strategy depending on the desired RNA construct.

Enzymatic Assays to Explore Viral mRNA Capping Machinery

In eukaryotes, mRNA is modified by the addition of the 7-methylguanosine (m⁷ G) 5' cap to protect mRNA from premature degradation, thereby enhancing translation and enabling differentiation between self (endogenous) and non-self RNAs (e. g., viral ones). Viruses often develop their own mRNA capping pathways to augment the expression of their proteins and escape host innate immune response. Insights into this capping system may provide new ideas for therapeutic interventions and facilitate drug discovery, e. g., against viruses that cause pandemic outbreaks, such as beta-coronaviruses SARS-CoV (2002), MARS-CoV (2012), and the most recent SARS-CoV-2. Thus, proper methods for the screening of large compound libraries are required to identify lead structures that could serve as a basis for rational antiviral drug design. This review summarizes the methods that allow the monitoring of the activity and inhibition of enzymes involved in mRNA capping.

SARS-CoV-2 Nsp16 activation mechanism and a cryptic pocket with pan-coronavirus antiviral potential

Coronaviruses have caused multiple epidemics in the past two decades, in addition to the current COVID-19 pandemic that is severely damaging global health and the economy. Coronaviruses employ between 20 and 30 proteins to carry out their viral replication cycle, including infection, immune evasion, and replication. Among these, nonstructural protein 16 (Nsp16), a 2'-O-methyltransferase, plays an essential role in immune evasion. Nsp16 achieves this by mimicking its human homolog, CMTr1, which methylates mRNA to enhance translation efficiency and distinguish self from other. Unlike human CMTr1, Nsp16 requires a binding partner, Nsp10, to activate its enzymatic activity. The requirement of this binding partner presents two questions that we investigate in this manuscript. First, how does Nsp10 activate Nsp16? Although experimentally derived structures of the active Nsp16/Nsp10 complex exist, structures of inactive, monomeric Nsp16 have yet to be solved. Therefore, it is unclear how Nsp10 activates Nsp16. Using over 1 ms of molecular dynamics simulations of both Nsp16 and its complex with Nsp10, we investigate how the presence of Nsp10 shifts Nsp16's conformational ensemble to activate it. Second, guided by this activation mechanism and Markov state models, we investigate whether Nsp16 adopts inactive structures with cryptic pockets that, if targeted with a small molecule, could inhibit Nsp16 by stabilizing its inactive state. After identifying such a pocket in SARS-CoV2 Nsp16, we show that this cryptic pocket also opens in SARS-CoV1 and MERS but not in human CMTr1. Therefore, it may be possible to develop pan-coronavirus antivirals that target this cryptic pocket.

Mn2+ coordinates Cap-0-RNA to align substrates for efficient 2'-O-methyl transfer by SARS-CoV-2 nsp16

Virally encoded 2′-O-methyltransferases catalyze the last step in the capping of viral RNAs, which protects the RNAs from degradation and prevents them from triggering host defenses. Minasov et al. report structures of the SARS-CoV-2 methyltransferase, a heterodimeric complex of the enzyme nsp16 and its coactivator nsp10, in complex with a short, capped RNA (instead of the RNA cap analogs used to generate previous structures), the methyl donor SAM, and divalent metal cations. The metal ions and a four-residue insert of nsp16 were important for precisely aligning the RNA substrate in the active site for efficient catalysis. This insert is present in coronavirus but not in mammalian methyltransferases, suggesting this site as a potential target for the design of coronavirus-specific methyltransferase inhibitors.

Capping of viral messenger RNAs is essential for efficient translation, for virus replication, and for preventing detection by the host cell innate response system. The SARS-CoV-2 genome encodes the 2′-O-methyltransferase nsp16, which, when bound to the coactivator nsp10, uses S-adenosylmethionine (SAM) as a donor to transfer a methyl group to the first ribonucleotide of the mRNA in the final step of viral mRNA capping. Here, we provide biochemical and structural evidence that this reaction requires divalent cations, preferably Mn²⁺, and a coronavirus-specific four-residue insert. We determined the x-ray structures of the SARS-CoV-2 2′-O-methyltransferase (the nsp16-nsp10 heterodimer) in complex with its reaction substrates, products, and divalent metal cations. These structural snapshots revealed that metal ions and the insert stabilize interactions between the capped RNA and nsp16, resulting in the precise alignment of the ribonucleotides in the active site. Comparison of available structures of 2′-O-methyltransferases with capped RNAs from different organisms revealed that the four-residue insert unique to coronavirus nsp16 alters the backbone conformation of the capped RNA in the binding groove, thereby promoting catalysis. This insert is highly conserved across coronaviruses, and its absence in mammalian methyltransferases makes this region a promising site for structure-guided drug design of selective coronavirus inhibitors.

Capsid Structure of Leishmania RNA Virus 1

Twelve million people worldwide suffer from leishmaniasis, resulting in more than 30 thousand deaths annually. The disease has several variants that differ in their symptoms.

Leishmania parasites cause a variety of symptoms, including mucocutaneous leishmaniasis, which results in the destruction of the mucous membranes of the nose, mouth, and throat. The species of Leishmania carrying Leishmania RNA virus 1 (LRV1), from the family Totiviridae, are more likely to cause severe disease and are less sensitive to treatment than those that do not contain the virus. Although the importance of LRV1 for the severity of leishmaniasis was discovered a long time ago, the structure of the virus remained unknown. Here, we present a cryo-electron microscopy reconstruction of the virus-like particle of LRV1 determined to a resolution of 3.65 Å. The capsid has icosahedral symmetry and is formed by 120 copies of a capsid protein assembled in asymmetric dimers. RNA genomes of viruses from the family Totiviridae are synthetized, but not capped at the 5′ end, by virus RNA polymerases. To protect viral RNAs from degradation, capsid proteins of the L-A totivirus cleave the 5′ caps of host mRNAs, creating decoys to overload the cellular RNA quality control system. Capsid proteins of LRV1 form positively charged clefts, which may be the cleavage sites for the 5′ cap of Leishmania mRNAs. The putative RNA binding site of LRV1 is distinct from that of the related L-A virus. The structure of the LRV1 capsid enables the rational design of compounds targeting the putative decapping site. Such inhibitors may be developed into a treatment for mucocutaneous leishmaniasis caused by LRV1-positive species of Leishmania.

IMPORTANCE Twelve million people worldwide suffer from leishmaniasis, resulting in more than 30 thousand deaths annually. The disease has several variants that differ in their symptoms. The mucocutaneous form, which leads to disintegration of the nasal septum, lips, and palate, is caused predominantly by Leishmania parasites carrying Leishmania RNA virus 1 (LRV1). Here, we present the structure of the LRV1 capsid determined using cryo-electron microscopy. Capsid proteins of a related totivirus, L-A virus, protect viral RNAs from degradation by cleaving the 5′ caps of host mRNAs. Capsid proteins of LRV1 may have the same function. We show that the LRV1 capsid contains positively charged clefts that may be sites for the cleavage of mRNAs of Leishmania cells. The structure of the LRV1 capsid enables the rational design of compounds targeting the putative mRNA cleavage site. Such inhibitors may be used as treatments for mucocutaneous leishmaniasis.

Functional and computational identification of a rescue mutation near the active site of an mRNA methyltransferase

RNA-based drugs are an emerging class of therapeutics combining the immense potential of DNA gene-therapy with the absence of genome integration-associated risks. While the synthesis of such molecules is feasible, large scale in vitro production of humanised mRNA remains a biochemical and economical challenge. Human mRNAs possess two post-transcriptional modifications at their 5' end: an inverted methylated guanosine and a unique 2'O-methylation on the ribose of the penultimate nucleotide. One strategy to precisely methylate the 2' oxygen is to use viral mRNA methyltransferases that have evolved to escape the host's cell immunity response following virus infection. However, these enzymes are ill-adapted to industrial processes and suffer from low turnovers. We have investigated the effects of homologous and orthologous active-site mutations on both stability and transferase activity, and identified new functional motifs in the interaction network surrounding the catalytic lysine. Our findings suggest that despite their low catalytic efficiency, the active-sites of viral mRNA methyltransferases have low mutational plasticity, while mutations in a defined third shell around the active site have strong effects on folding, stability and activity in the variant enzymes, mostly via network-mediated effects.

SARS-CoV-2 Nsp16 activation mechanism and a cryptic pocket with pan-coronavirus antiviral potential

Coronaviruses have caused multiple epidemics in the past two decades, in addition to the current COVID-19 pandemic that is severely damaging global health and the economy. Coronaviruses employ between twenty and thirty proteins to carry out their viral replication cycle including infection, immune evasion, and replication. Among these, nonstructural protein 16 (Nsp16), a 2'-O-methyltransferase, plays an essential role in immune evasion. Nsp16 achieves this by mimicking its human homolog, CMTr1, which methylates mRNA to enhance translation efficiency and distinguish self from other. Unlike human CMTr1, Nsp16 requires a binding partner, Nsp10, to activate its enzymatic activity. The requirement of this binding partner presents two questions that we investigate in this manuscript. First, how does Nsp10 activate Nsp16? While experimentally-derived structures of the active Nsp16/Nsp10 complex exist, structures of inactive, monomeric Nsp16 have yet to be solved. Therefore, it is unclear how Nsp10 activates Nsp16. Using over one millisecond of molecular dynamics simulations of both Nsp16 and its complex with Nsp10, we investigate how the presence of Nsp10 shifts Nsp16's conformational ensemble in order to activate it. Second, guided by this activation mechanism and Markov state models (MSMs), we investigate if Nsp16 adopts inactive structures with cryptic pockets that, if targeted with a small molecule, could inhibit Nsp16 by stabilizing its inactive state. After identifying such a pocket in SARS-CoV-2 Nsp16, we show that this cryptic pocket also opens in SARS-CoV-1 and MERS, but not in human CMTr1. Therefore, it may be possible to develop pan-coronavirus antivirals that target this cryptic pocket.

STATEMENT OF SIGNIFICANCE

Coronaviruses are a major threat to human health. These viruses employ molecular machines, called proteins, to infect host cells and replicate. Characterizing the structure and dynamics of these proteins could provide a basis for designing small molecule antivirals. In this work, we use computer simulations to understand the moving parts of an essential SARS-CoV-2 protein, understand how a binding partner turns it on and off, and identify a novel pocket that antivirals could target to shut this protein off. The pocket is also present in other coronaviruses but not in the related human protein, so it could be a valuable target for pan-coronavirus antivirals.

The multifaceted eukaryotic cap structure

The 5' cap structure is added onto RNA polymerase II transcripts soon after initiation of transcription and modulates several post-transcriptional regulatory events involved in RNA maturation. It is also required for stimulating translation initiation of many cytoplasmic mRNAs and serves to protect mRNAs from degradation. These functional properties of the cap are mediated by several cap binding proteins (CBPs) involved in nuclear and cytoplasmic gene expression steps. The role that CBPs play in gene regulation, as well as the biophysical nature by which they recognize the cap, is quite intricate. Differences in mechanisms of capping as well as nuances in cap recognition speak to the potential of targeting these processes for drug development. In this review, we focus on recent findings concerning the cap epitranscriptome, our understanding of cap binding by different CBPs, and explore therapeutic targeting of CBP-cap interaction. This article is categorized under: RNA Interactions with Proteins and Other Molecules > Protein-RNA Recognition RNA Processing > Capping and 5' End Modifications Translation > Translation Mechanisms.

Structural insight into the recognition of S-adenosyl-L-homocysteine and sinefungin in SARS-CoV-2 Nsp16/Nsp10 RNA cap 2'-O-Methyltransferase

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The binding affinity towards SARS-CoV-2 nsp16 of SFG is higher than that of SAH.

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Asp99 is a key binding residue for SAH and SFG via charge-charge attraction.

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SFG could electrostatically interact with the 2′-OH and N3 groups of adenosine moiety of RNA substrate.

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The distance between 2′-OH of RNA and –NH₃⁺ (at 6′ position) of SFG mimics the methyl transfer reaction.

The recent ongoing coronavirus disease 2019 (COVID-19) pandemic caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) continues to rapidly spread across the world. To date, neither a specific antiviral drug nor a clinically effective vaccine is available. Among the 15 viral non-structural proteins (nsps), nsp16 methyltransferase has been considered as a potential target due to its crucial role in RNA cap 2′-O-methylation process, preventing the virus detection by cell innate immunity mechanisms. In the present study, molecular recognition between the two natural nucleoside analogs (S-adenosyl-l-homocysteine (SAH) and sinefungin (SFG)) and the SARS-CoV-2 nsp16/nsp10/^m7G_pppAC₅ was studied using all-atom molecular dynamics simulations and free energy calculations based on MM/GBSA and WaterSwap approaches. The binding affinity and the number of hot-spot residues, atomic contacts, and H-bond formations of SFG/nsp16 complex were distinctly higher than those of SAH/nsp16 system, consistent with the lower water accessibility at the enzyme active site. Notably, only SFG could electrostatically interact with the 2′-OH and N3 of RNA’s adenosine moiety, mimicking the methyl transfer reaction of S-adenosyl-l-methionine substrate. The atomistic binding mechanism obtained from this work paves the way for further optimizations and designs of more specific SARS-CoV-2 nsp16 inhibitors in the fight against COVID-19.

Structural basis of RNA cap modification by SARS-CoV-2

The severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2), the causative agent of COVID-19 illness, has caused millions of infections worldwide. In SARS coronaviruses, the non-structural protein 16 (nsp16), in conjunction with nsp10, methylates the 5'-end of virally encoded mRNAs to mimic cellular mRNAs, thus protecting the virus from host innate immune restriction. We report here the high-resolution structure of a ternary complex of SARS-CoV-2 nsp16 and nsp10 in the presence of cognate RNA substrate analogue and methyl donor, S-adenosyl methionine (SAM). The nsp16/nsp10 heterodimer is captured in the act of 2'-O methylation of the ribose sugar of the first nucleotide of SARS-CoV-2 mRNA. We observe large conformational changes associated with substrate binding as the enzyme transitions from a binary to a ternary state. This induced fit model provides mechanistic insights into the 2'-O methylation of the viral mRNA cap. We also discover a distant (25 Å) ligand-binding site unique to SARS-CoV-2, which can alternatively be targeted, in addition to RNA cap and SAM pockets, for antiviral development.

A Comprehensive Mapping of the Druggable Cavities within the SARS-CoV-2 Therapeutically Relevant Proteins by Combining Pocket and Docking Searches as Implemented in Pockets 2.0

(1) Background: Virtual screening studies on the therapeutically relevant proteins of the severe acute respiratory syndrome Coronavirus 2 (SARS-CoV-2) require a detailed characterization of their druggable binding sites, and, more generally, a convenient pocket mapping represents a key step for structure-based in silico studies; (2) Methods: Along with a careful literature search on SARS-CoV-2 protein targets, the study presents a novel strategy for pocket mapping based on the combination of pocket (as performed by the well-known FPocket tool) and docking searches (as performed by PLANTS or AutoDock/Vina engines); such an approach is implemented by the Pockets 2.0 plug-in for the VEGA ZZ suite of programs; (3) Results: The literature analysis allowed the identification of 16 promising binding cavities within the SARS-CoV-2 proteins and the here proposed approach was able to recognize them showing performances clearly better than those reached by the sole pocket detection; and (4) Conclusions: Even though the presented strategy should require more extended validations, this proved successful in precisely characterizing a set of SARS-CoV-2 druggable binding pockets including both orthosteric and allosteric sites, which are clearly amenable for virtual screening campaigns and drug repurposing studies. All results generated by the study and the Pockets 2.0 plug-in are available for download.

Structural Basis of RNA Cap Modification by SARS-CoV-2 Coronavirus

The novel severe acute respiratory syndrome coronoavirus-2 (SARS-CoV-2), the causative agent of COVID-19 illness, has caused over 2 million infections worldwide in four months. In SARS coronaviruses, the non-structural protein 16 (nsp16) methylates the 5'-end of virally encoded mRNAs to mimic cellular mRNAs, thus protecting the virus from host innate immune restriction. We report here the high-resolution structure of a ternary complex of full-length nsp16 and nsp10 of SARS-CoV-2 in the presence of cognate RNA substrate and a methyl donor, S-adenosyl methionine. The nsp16/nsp10 heterodimer was captured in the act of 2'-O methylation of the ribose sugar of the first nucleotide of SARS-CoV-2 mRNA. We reveal large conformational changes associated with substrate binding as the enzyme transitions from a binary to a ternary state. This structure provides new mechanistic insights into the 2'-O methylation of the viral mRNA cap. We also discovered a distantly located ligand-binding site unique to SARS-CoV-2 that may serve as an alternative target site for antiviral development.

The methyltransferase domain of the Sudan ebolavirus L protein specifically targets internal adenosines of RNA substrates, in addition to the cap structure

Mononegaviruses, such as Ebola virus, encode an L (large) protein that bears all the catalytic activities for replication/transcription and RNA capping. The C-terminal conserved region VI (CRVI) of L protein contains a K-D-K-E catalytic tetrad typical for 2'O methyltransferases (MTase). In mononegaviruses, cap-MTase activities have been involved in the 2'O methylation and N7 methylation of the RNA cap structure. These activities play a critical role in the viral life cycle as N7 methylation ensures efficient viral mRNA translation and 2'O methylation hampers the detection of viral RNA by the host innate immunity. The functional characterization of the MTase+CTD domain of Sudan ebolavirus (SUDV) revealed cap-independent methyltransferase activities targeting internal adenosine residues. Besides this, the MTase+CTD also methylates, the N7 position of the cap guanosine and the 2'O position of the n1 guanosine provided that the RNA is sufficiently long. Altogether, these results suggest that the filovirus MTases evolved towards a dual activity with distinct substrate specificities. Whereas it has been well established that cap-dependent methylations promote protein translation and help to mimic host RNA, the characterization of an original cap-independent methylation opens new research opportunities to elucidate the role of RNA internal methylations in the viral replication.

Functional and evolutionary analysis of viral proteins containing a Rossmann-like fold

Viruses are the most abundant life form and infect practically all organisms. Consequently, these obligate parasites are a major cause of human suffering and economic loss. Rossmann-like fold is the most populated fold among α/β-folds in the Protein Data Bank and proteins containing Rossmann-like fold constitute 22% of all known proteins 3D structures. Thus, analysis of viral proteins containing Rossmann-like domains could provide an understanding of viral biology and evolution as well as could propose possible targets for antiviral therapy. We provide functional and evolutionary analysis of viral proteins containing a Rossmann-like fold found in the evolutionary classification of protein domains (ECOD) database developed in our lab. We identified 81 protein families of bacterial, archeal, and eukaryotic viruses in light of their evolution-based ECOD classification and Pfam taxonomy. We defined their functional significance using enzymatic EC number assignments as well as domain-level family annotations.

Binding of the Methyl Donor S-Adenosyl-l-Methionine to Middle East Respiratory Syndrome Coronavirus 2'-O-Methyltransferase nsp16 Promotes Recruitment of the Allosteric Activator nsp10

The Middle East respiratory syndrome coronavirus (MERS-CoV) nonstructural protein 16 (nsp16) is an S-adenosyl-l-methionine (SAM)-dependent 2'-O-methyltransferase (2'-O-MTase) that is thought to methylate the ribose 2'-OH of the first transcribed nucleotide (N₁) of viral RNA cap structures. This 2'-O-MTase activity is regulated by nsp10. The 2'-O methylation prevents virus detection by cell innate immunity mechanisms and viral translation inhibition by the interferon-stimulated IFIT-1 protein. To unravel the regulation of nsp10/nsp16 2'-O-MTase activity, we used purified MERS-CoV nsp16 and nsp10. First, we showed that nsp16 recruited N7-methylated capped RNA and SAM. The SAM binding promotes the assembly of the enzymatically active nsp10/nsp16 complex that converted ^7mGpppG (cap-0) into ^7mGpppG_2'Om (cap-1) RNA by 2'-OH methylation of N₁ in a SAM-dependent manner. The subsequent release of SAH speeds up nsp10/nsp16 dissociation that stimulates the reaction turnover. Alanine mutagenesis and RNA binding assays allowed the identification of the nsp16 residues involved in RNA recognition forming the RNA binding groove (K46, K170, E203, D133, R38, Y47, and Y181) and the cap-0 binding site (Y30, Y132, and H174). Finally, we found that nsp10/nsp16 2'-O-MTase activity is sensitive to known MTase inhibitors, such as sinefungin and cap analogues. This characterization of the MERS-CoV 2'-O-MTase is a preliminary step toward the development of molecules to inhibit cap 2'-O methylation and to restore the host antiviral response. IMPORTANCE MERS-CoV codes for a cap 2'-O-methyltransferase that converts cap-0 into cap-1 structure in order to prevent virus detection by cell innate immunity mechanisms. We report the biochemical properties of MERS-CoV 2'O-methyltransferase, which is stimulated by nsp10 acting as an allosteric activator of the nsp16 2'-O-methyltransferase possibly through enhanced RNA binding affinity. In addition, we show that SAM promotes the formation of the active nsp10/nsp16 complex. Conversely, after cap methylation, the reaction turnover is speeded up by cap-1 RNA release and nsp10/nsp16 complex dissociation, at the low intracellular SAH concentration. These results suggest that SAM/SAH balance is a regulator of the 2'-O-methyltransferase activity and raises the possibility that SAH hydrolase inhibitors might interfere with CoV replication cycle. The enzymatic and RNA binding assays developed in this work were also used to identify nsp16 residues involved in cap-0 RNA recognition and to understand the action mode of known methyltransferase inhibitors.

Applications of Phosphate Modification and Labeling to Study (m)RNA Caps

The cap is a natural modification present at the 5' ends of eukaryotic messenger RNA (mRNA), which because of its unique structural features, mediates essential biological functions during the process of gene expression. The core structural feature of the mRNA cap is an N7-methylguanosine moiety linked by a 5'-5' triphosphate chain to the first transcribed nucleotide. Interestingly, other RNA 5' end modifications structurally and functionally resembling the m7G cap have been discovered in different RNA types and in different organisms. All these structures contain the 'inverted' 5'-5' oligophosphate bridge, which is necessary for interaction with specific proteins and also serves as a cleavage site for phosphohydrolases regulating RNA turnover. Therefore, cap analogs containing oligophosphate chain modifications or carrying spectroscopic labels attached to phosphate moieties serve as attractive molecular tools for studies on RNA metabolism and modification of natural RNA properties. Here, we review chemical, enzymatic, and chemoenzymatic approaches that enable preparation of modified cap structures and RNAs carrying such structures, with emphasis on phosphate-modified mRNA cap analogs and their potential applications.

A general method for rapid and cost-efficient large-scale production of 5' capped RNA

The eukaryotic mRNA 5' cap structure is indispensible for pre-mRNA processing, mRNA export, translation initiation, and mRNA stability. Despite this importance, structural and biophysical studies that involve capped RNA are challenging and rare due to the lack of a general method to prepare mRNA in sufficient quantities. Here, we show that the vaccinia capping enzyme can be used to produce capped RNA in the amounts that are required for large-scale structural studies. We have therefore designed an efficient expression and purification protocol for the vaccinia capping enzyme. Using this approach, the reaction scale can be increased in a cost-efficient manner, where the yields of the capped RNA solely depend on the amount of available uncapped RNA target. Using a large number of RNA substrates, we show that the efficiency of the capping reaction is largely independent of the sequence, length, and secondary structure of the RNA, which makes our approach generally applicable. We demonstrate that the capped RNA can be directly used for quantitative biophysical studies, including fluorescence anisotropy and high-resolution NMR spectroscopy. In combination with (13)C-methyl-labeled S-adenosyl methionine, the methyl groups in the RNA can be labeled for methyl TROSY NMR spectroscopy. Finally, we show that our approach can produce both cap-0 and cap-1 RNA in high amounts. In summary, we here introduce a general and straightforward method that opens new means for structural and functional studies of proteins and enzymes in complex with capped RNA.

Identification and Characterization of a Ribose 2'-O-Methyltransferase Encoded by the Ronivirus Branch of Nidovirales

The order Nidovirales currently comprises four virus families: Arteriviridae, Coronaviridae (divided into the subfamilies Coronavirinae and Torovirinae), Roniviridae, and the recently recognized Mesoniviridae RNA cap formation and methylation have been best studied for coronaviruses, with emphasis on the identification and characterization of two virus-encoded methyltransferases (MTases) involved in RNA capping, a guanine-N7-MTase and a ribose-2'-O-MTase. Although bioinformatics analyses suggest that these MTases may also be encoded by other nidoviruses with large genomes, such as toroviruses and roniviruses, no experimental evidence has been reported thus far. In this study, we show that a ronivirus, gill-associated virus (GAV), encodes the 2'-O-MTase activity, although we could not detect 2'-O-MTase activity for the homologous protein of a torovirus, equine torovirus, which is more closely related to coronaviruses. Like the coronavirus 2'-O-MTase, the roniviral 2'-O-MTase harbors a catalytic K-D-K-E tetrad that is conserved among 2'-O-MTases and can target only the N7-methylated cap structure of adenylate-primed RNA substrates. However, in contrast with the coronavirus protein, roniviral 2'-O-MTase does not require a protein cofactor for stimulation of its activity and differs in its preference for several biochemical parameters, such as reaction temperature and pH. Furthermore, the ronivirus 2'-O-MTase can be targeted by MTase inhibitors. These results extend our current understanding of nidovirus RNA cap formation and methylation beyond the coronavirus family.

IMPORTANCE

Methylation of the 5'-cap structure of viral RNAs plays important roles in genome replication and evasion of innate recognition of viral RNAs by cellular sensors. It is known that coronavirus nsp14 acts as an N7-(guanine)-methyltransferase (MTase) and nsp16 as a 2'-O-MTase, which are involved in the modification of RNA cap structure. However, these enzymatic activities have not been shown for any other nidoviruses beyond coronaviruses in the order Nidovirales In this study, we identified a 2'-O-methyltransferase encoded by ronivirus that shows common and unique features in comparison with that of coronaviruses. Ronivirus 2'-O-MTase does not need a protein cofactor for MTase activity, whereas coronavirus nsp16 needs the stimulating factor nsp10 for its full activity. The conserved K-D-K-E catalytic tetrad is identified in ronivirus 2'-O-MTase. These results extend our understanding of nidovirus RNA capping and methylation beyond coronaviruses and also strengthen the evolutionary and functional links between roniviruses and coronaviruses.

Molecular basis for specific viral RNA recognition and 2'-O-ribose methylation by the dengue virus nonstructural protein 5 (NS5)

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 (GpppAG-RNA), leading to the formation of the 5' RNA cap: G0pppAG-RNA → (m7)G0pppAG-RNA ("cap-0")→(m7)G0pppAm2'-O-G-RNA ("cap-1"). 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)G0pppA1G2U3U4G5U6U7-3'), 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.

X-ray structure and activities of an essential Mononegavirales L-protein domain

The L protein of mononegaviruses harbours all catalytic activities for genome replication and transcription. It contains six conserved domains (CR-I to -VI; Fig. 1a). CR-III has been linked to polymerase and polyadenylation activity, CR-V to mRNA capping and CR-VI to cap methylation. However, how these activities are choreographed is poorly understood. Here we present the 2.2-Å X-ray structure and activities of CR-VI+, a portion of human Metapneumovirus L consisting of CR-VI and the poorly conserved region at its C terminus, the +domain. The CR-VI domain has a methyltransferase fold, which besides the typical S-adenosylmethionine-binding site ((SAM)P) also contains a novel pocket ((NS)P) that can accommodate a nucleoside. CR-VI lacks an obvious cap-binding site, and the (SAM)P-adjoining site holding the nucleotides undergoing methylation ((SUB)P) is unusually narrow because of the overhanging +domain. CR-VI+ sequentially methylates caps at their 2'O and N7 positions, and also displays nucleotide triphosphatase activity.

Structural analysis of human 2'-O-ribose methyltransferases involved in mRNA cap structure formation

The 5′ cap of human messenger RNA contains 2′-O-methylation of the first and often second transcribed nucleotide that is important for its processing, translation and stability. Human enzymes that methylate these nucleotides, termed CMTr1 and CMTr2, respectively, have recently been identified. However, the structures of these enzymes and their mechanisms of action remain unknown. In the present study, we solve the crystal structures of the active CMTr1 catalytic domain in complex with a methyl group donor and a capped oligoribonucleotide, thereby revealing the mechanism of specific recognition of capped RNA. This mechanism differs significantly from viral enzymes, thus providing a framework for their specific targeting. Based on the crystal structure of CMTr1, a comparative model of the CMTr2 catalytic domain is generated. This model, together with mutational analysis, leads to the identification of residues involved in RNA and methyl group donor binding.

Human mRNA transcripts possess a 5' cap structure that is modified by methylation. Here, Smietanski et al. present the structures of human methyltransferases responsible for this reaction, revealing key differences to their viral counterparts and thereby providing a framework for targeted drug design.

RNA methyltransferases involved in 5' cap biosynthesis

In eukaryotes and viruses that infect them, the 5′ end of mRNA molecules, and also many other functionally important RNAs, are modified to form a so-called cap structure that is important for interactions of these RNAs with many nuclear and cytoplasmic proteins. The RNA cap has multiple roles in gene expression, including enhancement of RNA stability, splicing, nucleocytoplasmic transport, and translation initiation. Apart from guanosine addition to the 5′ end in the most typical cap structure common to transcripts produced by RNA polymerase II (in particular mRNA), essentially all cap modifications are due to methylation. The complexity of the cap structure and its formation can range from just a single methylation of the unprocessed 5′ end of the primary transcript, as in mammalian U6 and 7SK, mouse B2, and plant U3 RNAs, to an elaborate m⁷Gpppm^6,6AmpAmpCmpm³Um structure at the 5′ end of processed RNA in trypanosomes, which are formed by as many as 8 methylation reactions. While all enzymes responsible for methylation of the cap structure characterized to date were found to belong to the same evolutionarily related and structurally similar Rossmann Fold Methyltransferase superfamily, that uses the same methyl group donor, S-adenosylmethionine; the enzymes also exhibit interesting differences that are responsible for their distinct functions. This review focuses on the evolutionary classification of enzymes responsible for cap methylation in RNA, with a focus on the sequence relationships and structural similarities and dissimilarities that provide the basis for understanding the mechanism of biosynthesis of different caps in cellular and viral RNAs. Particular attention is paid to the similarities and differences between methyltransferases from human cells and from human pathogens that may be helpful in the development of antiviral and antiparasitic drugs.

Towards novel efficient and stable nuclear import signals: synthesis and properties of trimethylguanosine cap analogs modified within the 5',5'-triphosphate bridge

A trimethylguanosine (TMG) cap is present at the 5' end of several small nuclear and nucleolar RNAs. Recently, it has been reported that the TMG cap is a potential nuclear import signal for nucleus-targeting therapeutic nucleic acids and proteins. The import is mediated by recognition of the TMG cap by the snRNA transporting protein, snurportin1. This work describes the synthesis and properties of a series of dinucleotide TMG cap (m3(2,2,7)GpppG) analogs modified in the 5',5'-triphosphate bridge as tools to study TMG cap-dependent biological processes. The bridge was altered at different positions by introducing either bridging (imidodiphosphate, O to NH and methylenebisphosphonate, O to CH2) or non-bridging (phosphorothioate, O to S and boranophosphate, O to BH3) modifications, or by elongation to tetraphosphate. The stability of novel analogs in blood serum was studied to reveal that the α,β-bridging O to NH substitution (m3(2,2,7)GppNHpG) confers the highest resistance. Short RNAs capped with analogs containing α,β-bridging (m3(2,2,7)GppNHpG) or β-non-bridging (m3(2,2,7)GppSpG D2) modifications were resistant to decapping pyrophosphatase, hNudt16. Preliminary studies on binding by human snurportin1 revealed that both O to NH and O to S substitutions support this binding. Due to favorable properties in all three assays, m3(2,2,7)GppNHpG was selected as a promising candidate for further studies on the efficiency of the TMG cap as a nuclear import signal.

mRNA maturation in giant viruses: variation on a theme

Giant viruses from the Mimiviridae family replicate entirely in their host cytoplasm where their genes are transcribed by a viral transcription apparatus. mRNA polyadenylation uniquely occurs at hairpin-forming palindromic sequences terminating viral transcripts. Here we show that a conserved gene cluster both encode the enzyme responsible for the hairpin cleavage and the viral polyA polymerases (vPAP). Unexpectedly, the vPAPs are homodimeric and uniquely self-processive. The vPAP backbone structures exhibit a symmetrical architecture with two subdomains sharing a nucleotidyltransferase topology, suggesting that vPAPs originate from an ancestral duplication. A Poxvirus processivity factor homologue encoded by Megavirus chilensis displays a conserved 5'-GpppA 2'O methyltransferase activity but is also able to internally methylate the mRNAs' polyA tails. These findings elucidate how the arm wrestling between hosts and their viruses to access the translation machinery is taking place in Mimiviridae.

Innate immune restriction and antagonism of viral RNA lacking 2׳-O methylation

N-7 and 2′-O methylation of host cell mRNA occurs in the nucleus and results in the generation of cap structures (cap 0, m⁷GpppN; cap 1, m⁷GpppNm) that control gene expression by modulating nuclear export, splicing, turnover, and protein synthesis. Remarkably, RNA cap modification also contributes to mammalian cell host defense as viral RNA lacking 2′-O methylation is sensed and inhibited by IFIT1, an interferon (IFN) stimulated gene (ISG). Accordingly, pathogenic viruses that replicate in the cytoplasm have evolved mechanisms to circumvent IFIT1 restriction and facilitate infection of mammalian cells. These include: (a) generating cap 1 structures on their RNA through cap-snatching or virally-encoded 2′-O methyltransferases, (b) using cap-independent means of translation, or (c) using RNA secondary structural motifs to antagonize IFIT1 binding. This review will discuss new insights as to how specific modifications at the 5′-end of viral RNA modulate host pathogen recognition responses to promote infection and disease.

Chemistry enters nucleic acids biology: enzymatic mechanisms of RNA modification

Modified nucleotides are universally conserved in all living kingdoms and are present in almost all types of cellular RNAs, including tRNA, rRNA, sn(sno)RNA, and mRNA and in recently discovered regulatory RNAs. Altogether, over 110 chemically distinct RNA modifications have been characterized and localized in RNA by various analytical methods. However, this impressive list of known modified nucleotides is certainly incomplete, mainly due to difficulties in identification and characterization of these particular residues in low abundance cellular RNAs. In DNA, modified residues are formed by both enzymatic reactions (like DNA methylations, for example) and by spontaneous chemical reactions resulting from oxidative damage. In contrast, all modified residues characterized in cellular RNA molecules are formed by specific action of dedicated RNA-modification enzymes, which recognize their RNA substrate with high specificity. These RNA-modification enzymes display a great diversity in terms of the chemical reaction and use various low molecular weight cofactors (or co-substrates) in enzymatic catalysis. Depending on the nature of the target base and of the co-substrate, precise chemical mechanisms are used for appropriate activation of the base and the co-substrate in the enzyme active site. In this review, we give an extended summary of the enzymatic mechanisms involved in formation of different methylated nucleotides in RNA, as well as pseudouridine residues, which are almost universally conserved in all living organisms. Other interesting mechanisms include thiolation of uridine residues by ThiI and the reaction of guanine exchange catalyzed by TGT. The latter implies the reversible cleavage of the N-glycosidic bond in order to replace the initially encoded guanine by an aza-guanosine base. Despite the extensive studies of RNA modification and RNA-modification machinery during the last 20 years, our knowledge on the exact chemical steps involved in catalysis of RNA modification remains very limited. Recent discoveries of radical mechanisms involved in base methylation clearly demonstrate that numerous possibilities are used in Nature for these difficult reactions. Future studies are certainly required for better understanding of the enzymatic mechanisms of RNA modification, and this knowledge is crucial not only for basic research, but also for development of new therapeutic molecules.

Identification of the active sites in the methyltransferases of a transcribing dsRNA virus

Many double-stranded RNA (dsRNA) viruses are capable of transcribing and capping RNA within a stable icosahedral viral capsid. The turret of turreted dsRNA viruses belonging to the family Reoviridae is formed by five copies of the turret protein, which contains domains with both 7-N-methyltransferase and 2′-O-methyltransferase activities, and serves to catalyze the methylation reactions during RNA capping. Cypovirus of the family Reoviridae provides a good model system for studying the methylation reactions in dsRNA viruses. Here, we present the structure of a transcribing cypovirus to a resolution of ~ 3.8 Å by cryo-electron microscopy. The binding sites for both S-adenosyl-l-methionine and RNA in the two methyltransferases of the turret were identified. Structural analysis of the turret in complex with RNA revealed a pathway through which the RNA molecule reaches the active sites of the two methyltransferases before it is released into the cytoplasm. The pathway shows that RNA capping reactions occur in the active sites of different turret protein monomers, suggesting that RNA capping requires concerted efforts by at least three turret protein monomers. Thus, the turret structure provides novel insights into the precise mechanisms of RNA methylation.

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Structure of methyltransferases (MTases) and RNA in a transcribing dsRNA virus.

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S-Adenosyl-l-methionine/S-adenosyl-l-homocysteine was observed in the two MTases.

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A pathway was identified through which RNA reaches active sites of the two MTase.

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Methylation reactions require concerted efforts by turret protein monomers.

FTSJ2, a heat shock-inducible mitochondrial protein, suppresses cell invasion and migration

Ribosomal RNA large subunit methyltransferase J (RrmJ), an Escherichia coli heat shock protein, is responsible for 2′-O-ribose methylation in 23S rRNA. In mammals, three close homologs of RrmJ have been identified and have been designated as FTSJ1, FTSJ2 and FTSJ3; however, little is known about these genes. In this study, we characterized the mammalian FTSJ2, which was the most related protein to RrmJ in a phylogenetic analysis that had similar amino acid sequence features and tertiary protein structures of RrmJ. FTSJ2 was first identified in this study as a nucleus encoded mitochondrial protein that preserves the heat shock protein character in mammals in which the mRNA expressions was increased in porcine lung tissues and A549 cells after heat shock treatment. In addition, a recent study in non-small cell lung cancer (NSCLC) suggested that the FTSJ2 gene is located in a novel oncogenic locus. However, our results demonstrate that the expression of FTSJ2 mRNA was decreased in the more invasive subline (CL1-5) of the lung adenocarcinoma cells (CL1) compared with the less invasive subline (CL1-0), and overexpression of FTSJ2 resulted in the inhibition of cell invasion and migration in the rhabdomyosarcoma cell (TE671). In conclusion, our findings indicate that mammalian FTSJ2 is a mitochondrial ortholog of E. coli RrmJ and conserves the heat shock protein properties. Moreover, FTSJ2 possesses suppressive effects on the invasion and migration of cancer cells.

Domain-level rocking motion within a polymerase that translocates on single-stranded nucleic acid

Vaccinia virus poly(A) polymerase (VP55) is the only known polymerase that can translocate independently with respect to single-stranded nucleic acid (ssNA). Previously, its structure has only been solved in the context of the VP39 processivity factor. Here, a crystal structure of unliganded monomeric VP55 has been solved to 2.86 Å resolution, showing the first backbone structural isoforms among either VP55 or its processivity factor (VP39). Backbone differences between the two molecules of VP55 in the asymmetric unit indicated that unliganded monomeric VP55 can undergo a `rocking' motion of the N-terminal domain with respect to the other two domains, which may be `rigidified' upon VP39 docking. This observation is consistent with previously demonstrated experimental molecular dynamics of the monomer during translocation with respect to nucleic acid and with different mechanisms of translocation in the presence and absence of processivity factor VP39. Side-chain conformational changes in the absence of ligand were observed at a key primer contact site and at the catalytic center of VP55. The current structure completes the trio of possible structural forms for VP55 and VP39, namely the VP39 monomer, the VP39-VP55 heterodimer and the VP55 monomer.

Poly(A)-specific ribonuclease (PARN): an allosterically regulated, processive and mRNA cap-interacting deadenylase

Deadenylation of eukaryotic mRNA is a mechanism critical for mRNA function by influencing mRNA turnover and efficiency of protein synthesis. Here, we review poly(A)-specific ribonuclease (PARN), which is one of the biochemically best characterized deadenylases. PARN is unique among the currently known eukaryotic poly(A) degrading nucleases, being the only deadenylase that has the capacity to directly interact during poly(A) hydrolysis with both the m(7)G-cap structure and the poly(A) tail of the mRNA. In short, PARN is a divalent metal-ion dependent poly(A)-specific, processive and cap-interacting 3'-5' exoribonuclease that efficiently degrades poly(A) tails of eukaryotic mRNAs. We discuss in detail the mechanisms of its substrate recognition, catalysis, allostery and processive mode of action. On the basis of biochemical and structural evidence, we present and discuss a working model for PARN action. Models of regulation of PARN activity by trans-acting factors are discussed as well as the physiological relevance of PARN.

First structural evidence of sequestration of mRNA cap structures by type 1 ribosome inactivating protein from Momordica balsamina

This is the first structural evidence of recognition of mRNA cap structures by a ribosome inactivating protein. It is well known that a unique cap structure is formed at the 5' end of mRNA for carrying out various processes including mRNA maturation, translation initiation, and RNA turnover. The binding studies and crystal structure determinations of type 1 ribosome inactivating protein (RIP-1) from Momordica balsamina (MbRIP-1) were carried out with mRNA cap structures including (i) N7-methyl guanine (m7G), (ii) N7-methyl guanosine diphosphate (m7GDP), and (iii) N7-methyl guanosine triphosphate (m7GTP). These compounds showed affinities to MbRIP-1 at nanomolar concentrations. The structure determinations of the complexes of MbRIP-1 with m7G, m7GDP, and m7GTP at 2.65, 1.77, and 1.75 Å resolutions revealed that all the three compounds bound to MbRIP-1 in the substrate binding site at the positions which are slightly shifted towards Glu85 as compared to those of rRNA substrates. In this position, Glu85 forms several hydrogen bonds with guanine moiety while N-7 methyl group forms van der Waals contacts. However, the guanine rings are poorly stacked in these complexes. Thus, the mode of binding by MbRIP-1 to mRNA cap structures is different which results in the inhibition of depurination. Since some viruses are known to exploit the capping property of the host, this action of MbRIP-1 may have implications for the antiviral activity of this protein in vivo. The understanding of the mode of binding of MbRIP-1 to cap structures may also assist in the design of anti-viral agents.

eIF4E3 acts as a tumor suppressor by utilizing an atypical mode of methyl-7-guanosine cap recognition

Recognition of the methyl-7-guanosine (m(7)G) cap structure on mRNA is an essential feature of mRNA metabolism and thus gene expression. Eukaryotic translation initiation factor 4E (eIF4E) promotes translation, mRNA export, proliferation, and oncogenic transformation dependent on this cap-binding activity. eIF4E-cap recognition is mediated via complementary charge interactions of the positively charged m(7)G cap between the negative π-electron clouds from two aromatic residues. Here, we demonstrate that a variant subfamily, eIF4E3, specifically binds the m(7)G cap in the absence of an aromatic sandwich, using instead a different spatial arrangement of residues to provide the necessary electrostatic and van der Waals contacts. Contacts are much more extensive between eIF4E3-cap than other family members. Structural analyses of other cap-binding proteins indicate this recognition mode is atypical. We demonstrate that eIF4E3 relies on this cap-binding activity to act as a tumor suppressor, competing with the growth-promoting functions of eIF4E. In fact, reduced eIF4E3 in high eIF4E cancers suggests that eIF4E3 underlies a clinically relevant inhibitory mechanism that is lost in some malignancies. Taken together, there is more structural plasticity in cap recognition than previously thought, and this is physiologically relevant.

Computational studies on the substrate interactions of influenza A virus PB2 subunit

Influenza virus, which spreads around the world in seasonal epidemics and leads to large numbers of deaths every year, has several ribonucleoproteins in the central core of the viral particle. These viral ribonucleoproteins can specifically bind the conserved 3' and 5' caps of the viral RNAs with responsibility for replication and transcription of the viral RNA in the nucleus of infected cells. A fundamental question of most importance is that how the cap-binding proteins in the influenza virus discriminates between capped RNAs and non-capped ones. To get an answer, we performed molecular dynamics simulations and free energy calculations on the influenza A virus PB2 subunit, an important component of the RNP complexes, with a cap analog m7GTP. Our calculations showed that some key residues in the active site, such as Arg355, His357, Glu361 as well as Gln406, could offer significant hydrogen bonding and hydrophobic interactions with the guanine ring of the cap analog m7GTP to form an aromatic sandwich mechanism for the cap recognition and positioning in the active site. Subsequently, we applied this idea to a virtual screening procedure and identified 5 potential candidates that might be inhibitors against the PB2 subunit. Interestingly, 2 candidates Cpd1 and Cpd2 have been already reported to have inhibitory activities to the influenza virus cap-binding proteins. Further calculation also showed that they had comparatively higher binding affinities to the PB2 subunit than that of m7GTP. We believed that our findings could give an atomic insight into the deeper understanding of the cap recognition and binding mechanism, providing useful information for searching or designing novel drugs against influenza viruses.

2'-O methylation of internal adenosine by flavivirus NS5 methyltransferase

RNA modification plays an important role in modulating host-pathogen interaction. Flavivirus NS5 protein encodes N-7 and 2′-O methyltransferase activities that are required for the formation of 5′ type I cap (m⁷GpppAm) of viral RNA genome. Here we reported, for the first time, that flavivirus NS5 has a novel internal RNA methylation activity. Recombinant NS5 proteins of West Nile virus and Dengue virus (serotype 4; DENV-4) specifically methylates polyA, but not polyG, polyC, or polyU, indicating that the methylation occurs at adenosine residue. RNAs with internal adenosines substituted with 2′-O-methyladenosines are not active substrates for internal methylation, whereas RNAs with adenosines substituted with N⁶-methyladenosines can be efficiently methylated, suggesting that the internal methylation occurs at the 2′-OH position of adenosine. Mass spectroscopic analysis further demonstrated that the internal methylation product is 2′-O-methyladenosine. Importantly, genomic RNA purified from DENV virion contains 2′-O-methyladenosine. The 2′-O methylation of internal adenosine does not require specific RNA sequence since recombinant methyltransferase of DENV-4 can efficiently methylate RNAs spanning different regions of viral genome, host ribosomal RNAs, and polyA. Structure-based mutagenesis results indicate that K61-D146-K181-E217 tetrad of DENV-4 methyltransferase forms the active site of internal methylation activity; in addition, distinct residues within the methyl donor (S-adenosyl-L-methionine) pocket, GTP pocket, and RNA-binding site are critical for the internal methylation activity. Functional analysis using flavivirus replicon and genome-length RNAs showed that internal methylation attenuated viral RNA translation and replication. Polymerase assay revealed that internal 2′-O-methyladenosine reduces the efficiency of RNA elongation. Collectively, our results demonstrate that flavivirus NS5 performs 2′-O methylation of internal adenosine of viral RNA in vivo and host ribosomal RNAs in vitro.

We report that flavivirus NS5 has a novel internal RNA methylation activity. Recombinant proteins of NS5 and its N-terminal methyltransferase domain of West Nile virus and Dengue virus (DENV) specifically methylates polyA, but not polyG, polyC, or polyU. RNAs with internal adenosines substituted with 2′-O-methyladenosines are not active substrates for internal methylation, suggesting that the internal methylation occurs at the 2′-OH position of adenosine. Mass spectroscopic analysis confirmed that the internal methylation product is 2′-O-methyladenosine. Furthermore, the 2′-O-methyladenosine could also be detected in DENV genomic RNA. The 2′-O methylation of internal adenosine does not require specific RNA sequence context because the DENV methyltransferase can methylate RNAs spanning different regions of viral genome and host ribosomal RNAs at equal efficiencies. Mutagenesis analysis showed that K61-D146-K181-E217 motif of the DENV methyltransferase forms the active site of internal methylation activity; in addition, distinct residues on the surface of the enzyme are critical for the internal methylation activity. Functional analysis showed that internal methylation attenuated viral RNA translation and replication. Overall, our results demonstrate that flavivirus NS5 performs 2′-O methylation of internal adenosine of viral RNA in vivo and host ribosomal RNA in vitro. Such 2′-O-methyladenosine modification may modulate virus-host interaction.

Synthesis of 5' cap-0 and cap-1 RNAs using solid-phase chemistry coupled with enzymatic methylation by human (guanine-N⁷)-methyl transferase

The 5' end of eukaryotic mRNA carries a N(7)-methylguanosine residue linked by a 5'-5' triphosphate bond. This cap moiety ((7m)GpppN) is an essential RNA structural modification allowing its efficient translation, limiting its degradation by cellular 5' exonucleases and avoiding its recognition as "nonself" by the innate immunity machinery. In vitro synthesis of capped RNA is an important bottleneck for many biological studies. Moreover, the lack of methods allowing the synthesis of large amounts of RNA starting with a specific 5'-end sequence have hampered biological and structural studies of proteins recognizing the cap structure or involved in the capping pathway. Due to the chemical nature of N(7)-methylguanosine, the synthesis of RNAs possessing a cap structure at the 5' end is still a significant challenge. In the present work, we combined a chemical synthesis method and an enzymatic methylation assay in order to produce large amounts of RNA oligonucleotides carrying a cap-0 or cap-1. Short RNAs were synthesized on solid support by the phosphoramidite 2'-O-pivaloyloxymethyl chemistry. The cap structure was then coupled by the addition of GDP after phosphorylation of the terminal 5'-OH and activation by imidazole. After deprotection and release from the support, GpppN-RNAs or GpppN(2'-Om)-RNAs were purified before the N(7)-methyl group was added by enzymatic means using the human (guanine-N(7))-methyl transferase to yield (7m)GpppN-RNAs (cap-0) or (7m)GpppN(2'-Om)-RNAs (cap-1). The RNAs carrying different cap structures (cap, cap-0 or, cap-1) act as bona fide substrates mimicking cellular capped RNAs and can be used for biochemical and structural studies.

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.

Analysis of RNA binding by the dengue virus NS5 RNA capping enzyme

Flaviviruses are small, capped positive sense RNA viruses that replicate in the cytoplasm of infected cells. Dengue virus and other related flaviviruses have evolved RNA capping enzymes to form the viral RNA cap structure that protects the viral genome and directs efficient viral polyprotein translation. The N-terminal domain of NS5 possesses the methyltransferase and guanylyltransferase activities necessary for forming mature RNA cap structures. The mechanism for flavivirus guanylyltransferase activity is currently unknown, and how the capping enzyme binds its diphosphorylated RNA substrate is important for deciphering how the flavivirus guanylyltransferase functions. In this report we examine how flavivirus NS5 N-terminal capping enzymes bind to the 5′ end of the viral RNA using a fluorescence polarization-based RNA binding assay. We observed that the K_D for RNA binding is approximately 200 nM Dengue, Yellow Fever, and West Nile virus capping enzymes. Removal of one or both of the 5′ phosphates reduces binding affinity, indicating that the terminal phosphates contribute significantly to binding. RNA binding affinity is negatively affected by the presence of GTP or ATP and positively affected by S-adensyl methoninine (SAM). Structural superpositioning of the dengue virus capping enzyme with the Vaccinia virus VP39 protein bound to RNA suggests how the flavivirus capping enzyme may bind RNA, and mutagenesis analysis of residues in the putative RNA binding site demonstrate that several basic residues are critical for RNA binding. Several mutants show differential binding to 5′ di-, mono-, and un-phosphorylated RNAs. The mode of RNA binding appears similar to that found with other methyltransferase enzymes, and a discussion of diphosphorylated RNA binding is presented.

Biochemical and structural insights into the mechanisms of SARS coronavirus RNA ribose 2'-O-methylation by nsp16/nsp10 protein complex

The 5'-cap structure is a distinct feature of eukaryotic mRNAs, and eukaryotic viruses generally modify the 5'-end of viral RNAs to mimic cellular mRNA structure, which is important for RNA stability, protein translation and viral immune escape. SARS coronavirus (SARS-CoV) encodes two S-adenosyl-L-methionine (SAM)-dependent methyltransferases (MTase) which sequentially methylate the RNA cap at guanosine-N7 and ribose 2'-O positions, catalyzed by nsp14 N7-MTase and nsp16 2'-O-MTase, respectively. A unique feature for SARS-CoV is that nsp16 requires non-structural protein nsp10 as a stimulatory factor to execute its MTase activity. Here we report the biochemical characterization of SARS-CoV 2'-O-MTase and the crystal structure of nsp16/nsp10 complex bound with methyl donor SAM. We found that SARS-CoV nsp16 MTase methylated m7GpppA-RNA but not m7GpppG-RNA, which is in contrast with nsp14 MTase that functions in a sequence-independent manner. We demonstrated that nsp10 is required for nsp16 to bind both m7GpppA-RNA substrate and SAM cofactor. Structural analysis revealed that nsp16 possesses the canonical scaffold of MTase and associates with nsp10 at 1∶1 ratio. The structure of the nsp16/nsp10 interaction interface shows that nsp10 may stabilize the SAM-binding pocket and extend the substrate RNA-binding groove of nsp16, consistent with the findings in biochemical assays. These results suggest that nsp16/nsp10 interface may represent a better drug target than the viral MTase active site for developing highly specific anti-coronavirus drugs.

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 Ų 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.

A novel coronavirus emerged in 2003 and was identified as the etiological agent of the deadly disease called Severe Acute Respiratory Syndrome. This coronavirus replicates and transcribes its giant genome using sixteen non-structural proteins (nsp1-16). Viral RNAs are capped to ensure stability, efficient translation, and evading the innate immunity system of the host cell. The nsp16 protein is a RNA cap modifying enzyme only active in the presence of its activating partner nsp10. We have crystallized the nsp10/16 complex and report its crystal structure at atomic resolution. Nsp10 binds to nsp16 through a ∼930 Ų activation surface area in nsp10, and the resulting complex exhibits RNA cap (nucleoside-2′-O)-methyltransferase activity. We have performed mutational and functional assays to identify key residues involved in catalysis and/or in RNA binding, and in the association of nsp10 to nsp16. We present two additional crystal structures, that of the known inhibitor Sinefungin bound in the SAM binding pocket, and that of a tighter complex made of the mutant nsp10(Y96F) bound to nsp16. Our study provides a basis for antiviral drug design as well as the first structural insight into the regulation of RNA capping enzymes in (+)RNA viruses.

Identification of aromatic amino acid residues in conserved region VI of the large polymerase of vesicular stomatitis virus is essential for both guanine-N-7 and ribose 2'-O methyltransferases

Non-segmented negative-sense RNA viruses possess a unique mechanism for mRNA cap methylation. For vesicular stomatitis virus, conserved region VI in the large (L) polymerase protein catalyzes both guanine-N-7 (G-N-7) and ribose 2'-O (2'-O) methyltransferases, and the two methylases share a binding site for the methyl donor S-adenosyl-l-methionine. Unlike conventional mRNA cap methylation, the 2'-O methylation of VSV precedes subsequent G-N-7 methylation. In this study, we found that individual alanine substitutions in two conserved aromatic residues (Y1650 and F1691) in region VI of L protein abolished both G-N-7 and 2'-O methylation. However, replacement of one aromatic residue with another aromatic residue did not significantly affect the methyltransferase activities. Our studies provide genetic and biochemical evidence that conserved aromatic residues in region VI of L protein essential for both G-N-7 and 2'-O methylations. In combination with the structural prediction, our results suggest that these aromatic residues may participate in RNA recognition.