mRNA:guanine-N7 cap methyltransferases: identification of novel members of the family, evolutionary analysis, homology modeling, and analysis of sequence-structure-function relationships

Advances in non-viral mRNA delivery to the spleen

Developing safe and effective delivery strategies for localizing messenger RNA (mRNA) payloads to the spleen is an important goal in the field of genetic medicine. Accomplishing this goal is challenging due to the instability, size, and charge of mRNA payloads. Here, we provide an analysis of non-viral delivery technologies that have been developed to deliver mRNA payloads to the spleen. Specifically, our review begins by outlining the unique anatomy and potential targets for mRNA delivery within the spleen. Next, we describe approaches in mRNA sequence engineering that can be used to improve mRNA delivery to the spleen. Then, we describe advances in non-viral carrier systems that can package and deliver mRNA payloads to the spleen, highlighting key advances in the literature in lipid nanoparticle (LNP) and polymer nanoparticle (PNP) technology platforms. Finally, we provide commentary and outlook on how splenic mRNA delivery may afford next-generation treatments for autoimmune disorders and cancers. In undertaking this approach, our goal with this review is to both establish a fundamental understanding of drug delivery challenges associated with localizing mRNA payloads to the spleen, while also broadly highlighting the potential to use these genetic medicines to treat disease.

Epigenetic Signaling and RNA Regulation in Cardiovascular Diseases

RNA epigenetics is perhaps the most recent field of interest for translational epigeneticists. RNA modifications create such an extensive network of epigenetically driven combinations whose role in physiology and pathophysiology is still far from being elucidated. Not surprisingly, some of the players determining changes in RNA structure are in common with those involved in DNA and chromatin structure regulation, while other molecules seem very specific to RNA. It is envisaged, then, that new small molecules, acting selectively on RNA epigenetic changes, will be reported soon, opening new therapeutic interventions based on the correction of the RNA epigenetic landscape. In this review, we shall summarize some aspects of RNA epigenetics limited to those in which the potential clinical translatability to cardiovascular disease is emerging.

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.

Mutational analysis of vaccinia virus mRNA cap (guanine-N7) methyltransferase reveals essential contributions of the N-terminal peptide that closes over the active site

RNA guanine-N7 methyltransferase catalyzes the third step of eukaryal mRNA capping, the transfer of a methyl group from AdoMet to GpppRNA to form m(7)GpppRNA. Mutational and crystallographic analyses of cellular and poxvirus cap methyltransferases have yielded a coherent picture of a conserved active site and determinants of substrate specificity. Models of the Michaelis complex suggest a direct in-line mechanism of methyl transfer. Because no protein contacts to the guanine-N7 nucleophile, the AdoMet methyl carbon (Cepsilon) or the AdoHcy sulfur (Sdelta) leaving group were observed in ligand-bound structures of cellular cap methyltransferase, it was initially thought that the enzyme facilitates catalysis by optimizing proximity and geometry of the donor and acceptor. However, the structure of AdoHcy-bound vaccinia virus cap methyltransferase revealed the presence of an N-terminal "lid peptide" that closes over the active site and makes multiple contacts with the substrates, including the AdoMet sulfonium. This segment is disordered in the vaccinia apoenzyme and is not visible in the available structures of cellular cap methyltransferase. Here, we conducted a mutational analysis of the vaccinia virus lid peptide ((545)DKFRLNPEVSYFTNKRTRG(563)) entailing in vivo and in vitro readouts of the effects of alanine and conservative substitutions. We thereby identified essential functional groups that interact with the AdoMet sulfonium (Tyr555, Phe556), the AdoMet adenine (Asn550), and the cap triphosphate bridge (Arg560, Arg562). The results suggest that van der Waals contacts of Tyr555 and Phe556 to the AdoMet Sdelta and C epsilon atoms, and the electron-rich environment around the sulfonium, serve to stabilize the transition state of the transmethylation reaction.

Structure-function analysis of vaccinia virus mRNA cap (guanine-N7) methyltransferase

The guanine-N7 methyltransferase domain of vaccinia virus mRNA capping enzyme is a heterodimer composed of a catalytic subunit and a stimulatory subunit. Structure-function analysis of the catalytic subunit by alanine scanning and conservative substitutions (49 mutations at 25 amino acids) identified 12 functional groups essential for methyltransferase activity in vivo, most of which were essential for cap methylation in vitro. Defects in cap binding were demonstrated for a subset of lethal mutants that displayed residual activity in vitro. We discuss our findings in light of a model of the Michaelis complex derived from crystal structures of AdoHcy-bound vaccinia cap methyltransferase and GTP-bound cellular cap methyltransferase. The structure-function data yield a coherent picture of the vaccinia cap methyltransferase active site and the determinants of substrate specificity and affinity.

Modeling and experimental analyses reveal a two-domain structure and amino acids important for the activity of aminoglycoside resistance methyltransferase Sgm

Methyltransferases that carry out posttranscriptional N7-methylation of G1405 in 16S rRNA confer bacterial resistance to aminoglycoside antibiotics, including kanamycin and gentamicin. Genes encoding enzymes from this family (hereafter referred to as Arm, for aminoglycoside resistance methyltransferases) have been recently found to spread by horizontal gene transfer between various human pathogens. The knowledge of the Arm protein structure would lay the groundwork for the development of potential resistance inhibitors, which could be used to restore the potential of aminoglycosides to act against the resistant pathogens. We analyzed the sequence-function relationships of Sgm MTase, a member of the Arm family, by limited proteolysis and site-directed and random mutagenesis. We also modeled the structure of Sgm using bioinformatics techniques and used the model to provide a structural context for experimental results. We found that Sgm comprises two domains and we characterized a number of functionally compromised point mutants with substitutions of invariant or conserved residues. Our study provides a low-resolution (residue-level) model of sequence-structure-function relationships in the Arm family of enzymes and reveals the cofactor-binding and substrate-binding sites. These functional regions will be prime targets for further experimental and theoretical studies aimed at defining the reaction mechanism of m7 G1405 methylation, increasing the resolution of the model and developing Arm-specific inhibitors.

Structural bases for substrate recognition and activity in Meaban virus nucleoside-2'-O-methyltransferase

Viral methyltransferases are involved in the mRNA capping process, resulting in the transfer of a methyl group from S-adenosyl-L-methionine to capped RNA. Two groups of methyltransferases (MTases) are known: (guanine-N7)-methyltransferases (N7MTases), adding a methyl group onto the N7 atom of guanine, and (nucleoside-2'-O-)-methyltransferases (2'OMTases), adding a methyl group to a ribose hydroxyl. We have expressed and purified two constructs of Meaban virus (MV; genus Flavivirus) NS5 protein MTase domain (residues 1-265 and 1-293, respectively). We report here the three-dimensional structure of the shorter MTase construct in complex with the cofactor S-adenosyl-L-methionine, at 2.9 angstroms resolution. Inspection of the refined crystal structure, which highlights structural conservation of specific active site residues, together with sequence analysis and structural comparison with Dengue virus 2'OMTase, suggests that the crystallized enzyme belongs to the 2'OMTase subgroup. Enzymatic assays show that the short MV MTase construct is inactive, but the longer construct expressed can transfer a methyl group to the ribose 2'O atom of a short GpppAC(5) substrate. West Nile virus MTase domain has been recently shown to display both N7 and 2'O MTase activity on a capped RNA substrate comprising the 5'-terminal 190 nt of the West Nile virus genome. The lack of N7 MTase activity here reported for MV MTase may be related either to the small size of the capped RNA substrate, to its sequence, or to different structural properties of the C-terminal regions of West Nile virus and MV MTase-domains.

Molecular phylogenetics and comparative modeling of HEN1, a methyltransferase involved in plant microRNA biogenesis

Background

Recently, HEN1 protein from Arabidopsis thaliana was discovered as an essential enzyme in plant microRNA (miRNA) biogenesis. HEN1 transfers a methyl group from S-adenosylmethionine to the 2'-OH or 3'-OH group of the last nucleotide of miRNA/miRNA* duplexes produced by the nuclease Dicer. Previously it was found that HEN1 possesses a Rossmann-fold methyltransferase (RFM) domain and a long N-terminal extension including a putative double-stranded RNA-binding motif (DSRM). However, little is known about the details of the structure and the mechanism of action of this enzyme, and about its phylogenetic origin.

Results

Extensive database searches were carried out to identify orthologs and close paralogs of HEN1. Based on the multiple sequence alignment a phylogenetic tree of the HEN1 family was constructed. The fold-recognition approach was used to identify related methyltransferases with experimentally solved structures and to guide the homology modeling of the HEN1 catalytic domain. Additionally, we identified a La-like predicted RNA binding domain located C-terminally to the DSRM domain and a domain with a peptide prolyl cis/trans isomerase (PPIase) fold, but without the conserved PPIase active site, located N-terminally to the catalytic domain.

Conclusion

The bioinformatics analysis revealed that the catalytic domain of HEN1 is not closely related to any known RNA:2'-OH methyltransferases (e.g. to the RrmJ/fibrillarin superfamily), but rather to small-molecule methyltransferases. The structural model was used as a platform to identify the putative active site and substrate-binding residues of HEN and to propose its mechanism of action.

Cell-based assays to detect inhibitors of fungal mRNA capping enzymes and characterization of sinefungin as a cap methyltransferase inhibitor

The m7GpppN cap at the 5' end of eukaryotic mRNAs is important for transcript stability and translation. Three enzymatic activities that generate the mRNA cap include an RNA 5'-triphosphatase, an RNA guanylyltransferase, and an RNA (guanine-7-) -methyltransferase. The physical organization of the genes encoding these enzymes differs between mammalian cells and yeast, fungi, or viruses. The catalytic mechanism used by the RNA triphosphatases of mammalian cells also differs from that used by the yeast, fungal, or viral enzymes. These structural and functional differences suggest that inhibitors of mRNA capping might be useful antifungal or antiviral agents. The authors describe several whole-cell yeast-based assays developed to identify and characterize inhibitors of fungal mRNA capping. They also report the identification and characterization of the natural product sinefungin in the assays. Their characterization of this S-adenosylmethionine analog suggests that it inhibits mRNA cap methyltransferases and exhibits approximately 5- to 10-fold specificity for the yeast ABD1 and fungal CCM1 enzymes over the human Hcm1 enzyme expressed in yeast cells.

Improving sequence-based fold recognition by using 3D model quality assessment

MOTIVATION

The ability of a simple method (MODCHECK) to determine the sequence-structure compatibility of a set of structural models generated by fold recognition is tested in a thorough benchmark analysis. Four Model Quality Assessment Programs (MQAPs) were tested on 188 targets from the latest LiveBench-9 automated structure evaluation experiment. We systematically test and evaluate whether the MQAP methods can successfully detect native-like models.

RESULTS

We show that compared with the other three methods tested MODCHECK is the most reliable method for consistently performing the best top model selection and for ranking the models. In addition, we show that the choice of model similarity score used to assess a model's similarity to the experimental structure can influence the overall performance of these tools. Although these MQAP methods fail to improve the model selection performance for methods that already incorporate protein three dimension (3D) structural information, an improvement is observed for methods that are purely sequence-based, including the best profile-profile methods. This suggests that even the best sequence-based fold recognition methods can still be improved by taking into account the 3D structural information.

CONTACT

d.jones@cs.ucl.ac.uk

Sequence-structure-function relationships of a tRNA (m7G46) methyltransferase studied by homology modeling and site-directed mutagenesis

The Escherichia coli TrmB protein and its Saccharomyces cerevisiae ortholog Trm8p catalyze the S-adenosyl-L-methionine-dependent formation of 7-methylguanosine at position 46 (m7G46) in tRNA. To learn more about the sequence-structure-function relationships of these enzymes we carried out a thorough bioinformatics analysis of the tRNA:m7G methyltransferase (MTase) family to predict sequence regions and individual amino acid residues that may be important for the interactions between the MTase and the tRNA substrate, in particular the target guanosine 46. We used site-directed mutagenesis to construct a series of alanine substitutions and tested the activity of the mutants to elucidate the catalytic and tRNA-recognition mechanism of TrmB. The functional analysis of the mutants, together with the homology model of the TrmB structure and the results of the phylogenetic analysis, revealed the crucial residues for the formation of the substrate-binding site and the catalytic center in tRNA:m7G MTases.

Yeast-like mRNA capping apparatus in Giardia lamblia

A scheme of eukaryotic phylogeny has been suggested based on the structure and physical linkage of the RNA triphosphatase and RNA guanylyltransferase enzymes that catalyze mRNA cap formation. Here we show that the unicellular pathogen Giardia lamblia encodes an mRNA capping apparatus consisting of separate triphosphatase and guanylyltransferase components, which we characterize biochemically. We also show that native Giardia mRNAs have blocked 5'-ends and that 7-methylguanosine caps promote translation of transfected mRNAs in Giardia in vivo. The Giardia triphosphatase belongs to the tunnel family of metal-dependent phosphohydrolases that includes the RNA triphosphatases of fungi, microsporidia, and protozoa such as Plasmodium and Trypanosoma. The tunnel enzymes adopt a unique active-site fold and are structurally and mechanistically unrelated to the cysteine-phosphatase-type RNA triphosphatases found in metazoans and plants, which comprise part of a bifunctional triphosphataseguanylyltransferase fusion protein. All available evidence now points to the separate tunnel-type triphosphatase and guanylyltransferase as the aboriginal state of the capping apparatus. We identify a putative tunnel-type triphosphatase and a separate guanylyltransferase encoded by the red alga Cyanidioschyzon merolae. These findings place fungi, protozoa, and red algae in a common lineage distinct from that of metazoa and plants.

Enzyme mechanisms for sterol C-methylations

The mechanisms by which sterol methyl transferases (SMT) transform olefins into structurally different C-methylated products are complex, prompting over 50 years of intense research. Recent enzymological studies, together with the latest discoveries in the fossil record, functional analyses and gene cloning, establish new insights into the enzymatic mechanisms of sterol C-methylation and form a basis for understanding regulation and evolution of the sterol pathway. These studies suggest that SMTs, originated shortly after life appeared on planet earth. SMTs, including those which ultimately give rise to 24 alpha- and 24 beta-alkyl sterols, align the si(beta)-face pi-electrons of the Delta(24)-double bond with the S-methyl group of AdoMet relative to a set of deprotonation bases in the active site. From the orientation of the conformationally flexible side chain in the SMT Michaelis complex, it has been found that either a single product is formed or cationic intermediates are partitioned into multiple olefins. The product structure and stereochemistry of SMT action is phylogenetically distinct and physiologically significant. SMTs control phytosterol homeostasis and their activity is subject to feedback regulation by specific sterol inserts in the membrane. A unified conceptual framework has been formulated in the steric-electric plug model that posits SMT substrate acceptability on the generation of single or double 24-alkylated side chains, which is the basis for binding order, stereospecificity and product diversity in this class of AdoMet-dependent methyl transferase enzymes. The focus of this review is the mechanism of the C-methylation process which, as discussed, can be altered by point mutations in the enzyme to direct the shape of sterol structure to optimize function.

Sequence-structure-function relationships of Tgs1, the yeast snRNA/snoRNA cap hypermethylase

The Saccharomyces cerevisiae Tgs1 methyltransferase (MTase) is responsible for conversion of the m(7)G caps of snRNAs and snoRNAs to a 2,2,7- trimethylguanosine structure. To learn more about the evolutionary origin of Tgs1 and to identify structural features required for its activity, we performed a structure-function study. By using sequence comparison and phylogenetic analysis, we found that Tgs1 shows strongest similarity to Mj0882, a protein related to a family comprised of bacterial rRNA:m(2)G MTases RsmC and RsmD. The structural information of Mj0882 was used to build a homology model of Tgs1p which allowed us to predict the range of the minimal globular MTase domain and the localization of other residues that may be important for enzyme function. To further characterize functional domains of Tgs1, mutants were constructed and tested for their effects on cell viability, subcellular localization and binding to the small nuclear ribonucleoproteins (snRNPs) and small nucleolar RNPs (snoRNPs). We found that the N-terminal domain of the hypermethylase is dispensable for binding to the common snRNPs and snoRNPs proteins but essential for correct nucleolar localization. Site- directed mutagenesis of Tgs1 allowed also the identification of the residues likely to be involved in the formation of the m7G-binding site and the catalytic center.

Yeast-based genetic system for functional analysis of poxvirus mRNA cap methyltransferase

Structural differences between poxvirus and human mRNA capping enzymes recommend cap formation as a target for antipoxviral drug discovery. Genetic and pharmacologic analysis of the poxvirus capping enzymes requires in vivo assays in which the readout depends on the capacity of the viral enzyme to catalyze cap synthesis. Here we have used the budding yeast Saccharomyces cerevisiae as a genetic model for the study of poxvirus cap guanine-N7 methyltransferase. The S. cerevisiae capping system consists of separate triphosphatase (Cet1), guanylyltransferase (Ceg1), and methyltransferase (Abd1) components. All three activities are essential for cell growth. We report that the methyltransferase domain of vaccinia virus capping enzyme (composed of catalytic vD1-C and stimulatory vD12 subunits) can function in lieu of yeast Abd1. Coexpression of both vaccinia virus subunits is required for complementation of the growth of abd1Delta cells. Previously described mutations of vD1-C and vD12 that eliminate or reduce methyltransferase activity in vitro either abolish abd1Delta complementation or elicit conditional growth defects. We have used the yeast complementation assay as the primary screen in a new round of alanine scanning of the catalytic subunit. We thereby identified several new amino acids that are critical for cap methylation activity in vivo. Studies of recombinant proteins show that the lethal vD1-C mutations do not preclude heterodimerization with vD12 but either eliminate or reduce cap methyltransferase activity in vitro.

The yggH gene of Escherichia coli encodes a tRNA (m7G46) methyltransferase

We cloned, expressed, and purified the Escherichia coli YggH protein and show that it catalyzes the S-adenosyl-L-methionine-dependent formation of N(7)-methylguanosine at position 46 (m(7)G46) in tRNA. Additionally, we generated an E. coli strain with a disrupted yggH gene and show that the mutant strain lacks tRNA (m(7)G46) methyltransferase activity.

An RNA cap (nucleoside-2'-O-)-methyltransferase in the flavivirus RNA polymerase NS5: crystal structure and functional characterization

Viruses represent an attractive system with which to study the molecular basis of mRNA capping and its relation to the RNA transcription machinery. The RNA-dependent RNA polymerase NS5 of flaviviruses presents a characteristic motif of S-adenosyl-L-methionine-dependent methyltransferases at its N-terminus, and polymerase motifs at its C-terminus. The crystal structure of an N-terminal fragment of Dengue virus type 2 NS5 is reported at 2.4 A resolution. We show that this NS5 domain includes a typical methyltransferase core and exhibits a (nucleoside-2'-O-)-methyltransferase activity on capped RNA. The structure of a ternary complex comprising S-adenosyl-L-homocysteine and a guanosine triphosphate (GTP) analogue shows that 54 amino acids N-terminal to the core provide a novel GTP-binding site that selects guanine using a previously unreported mechanism. Binding studies using GTP- and RNA cap-analogues, as well as the spatial arrangement of the methyltransferase active site relative to the GTP-binding site, suggest that the latter is a specific cap-binding site. As RNA capping is an essential viral function, these results provide a structural basis for the rational design of drugs against the emerging flaviviruses.

Viral RNA-polymerases -- a predicted 2'-O-ribose methyltransferase domain shared by all Mononegavirales

The Mononegavirales virus group comprises several major human pathogens, including measles, rabies and Ebola viruses. This article reports a computational analysis of the C-terminal region of RNA-dependent RNA-polymerases from Mononegavirales. Using a combination of sequence similarity and threading analysis, a 2'-O-ribose methyltransferase domain was identified that is involved in the capping of viral mRNAs.

RNA:(guanine-N2) methyltransferases RsmC/RsmD and their homologs revisited--bioinformatic analysis and prediction of the active site based on the uncharacterized Mj0882 protein structure

BACKGROUND

Escherichia coli guanine-N2 (m2G) methyltransferases (MTases) RsmC and RsmD modify nucleosides G1207 and G966 of 16S rRNA. They possess a common MTase domain in the C-terminus and a variable region in the N-terminus. Their C-terminal domain is related to the YbiN family of hypothetical MTases, but nothing is known about the structure or function of the N-terminal domain.

RESULTS

Using a combination of sequence database searches and fold recognition methods it has been demonstrated that the N-termini of RsmC and RsmD are related to each other and that they represent a "degenerated" version of the C-terminal MTase domain. Novel members of the YbiN family from Archaea and Eukaryota were also indentified. It is inferred that YbiN and both domains of RsmC and RsmD are closely related to a family of putative MTases from Gram-positive bacteria and Archaea, typified by the Mj0882 protein from M. jannaschii (1dus in PDB). Based on the results of sequence analysis and structure prediction, the residues involved in cofactor binding, target recognition and catalysis were identified, and the mechanism of the guanine-N2 methyltransfer reaction was proposed.

CONCLUSIONS

Using the known Mj0882 structure, a comprehensive analysis of sequence-structure-function relationships in the family of genuine and putative m2G MTases was performed. The results provide novel insight into the mechanism of m2G methylation and will serve as a platform for experimental analysis of numerous uncharacterized N-MTases.

Characterization of the mRNA capping apparatus of the microsporidian parasite Encephalitozoon cuniculi

A scheme of eukaryotic phylogeny has been suggested based on the structure and physical linkage of the enzymes that catalyze mRNA cap formation. Here we show that the intracellular parasite Encephalitozoon cuniculi encodes a complete mRNA capping apparatus consisting of separate triphosphatase (EcCet1), guanylyltransferase (EcCeg1), and methyltransferase (Ecm1) enzymes, which we characterize biochemically and genetically. The triphosphatase EcCet1 belongs to a metal-dependent phosphohydrolase family that includes the triphosphatase components of the capping apparatus of fungi, DNA viruses, and the malaria parasite Plasmodium falciparum. These enzymes are structurally and mechanistically unrelated to the metal-independent cysteine phosphatase-type RNA triphosphatases found in metazoans and plants. Our findings support the proposed evolutionary connection between microsporidia and fungi, and they place fungi and protozoa in a common lineage distinct from that of metazoans and plants. RNA triphosphatase presents an attractive target for antiprotozoal/antifungal drug development.

In silico analysis of the tRNA:m1A58 methyltransferase family: homology-based fold prediction and identification of new members from Eubacteria and Archaea

The amino acid sequences of Gcd10p and Gcd14p, the two subunits of the tRNA:(1-methyladenosine-58; m(1)A58) methyltransferase (MTase) of Saccharomyces cerevisiae, have been analyzed using iterative sequence database searches and fold recognition programs. The results suggest that the 'catalytic' Gcd14p and 'substrate binding' Gcd10p are related to each other and to a group of prokaryotic open reading frames, which were previously annotated as hypothetical protein isoaspartate MTases in sequence databases. It is predicted that the prokaryotic proteins are genuine tRNA:m(1)A MTases based on similarity of their predicted active site to the Gcd14p family. In addition to the MTase domain, an additional domain was identified in the N-terminus of all these proteins that may be involved in interaction with tRNA. These results suggest that the eukaryotic tRNA:m(1)A58 MTase is a product of gene duplication and divergent evolution of a possibly homodimeric prokaryotic enzyme.

Reassignment of specificities of two cap methyltransferase domains in the reovirus lambda 2 protein

BACKGROUND

The reovirus lambda2 protein catalyzes mRNA capping, that is, addition of a guanosine to the 5' end of each transcript in a 5'-to-5' orientation, as well as transfer of a methyl group from S-adenosyl-L-methionine (AdoMet) to the N7 atom of the added guanosyl moiety and subsequently to the ribose 2'-O atom of the first template-encoded nucleotide. The structure of the human reovirus core has been solved at 3.6 A resolution, revealing a series of domains that include a putative guanylyltransferase domain and two putative methyltransferase (MTase) domains. It has been suggested that the order of domains in the lambda2 protein corresponds to the order of reactions in the pathway and that the m7G (cap 0) and the 2'-O-ribose (cap 1) MTase activities may be exerted by the MTase 1 and the MTase 2 domains, respectively.

RESULTS

We show that the reovirus MTase 1 domain shares a putative active site with the structurally characterized 2'-O-ribose MTases, including vaccinia virus cap 1 MTase, whereas the MTase 2 domain is structurally similar to glycine N-MTase.

CONCLUSIONS

On the basis of our analysis of the structural details we propose that the previously suggested functional assignments of the MTase 1 and MTase 2 domains should be swapped.