Yeast mRNA cap methyltransferase is a 50-kilodalton protein encoded by an essential gene

eIF4E orchestrates mRNA processing, RNA export and translation to modify specific protein production

The eukaryotic translation initiation factor eIF4E acts as a multifunctional factor that simultaneously influences mRNA processing, export, and translation in many organisms. Its multifactorial effects are derived from its capacity to bind to the methyl-7-guanosine cap on the 5'end of mRNAs and thus can act as a cap chaperone for transcripts in the nucleus and cytoplasm. In this review, we describe the multifactorial roles of eIF4E in major mRNA-processing events including capping, splicing, cleavage and polyadenylation, nuclear export and translation. We discuss the evidence that eIF4E acts at two levels to generate widescale changes to processing, export and ultimately the protein produced. First, eIF4E alters the production of components of the mRNA processing machinery, supporting a widescale reprogramming of multiple mRNA processing events. In this way, eIF4E can modulate mRNA processing without physically interacting with target transcripts. Second, eIF4E also physically interacts with both capped mRNAs and components of the RNA processing or translation machineries. Further, specific mRNAs are sensitive to eIF4E only in particular mRNA processing events. This selectivity is governed by the presence of cis-acting elements within mRNAs known as USER codes that recruit relevant co-factors engaging the appropriate machinery. In all, we describe the molecular bases for eIF4E's multifactorial function and relevant regulatory pathways, discuss the basis for selectivity, present a compendium of ~80 eIF4E-interacting factors which play roles in these activities and provide an overview of the relevance of its functions to its oncogenic potential. Finally, we summarize early-stage clinical studies targeting eIF4E in cancer.

Biochemical characterization of mRNA capping enzyme from Faustovirus

The mammalian mRNA 5' cap structures play important roles in cellular processes such as nuclear export, efficient translation, and evading cellular innate immune surveillance and regulating 5'-mediated mRNA turnover. Hence, installation of the proper 5' cap is crucial in therapeutic applications of synthetic mRNA. The core 5' cap structure, Cap-0, is generated by three sequential enzymatic activities: RNA 5' triphosphatase, RNA guanylyltransferase, and cap N7-guanine methyltransferase. Vaccinia virus RNA capping enzyme (VCE) is a heterodimeric enzyme that has been widely used in synthetic mRNA research and manufacturing. The large subunit of VCE D1R exhibits a modular structure where each of the three structural domains possesses one of the three enzyme activities, whereas the small subunit D12L is required to activate the N7-guanine methyltransferase activity. Here, we report the characterization of a single-subunit RNA capping enzyme from an amoeba giant virus. Faustovirus RNA capping enzyme (FCE) exhibits a modular array of catalytic domains in common with VCE and is highly efficient in generating the Cap-0 structure without an activation subunit. Phylogenetic analysis suggests that FCE and VCE are descended from a common ancestral capping enzyme. We found that compared to VCE, FCE exhibits higher specific activity, higher activity toward RNA containing secondary structures and a free 5' end, and a broader temperature range, properties favorable for synthetic mRNA manufacturing workflows.

Characterization, Directed Evolution, and Targeting of DNA Virus-Encoded RNA Capping Enzymes Using Phenotypic Yeast Platforms

The constant and the sudden emergence of zoonotic human and animal viruses is a significant threat to human health, the world economy, and the world food supply. This has necessitated the development of broad-spectrum therapeutic strategies to combat these emerging pathogens. Mechanisms that are essential for viral replication and propagation have been successfully targeted in the past to develop broad-spectrum therapeutics that can be readily repurposed to combat new zoonotic pathogens. Because of the importance of viral RNA capping enzymes to viral replication and pathogenesis, as well as their presence in both DNA and RNA viruses, these viral proteins have been a long-standing therapeutic target. Here, we use genome sequencing information and yeast-based platforms (YeRC0M) to identify, characterize, and target viral genome-encoded essential RNA capping enzymes from emerging strains of DNA viruses, i.e., Monkeypox virus and African Swine Fever Virus, which are a significant threat to human and domestic animal health. We first identified and biochemically characterized these viral RNA capping enzymes and their necessary protein domains. We observed significant differences in functional protein domains and organization for RNA capping enzymes from emerging DNA viruses in comparison to emerging RNA viruses. We also observed several differences in the biochemical properties of these viral RNA capping enzymes using our phenotypic yeast-based approaches (YeRC0M) as compared to the previous in vitro studies. Further, using directed evolution, we were able to identify inactivation and attenuation mutations in these essential viral RNA capping enzymes; these data could have implications on virus biocontainment as well as live attenuated vaccine development. We also developed methods that would facilitate high-throughput phenotypic screening to identify broad-spectrum inhibitors that selectively target viral RNA capping enzymes over host RNA capping enzymes. As demonstrated here, our approaches to identify, characterize, and target viral genome-encoded essential RNA capping enzymes are highly modular and can be readily adapted for targeting emerging viral pathogens as well as their variants that emerge in the future.

RNA modifications in cancer

Currently, more than 170 modifications have been identified on RNA. Among these RNA modifications, various methylations account for two-thirds of total cases and exist on almost all RNAs. Roles of RNA modifications in cancer are garnering increasing interest. The research on m6A RNA methylation in cancer is in full swing at present. However, there are still many other popular RNA modifications involved in the regulation of gene expression post-transcriptionally besides m6A RNA methylation. In this review, we focus on several important RNA modifications including m1A, m5C, m7G, 2'-O-Me, Ψ and A-to-I editing in cancer, which will provide a new perspective on tumourigenesis by peeking into the complex regulatory network of epigenetic RNA modifications, transcript processing, and protein translation.

Profiling The Compendium Of Changes In Saccharomyces cerevisiae Due To Mutations That Alter Availability Of The Main Methyl Donor S-Adenosylmethionine

The SAM1 and SAM2 genes encode for S-AdenosylMethionine (AdoMet) synthetase enzymes, with AdoMet serving as the main methyl donor. We have previously shown that independent deletion of these genes alters chromosome stability and AdoMet concentrations in opposite ways in S. cerevisiae. To characterize other changes occurring in these mutants, we grew wildtype, sam1∆/sam1∆, and sam2∆/sam2∆ strains in 15 different Phenotypic Microarray plates with different components, equal to 1440 wells, and measured for growth variations. RNA-Sequencing was also carried out on these strains and differential gene expression determined for each mutant. In this study, we explore how the phenotypic growth differences are linked to the altered gene expression, and thereby predict the mechanisms by which loss of the SAM genes and subsequent AdoMet level changes, impact S. cerevisiae pathways and processes. We present six stories, discussing changes in sensitivity or resistance to azoles, cisplatin, oxidative stress, arginine biosynthesis perturbations, DNA synthesis inhibitors, and tamoxifen, to demonstrate the power of this novel methodology to broadly profile changes due to gene mutations. The large number of conditions that result in altered growth, as well as the large number of differentially expressed genes with wide-ranging functionality, speaks to the broad array of impacts that altering methyl donor abundance can impart, even when the conditions tested were not specifically selected as targeting known methyl involving pathways. Our findings demonstrate that some cellular changes are directly related to AdoMet-dependent methyltransferases and AdoMet availability, some are directly linked to the methyl cycle and its role is production of several important cellular components, and others reveal impacts of SAM gene mutations on previously unconnected pathways.

Synthetic Platforms for Characterizing and Targeting of SARS-CoV-2 Genome Capping Enzymes

Essential viral enzymes have been successfully targeted to combat the diseases caused by emerging pathogenic RNA viruses (e.g., viral RNA-dependent RNA polymerase). Because of the conserved nature of such viral enzymes, therapeutics targeting these enzymes have the potential to be repurposed to combat emerging diseases, e.g., remdesivir, which was initially developed as a potential Ebola treatment, then was repurposed for COVID-19. Our efforts described in this study target another essential and highly conserved, but relatively less explored, step in RNA virus translation and replication, i.e., capping of the viral RNA genome. The viral genome cap structure disguises the genome of most RNA viruses to resemble the mRNA cap structure of their host and is essential for viral translation, propagation, and immune evasion. Here, we developed a synthetic, phenotypic yeast-based complementation platform (YeRC0M) for molecular characterization and targeting of SARS-CoV-2 genome-encoded RNA cap-0 (guanine-N7)-methyltransferase (N7-MTase) enzyme (nsp14). In YeRC0M, the lack of yeast mRNA capping N7-MTase in yeast, which is an essential gene in yeast, is complemented by the expression of functional viral N7-MTase or its variants. Using YeRC0M, we first identified important protein domains and amino acid residues that are essential for SARS-CoV-2 nsp14 N7-MTase activity. We also expanded YeRC0M to include key nsp14 variants observed in emerging variants of SARS-CoV-2 (e.g., delta variant of SARS-CoV-2 encodes nsp14 A394V and nsp14 P46L). We also combined YeRC0M with directed evolution to identify attenuation mutations in SARS-CoV-2 nsp14. Because of the high sequence similarity of nsp14 in emerging coronaviruses, these observations could have implications on live attenuated vaccine development strategies. These data taken together reveal key domains in SARS-CoV-2 nsp14 that can be targeted for therapeutic strategies. We also anticipate that these readily tractable phenotypic platforms can also be used for the identification of inhibitors of viral RNA capping enzymes as antivirals.

Towards T7 RNA polymerase (T7RNAP)-based expression system in yeast: challenges and opportunities

During the last decades, we have witnessed unprecedented advances in biological engineering and synthetic biology. These disciplines aim to take advantage of gene pathway regulation and gene expression in different organisms, to enable cells to perform desired functions. Yeast has been widely utilized as a model for the study of eukaryotic protein expression while bacteriophage T7RNAP and its promoter constitute the preferred system for prokaryotic protein expression (such as pET-based expression systems). The ability to integrate a T7RNAP-based expression system in yeast could allow for a better understanding of gene regulation in eukaryotic cells, and potentially increase the efficiency and processivity of yeast as an expression system. However, the attempts for the creation of such a system have been unsuccessful to date. This review aims to: (i) summarize the efforts that, for many years, have been devoted to the creation of a T7RNAP-based yeast expression system and ii) provide an overview of the latest advances in knowledge of eukaryotic transcription and translation that could lead to the construction of a successful T7RNAP expression system in yeast. The completion of this new expression system would allow to further expand the toolkit of yeast in synthetic biology and ultimately contribute to boost yeast usage as a key cell factory in sustainable biorefinery and circular economy.

Anomalous HIV-1 RNA, How Cap-Methylation Segregates Viral Transcripts by Form and Function

The acquisition of m7G-cap-binding proteins is now recognized as a major variable driving the form and function of host RNAs. This manuscript compares the 5'-cap-RNA binding proteins that engage HIV-1 precursor RNAs, host mRNAs, small nuclear (sn)- and small nucleolar (sno) RNAs and sort into disparate RNA-fate pathways. Before completion of the transcription cycle, the transcription start site of nascent class II RNAs is appended to a non-templated guanosine that is methylated (m7G-cap) and bound by hetero-dimeric CBP80-CBP20 cap binding complex (CBC). The CBC is a nexus for the co-transcriptional processing of precursor RNAs to mRNAs and the snRNA and snoRNA of spliceosomal and ribosomal ribonucleoproteins (RNPs). Just as sn/sno-RNAs experience hyper-methylation of m7G-cap to trimethylguanosine (TMG)-cap, so do select HIV RNAs and an emerging cohort of mRNAs. TMG-cap is blocked from Watson:Crick base pairing and disqualified from participating in secondary structure. The HIV TMG-cap has been shown to license select viral transcripts for specialized cap-dependent translation initiation without eIF4E that is dependent upon CBP80/NCBP3. The exceptional activity of HIV precursor RNAs secures their access to maturation pathways of sn/snoRNAs, canonical and non-canonical host mRNAs in proper stoichiometry to execute the retroviral replication cycle.

Identification and characterization of the interaction between the methyl-7-guanosine cap maturation enzyme RNMT and the cap-binding protein eIF4E

The control of RNA metabolism is an important aspect of molecular biology with wide-ranging impacts on cells. Central to processing of coding RNAs is the addition of the methyl-7 guanosine (m⁷G) "cap" on their 5' end. The eukaryotic translation initiation factor eIF4E directly binds the m⁷G cap and through this interaction plays key roles in many steps of RNA metabolism including nuclear RNA export and translation. eIF4E also stimulates capping of many transcripts through its ability to drive the production of the enzyme RNMT which methylates the G-cap to form the mature m⁷G cap. Here, we found that eIF4E also physically associated with RNMT in human cells. Moreover, eIF4E directly interacted with RNMT in vitro. eIF4E is only the second protein reported to directly bind the methyltransferase domain of RNMT, the first being its co-factor RAM. We combined high-resolution NMR methods with biochemical studies to define the binding interfaces for the RNMT-eIF4E complex. Further, we found that eIF4E competes for RAM binding to RNMT and conversely, RNMT competes for binding of well-established eIF4E-binding partners such as the 4E-BPs. RNMT uses novel structural means to engage eIF4E. Finally, we observed that m⁷G cap-eIF4E-RNMT trimeric complexes form, and thus RNMT-eIF4E complexes may be employed so that eIF4E captures newly capped RNA. In all, we show for the first time that the cap-binding protein eIF4E directly binds to the cap-maturation enzyme RNMT.

The Cap-Binding Complex CBC and the Eukaryotic Translation Factor eIF4E: Co-Conspirators in Cap-Dependent RNA Maturation and Translation

The translation of RNA into protein is a dynamic process which is heavily regulated during normal cell physiology and can be dysregulated in human malignancies. Its dysregulation can impact selected groups of RNAs, modifying protein levels independently of transcription. Integral to their suitability for translation, RNAs undergo a series of maturation steps including the addition of the m7G cap on the 5' end of RNAs, splicing, as well as cleavage and polyadenylation (CPA). Importantly, each of these steps can be coopted to modify the transcript signal. Factors that bind the m7G cap escort these RNAs through different steps of maturation and thus govern the physical nature of the final transcript product presented to the translation machinery. Here, we describe these steps and how the major m7G cap-binding factors in mammalian cells, the cap binding complex (CBC) and the eukaryotic translation initiation factor eIF4E, are positioned to chaperone transcripts through RNA maturation, nuclear export, and translation in a transcript-specific manner. To conceptualize a framework for the flow and integration of this genetic information, we discuss RNA maturation models and how these integrate with translation. Finally, we discuss how these processes can be coopted by cancer cells and means to target these in malignancy.

To cap it all off, again: dynamic capping and recapping of coding and non-coding RNAs to control transcript fate and biological activity

The addition of the methyl-7-guanosine (m⁷G) “cap” on the 5' ends of coding and some non-coding RNAs is essential for their protein coding capacity and biochemical activity, respectively. It was previously considered that capping was a constitutive process that generates a complete cap on all transcripts at steady-state. However, development of new methodologies demonstrated that steady-state capping is a dynamic and regulatable feature of many coding and non-coding RNAs. Indeed, capping status of specific RNAs can flux during differentiation and development, thereby impacting on their protein-coding capacity and activity. Moreover, in some primary cancer specimens, capping can be elevated for transcripts encoding proteins involved in proliferation and survival corresponding to their increased protein levels. Overexpression of one of the capping enzymes (RNMT), the transcription factor MYC or the eukaryotic translation initiation factor eIF4E all led to increased levels of steady-state capping of selected transcripts. Additionally, transcripts can be decapped and recapped, allowing these to be sequestered until needed. This review provides a summary of the major advances in enzymatic and affinity-based approaches to quantify m⁷G capping. Further, we summarize the evidence for regulation of capping. Capping has emerged as a significant regulatory step in RNA metabolism which is poised to impact a myriad of biological processes.

The Mammalian Cap-Specific m6Am RNA Methyltransferase PCIF1 Regulates Transcript Levels in Mouse Tissues

The 5' end of eukaryotic mRNAs is protected by the m⁷G-cap structure. The transcription start site nucleotide is ribose methylated (Nm) in many eukaryotes, whereas an adenosine at this position is further methylated at the N⁶ position (m⁶A) by the mammalian Phosphorylated C-terminal domain (CTD)-interacting Factor 1 (PCIF1) to generate m⁶Am. Here, we show that although the loss of cap-specific m⁶Am in mice does not affect viability or fertility, the Pcif1 mutants display reduced body weight. Transcriptome analyses of mutant mouse tissues support a role for the cap-specific m⁶Am modification in stabilizing transcripts. In contrast, the Drosophila Pcif1 is catalytically dead, but like its mammalian counterpart, it retains the ability to associate with the Ser5-phosphorylated CTD of RNA polymerase II (RNA Pol II). Finally, we show that the Trypanosoma Pcif1 is an m⁶Am methylase that contributes to the N⁶,N⁶,2'-O-trimethyladenosine (m⁶₂Am) in the hypermethylated cap4 structure of trypanosomatids. Thus, PCIF1 has evolved to function in catalytic and non-catalytic roles.

Thermostability, Tunability, and Tenacity of RNA as Rubbery Anionic Polymeric Materials in Nanotechnology and Nanomedicine-Specific Cancer Targeting with Undetectable Toxicity

RNA nanotechnology is the bottom-up self-assembly of nanometer-scale architectures, resembling LEGOs, composed mainly of RNA. The ideal building material should be (1) versatile and controllable in shape and stoichiometry, (2) spontaneously self-assemble, and (3) thermodynamically, chemically, and enzymatically stable with a long shelf life. RNA building blocks exhibit each of the above. RNA is a polynucleic acid, making it a polymer, and its negative-charge prevents nonspecific binding to negatively charged cell membranes. The thermostability makes it suitable for logic gates, resistive memory, sensor set-ups, and NEM devices. RNA can be designed and manipulated with a level of simplicity of DNA while displaying versatile structure and enzyme activity of proteins. RNA can fold into single-stranded loops or bulges to serve as mounting dovetails for intermolecular or domain interactions without external linking dowels. RNA nanoparticles display rubber- and amoeba-like properties and are stretchable and shrinkable through multiple repeats, leading to enhanced tumor targeting and fast renal excretion to reduce toxicities. It was predicted in 2014 that RNA would be the third milestone in pharmaceutical drug development. The recent approval of several RNA drugs and COVID-19 mRNA vaccines by FDA suggests that this milestone is being realized. Here, we review the unique properties of RNA nanotechnology, summarize its recent advancements, describe its distinct attributes inside or outside the body and discuss potential applications in nanotechnology, medicine, and material science.

Synthesis of 5'-Thiamine-Capped RNA

RNA 5'-modifications are known to extend the functional spectrum of ribonucleotides. In recent years, numerous non-canonical 5'-modifications, including adenosine-containing cofactors from the group of B vitamins, have been confirmed in all kingdoms of life. The structural component of thiamine adenosine triphosphate (thiamine-ATP), a vitamin B1 derivative found to accumulate in Escherichia coli and other organisms in response to metabolic stress conditions, suggests an analogous function as a 5'-modification of RNA. Here, we report the synthesis of thiamine adenosine dinucleotides and the preparation of pure 5'-thiamine-capped RNAs based on phosphorimidazolide chemistry. Furthermore, we present the incorporation of thiamine-ATP and thiamine adenosine diphosphate (thiamine-ADP) as 5'-caps of RNA by T7 RNA polymerase. Transcripts containing the thiamine modification were modified specifically with biotin via a combination of thiazole ring opening, nucleophilic substitution and copper-catalyzed azide-alkyne cycloaddition. The highlighted methods provide easy access to 5'-thiamine RNA, which may be applied in the development of thiamine-specific RNA capture protocols as well as the discovery and confirmation of 5'-thiamine-capped RNAs in various organisms.

The eukaryotic translation initiation factor eIF4E elevates steady-state m7G capping of coding and noncoding transcripts

Methyl-7-guanosine (m7G) "capping" of coding and some noncoding RNAs is critical for their maturation and subsequent activity. Here, we discovered that eukaryotic translation initiation factor 4E (eIF4E), itself a cap-binding protein, drives the expression of the capping machinery and increased capping efficiency of ∼100 coding and noncoding RNAs. To quantify this, we developed enzymatic (cap quantification; CapQ) and quantitative cap immunoprecipitation (CapIP) methods. The CapQ method has the further advantage that it captures information about capping status independent of the type of 5' cap, i.e., it is not restricted to informing on m7G caps. These methodological advances led to unanticipated revelations: 1) Many RNA populations are inefficiently capped at steady state (∼30 to 50%), and eIF4E overexpression increased this to ∼60 to 100%, depending on the RNA; 2) eIF4E physically associates with noncoding RNAs in the nucleus; and 3) approximately half of eIF4E-capping targets identified are noncoding RNAs. eIF4E's association with noncoding RNAs strongly positions it to act beyond translation. Coding and noncoding capping targets have activities that influence survival, cell morphology, and cell-to-cell interaction. Given that RNA export and translation machineries typically utilize capped RNA substrates, capping regulation provides means to titrate the protein-coding capacity of the transcriptome and, for noncoding RNAs, to regulate their activities. We also discovered a cap sensitivity element (CapSE) which conferred eIF4E-dependent capping sensitivity. Finally, we observed elevated capping for specific RNAs in high-eIF4E leukemia specimens, supporting a role for cap dysregulation in malignancy. In all, levels of capping RNAs can be regulated by eIF4E.

Arabidopsis DXO1 possesses deNADding and exonuclease activities and its mutation affects defense-related and photosynthetic gene expression

RNA capping and decapping tightly coordinate with transcription, translation, and RNA decay to regulate gene expression. Proteins in the DXO/Rai1 family have been implicated in mRNA decapping and decay, and mammalian DXO was recently found to also function as a decapping enzyme for NAD⁺ -capped RNAs (NAD-RNA). The Arabidopsis genome contains a single gene encoding a DXO/Rai1 protein, AtDXO1. Here we show that AtDXO1 possesses both NAD-RNA decapping activity and 5'-3' exonuclease activity but does not hydrolyze the m⁷ G cap. The atdxo1 mutation increased the stability of NAD-RNAs and led to pleiotropic phenotypes, including severe growth retardation, pale color, and multiple developmental defects. Transcriptome profiling analysis showed that the atdxo1 mutation resulted in upregulation of defense-related genes but downregulation of photosynthesis-related genes. The autoimmunity phenotype of the mutant could be suppressed by either eds1 or npr1 mutation. However, the various phenotypes associated with the atdxo1 mutant could be complemented by an enzymatically inactive AtDXO1. The atdxo1 mutation apparently enhances post-transcriptional gene silencing by elevating levels of siRNAs. Our study indicates that AtDXO1 regulates gene expression in various biological and physiological processes through its pleiotropic molecular functions in mediating RNA processing and decay.

The capping enzyme facilitates promoter escape and assembly of a follow-on preinitiation complex for reinitiation

After synthesis of a short nascent RNA, RNA polymerase II (pol II) dissociates general transcription factors (GTFs; TFIIA, TFIIB, TBP, TFIIE, TFIIF, and TFIIH) and escapes the promoter, but many of the mechanistic details of this process remain unclear. Here we developed an in vitro transcription system from the yeast Saccharomyces cerevisiae that allows conversion of the preinitiation complex (PIC) to bona fide initially transcribing complex (ITC), elongation complex (EC), and reinitiation complex (EC+ITC). By biochemically isolating postinitiation complexes stalled at different template positions, we have determined the timing of promoter escape and the composition of protein complexes associated with different lengths of RNA. Almost all of the postinitiation complexes retained the GTFs when pol II was stalled at position +27 relative to the transcription start site, whereas most complexes had completed promoter escape when stalled at +49. This indicates that GTFs remain associated with pol II much longer than previously expected. Nevertheless, the long-persisting transcription complex containing RNA and all of the GTFs is unstable and is susceptible to extensive backtracking of pol II. Addition of the capping enzyme and/or Spt4/5 significantly increased the frequency of promoter escape as well as assembly of a follow-on PIC at the promoter for reinitiation. These data indicate that elongation factors play an important role in promoter escape and that ejection of TFIIB from the RNA exit tunnel of pol II by the growing nascent RNA is not sufficient to complete promoter escape.

7-Methylguanosine Modifications in Transfer RNA (tRNA)

More than 90 different modified nucleosides have been identified in tRNA. Among the tRNA modifications, the 7-methylguanosine (m⁷G) modification is found widely in eubacteria, eukaryotes, and a few archaea. In most cases, the m⁷G modification occurs at position 46 in the variable region and is a product of tRNA (m⁷G46) methyltransferase. The m⁷G46 modification forms a tertiary base pair with C13-G22, and stabilizes the tRNA structure. A reaction mechanism for eubacterial tRNA m⁷G methyltransferase has been proposed based on the results of biochemical, bioinformatic, and structural studies. However, an experimentally determined mechanism of methyl-transfer remains to be ascertained. The physiological functions of m⁷G46 in tRNA have started to be determined over the past decade. For example, tRNA m⁷G46 or tRNA (m⁷G46) methyltransferase controls the amount of other tRNA modifications in thermophilic bacteria, contributes to the pathogenic infectivity, and is also associated with several diseases. In this review, information of tRNA m⁷G modifications and tRNA m⁷G methyltransferases is summarized and the differences in reaction mechanism between tRNA m⁷G methyltransferase and rRNA or mRNA m⁷G methylation enzyme are discussed.

Multicopy suppressors of temperature-sensitive mutations of yeast mRNA capping enzyme

We have isolated three Saccharomyces cerevisiae genes-CES1, CES2, and CES3-- that, when present in high copy, suppress the ts growth defect caused by mutations in the CEG1 gene encoding mRNA guanylyltransferase (capping enzyme). Molecular characterization of the capping enzyme suppressor genes reveals the following. CES2 is identical to ESP1, a gene required for proper nuclear division. We show by deletion analysis that the 1573-amino acid ESP1 polypeptide is composed of distinct functional domains. The C-terminal portion of ESP1 is essential for cell growth, but dispensable for CES2 activity. The N-terminal half of ESP1, which is sufficient for CES2 function, displays local sequence similarity to the small subunit of the vaccinia virus RNA capping enzyme. This suggests a basis for suppression by physical or functional interaction between the CES2 domain of ESP1 and the yeast guanylyltransferase. CES1 encodes a novel hydrophilic 915-amino acid protein. The amino acid sequence of CES1 is uninformative, except for its extensive similarity to another yeast gene product of unknown function. The CES1 homologue (designated CES4) is also a multicopy suppressor of capping enzyme ts mutations. Neither CES1 nor CES4 is essential for cell growth, and a double deletion mutant is viable. CES3 corresponds to BUD5, which encodes a putative guanine nucleotide exchange factor. We hypothesize that CES1, CES4, and BUD5 may impact on RNA transactions downstream of cap synthesis that are cap dependent in vivo.

Above the Epitranscriptome: RNA Modifications and Stem Cell Identity

Sequence databases and transcriptome-wide mapping have revealed different reversible and dynamic chemical modifications of the nitrogen bases of RNA molecules. Modifications occur in coding RNAs and noncoding-RNAs post-transcriptionally and they can influence the RNA structure, metabolism, and function. The result is the expansion of the variety of the transcriptome. In fact, depending on the type of modification, RNA molecules enter into a specific program exerting the role of the player or/and the target in biological and pathological processes. Many research groups are exploring the role of RNA modifications (alias epitranscriptome) in cell proliferation, survival, and in more specialized activities. More recently, the role of RNA modifications has been also explored in stem cell biology. Our understanding in this context is still in its infancy. Available evidence addresses the role of RNA modifications in self-renewal, commitment, and differentiation processes of stem cells. In this review, we will focus on five epitranscriptomic marks: N6-methyladenosine, N1-methyladenosine, 5-methylcytosine, Pseudouridine (Ψ) and Adenosine-to-Inosine editing. We will provide insights into the function and the distribution of these chemical modifications in coding RNAs and noncoding-RNAs. Mainly, we will emphasize the role of epitranscriptomic mechanisms in the biology of naïve, primed, embryonic, adult, and cancer stem cells.

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.

Molecular Basis of Transcription-Coupled Pre-mRNA Capping

Capping is the first step in pre-mRNA processing, and the resulting 5'-RNA cap is required for mRNA splicing, export, translation, and stability. Capping is functionally coupled to transcription by RNA polymerase (Pol) II, but the coupling mechanism remains unclear. We show that efficient binding of the capping enzyme (CE) to transcribing, phosphorylated yeast Pol II (Pol IIp) requires nascent RNA with an unprocessed 5'-triphosphate end. The transcribing Pol IIp-CE complex catalyzes the first two steps of capping, and its analysis by mass spectrometry, cryo-electron microscopy, and protein crosslinking revealed the molecular basis for transcription-coupled pre-mRNA capping. CE docks to the Pol II wall and spans the end of the RNA exit tunnel to position the CE active sites for sequential binding of the exiting RNA 5' end. Thus, the RNA 5' end triggers its own capping when it emerges from Pol II, to ensure seamless RNA protection from 5'-exonucleases during early transcription.

Yeast-based assays for the high-throughput screening of inhibitors of coronavirus RNA cap guanine-N7-methyltransferase

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A yeast-based system for screening of coronavirus N7-MTases inhibitors was developed.

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Sinefungin is not an ideal antiviral inhibitor.

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Three natural extracts were observed to specifically inhibit the N7-MTases of coronavirus.

The 5′-cap structure is a distinct feature of eukaryotic mRNAs and is important for RNA stability and protein translation by providing a molecular signature for the distinction of self or non-self mRNA. Eukaryotic viruses generally modify the 5′-end of their RNAs to mimic the cellular mRNA structure, thereby facilitating viral replication in host cells. However, the molecular organization and biochemical mechanisms of the viral capping apparatus typically differ from its cellular counterpart, which makes viral capping enzymes attractive targets for drug discovery. Our previous work showed that SARS coronavirus (SARS-CoV) non-structural protein 14 represents a structurally novel and unique guanine-N7-methyltransferase (N7-MTase) that is able to functionally complement yeast cellular N7-MTase. In the present study, we developed a yeast-based system for identifying and screening inhibitors against coronavirus N7-MTase using both 96-well and 384-well microtiter plates. The MTase inhibitors previously identified by in vitro biochemical assays were tested, and some, such as sinefungin, effectively suppressed N7-MTase in the yeast system. However, other compounds, such as ATA and AdoHcy, did not exert an inhibitory effect within a cellular context. These results validated the yeast assay system for inhibitor screening yet also demonstrated the difference between cell-based and in vitro biochemical assays. The yeast system was applied to the screening of 3000 natural product extracts, and three were observed to more potently inhibit the activity of coronavirus than human N7-MTase.

The essential role for the RNA triphosphatase Cet1p in nuclear import of the mRNA capping enzyme Cet1p-Ceg1p complex of Saccharomyces cerevisiae

mRNA capping is the first cotranscriptional modification of mRNA in the nucleus. In Saccharomyces cerevisiae, the first two steps of mRNA capping are catalyzed by the RNA triphosphatase Cet1p and the RNA guanylyltransferase Ceg1p. Cet1p and Ceg1p interact to form a mRNA capping enzyme complex and the guanylyltransferase activity of Ceg1p is stimulated by binding with Cet1p. The Cet1p-Ceg1p complex needs to be transported into the nucleus, where mRNA capping occurs. However, the molecular mechanism of nuclear transport of the Cet1p-Ceg1p complex is not known. Here, we show that Cet1p is responsible and that the Cet1p-Ceg1p interaction is essential for the nuclear localization of the Cet1p-Ceg1p complex. The results indicate that the Cet1p-Ceg1p interaction is important not only for the activation of Ceg1p, but also for nuclear import of the complex.

Methylation modifications in eukaryotic messenger RNA

RNA methylation modifications have been found for decades of years, which occur at different RNA types of numerous species, and their distribution is species-specific. However, people rarely know their biological functions. There are several identified methylation modifications in eukaryotic messenger RNA (mRNA), such as N(7)-methylguanosine (m(7)G) at the cap, N(6)-methyl-2'-O-methyladenosine (m(6)Am), 2'-O-methylation (Nm) within the cap and the internal positions, and internal N(6)-methyladenosine (m(6)A) and 5-methylcytosine (m(5)C). Among them, m(7)G cap was studied more clearly and found to have vital roles in several important mRNA processes like mRNA translation, stability and nuclear export. m(6)A as the most abundant modification in mRNA was found in the 1970s and has been proposed to function in mRNA splicing, translation, stability, transport and so on. m(6)A has been discovered as the first RNA reversible modification which is demethylated directly by human fat mass and obesity associated protein (FTO) and its homolog protein, alkylation repair homolog 5 (ALKBH5). FTO has a special demethylation mechanism that demethylases m(6)A to A through two over-oxidative intermediate states: N(6)-hydroxymethyladenosine (hm(6)A) and N(6)-formyladenosine (f(6)A). The two newly discovered m(6)A demethylases, FTO and ALKBH5, significantly control energy homeostasis and spermatogenesis, respectively, indicating that the dynamic and reversible m(6)A, analogous to DNA and histone modifications, plays broad roles in biological kingdoms and brings us an emerging field "RNA Epigenetics". 5-methylcytosine (5mC) as an epigenetic mark in DNA has been studied widely, but m(5)C in mRNA is seldom explored. The bisulfide sequencing showed m(5)C is another abundant modification in mRNA, suggesting that it might be another RNA epigenetic mark. This review focuses on the main methylation modifications in mRNA to describe their formation, distribution, function and demethylation from the current knowledge and to provide future perspectives on functional studies.

RNA interference of a putative S-adenosyl-L-homocysteine hydrolase gene affects larval performance in Leptinotarsa decemlineata (Say)

In Leptinotarsa decemlineata, juvenile hormones (JHs) play primary roles in the regulation of metamorphosis, reproduction and diapause. In JH biosynthetic pathway in insect corpora allata, methylation of farnesoic acid or JH acid using S-adenosyl-L-methionine generates a potent feedback inhibitor S-adenosyl-L-homocysteine (AdoHcy). Rapid removal of AdoHcy is hypothesized to be essential for JH synthesis. AdoHcy hydrolase (SAHase) is the only eukaryotic enzyme catalyzing the removal. In the present paper, we firstly cloned a putative LdSAHase gene from L. decemlineata. The cDNA consists of 1806 bp and encodes a 525 amino acid protein. LdSAHase was expressed in all developmental stages. The gene had the highest and the lowest level of transcription respectively in the 3rd- and 4th-instars' heads that contain corpora allata, which was positively correlated with JH titer in the haemolymph and the mRNA level of a JH early-inducible gene, the Krüppel homolog 1 gene (Kr-h1). Secondly, dietary ingestion of bacterially-expressed LdSAHase-dsRNA significantly decreased LdSAHase and LdKr-h1 mRNA levels, reduced JH titer, and caused the death of the larvae, and the failure of pupation and adult emergence. After continuous exposure for 12 days, 42% of the larvae died, 65% of the prepupae failed to pupate and 100% of the pupae failed to emerge. Moreover, RNAi-mediated LdSAHase knockdown also reduced larval developing time, and decreased larval weight. Lastly, application of JH analogue pyriproxyfen to LdSAHase-dsRNA-exposed larvae did not greatly increase LdSAHase expression level and JH content, but up-regulated LdKr-h1 mRNA level. Expectedly, pyriproxyfen application could partially rescue the negative effects on the survival and the development. Thus, our results support the hypothesis that SAHase plays a critical role in JH biosynthesis in insects.

Magnesium-induced nucleophile activation in the guanylyltransferase mRNA capping enzyme

The mRNA guanylyltransferase, or mRNA capping enzyme, cotranscriptionally caps the 5'-end of nascent mRNA with GMP during the second reaction in a set of three enzymatic reactions that result in the formation of an N7-methylguanosine cap during mRNA maturation. The mRNA capping enzyme is characterized, in part, by a conserved lysine nucleophile that attacks the α-phosphorus atom of GTP, forming a lysine-GMP intermediate. Experiments have firmly established that magnesium is required for efficient intermediate formation but have provided little insight into the requirement's molecular origins. Using empirical and thermodynamic integration pK(a) estimates, along with conventional molecular dynamics simulations, we show that magnesium binding likely activates the lysine nucleophile by increasing its acidity and by biasing the deprotonated nucleophile into conformations conducive to intermediate formation. These results provide additional functional understanding of an important enzyme in the mRNA transcript life cycle and allow functional analogies to be drawn that affect our understanding of the metal dependence of related superfamily members.

S-adenosyl-L-homocysteine hydrolase and methylation disorders: yeast as a model system

S-adenosyl-L-methionine (AdoMet)-dependent methylation is central to the regulation of many biological processes: more than 50 AdoMet-dependent methyltransferases methylate a broad spectrum of cellular compounds including nucleic acids, proteins and lipids. Common to all AdoMet-dependent methyltransferase reactions is the release of the strong product inhibitor S-adenosyl-L-homocysteine (AdoHcy), as a by-product of the reaction. S-adenosyl-L-homocysteine hydrolase is the only eukaryotic enzyme capable of reversible AdoHcy hydrolysis to adenosine and homocysteine and, thus, relief from AdoHcy inhibition. Impaired S-adenosyl-L-homocysteine hydrolase activity in humans results in AdoHcy accumulation and severe pathological consequences. Hyperhomocysteinemia, which is characterized by elevated levels of homocysteine in blood, also exhibits a similar phenotype of AdoHcy accumulation due to the reversal of the direction of the S-adenosyl-L-homocysteine hydrolase reaction. Inhibition of S-adenosyl-L-homocysteine hydrolase is also linked to antiviral effects. In this review the advantages of yeast as an experimental system to understand pathologies associated with AdoHcy accumulation will be discussed.

RNA biology in a test tube--an overview of in vitro systems/assays

In vitro systems have provided a wealth of information in the field of RNA biology, as they constitute a superior and sometimes the unique approach to address many important questions. Such cell-free methods can be sorted by the degree of complexity of the preparation of enzymatic and/or regulatory activity. Progress in the study of pre-mRNA processing has largely relied on traditional in vitro methods, as these reactions have been recapitulated in cell-free systems. The pre-mRNA capping, editing, and cleavage/polyadenylation reactions have even been reconstituted using purified components, and the enzymes responsible for catalysis have been characterized by such techniques. In vitro splicing using nuclear or cytoplasmic extracts has yielded clues on spliceosome assembly, kinetics, and mechanisms of splicing and has been essential to elucidate the function of splicing factors. Coupled systems have been important to functionally connect distinct processes, like transcription and splicing. Extract preparation has also been adapted to cells from a variety of tissues and species, revealing general versus species-specific mechanisms. Cell-free assays have also been applied to newly discovered pathways such as those involving small RNAs, including microRNAs (miRNAs), small interfering RNAs (siRNAs), and Piwi-interacting RNAs (piRNAs). The first two pathways have been well characterized largely by in vitro methods, which need to be developed for piRNAs. Finally, new techniques, such as single-molecule studies, are continuously being established, providing new and important insights into the field. Thus, in vitro approaches have been, are, and will continue being at the forefront of RNA research.

Structure of the guanylyltransferase domain of human mRNA capping enzyme

The enzyme guanylyltransferase (GTase) plays a central role in the three-step catalytic process of adding an (m7)GpppN cap cotranscriptionally to nascent mRNA (pre-mRNAs). The 5'-mRNA capping process is functionally and evolutionarily conserved from unicellular organisms to human. However, the GTases from viruses and yeast have low amino acid sequence identity (∼25%) with GTases from mammals that, in contrast, are highly conserved (∼98%). We have defined by limited proteolysis of human capping enzyme residues 229-567 as comprising the minimum enzymatically active human GTase (hGTase) domain and have determined the structure by X-ray crystallography. Seven related conformational states of hGTase exist in the crystal. The GTP-binding site is evolutionarily and structurally conserved. The positional variations of the oligonucleotide/oligosaccharide binding fold lid domain over the GTP-binding site provide snapshots of the opening and closing of the active site cleft through a swivel motion. The pattern of conserved surface residues in mammals, but not in yeast, supports the finding that the recognition of the capping apparatus by RNA polymerase II and associated transcription factors is highly conserved in mammals, and the mechanism may differ somewhat from that in yeast. The hGTase structure should help in the design of biochemical and molecular biology experiments to explore the proteinprotein and proteinRNA interactions that ensure regulated transcription of genes in humans and other mammals.

2'-O-ribose methylation of cap2 in human: function and evolution in a horizontally mobile family

The 5′ cap of human messenger RNA consists of an inverted 7-methylguanosine linked to the first transcribed nucleotide by a unique 5′–5′ triphosphate bond followed by 2′-O-ribose methylation of the first and often the second transcribed nucleotides, likely serving to modify efficiency of transcript processing, translation and stability. We report the validation of a human enzyme that methylates the ribose of the second transcribed nucleotide encoded by FTSJD1, henceforth renamed HMTR2 to reflect function. Purified recombinant hMTr2 protein transfers a methyl group from S-adenosylmethionine to the 2′-O-ribose of the second nucleotide of messenger RNA and small nuclear RNA. Neither N⁷ methylation of the guanosine cap nor 2′-O-ribose methylation of the first transcribed nucleotide are required for hMTr2, but the presence of cap1 methylation increases hMTr2 activity. The hMTr2 protein is distributed throughout the nucleus and cytosol, in contrast to the nuclear hMTr1. The details of how and why specific transcripts undergo modification with these ribose methylations remains to be elucidated. The 2′-O-ribose RNA cap methyltransferases are present in varying combinations in most eukaryotic and many viral genomes. With the capping enzymes in hand their biological purpose can be ascertained.

Cap and cap-binding proteins in the control of gene expression

The 5' mRNA cap structure is essential for efficient gene expression from yeast to human. It plays a critical role in all aspects of the life cycle of an mRNA molecule. Capping occurs co-transcriptionally on the nascent pre-mRNA as it emerges from the RNA exit channel of RNA polymerase II. The cap structure protects mRNAs from degradation by exonucleases and promotes transcription, polyadenylation, splicing, and nuclear export of mRNA and U-rich, capped snRNAs. In addition, the cap structure is required for the optimal translation of the vast majority of cellular mRNAs, and it also plays a prominent role in the expression of eukaryotic, viral, and parasite mRNAs. Cap-binding proteins specifically bind to the cap structure and mediate its functions in the cell. Two major cellular cap-binding proteins have been described to date: eukaryotic translation initiation factor 4E (eIF4E) in the cytoplasm and nuclear cap binding complex (nCBC), a nuclear complex consisting of a cap-binding subunit cap-binding protein 20 (CBP 20) and an auxiliary protein cap-binding protein 80 (CBP 80). nCBC plays an important role in various aspects of nuclear mRNA metabolism such as pre-mRNA splicing and nuclear export, whereas eIF4E acts primarily as a facilitator of mRNA translation. In this review, we highlight recent findings on the role of the cap structure and cap-binding proteins in the regulation of gene expression. We also describe emerging regulatory pathways that control mRNA capping and cap-binding proteins in the cell.

Myc Regulation of mRNA Cap Methylation

The c-myc proto-oncogene regulates the expression of 15% to 20% of all genes, depending on the cell type, and the regulation is usually modest (1.5- to 2.0-fold). The authors discovered that in addition to regulating mRNA abundance, c-Myc regulates the formation of the 7-methylguanosine cap on many mRNAs, including transcriptional target genes and others not transcriptionally activated. Because the 7-methylguanosine cap is required for effective translation, enhanced methyl cap formation leads to increased protein production from Myc-responsive genes that exceeds the transcriptional induction. Increased cap methylation is linked to Myc-dependent enhanced activity of 2 critical kinases, TFIIH and p-TEFb, which phosphorylate the RNA polymerase II carboxy-terminal domain (CTD). Phosphorylation of the CTD recruits RNGTT and RNMT, the enzymes involved in mRNA capping, to the nascent transcript. Evidence is accumulating that enhanced cap methylation makes a significant contribution to Myc-dependent gene regulation and protein production.

Regulation of mRNA cap methylation

The 7-methylguanosine cap added to the 5′ end of mRNA is essential for efficient gene expression and cell viability. Methylation of the guanosine cap is necessary for the translation of most cellular mRNAs in all eukaryotic organisms in which it has been investigated. In some experimental systems, cap methylation has also been demonstrated to promote transcription, splicing, polyadenylation and nuclear export of mRNA. The present review discusses how the 7-methylguanosine cap is synthesized by cellular enzymes, the impact that the 7-methylguanosine cap has on biological processes, and how the mRNA cap methylation reaction is regulated.

The flavivirus NS5 protein is a true RNA guanylyltransferase that catalyzes a two-step reaction to form the RNA cap structure

The 5'-end of the flavivirus genome harbors a methylated (m7)GpppA(2'OMe) cap structure, which is generated by the virus-encoded RNA triphosphatase, RNA (guanine-N7) methyltransferase, nucleoside 2'-O-methyltransferase, and RNA guanylyltransferase. The presence of the flavivirus guanylyltransferase activity in NS5 has been suggested by several groups but has not been empirically proven. Here we provide evidence that the N-terminus of the flavivirus NS5 protein is a true RNA guanylyltransferase. We demonstrate that GTP can be used as a substrate by the enzyme to form a covalent GMP-enzyme intermediate via a phosphoamide bond. Mutational studies also confirm the importance of a specific lysine residue in the GTP binding site for the enzymatic activity. We show that the GMP moiety can be transferred to the diphosphate end of an RNA transcript harboring an adenosine as the initiating residue. We also demonstrate that the flavivirus RNA triphosphatase (NS3 protein) stimulates the RNA guanylyltransferase activity of the NS5 protein. Finally, we show that both enzymes are sufficient and necessary to catalyze the de novo formation of a methylated RNA cap structure in vitro using a triphosphorylated RNA transcript. Our study provides biochemical evidence that flaviviruses encode a complete RNA capping machinery.

Enhanced mRNA cap methylation increases cyclin D1 expression and promotes cell transformation

Cap-dependent mRNA translation requires the methylation of the mRNA guanosine cap by RNA guanine-7-methyltransferase (RNMT). mRNA cap methylation was recently described to be rate-limiting for a subset of mRNAs, and to be enhanced by expression of c-Myc and E2F1, although the biological significance of this finding was not investigated. Here, it is reported that increased RNMT expression enhances cellular mRNA cap methyltransferase activity, promotes mammary epithelial cell transformation and cooperates with H-RasV12 or c-Myc to promote fibroblast cell transformation. Cyclin D1 is a prominent oncogene in epithelial tumours. A significant fraction of Cyclin D1 mRNA was found to be unmethylated on the mRNA cap and thus dormant in mammary epithelial cells. Cyclin D1 expression was increased by enhanced mRNA cap methylation. In summary, this report shows that mRNA cap methylation is rate-limiting for expression of an oncogene and cell transformation.

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.

Evidence for a capping enzyme with specificity for the trypanosome spliced leader RNA

Capping of the pre-mRNA 5' end by addition a monomethylated guanosine cap (m(7)G) is an essential and the earliest modification in the biogenesis of mRNA. The reaction is catalyzed by three enzymes: triphosphatase, guanylyltransferase, and (guanine N-7) methyltransferase. Whereas this modification occurs co-transcriptionally in most eukaryotic organisms, trypanosomatid protozoa mRNAs acquire the m(7)G cap by trans-splicing, which entails the transfer of the capped spliced leader (SL) from the SL RNA to the mRNA. Intriguingly, the genomes of all trypanosomatid protozoa sequenced to date possess two distinct proteins with the signature motifs of guanylyltransferases: TbCGM1 and the previously characterized TbCE1. Here we provide biochemical evidence that TbCgm1 is a capping enzyme. Whereas RNAi-induced downregulation of TbCe1 had no phenotypic consequences, we found that TbCGM1 is essential for trypanosome viability and is required for SL RNA capping. Furthermore, consistent with co-transcriptional addition of the m(7)G cap, chromatin immunoprecipitation revealed recruitment of TbCgm1 to the SL RNA genes.

Distinct RNA elements confer specificity to flavivirus RNA cap methylation events

The 5' end of the flavivirus plus-sense RNA genome contains a type 1 cap (m(7)GpppAmG), followed by a conserved stem-loop structure. We report that nonstructural protein 5 (NS5) from four serocomplexes of flaviviruses specifically methylates the cap through recognition of the 5' terminus of viral RNA. Distinct RNA elements are required for the methylations at guanine N-7 on the cap and ribose 2'-OH on the first transcribed nucleotide. In a West Nile virus (WNV) model, N-7 cap methylation requires specific nucleotides at the second and third positions and a 5' stem-loop structure; in contrast, 2'-OH ribose methylation requires specific nucleotides at the first and second positions, with a minimum 5' viral RNA of 20 nucleotides. The cap analogues GpppA and m(7)GpppA are not active substrates for WNV methytransferase. Footprinting experiments using Gppp- and m(7)Gppp-terminated RNAs suggest that the 5' termini of RNA substrates interact with NS5 during the sequential methylation reactions. Cap methylations could be inhibited by an antisense oligomer targeting the first 20 nucleotides of WNV genome. The viral RNA-specific cap methylation suggests methyltransferase as a novel target for flavivirus drug discovery.

Biochemical and genetic analysis of RNA cap guanine-N2 methyltransferases from Giardia lamblia and Schizosaccharomyces pombe

RNA cap guanine-N2 methyltransferases such as Schizosaccharomyces pombe Tgs1 and Giardia lamblia Tgs2 catalyze methylation of the exocyclic N2 amine of 7-methylguanosine. Here we performed a mutational analysis of Giardia Tgs2, entailing an alanine scan of 17 residues within the minimal active domain. Alanine substitutions at Phe18, Thr40, Asp76, Asn103 and Asp140 reduced methyltransferase specific activity to <3% of wild-type Tgs2, thereby defining these residues as essential. Alanines at Pro142, Tyr148 and Pro185 reduced activity to 7–12% of wild-type. Structure–activity relationships at Phe18, Thr40, Asp76, Asn103, Asp140 and Tyr148, and at three other essential residues defined previously (Asp68, Glu91 and Trp143) were gleaned by testing the effects of 18 conservative substitutions. Our results engender a provisional map of the Tgs2 active site, which we discuss in light of crystal structures of related methyltransferases. A genetic analysis of S. pombe Tgs1 showed that it is nonessential. An S. pombe tgs1Δ strain grows normally, notwithstanding the absence of 2,2,7-trimethylguanosine caps on its U1, U2, U4 and U5 snRNAs. However, we find that S. pombe requires cap guanine-N7 methylation catalyzed by the enzyme Pcm1. Deletion of the pcm1⁺ gene was lethal, as were missense mutations in the Pcm1 active site. Thus, whereas m⁷G caps are essential in both S. pombe and S. cerevisiae, m^2,2,7G caps are not.

Functional characterization of a 48 kDa Trypanosoma brucei cap 2 RNA methyltransferase

Kinetoplastid mRNAs possess a unique hypermethylated cap 4 structure derived from the standard m⁷GpppN cap structure, with 2′-O methylations on the first four ribose sugars and additional base methylations on the first adenine and the fourth uracil. While the enzymes responsible for m⁷GpppN cap 0 formations has been characterized in Trypanosoma brucei, the mechanism of cap 4 methylation and the role of the hypermethylated structure remain unclear. Here, we describe the characterization of a 48 kDa T.brucei 2′-O nucleoside methyltransferase (TbCom1). Recombinant TbCom1 transfers the methyl group from S-adenosylmethionine (AdoMet) to the 2′-OH of the second nucleoside of m⁷GpppNpNp-RNA to form m⁷GpppNpNmp-RNA. TbCom1 is also capable of converting cap 1 RNA to cap 2 RNA. The methyl transfer reaction is dependent on the m⁷GpppN cap, as the enzyme does not form a stable interaction with GpppN-terminated RNA. Mutational analysis establishes that the TbCom1 and vaccinia virus VP39 methyltransferases share mechanistic similarities in AdoMet- and cap-recognition. Two aromatic residues, Tyr18 and Tyr187, may participate in base-stacking interactions with the guanine ring of the cap, as the removal of each of these aromatic side-chains abolishes cap-specific RNA-binding.

Mutational analysis of Encephalitozoon cuniculi mRNA cap (guanine-N7) methyltransferase, structure of the enzyme bound to sinefungin, and evidence that cap methyltransferase is the target of sinefungin's antifungal activity

Cap (guanine-N7) methylation is an essential step in eukaryal mRNA synthesis and a potential target for antiviral, antifungal, and antiprotozoal drug discovery. Previous mutational and structural analyses of Encephalitozoon cuniculi Ecm1, a prototypal cellular cap methyltransferase, identified amino acids required for cap methylation in vivo, but also underscored the nonessentiality of many side chains that contact the cap and AdoMet substrates. Here we tested new mutations in residues that comprise the guanine-binding pocket, alone and in combination. The outcomes indicate that the shape of the guanine binding pocket is more crucial than particular base edge interactions, and they highlight the contributions of the aliphatic carbons of Phe-141 and Tyr-145 that engage in multiple van der Waals contacts with guanosine and S-adenosylmethionine (AdoMet), respectively. We purified 45 Ecm1 mutant proteins and assayed them for methylation of GpppA in vitro. Of the 21 mutations that resulted in unconditional lethality in vivo,14 reduced activity in vitro to < or = 2% of the wild-type level and 5 reduced methyltransferase activity to between 4 and 9% of wild-type Ecm1. The natural product antibiotic sinefungin is an AdoMet analog that inhibits Ecm1 with modest potency. The crystal structure of an Ecm1-sinefungin binary complex reveals sinefungin-specific polar contacts with main-chain and side-chain atoms that can explain the 3-fold higher affinity of Ecm1 for sinefungin versus AdoMet or S-adenosylhomocysteine (AdoHcy). In contrast, sinefungin is an extremely potent inhibitor of the yeast cap methyltransferase Abd1, to which sinefungin binds 900-fold more avidly than AdoHcy or AdoMet. We find that the sensitivity of Saccharomyces cerevisiae to growth inhibition by sinefungin is diminished when Abd1 is overexpressed. These results highlight cap methylation as a principal target of the antifungal activity of sinefungin.

Yeast transcripts cleaved by an internal ribozyme provide new insight into the role of the cap and poly(A) tail in translation and mRNA decay

It has been proposed that the 7-methylguanosine cap and poly(A) tail of mRNAs have important functions in translation and transcript stability. To directly test these roles of the cap and poly(A) tail, we have constructed plasmids with a ribozyme within the coding region or 3' UTR of reporter genes. We show that the unadenylated 5' cleavage product is translated and is rapidly degraded by the cytoplasmic exosome. This exosome-mediated decay is independent of the nonstop mRNA decay pathway, and, thus, reveals an additional substrate for exosome-mediated decay that may have physiological equivalents. The rapid decay of this transcript in the cytoplasm indicates that this unadenylated cleavage product is rapidly exported from the nucleus. We also show that this cleavage product is not subject to rapid decapping; thus, the lack of a poly(A) tail does not always trigger rapid decapping of the transcript. We show that the 3' cleavage product is rapidly degraded by Xrn1p in the cytoplasm. We cannot detect any protein from this 3' cleavage product, which supports previous data concluding that the 5' cap is required for translation. The reporter genes we have utilized in these studies should be generally useful tools in studying the importance of the poly(A) tail and 5' cap of a transcript for export, translation, mRNA decay, and other aspects of mRNA metabolism in vivo.

Genetic analysis of poxvirus mRNA cap methyltransferase: suppression of conditional mutations in the stimulatory D12 subunit by second-site mutations in the catalytic D1 subunit

The guanine-N7 methyltransferase domain of vaccinia virus mRNA capping enzyme, composed of catalytic vD1(498-844) and stimulatory vD12 subunits, can function in vivo in Saccharomyces cerevisiae in lieu of the essential cellular cap methyltransferase Abd1. Coexpression of both poxvirus subunits is required to complement the growth of abd1Delta cells. A double-alanine scan of the vD12 protein identified lethal and temperature-sensitive vD12 alleles. We used this mutant collection to perform a forward genetic screen for compensatory changes in the catalytic subunit that suppressed the growth phenotypes of the vD12 mutants. The screen reiteratively defined a small ensemble of amino acids in vD1(498-844) at which mutations restored methyltransferase function in conjunction with defective vD12 proteins. Reference to the crystal structure of the microsporidian cap methyltransferase suggests that distinct functional classes of suppressors were selected, including: (i) those that map to surface-exposed loops, which likely comprise the physical subunit interface; (ii) those in or near the substrate binding sites, which presumably affect or mimic inter-subunit allostery.

Poxvirus mRNA cap methyltransferase. Bypass of the requirement for the stimulatory subunit by mutations in the catalytic subunit and evidence for intersubunit allostery

The guanine-N7 methyltransferase domain of vaccinia virus mRNA capping enzyme is a heterodimer composed of a catalytic subunit vD1-(540-844) and a stimulatory subunit vD12. The poxvirus enzyme can function in vivo in Saccharomyces cerevisiae in lieu of the essential cellular cap methyltransferase Abd1. Coexpression of both poxvirus subunits is required to complement the growth of abd1delta cells. We performed a genetic screen for mutations in the catalytic subunit that bypassed the requirement for the stimulatory subunit in vivo. We thereby identified missense changes in vicinal residues Tyr-752 (to Ser, Cys, or His) and Asn-753 (to Ile), which are located in the cap guanine-binding pocket. Biochemical experiments illuminated a mechanism of intersubunit allostery, whereby the vD12 subunit enhances the affinity of the catalytic subunit for AdoMet and the cap guanine methyl acceptor by 6- and 14-fold, respectively, and increases kcat by a factor of 4. The bypass mutations elicited gains of function in both vD12-independent and vD12-dependent catalysis of cap methylation in vitro when compared with wild-type vD1-(540-844). These results highlight the power of yeast as a surrogate model for the genetic analysis of interacting poxvirus proteins and demonstrate that the activity of an RNA processing enzyme can be augmented through selection and protein engineering.

Characterization of a Trypanosoma brucei RNA cap (guanine N-7) methyltransferase

The m7GpppN cap structure of eukaryotic mRNA is formed by the sequential action of RNA triphosphatase, guanylyltransferase, and (guanine N-7) methyltransferase. In trypanosomatid protozoa, the m7GpppN is further modified by seven methylation steps within the first four transcribed nucleosides to form the cap 4 structure. The RNA triphosphatase and guanylyltransferase components have been characterized in Trypanosoma brucei. Here we describe the identification and characterization of a T. brucei (guanine N-7) methyltransferase (TbCmt1). Sequence alignment of the 324-amino acid TbCmt1 with the corresponding enzymes from human (Hcm1), fungal (Abd1), and microsporidian (Ecm1) revealed the presence of conserved residues known to be essential for methyltransferase activity. Purified recombinant TbCmt1 catalyzes the transfer of a methyl group from S-adenosylmethionine to the N-7 position of the cap guanine in GpppN-terminated RNA to form the m7GpppN cap. TbCmt1 also methylates GpppG and GpppA but not GTP or dGTP. Mutational analysis of individual residues of TbCmt1 that were predicted-on the basis of the crystal structure of Ecm1--to be located at or near the active site identified six conserved residues in the putative AdoMet- or cap-binding pocket that caused significant reductions in TbCmt1 methyltransferase activity. We also report the identification of a second T. brucei RNA (guanine N-7) cap methyltransferase (named TbCgm1). The 1050-amino acid TbCgm1 consists of a C-terminal (guanine N-7) methyltransferase domain, which is homologous with TbCmt1, and an N-terminal guanylyltransferase domain, which contains signature motifs found in the nucleotidyl transferase superfamily.

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.