Many paths to methyltransfer: a chronicle of convergence

Structural basis for substrate flexibility of the O-methyltransferase MpaG' involved in mycophenolic acid biosynthesis

MpaG' is an S-adenosyl-L-methionine (SAM)-dependent methyltransferase involved in the compartmentalized biosynthesis of mycophenolic acid (MPA), a first-line immunosuppressive drug for organ transplantations and autoimmune diseases. MpaG' catalyzes the 5-O-methylation of three precursors in MPA biosynthesis including demethylmycophenolic acid (DMMPA), 4-farnesyl-3,5-dihydroxy-6-methylphthalide (FDHMP), and an intermediate containing three fewer carbon atoms compared to FDHMP (FDHMP-3C) with different catalytic efficiencies. Here, we report the crystal structures of S-adenosyl-L-homocysteine (SAH)/DMMPA-bound MpaG', SAH/FDHMP-3C-bound MpaG', and SAH/FDHMP-bound MpaG' to understand the catalytic mechanism of MpaG' and structural basis for its substrate flexibility. Structural and biochemical analyses reveal that MpaG' utilizes the catalytic dyad H306-E362 to deprotonate the C5 hydroxyl group of the substrates for the following methylation. The three substrates with differently modified farnesyl moieties are well accommodated in a large semi-open substrate binding pocket with the orientation of their phthalide moiety almost identical. Based on the structure-directed mutagenesis, a single mutant MpaG'Q267A is engineered with significantly improved catalytic efficiency for all three substrates. This study expands the mechanistic understanding and the pocket engineering strategy for O-methyltransferases involved in fungal natural product biosynthesis. Our research also highlights the potential of O-methyltransferases to modify diverse substrates by protein design and engineering.

Characterization of a unique catechol-O-methyltransferase as a molecular drug target in parasitic filarial nematodes

BACKGROUND

Filarial nematodes cause severe illnesses in humans and canines including limb deformities and disfigurement, heart failure, blindness, and death, among others. There are no vaccines, and current drugs against filarial nematodes infections have only modest effects and are prone to complications.

METHODOLOGY/PRINCIPAL FINDINGS

We identified a gene (herein called DiMT) encoding an S-adenosyl-L-methionine (SAM)-dependent methyltransferase with orthologs in parasite filarial worms but not in mammals. By in silico analysis, DiMT possesses catalytic sites for binding SAM and catecholamines with high affinity. We expressed and purified recombinant DiMT protein and used it as an enzyme in a series of SAM-dependent methylation assays. DiMT acted specifically as a catechol-O-methyltransferase (COMT), catalyzing catabolic methylation of dopamine, and depicted Michaelis Menten kinetics on substrate and co-substrate. Among a set of SAM-dependent methyltransferase inhibitors, we identified compounds that bound with high affinity to DiMT's catalytic sites and inhibited its enzymatic activity. By testing the efficacy of DiMT inhibitors against microfilariae of Dirofilaria immitis in culture, we identified three inhibitors with concentration- and time-dependent effect of killing D. immitis microfilariae. Importantly, RNAi silencing of a DiMT ortholog in Caenorhabditis elegans has been shown to be lethal, likely as a result of excessive accumulation of active catecholamines that inhibit worm locomotion, pharyngeal pumping and fecundity.

CONCLUSIONS/SIGNIFICANCE

Together, we have unveiled DiMT as an essential COMT that is conserved in parasitic filarial nematodes, but is significantly different from mammalian COMTs and, therefore, is a viable target for development of novel drugs against filarial nematode infections.

METTL Family in Healthy and Disease

Transcription, RNA splicing, RNA translation, and post-translational protein modification are fundamental processes of gene expression. Epigenetic modifications, such as DNA methylation, RNA modifications, and protein modifications, play a crucial role in regulating gene expression. The methyltransferase-like protein (METTL) family, a constituent of the 7-β-strand (7BS) methyltransferase subfamily, is broadly distributed across the cell nucleus, cytoplasm, and mitochondria. Members of the METTL family, through their S-adenosyl methionine (SAM) binding domain, can transfer methyl groups to DNA, RNA, or proteins, thereby impacting processes such as DNA replication, transcription, and mRNA translation, to participate in the maintenance of normal function or promote disease development. This review primarily examines the involvement of the METTL family in normal cell differentiation, the maintenance of mitochondrial function, and its association with tumor formation, the nervous system, and cardiovascular diseases. Notably, the METTL family is intricately linked to cellular translation, particularly in its regulation of translation factors. Members represent important molecules in disease development processes and are associated with patient immunity and tolerance to radiotherapy and chemotherapy. Moreover, future research directions could include the development of drugs or antibodies targeting its structural domains, and utilizing nanomaterials to carry miRNA corresponding to METTL family mRNA. Additionally, the precise mechanisms underlying the interactions between the METTL family and cellular translation factors remain to be clarified.

Deciphering the SAM- and metal-dependent mechanism of O-methyltransferases in cystargolide and belactosin biosynthesis: A structure-activity relationship study

Cystargolides and belactosins are natural products with a distinct dipeptide structure and an electrophilic β-lactone warhead. They are known to inhibit proteases such as the proteasome or caseinolytic protease P, highlighting their potential in treating cancers and neurodegenerative diseases. Recent genetic analyses have shown homology between the biosynthetic pathways of the two inhibitors. Here, we characterize the O-methyltransferases BelI and CysG, which catalyze the initial step of β-lactone formation. Employing techniques such as crystallography, computational analysis, mutagenesis, and activity assays, we identified a His-His-Asp (HHD) motif in the active sites of the two enzymes, which is crucial for binding a catalytically active calcium ion. Our findings thus elucidate a conserved divalent metal-dependent mechanism in both biosynthetic pathways that distinguish BelI and CysG from previously characterized O-methyltransferases.

Metabolomics comparison of four varieties apple with different browning characters in response to pretreatment during pulp processing

Browning commonly appeared in apple processing, which varied in different apple varieties. Present work investigated the metabolomics of four varieties apple of Yataka, Gala, Sansa, and Fuji, which possessed different browning characteristics and related enzymes. Sansa as browning insensitive apple variety, exhibited the least chroma change with the lowest PPO activity and the highest SOD activity among the four apple varieties. Browning inhibition pretreatment increased the activity of SOD and PAL and decreased PPO and POD activity. In addition, metabolomic variances among the four apple varieties (FC), their browning pulp (BR) and browning inhibition pulp (CM) were compared. And the key metabolites were in-depth analyzed to match the relevant KEGG pathways and speculated metabolic networks. There were 487, 644, and 494 significant differential metabolites detected in FC, BR and CM, which were consisted of lipids, benzenoids, phenylpropanoids, organheterocyclic compounds, organic acids, nucleosides, accounting for 23 %, 11 %, 15 %, 16 %, 11 % of the total metabolites. The differential metabolites were matched with 39, 49, and 36 KEGG pathways in FC, BR, and CM, respectively, in which other secondary metabolites biosynthesis metabolism was the most significant in FC, lipid metabolism was the most significant in BR and CM, and energy metabolism was markedly annotated in CM. Notably, Sansa displayed the highest number of differential metabolites in both its BR (484) and CM (342). The BR of Sansa was characterized by flavonoid biosynthesis, while the other three apple varieties were associated with α-linolenic acid metabolism. Furthermore, in browning sensitive apple varieties, the flavonoid and phenylpropanoid biosynthesis pathway was significantly activated by browning inhibition pretreatment. Phenolic compounds, lipids, sugars, organic acids, nucleotides, and adenosine were regulated differently in the four apple varieties, potentially serving as key regulatory sites. Overall, this work provides novel insight for browning prevention in different apple varieties.

Roles of Lysine Methylation in Glucose and Lipid Metabolism: Functions, Regulatory Mechanisms, and Therapeutic Implications

Glucose and lipid metabolism are essential energy sources for the body. Dysregulation in these metabolic pathways is a significant risk factor for numerous acute and chronic diseases, including type 2 diabetes (T2DM), Alzheimer's disease (AD), obesity, and cancer. Post-translational modifications (PTMs), which regulate protein structure, localization, function, and activity, play a crucial role in managing cellular glucose and lipid metabolism. Among these PTMs, lysine methylation stands out as a key dynamic modification vital for the epigenetic regulation of gene transcription. Emerging evidence indicates that lysine methylation significantly impacts glucose and lipid metabolism by modifying key enzymes and proteins. This review summarizes the current understanding of lysine methylation's role and regulatory mechanisms in glucose and lipid metabolism. We highlight the involvement of methyltransferases (KMTs) and demethylases (KDMs) in generating abnormal methylation signals affecting these metabolic pathways. Additionally, we discuss the chemical biology and pharmacology of KMT and KDM inhibitors and targeted protein degraders, emphasizing their clinical implications for diseases such as diabetes, obesity, neurodegenerative disorders, and cancers. This review suggests that targeting lysine methylation in glucose and lipid metabolism could be an ideal therapeutic strategy for treating these diseases.

Methyltransferases from RiPP pathways: shaping the landscape of natural product chemistry

This review article aims to highlight the role of methyltransferases within the context of ribosomally synthesised and post-translationally modified peptide (RiPP) natural products. Methyltransferases play a pivotal role in the biosynthesis of diverse natural products with unique chemical structures and bioactivities. They are highly chemo-, regio-, and stereoselective allowing methylation at various positions. The different possible acceptor regions in ribosomally synthesised peptides are described in this article. Furthermore, we will discuss the potential application of these methyltransferases as powerful biocatalytic tools in the synthesis of modified peptides and other bioactive compounds. By providing an overview of the various methylation options available, this review is intended to emphasise the biocatalytic potential of RiPP methyltransferases and their impact on the field of natural product chemistry.

Engineered Methionine Adenosyltransferase Cascades for Metabolic Labeling of Individual DNA Methylomes in Live Cells

Methylation, a widely occurring natural modification serving diverse regulatory and structural functions, is carried out by a myriad of S-adenosyl-l-methionine (AdoMet)-dependent methyltransferases (MTases). The AdoMet cofactor is produced from l-methionine (Met) and ATP by a family of multimeric methionine adenosyltransferases (MAT). To advance mechanistic and functional studies, strategies for repurposing the MAT and MTase reactions to accept extended versions of the transferable group from the corresponding precursors have been exploited. Here, we used structure-guided engineering of mouse MAT2A to enable biocatalytic production of an extended AdoMet analogue, Ado-6-azide, from a synthetic methionine analogue, S-(6-azidohex-2-ynyl)-l-homocysteine (N3-Met). Three engineered MAT2A variants showed catalytic proficiency with the extended analogues and supported DNA derivatization in cascade reactions with M.TaqI and an engineered variant of mouse DNMT1 both in the absence and presence of competing Met. We then installed two of the engineered variants as MAT2A-DNMT1 cascades in mouse embryonic stem cells by using CRISPR-Cas genome editing. The resulting cell lines maintained normal viability and DNA methylation levels and showed Dnmt1-dependent DNA modification with extended azide tags upon exposure to N3-Met in the presence of physiological levels of Met. This for the first time demonstrates a genetically stable system for biosynthetic production of an extended AdoMet analogue, which enables mild metabolic labeling of a DNMT-specific methylome in live mammalian cells.

Structures and Protein Engineering of the α-Keto Acid C-Methyltransferases SgvM and MrsA for Rational Substrate Transfer

S-adenosyl-l-methionine-dependent methyltransferases (MTs) are involved in the C-methylation of a variety of natural products. The MTs SgvM from Streptomyces griseoviridis and MrsA from Pseudomonas syringae pv. syringae catalyze the methylation of the β-carbon atom of α-keto acids in the biosynthesis of the antibiotic natural products viridogrisein and 3-methylarginine, respectively. MrsA shows high substrate selectivity for 5-guanidino-2-oxovalerate, while other α-keto acids, such as the SgvM substrates 4-methyl-2-oxovalerate, 2-oxovalerate, and phenylpyruvate, are not accepted. Here we report the crystal structures of SgvM and MrsA in the apo form and bound with substrate or S-adenosyl-l-methionine. By investigating key residues for substrate recognition in the active sites of both enzymes and engineering MrsA by site-directed mutagenesis, the substrate range of MrsA was extended to accept α-keto acid substrates of SgvM with uncharged and lipophilic β-residues. Our results showcase the transfer of the substrate scope of α-keto acid MTs from different biosynthetic pathways by rational design.

KMT2C and KMT2D aberrations in breast cancer

KMT2C and KMT2D are histone lysine methyltransferases responsible for the monomethylation of histone 3 lysine 4 (H3K4) residues at gene enhancer sites. KMT2C/D are the most frequently mutated histone methyltransferases (HMTs) in breast cancer, occurring at frequencies of 10-20% collectively. Frequent damaging and truncating somatic mutations indicate a tumour-suppressive role of KMT2C/D in breast oncogenesis. Recent studies using cell lines and mouse models to replicate KMT2C/D loss show that these genes contribute to oestrogen receptor (ER)-driven transcription in ER+ breast cancers through the priming of gene enhancer regions. This review provides an overview of the functions of KMT2C/D and outlines the recent clinical and experimental evidence of the roles of KMT2C and KMT2D in breast cancer development.

The discovery of novel noncoding RNAs in 50 bacterial genomes

Structured noncoding RNAs (ncRNAs) contribute to many important cellular processes involving chemical catalysis, molecular recognition and gene regulation. Few ncRNA classes are broadly distributed among organisms from all three domains of life, but the list of rarer classes that exhibit surprisingly diverse functions is growing. We previously developed a computational pipeline that enables the near-comprehensive identification of structured ncRNAs expressed from individual bacterial genomes. The regions between protein coding genes are first sorted based on length and the fraction of guanosine and cytidine nucleotides. Long, GC-rich intergenic regions are then examined for sequence and structural similarity to other bacterial genomes. Herein, we describe the implementation of this pipeline on 50 bacterial genomes from varied phyla. More than 4700 candidate intergenic regions with the desired characteristics were identified, which yielded 44 novel riboswitch candidates and numerous other putative ncRNA motifs. Although experimental validation studies have yet to be conducted, this rate of riboswitch candidate discovery is consistent with predictions that many hundreds of novel riboswitch classes remain to be discovered among the bacterial species whose genomes have already been sequenced. Thus, many thousands of additional novel ncRNA classes likely remain to be discovered in the bacterial domain of life.

The sterol C-24 methyltransferase encoding gene, erg6, is essential for viability of Aspergillus species

Triazoles, the most widely used class of antifungal drugs, inhibit the biosynthesis of ergosterol, a crucial component of the fungal plasma membrane. Inhibition of a separate ergosterol biosynthetic step, catalyzed by the sterol C-24 methyltransferase Erg6, reduces the virulence of pathogenic yeasts, but its effects on filamentous fungal pathogens like Aspergillus fumigatus remain unexplored. Here, we show that the lipid droplet-associated enzyme Erg6 is essential for the viability of A. fumigatus and other Aspergillus species, including A. lentulus, A. terreus, and A. nidulans. Downregulation of erg6 causes loss of sterol-rich membrane domains required for apical extension of hyphae, as well as altered sterol profiles consistent with the Erg6 enzyme functioning upstream of the triazole drug target, Cyp51A/Cyp51B. Unexpectedly, erg6-repressed strains display wild-type susceptibility against the ergosterol-active triazole and polyene antifungals. Finally, we show that erg6 repression results in significant reduction in mortality in a murine model of invasive aspergillosis. Taken together with recent studies, our work supports Erg6 as a potentially pan-fungal drug target.

A novel O-methyltransferase Cp4MP-OMT catalyses the final step in the biosynthesis of the volatile 1,4-dimethoxybenzene in pumpkin (Cucurbita pepo) flowers

BACKGROUND

Floral scents play a crucial role in attracting insect pollinators. Among the compounds attractive to pollinators is 1,4-dimethoxybenzene (1,4-DMB). It is a significant contributor to the scent profile of plants from various genera, including economically important Cucurbita species. Despite its importance, the biosynthetic pathway for the formation of 1,4-DMB was not elucidated so far.

RESULTS

In this study we showed the catalysis of 1,4-DMB in the presence of 4-methoxyphenol (4-MP) by protein extract from Styrian oil pumpkin (Cucurbita pepo) flowers. Based on this finding, we identified a novel O-methyltransferase gene, Cp4MP-OMT, whose expression is highly upregulated in the volatile-producing tissue of pumpkin flowers when compared to vegetative tissues. OMT activity was verified by purified recombinant Cp4MP-OMT, illustrating its ability to catalyse the methylation of 4-MP to 1,4-DMB in the presence of cofactor SAM (S-(5'-adenosyl)-L-methionine).

CONCLUSIONS

Cp4MP-OMT is a novel O-methyltransferase from C. pepo, responsible for the final step in the biosynthesis of the floral scent compound 1,4-DMB. Considering the significance of 1,4-DMB in attracting insects for pollination and in the further course fruit formation, enhanced understanding of its biosynthetic pathways holds great promise for both ecological insights and advancements in plant breeding initiatives.

Identification and virus-induced gene silencing (VIGS) analysis of methyltransferase affecting tomato (Solanum lycopersicum) fruit ripening

MAIN CONCLUSION

Six methyltransferase genes affecting tomato fruit ripening were identified through genome-wide screening, VIGS assay, and expression pattern analysis. The data provide the basis for understanding new mechanisms of methyltransferases. Fruit ripening is a critical stage for the formation of edible quality and seed maturation, which is finely modulated by kinds of factors, including genetic regulators, hormones, external signals, etc. Methyltransferases (MTases), important genetic regulators, play vital roles in plant development through epigenetic regulation, post-translational modification, or other mechanisms. However, the regulatory functions of numerous MTases except DNA methylation in fruit ripening remain limited so far. Here, six MTases, which act on different types of substrates, were identified to affect tomato fruit ripening. First, 35 MTase genes with relatively high expression at breaker (Br) stage of tomato fruit were screened from the tomato MTase gene database encompassing 421 genes totally. Thereafter, six MTase genes were identified as potential regulators of fruit ripening via virus-induced gene silencing (VIGS), including four genes with a positive regulatory role and two genes with a negative regulatory role, respectively. The expression of these six MTase genes exhibited diverse patterns during the fruit ripening process, and responded to various external ripening-related factors, including ethylene, 1-methylcyclopropene (1-MCP), temperature, and light exposure. These results help to further elaborate the biological mechanisms of MTase genes in tomato fruit ripening and enrich the understanding of the regulatory mechanisms of fruit ripening involving MTases, despite of DNA MTases.

Methyl transfer in psilocybin biosynthesis

Psilocybin, the natural hallucinogen produced by Psilocybe ("magic") mushrooms, holds great promise for the treatment of depression and several other mental health conditions. The final step in the psilocybin biosynthetic pathway, dimethylation of the tryptophan-derived intermediate norbaeocystin, is catalysed by PsiM. Here we present atomic resolution (0.9 Å) crystal structures of PsiM trapped at various stages of its reaction cycle, providing detailed insight into the SAM-dependent methylation mechanism. Structural and phylogenetic analyses suggest that PsiM derives from epitranscriptomic N6-methyladenosine writers of the METTL16 family, which is further supported by the observation that bound substrates physicochemically mimic RNA. Inherent limitations of the ancestral monomethyltransferase scaffold hamper the efficiency of psilocybin assembly and leave PsiM incapable of catalysing trimethylation to aeruginascin. The results of our study will support bioengineering efforts aiming to create novel variants of psilocybin with improved therapeutic properties.

Phylogeny of Transferable Oxazolidinone Resistance Genes and Homologs

Oxazolidinone resistance, especially transmissible resistance, is a major public health concern, and the origin of this resistance mechanism is not yet resolved. This study aims to delve into the phylogenetic origin of the transmissible oxazolidinone resistance mechanisms conferring cross-resistance to other drugs of human and veterinary importance. The amino acid sequences of the five cfr ribosomal methylases and optrA and poxtA were used as queries in searches against 219,549 bacterial proteomes in the NCBI RefSeq database. Hits with >40% amino acid identity and >80% query coverage were aligned, and phylogenetic trees were reconstructed. All five cfr genes yielded highly similar trees, with rlmN housekeeping ribosomal methylases located basal to the sister groups of S-adenosyl-methionine-dependent methyltransferases from various Deltaproteobacteria and Actinomycetia, including antibiotic-producing Streptomyces species, and the monophyletic group of cfr genes. The basal branches of the latter contained paenibacilli and other soil bacteria; they then could be split into the clades [cfr(C):cfr(E)] and [[cfr:cfr(B)]:cfr(D)], always with different Bacillaceae in their stems. Lachnospiraceae were encountered in the basal branches of both optrA and poxtA trees. The ultimate origin of the cfr genes is the rlmN housekeeping ribosomal methylases, which evolved into a suicide-avoiding methylase in antibiotic producers; a soil organism (Lachnospiraceae, Paenibacilli) probably acted as a transfer organism into pathogenic bacteria. In the case of optrA, the porcine pathogenic Streptococcus suis was present in all branches, while the proteins closest to poxtA originated from Clostridia.

A Novel O- and S-Methyltransferase from Pleurotus sapidus Is Involved in Flavor Formation

Increasing consumer aversion to non-natural flavoring substances is prompting a heightened interest in enzymatic processes for flavor production. This includes methylation reactions, which are often performed by using hazardous chemicals. By correlation of aroma profile data and transcriptomic analysis, a novel O-methyltransferase (OMT) catalyzing a respective reaction within the formation of p-anisaldehyde was identified in the mushroom Pleurotus sapidus. Heterologous expression in E. coli followed by purification allowed for further characterization of the enzyme. Besides p-hydroxybenzaldehyde, the proposed precursor of p-anisaldehyde, the enzyme catalyzed the methylation of further hydroxylated aromatic compounds at the meta- and para-position. The Km values determined for p-hydroxybenzaldehyde and S-adenosyl-l-methionine were 80 and 107 μM, respectively. Surprisingly, the studied enzyme enabled the transmethylation of thiol-nucleophiles, as indicated by the formation of 2-methyl-3-(methylthio)furan from 2-methyl-3-furanthiol. Moreover, the enzyme was crystallized at a resolution of 2.0 Å, representing the first published crystal structure of a basidiomycetous OMT.

An optimized purification protocol for enzymatically synthesized S-adenosyl-L-methionine (SAM) for applications in solution state infrared spectroscopic studies

S-adenosyl-L-methionine (SAM) is an abundant biomolecule used by methyltransferases to regulate a wide range of essential cellular processes such as gene expression, cell signaling, protein functions, and metabolism. Despite considerable effort, there remain many specificity challenges associated with designing small molecule inhibitors for methyltransferases, most of which exhibit off-target effects. Interestingly, NMR evidence suggests that SAM undergoes conformeric exchange between several states when free in solution. Infrared spectroscopy can detect different conformers of molecules if present in appreciable populations. When SAM is noncovalently bound within enzyme active sites, the nature and the number of different conformations of the molecule are likely to be altered from when it is free in solution. If there are unique structures or different numbers of conformers between different methyltransferase active sites, solution-state information may provide promising structural leads to increase inhibitor specificity for a particular methyltransferase. Toward this goal, frequencies measured in SAM's infrared spectra must be assigned to the motions of specific atoms via isotope incorporation at discrete positions. The incorporation of isotopes into SAM's structure can be accomplished via an established enzymatic synthesis using isotopically labeled precursors. However, published protocols produced an intense and highly variable IR signal which overlapped with many of the signals from SAM rendering comparison between isotopes challenging. We observed this intense absorption to be from co-purifying salts and the SAM counterion, producing a strong, broad signal at 1100 cm-1. Here, we report a revised SAM purification protocol that mitigates the contaminating salts and present the first IR spectra of isotopically labeled CD3-SAM. These results provide a foundation for isotopic labeling experiments of SAM that will define which atoms participate in individual molecular vibrations, as a means to detect specific molecular conformations.

Structural insights into the specific recognition of mitochondrial ribosome-binding factor hsRBFA and 12 S rRNA by methyltransferase METTL15

Mitochondrial rRNA modifications are essential for mitoribosome assembly and its proper function. The m4C methyltransferase METTL15 maintains mitochondrial homeostasis by catalyzing m4C839 located in 12 S rRNA helix 44 (h44). This modification is essential to fine-tuning the ribosomal decoding center and increasing decoding fidelity according to studies of a conserved site in Escherichia coli. Here, we reported a series of crystal structures of human METTL15-hsRBFA-h44-SAM analog, METTL15-hsRBFA-SAM, METTL15-SAM and apo METTL15. The structures presented specific interactions of METTL15 with different substrates and revealed that hsRBFA recruits METTL15 to mitochondrial small subunit for further modification instead of 12 S rRNA. Finally, we found that METTL15 deficiency caused increased reactive oxygen species, decreased membrane potential and altered cellular metabolic state. Knocking down METTL15 caused an elevated lactate secretion and increased levels of histone H4K12-lactylation and H3K9-lactylation. METTL15 might be a suitable model to study the regulation between mitochondrial metabolism and histone lactylation.

The Bacillus subtilis ywbD gene encodes RlmQ, the 23S rRNA methyltransferase forming m7G2574 in the A-site of the peptidyl transferase center

Ribosomal RNA contains many posttranscriptionally modified nucleosides, particularly in the functional parts of the ribosome. The distribution of these modifications varies from one organism to another. In Bacillus subtilis, the model organism for Gram-positive bacteria, mass spectrometry experiments revealed the presence of 7-methylguanosine (m7G) at position 2574 of the 23S rRNA, which lies in the A-site of the peptidyl transferase center of the large ribosomal subunit. Testing several m7G methyltransferase candidates allowed us to identify the RlmQ enzyme, encoded by the ywbD open reading frame, as the MTase responsible for this modification. The enzyme methylates free RNA and not ribosomal 50S or 70S particles, suggesting that modification occurs in the early steps of ribosome biogenesis.

Exploring Residue-Level Interactions between the Biofilm-Driving R2ab Protein and Polystyrene Nanoparticles

In biological systems, proteins can bind to nanoparticles to form a "corona" of adsorbed molecules. The nanoparticle corona is of significant interest because it impacts an organism's response to a nanomaterial. Understanding the corona requires knowledge of protein structure, orientation, and dynamics at the surface. A residue-level mapping of protein behavior on nanoparticle surfaces is needed, but this mapping is difficult to obtain with traditional approaches. Here, we have investigated the interaction between R2ab and polystyrene nanoparticles (PSNPs) at the level of individual residues. R2ab is a bacterial surface protein from Staphylococcus epidermidis and is known to interact strongly with polystyrene, leading to biofilm formation. We have used mass spectrometry after lysine methylation and hydrogen-deuterium exchange (HDX) NMR spectroscopy to understand how the R2ab protein interacts with PSNPs of different sizes. Lysine methylation experiments reveal subtle but statistically significant changes in methylation patterns in the presence of PSNPs, indicating altered protein surface accessibility. HDX rates become slower overall in the presence of PSNPs. However, some regions of the R2ab protein exhibit faster than average exchange rates in the presence of PSNPs, while others are slower than the average behavior, suggesting conformational changes upon binding. HDX rates and methylation ratios support a recently proposed "adsorbotope" model for PSNPs, wherein adsorbed proteins consist of unfolded anchor points interspersed with partially structured regions. Our data also highlight the challenges of characterizing complex protein-nanoparticle interactions using these techniques, such as fast exchange rates. While providing insights into how R2ab adsorbs onto PSNP surfaces, this research emphasizes the need for advanced methods to comprehend residue-level interactions in the nanoparticle corona.

Functional characterization of a naphthalene-O-methyltransferase from Nocardia sp. CS682

Methylation plays important roles in biosynthesis, metabolism, signal transduction, detoxification, protein sorting and repair, and nucleic acid processing. Generally the methyltransferases transfer methyl groups in various natural products using S-adenosyl methionine (SAM) as a cofactor. In this study, we examined and functionally characterized ThnM3 (enzyme), by testing various substrates with different chemical structures. Among the tested substrates, 1,8-dihydroxynaphthalene was the best substrate for methylation. Whole-cell biotransformation was performed using the enzyme in engineered Escherichia coli to produce 8-methoxynaphthalene-1-ol, and 1,8-dimethoxynaphthalene derivatives of 1,8-dihydroxynaphthalene. The products were confirmed using high-performance liquid chromatography, mass spectrometry, and nuclear magnetic resonance spectroscopic analyses. Therefore, this study is the first to amplify, express the thnM3 (gene), and functionally characterize theThnM3, which exhibits the regioselective modifications of 1,8-dihydroxynaphthalene.

Structure of methyltransferase RedM that forms the dimethylpyrrolinium of the bisindole reductasporine

Bisindoles are biologically active natural products that arise from the oxidative dimerization of two molecules of l-tryptophan. In bacterial bisindole pathways, a core set of transformations is followed by the action of diverse tailoring enzymes that catalyze reactions that lead to diverse bisindole products. Among bisindoles, reductasporine is distinct due to its dimethylpyrrolinium structure. Its previously reported biosynthetic gene cluster encodes two unique tailoring enzymes, the imine reductase RedE and the dimethyltransferase RedM, which were shown to produce reductasporine from a common bisindole intermediate in recombinant E. coli. To gain more insight into the unique tailoring enzymes in reductasporine assembly, we reconstituted the biosynthetic pathway to reductasporine in vitro and then solved the 1.7 Å resolution structure of RedM. Our work reveals RedM adopts a variety of conformational changes with distinct open and closed conformations, and site-directed mutagenesis alongside sequence analysis identifies important active site residues. Finally, our work sets the stage for understanding how RedM evolved to react with a pyrrolinium scaffold and may enable the development of new dimethyltransferase catalysts.

Methyltransferase-like (METTL) homologues participate in Nicotiana benthamiana antiviral responses

Methyltransferase (MTase) enzymes catalyze the addition of a methyl group to a variety of biological substrates. MTase-like (METTL) proteins are Class I MTases whose enzymatic activities contribute to the epigenetic and epitranscriptomic regulation of multiple cellular processes. N⁶-adenosine methylation (m⁶A) is a common chemical modification of eukaryotic and viral RNA whose abundance is jointly regulated by MTases and METTLs, demethylases, and m⁶A binding proteins. m⁶A affects various cellular processes including RNA degradation, post-transcriptional processing, and antiviral immunity. Here, we used Nicotiana benthamiana and plum pox virus (PPV), an RNA virus of the Potyviridae family, to investigated the roles of MTases in plant-virus interaction. RNA sequencing analysis identified MTase transcripts that are differentially expressed during PPV infection; among these, accumulation of a METTL gene was significantly downregulated. Two N. benthamiana METTL transcripts (NbMETTL1 and NbMETTL2) were cloned and further characterized. Sequence and structural analyses of the two encoded proteins identified a conserved S-adenosyl methionine (SAM) binding domain, showing they are SAM-dependent MTases phylogenetically related to human METTL16 and Arabidopsis thaliana FIONA1. Overexpression of NbMETTL1 and NbMETTL2 caused a decrease of PPV accumulation. In sum, our results indicate that METTL homologues participate in plant antiviral responses.

Structural and computational insights into the regioselectivity of SpnK involved in rhamnose methylation of spinosyn

Rhamnose methylation of spinosyn critical for insecticidal activity is orchestrated by substrate specificity of three S-adenosyl-L-methionine (SAM) dependent methyltransferases (MTs). Previous in vitro enzymatic assays indicate that 3'-O-MT SpnK accepts the rhamnosylated aglycone (RAGL) and 2'-O-methylated RAGL as substrates, but does not tolerate the presence of a methoxy moiety at the O-4' position of the rhamnose unit. Here we solved the crystal structures of apo and ligand-bound SpnK, and used molecular dynamic (MD) simulations to decipher the molecular basis of substrate specificity. SpnK assembles into a tetramer, with each set of three monomers forming an integrated substrate binding pocket. The MD simulations of SpnK complexed with RAGL or 2'-O-methylated RAGL revealed that the 4'-hydroxyl of the rhamnose unit formed a hydrogen bond with a conserved Asp299 of the catalytic center, which is disrupted in structures of SpnK complexed with 4'-O-methylated RAGL or 2',4'-di-O-methylated RAGL. Comparison with SpnI methylating the C2'-hydroxyl of RAGL reveals a correlation between a DLQT/DLWT motif and the selectivity of rhamnose O-MTs. Together, our structural and computational results revealed the structural basis of substrate specificity of rhamnose O-MTs and would potentially help the engineering of spinosyn derivatives.

Escherichia coli tRNA (Gm18) methyltransferase (TrmH) requires the correct localization of its methylation site (G18) in the D-loop for efficient methylation

TrmH is a eubacterial tRNA methyltransferase responsible for formation of 2'-O-methylguaosine at position 18 (Gm18) in tRNA. In Escherichia coli cells, only 14 tRNA species possess the Gm18 modification. To investigate the substrate tRNA selection mechanism of E. coli TrmH, we performed biochemical and structural studies. Escherichia coli TrmH requires a high concentration of substrate tRNA for efficient methylation. Experiments using native tRNA SerCGA purified from a trmH gene disruptant strain showed that modified nucleosides do not affect the methylation. A gel mobility-shift assay reveals that TrmH captures tRNAs without distinguishing between relatively good and very poor substrates. Methylation assays using wild-type and mutant tRNA transcripts revealed that the location of G18 in the D-loop is very important for efficient methylation by E. coli TrmH. In the case of tRNASer, tRNATyrand tRNALeu, the D-loop structure formed by interaction with the long variable region is important. For tRNAGln, the short distance between G18 and A14 is important. Thus, our biochemical study explains all Gm18 modification patterns in E. coli tRNAs. The crystal structure of E. coli TrmH has also been solved, and the tRNA binding mode of E. coli TrmH is discussed based on the structure.

Diversity in Biological Function and Mechanism of the tRNA Methyltransferase Trm10

Transfer ribonucleic acid (tRNA) is the most highly modified RNA species in the cell, and loss of tRNA modifications can lead to growth defects in yeast as well as metabolic, neurological, and mitochondrial disorders in humans. Significant progress has been made toward identifying the enzymes that are responsible for installing diverse modifications in tRNA, revealing a landscape of fascinating biological and mechanistic diversity that remains to be fully explored. Most early discoveries of tRNA modification enzymes were in model systems, where many enzymes were not strictly required for viability, an observation somewhat at odds with the extreme conservation of many of the same enzymes throughout multiple domains of life. Moreover, many tRNA modification enzymes act on more than one type of tRNA substrate, which is not necessarily surprising given the similar overall secondary and tertiary structures of tRNA, yet biochemical characterization has revealed interesting patterns of substrate specificity that can be challenging to rationalize on a molecular level. Questions about how many enzymes efficiently select a precise set of target tRNAs from among a structurally similar pool of molecules persist.The tRNA methyltransferase Trm10 provides an exciting paradigm to study the biological and mechanistic questions surrounding tRNA modifications. Even though the enzyme was originally characterized in Saccharomyces cerevisiae where its deletion causes no detectable phenotype under standard lab conditions, several more recently identified phenotypes provide insight into the requirement for this modification in the overall quality control of the tRNA pool. Studies of Trm10 in yeast also revealed another characteristic feature that has turned out to be a conserved feature of enzymes throughout the Trm10 family tree. We were initially surprised to see that purified S. cerevisiae Trm10 was capable of modifying tRNA substrates that were not detectably modified by the enzyme in vivo in yeast. This pattern has continued to emerge as we and others have studied Trm10 orthologs from Archaea and Eukarya, with enzymes exhibiting in vitro substrate specificities that can differ significantly from in vivo patterns of modification. While this feature complicates efforts to predict substrate specificities of Trm10 enzymes in the absence of appropriate genetic systems, it also provides an exciting opportunity for studying how enzyme activities can be regulated to achieve dynamic patterns of biological tRNA modification, which have been shown to be increasingly important for stress responses and human disease. Finally, the intriguing diversity in target nucleotide modification that has been revealed among Trm10 orthologs is distinctive among known tRNA modifying enzymes and necessitates unusual and likely novel catalytic strategies for methylation that are being revealed by biochemical and structural studies directed toward various family members. These efforts will no doubt yield more surprising discoveries in terms of tRNA modification enzymology.

tRNA m1G9 modification depends on substrate-specific RNA conformational changes induced by the methyltransferase Trm10

The methyltransferase Trm10 modifies a subset of tRNAs on the base N1 position of the ninth nucleotide in the tRNA core. Trm10 is conserved throughout Eukarya and Archaea, and mutations in the human gene (TRMT10A) have been linked to neurological disorders such as microcephaly and intellectual disability, as well as defects in glucose metabolism. Of the 26 tRNAs in yeast with guanosine at position 9, only 13 are substrates for Trm10. However, no common sequence or other posttranscriptional modifications have been identified among these substrates, suggesting the presence of some other tRNA feature(s) that allow Trm10 to distinguish substrate from nonsubstrate tRNAs. Here, we show that substrate recognition by Saccharomyces cerevisiae Trm10 is dependent on both intrinsic tRNA flexibility and the ability of the enzyme to induce specific tRNA conformational changes upon binding. Using the sensitive RNA structure-probing method SHAPE, conformational changes upon binding to Trm10 in tRNA substrates, but not nonsubstrates, were identified and mapped onto a model of Trm10-bound tRNA. These changes may play an important role in substrate recognition by allowing Trm10 to gain access to the target nucleotide. Our results highlight a novel mechanism of substrate recognition by a conserved tRNA modifying enzyme. Further, these studies reveal a strategy for substrate recognition that may be broadly employed by tRNA-modifying enzymes which must distinguish between structurally similar tRNA species.

Tumor-suppressive functions of protein lysine methyltransferases

Protein lysine methyltransferases (PKMTs) play crucial roles in histone and nonhistone modifications, and their dysregulation has been linked to the development and progression of cancer. While the majority of studies have focused on the oncogenic functions of PKMTs, extensive evidence has indicated that these enzymes also play roles in tumor suppression by regulating the stability of p53 and β-catenin, promoting α-tubulin-mediated genomic stability, and regulating the transcription of oncogenes and tumor suppressors. Despite their contradictory roles in tumorigenesis, many PKMTs have been identified as potential therapeutic targets for cancer treatment. However, PKMT inhibitors may have unintended negative effects depending on the specific cancer type and target enzyme. Therefore, this review aims to comprehensively summarize the tumor-suppressive effects of PKMTs and to provide new insights into the development of anticancer drugs targeting PKMTs.

Role of epigenetic regulation in glaucoma

Glaucoma is the world's leading irreversible blinding eye disease. Lowering intraocular pressure is currently the only effective clinical treatment. However, there is a lack of long-acting IOP-lowering drugs, and some patients still experience retinal ganglion cell loss even with good intraocular pressure control. Currently, there is no effective method for neuroprotection and regeneration in clinical practice for glaucoma. In recent years, epigenetics has been widely researched and reported for its role in glaucoma's neuroprotection and regeneration. This article reviews the changes in histone modifications, DNA methylation, non-coding RNA, and m6A methylation in glaucoma, aiming to provide new perspectives for glaucoma management, protection of retinal ganglion cells, and axon regeneration by understanding epigenetic alterations.

A feedback loop between heterochromatin and the nucleopore complex controls germ-cell-to-oocyte transition during Drosophila oogenesis

Germ cells differentiate into oocytes that launch the next generation upon fertilization. How the highly specialized oocyte acquires this distinct cell fate is poorly understood. During Drosophila oogenesis, H3K9me3 histone methyltransferase SETDB1 translocates from the cytoplasm to the nucleus of germ cells concurrently with oocyte specification. Here, we discovered that nuclear SETDB1 is required for silencing a cohort of differentiation-promoting genes by mediating their heterochromatinization. Intriguingly, SETDB1 is also required for upregulating 18 of the ∼30 nucleoporins (Nups) that compose the nucleopore complex (NPC), promoting NPC formation. NPCs anchor SETDB1-dependent heterochromatin at the nuclear periphery to maintain H3K9me3 and gene silencing in the egg chambers. Aberrant gene expression due to the loss of SETDB1 or Nups results in the loss of oocyte identity, cell death, and sterility. Thus, a feedback loop between heterochromatin and NPCs promotes transcriptional reprogramming at the onset of oocyte specification, which is critical for establishing oocyte identity.

Structural similarities between SAM and ATP recognition motifs and detection of ATP binding in a SAM binding DNA methyltransferase

S-adenosylmethionine (SAM) is a ubiquitous co-factor that serves as a donor for methylation reactions and additionally serves as a donor of other functional groups such as amino and ribosyl moieties in a variety of other biochemical reactions. Such versatility in function is enabled by the ability of SAM to be recognized by a wide variety of protein molecules that vary in their sequences and structural folds. To understand what gives rise to specific SAM binding in diverse proteins, we set out to study if there are any structural patterns at their binding sites. A comprehensive analysis of structures of the binding sites of SAM by all-pair comparison and clustering, indicated the presence of 4 different site-types, only one among them being well studied. For each site-type we decipher the common minimum principle involved in SAM recognition by diverse proteins and derive structural motifs that are characteristic of SAM binding. The presence of the structural motifs with precise three-dimensional arrangement of amino acids in SAM sites that appear to have evolved independently, indicates that these are winning arrangements of residues to bring about SAM recognition. Further, we find high similarity between one of the SAM site types and a well known ATP binding site type. We demonstrate using in vitro experiments that a known SAM binding protein, HpyAII.M1, a type 2 methyltransferase can bind and hydrolyse ATP. We find common structural motifs that explain this, further supported through site-directed mutagenesis. Observation of similar motifs for binding two of the most ubiquitous ligands in multiple protein families with diverse sequences and structural folds presents compelling evidence at the molecular level in favour of convergent evolution.

Elucidating the role of nicotinamide N-methyltransferase-p53 axis in the progression of chronic kidney disease

Background

Chronic kidney disease (CKD) is a significant global health issue characterized by progressive loss of kidney function. Renal interstitial fibrosis (TIF) is a common feature of CKD, but current treatments are seldom effective in reversing TIF. Nicotinamide N-methyltransferase (NNMT) has been found to increase in kidneys with TIF, but its role in renal fibrosis is unclear.

Methods

Using mice with unilateral ureteral obstruction (UUO) and cultured renal interstitial fibroblast cells (NRK-49F) stimulated with transforming growth factor-β1 (TGF-β1), we investigated the function of NNMT in vivo and in vitro.

Results

We performed single-cell transcriptome sequencing (scRNA-seq) on the kidneys of mice and found that NNMT increased mainly in fibroblasts of UUO mice compared to sham mice. Additionally, NNMT was positively correlated with the expression of renal fibrosis-related genes after UUO injury. Knocking down NNMT expression reduced fibroblast activation and was accompanied by an increase in DNA methylation of p53 and a decrease in its phosphorylation.

Conclusions

Our findings suggest that chronic kidney injury leads to an accumulation of NNMT, which might decrease p53 methylation, and increase the expression and activity of p53. We propose that NNMT promotes fibroblast activation and renal fibrosis, making NNMT a novel target for preventing and treating renal fibrosis.

Revealing Drivers for Carboxy-S-adenosyl-l-methionine Use by Neomorphic Variants of a DNA Methyltransferase

Methylation of DNA plays a key role in diverse biological processes spanning from bacteria to mammals. DNA methyltransferases (MTases) typically employ S-adenosyl-l-methionine (SAM) as a critical cosubstrate and the relevant methyl donor for modification of the C5 position of cytosine. Recently, work on the CpG-specific bacterial MTase, M.MpeI, has shown that a single N374K point mutation can confer the enzyme with the neomorphic ability to use the sparse, naturally occurring metabolite carboxy-S-adenosyl-l-methionine (CxSAM) in order to generate the unnatural DNA modification, 5-carboxymethylcytosine (5cxmC). Here, we aimed to investigate the mechanistic basis for this DNA carboxymethyltransferase (CxMTase) activity by employing a combination of computational modeling and in vitro characterization. Modeling of substrate interactions with the enzyme variant allowed us to identify a favorable salt bridge between CxSAM and N374K that helps to rationalize selectivity of the CxMTase. Unexpectedly, we also discovered a potential role for a key active site E45 residue that makes a bidentate interaction with the ribosyl sugar of CxSAM, located on the opposite face of the CxMTase active site. Prompted by these modeling results, we further explored the space-opening E45D mutation and found that the E45D/N374K double mutant in fact inverts selectivity, preferring CxSAM over SAM in biochemical assays. These findings provide new insight into CxMTase active site architecture and may offer broader utility given the numerous opportunities offered by using SAM analogs for selective molecular labeling in concert with nucleic acid or even protein-modifying MTases.

tRNA m1G9 modification depends on substrate-specific RNA conformational changes induced by the methyltransferase Trm10

The methyltransferase Trm10 modifies a subset of tRNAs on the base N1 position of the 9th nucleotide in the tRNA core. Trm10 is conserved throughout Eukarya and Archaea, and mutations in the human gene (TRMT10A) have been linked to neurological disorders such as microcephaly and intellectual disability, as well as defects in glucose metabolism. Of the 26 tRNAs in yeast with guanosine at position 9, only 14 are substrates for Trm10. However, no common sequence or other posttranscriptional modifications have been identified among these substrates, suggesting the presence of some other tRNA feature(s) which allow Trm10 to distinguish substrate from nonsubstrate tRNAs. Here, we show that substrate recognition by Saccharomyces cerevisiae Trm10 is dependent on both intrinsic tRNA flexibility and the ability of the enzyme to induce specific tRNA conformational changes upon binding. Using the sensitive RNA structure-probing method SHAPE, conformational changes upon binding to Trm10 in tRNA substrates, but not nonsubstrates, were identified and mapped onto a model of Trm10-bound tRNA. These changes may play an important role in substrate recognition by allowing Trm10 to gain access to the target nucleotide. Our results highlight a novel mechanism of substrate recognition by a conserved tRNA modifying enzyme. Further, these studies reveal a strategy for substrate recognition that may be broadly employed by tRNA-modifying enzymes which must distinguish between structurally similar tRNA species.

Discovery of a Druggable, Cryptic Pocket in SARS-CoV-2 nsp16 Using Allosteric Inhibitors

A collaborative, open-science team undertook discovery of novel small molecule inhibitors of the SARS-CoV-2 nsp16-nsp10 2'-O-methyltransferase using a high throughput screening approach with the potential to reveal new inhibition strategies. This screen yielded compound 5a, a ligand possessing an electron-deficient double bond, as an inhibitor of SARS-CoV-2 nsp16 activity. Surprisingly, X-ray crystal structures revealed that 5a covalently binds within a previously unrecognized cryptic pocket near the S-adenosylmethionine binding cleft in a manner that prevents occupation by S-adenosylmethionine. Using a multidisciplinary approach, we examined the mechanism of binding of compound 5a to the nsp16 cryptic pocket and developed 5a derivatives that inhibited nsp16 activity and murine hepatitis virus replication in rat lung epithelial cells but proved cytotoxic to cell lines canonically used to examine SARS-CoV-2 infection. Our study reveals the druggability of this newly discovered SARS-CoV-2 nsp16 cryptic pocket, provides novel tool compounds to explore the site, and suggests a new approach for discovery of nsp16 inhibition-based pan-coronavirus therapeutics through structure-guided drug design.

Identification of a Protein Arginine Methyltransferase 7 (PRMT7)/Protein Arginine Methyltransferase 9 (PRMT9) Inhibitor

Less studied than the other protein arginine methyltransferase isoforms, PRMT7 and PRMT9 have recently been identified as important therapeutic targets. Yet, most of their biological roles and functions are still to be defined, as well as the structural requirements that could drive the identification of selective modulators of their activity. We recently described the structural requirements that led to the identification of potent and selective PRMT4 inhibitors spanning both the substrate and the cosubstrate pockets. The reanalysis of the data suggested a PRMT7 preferential binding for shorter derivatives and prompted us to extend these structural studies to PRMT9. Here, we report the identification of the first potent PRMT7/9 inhibitor and its binding mode to the two PRMT enzymes. Label-free quantification mass spectrometry confirmed significant inhibition of PRMT activity in cells. We also report the setup of an effective AlphaLISA assay to screen small molecule inhibitors of PRMT9.

Establishment of a prognostic model toward lung squamous cell carcinoma based on m7G-related genes in the cancer genome atlas

Lung squamous cell carcinoma (LUSC) is a non-small cell lung cancer with a poor prognosis owing to late diagnosis. New molecular markers are urgently needed to improve the diagnosis and prognosis of LUSC. 7-Methylguanosine (m7G) modifications, a tRNA modification, are common in eubacteria, eukaryotes, and a few archaea. These modifications promote the turnover and stability of some mRNAs to prevent mRNA decay, improve translation efficiency, and reduce ribosomal pausing but are associated with poor survival in human cancer cells. However, expression of m7G-related genes in LUSC and their association with prognosis remain unclear. In the present study, we identified nine differentially expressed genes related to prognosis by comparing the expression profiles of tumor tissues (502 LUSC reports) with normal tissues (49 adjacent nontumor lung tissue reports). The genes included six upregulated genes (KLK7, LCE3E, AREG, KLK6, ZBED2, and MAPK4) and three downregulated genes (ADH1C, NTS, and ERLIN2). Based on these nine genes, patients with LUSC were classified into low- and high-risk groups to analyze the trends in prognosis. We found that the nine m7G-related genes play important roles in immune regulation, hormone regulation, and drug sensitivity through pathways including antigen processing and presentation, adherent plaques, extracellular matrix receptor interactions, drug metabolism of cytochrome P-450, and metabolism of cytochrome P-450 to xenobiotics; the functions of these genes are likely accomplished in part by m6A modifications. The effect of m7G-related genes on the diagnosis and prognosis of LUSC was further indicated by population analysis.NEW & NOTEWORTHY Based on the differential expression of 7-methylguanosine (m7G) modification-associated genes between normal and lung squamous cell carcinoma (LUSC) tissues, and considering the performance of our m7G-related gene risk profiles as independent risk factors in predicting overall survival, we conclude that m7G modification is closely linked to the development of LUSC. In addition, this study offers a new genetic marker for predicting the prognosis of patients with LUSC and presents a crucial theoretical foundation for future investigations on the relationship between m7G modification-related genes, immunity, and drug sensitivity in LUSC.

A Nucleolar Protein, MoRRP8 Is Required for Development and Pathogenicity in the Rice Blast Fungus

The nucleolus is the largest, membrane-less organelle within the nucleus of eukaryotic cell that plays a critical role in rRNA transcription and assembly of ribosomes. Recently, the nucleolus has been shown to be implicated in an array of processes including the formation of signal recognition particles and response to cellular stress. Such diverse functions of nucleolus are mediated by nucleolar proteins. In this study, we characterized a gene coding a putative protein containing a nucleolar localization sequence (NoLS) in the rice blast fungus, Magnaporthe oryzae. Phylogenetic and domain analysis suggested that the protein is orthologous to Rrp8 in Saccharomyces cerevisiae. MoRRP8-GFP (translational fusion of MoRRP8 with green fluorescence protein) co-localizes with a nucleolar marker protein, MoNOP1 fused to red fluorescence protein (RFP), indicating that MoRRP8 is a nucleolar protein. Deletion of the MoRRP8 gene caused a reduction in vegetative growth and impinged largely on asexual sporulation. Although the asexual spores of ΔMorrp8 were morphologically indistinguishable from those of wild-type, they showed delay in germination and reduction in appressorium formation. Our pathogenicity assay revealed that the MoRRP8 is required for full virulence and growth within host plants. Taken together, these results suggest that nucleolar processes mediated by MoRRP8 is pivotal for fungal development and pathogenesis.

Linking folding dynamics and function of SAM/SAH riboswitches at the single molecule level

Riboswitches are regulatory elements found in bacterial mRNAs that control downstream gene expression through ligand-induced conformational changes. Here, we used single-molecule FRET to map the conformational landscape of the translational SAM/SAH riboswitch and probe how co-transcriptional ligand-induced conformational changes affect its translation regulation function. Riboswitch folding is highly heterogeneous, suggesting a rugged conformational landscape that allows for sampling of the ligand-bound conformation even in the absence of ligand. The addition of ligand shifts the landscape, favoring the ligand-bound conformation. Mutation studies identified a key structural element, the pseudoknot helix, that is crucial for determining ligand-free conformations and their ligand responsiveness. We also investigated ribosomal binding site accessibility under two scenarios: pre-folding and co-transcriptional folding. The regulatory function of the SAM/SAH riboswitch involves kinetically favoring ligand binding, but co-transcriptional folding reduces this preference with a less compact initial conformation that exposes the Shine-Dalgarno sequence and takes min to redistribute to more compact conformations of the pre-folded riboswitch. Such slow equilibration decreases the effective ligand affinity. Overall, our study provides a deeper understanding of the complex folding process and how the riboswitch adapts its folding pattern in response to ligand, modulates ribosome accessibility and the role of co-transcriptional folding in these processes.

S-Adenosylmethionine: more than just a methyl donor

Covering: from 2000 up to the very early part of 2023S-Adenosyl-L-methionine (SAM) is a naturally occurring trialkyl sulfonium molecule that is typically associated with biological methyltransfer reactions. However, SAM is also known to donate methylene, aminocarboxypropyl, adenosyl and amino moieties during natural product biosynthetic reactions. The reaction scope is further expanded as SAM itself can be modified prior to the group transfer such that a SAM-derived carboxymethyl or aminopropyl moiety can also be transferred. Moreover, the sulfonium cation in SAM has itself been found to be critical for several other enzymatic transformations. Thus, while many SAM-dependent enzymes are characterized by a methyltransferase fold, not all of them are necessarily methyltransferases. Furthermore, other SAM-dependent enzymes do not possess such a structural feature suggesting diversification along different evolutionary lineages. Despite the biological versatility of SAM, it nevertheless parallels the chemistry of sulfonium compounds used in organic synthesis. The question thus becomes how enzymes catalyze distinct transformations via subtle differences in their active sites. This review summarizes recent advances in the discovery of novel SAM utilizing enzymes that rely on Lewis acid/base chemistry as opposed to radical mechanisms of catalysis. The examples are categorized based on the presence of a methyltransferase fold and the role played by SAM within the context of known sulfonium chemistry.

Exploring the Residue-Level Interactions between the R2ab Protein and Polystyrene Nanoparticles

In biological systems, proteins can bind to nanoparticles to form a "corona" of adsorbed molecules. The nanoparticle corona is of high interest because it impacts the organism's response to the nanomaterial. Understanding the corona requires knowledge of protein structure, orientation, and dynamics at the surface. Ultimately, a residue-level mapping of protein behavior on nanoparticle surfaces is needed, but this mapping is difficult to obtain with traditional approaches. Here, we have investigated the interaction between R2ab and polystyrene nanoparticles (PSNPs) at the level of individual residues. R2ab is a bacterial surface protein from Staphylococcus epidermidis and is known to interact strongly with polystyrene, leading to biofilm formation. We have used mass spectrometry after lysine methylation and hydrogen-deuterium exchange (HDX) NMR spectroscopy to understand how the R2ab protein interacts with PSNPs of different sizes. Through lysine methylation, we observe subtle but statistically significant changes in methylation patterns in the presence of PSNPs, indicating altered protein surface accessibility. HDX measurements reveal that certain regions of the R2ab protein undergo faster exchange rates in the presence of PSNPs, suggesting conformational changes upon binding. Both results support a recently proposed "adsorbotope" model, wherein adsorbed proteins consist of unfolded anchor points interspersed with regions of partial structure. Our data also highlight the challenges of characterizing complex protein-nanoparticle interactions using these techniques, such as fast exchange rates. While providing insights into how proteins respond to nanoparticle surfaces, this research emphasizes the need for advanced methods to comprehend these intricate interactions fully at the residue level.

Chemical probes and methods for the study of protein arginine methylation

Protein arginine methylation is a widespread post-translational modification (PTM) in eukaryotic cells. This chemical modification in proteins functionally modulates diverse cellular processes from signal transduction, gene expression, and DNA damage repair to RNA splicing. The chemistry of arginine methylation entails the transfer of the methyl group from S-adenosyl-l-methionine (AdoMet, SAM) onto a guanidino nitrogen atom of an arginine residue of a target protein. This reaction is catalyzed by about 10 members of protein arginine methyltransferases (PRMTs). With impacts on a variety of cellular processes, aberrant expression and activity of PRMTs have been shown in many disease conditions. Particularly in oncology, PRMTs are commonly overexpressed in many cancerous tissues and positively correlated with tumor initiation, development and progression. As such, targeting PRMTs is increasingly recognized as an appealing therapeutic strategy for new drug discovery. In the past decade, a great deal of research efforts has been invested in illuminating PRMT functions in diseases and developing chemical probes for the mechanistic study of PRMTs in biological systems. In this review, we provide a brief developmental history of arginine methylation along with some key updates in arginine methylation research, with a particular emphasis on the chemical aspects of arginine methylation. We highlight the research endeavors for the development and application of chemical approaches and chemical tools for the study of functions of PRMTs and arginine methylation in regulating biology and disease.

Polyketide Synthase-Mediated O-Methyloxime Formation in the Biosynthesis of the Oximidine Anticancer Agents

Bacterial trans-acyltransferase polyketide synthases (trans-AT PKSs) are modular megaenzymes that employ unusual catalytic domains to assemble diverse bioactive natural products. One such PKS is responsible for the biosynthesis of the oximidine anticancer agents, oxime-substituted benzolactone enamides that inhibit vacuolar H+ -ATPases. Here, we describe the identification of the oximidine gene cluster in Pseudomonas baetica and the characterization of four novel oximidine variants, including a structurally simpler intermediate that retains potent anticancer activity. Using a combination of in vivo, in vitro and computational approaches, we experimentally elucidate the oximidine biosynthetic pathway and reveal an unprecedented mechanism for O-methyloxime formation. We show that this process involves a specialized monooxygenase and methyltransferase domain and provide insight into their activity, mechanism and specificity. Our findings expand the catalytic capabilities of trans-AT PKSs and identify potential strategies for the production of novel oximidine analogues.

Nucleolar Essential Protein 1 (Nep1): Elucidation of enzymatic catalysis mechanism by molecular dynamics simulation and quantum mechanics study

The Nep1 protein is essential for the formation of eukaryotic and archaeal small ribosomal subunits, and it catalyzes the site-directed SAM-dependent methylation of pseudouridine (Ψ) during pre-rRNA processing. It possesses a non-trivial topology, namely, a 31 knot in the active site. Here, we address the issue of seemingly unfeasible deprotonation of Ψ in Nep1 active site by a distant aspartate residue (D101 in S. cerevisiae), using a combination of bioinformatics, computational, and experimental methods. We identified a conserved hydroxyl-containing amino acid (S233 in S. cerevisiae, T198 in A. fulgidus) that may act as a proton-transfer mediator. Molecular dynamics simulations, based on the crystal structure of S. cerevisiae, and on a complex generated by molecular docking in A. fulgidus, confirmed that this amino acid can shuttle protons, however, a water molecule in the active site may also serve this role. Quantum-chemical calculations based on density functional theory and the cluster approach showed that the water-mediated pathway is the most favorable for catalysis. Experimental kinetic and mutational studies reinforce the requirement for the aspartate D101, but not S233. These findings provide insight into the catalytic mechanisms underlying proton transfer over extended distances and comprehensively elucidate the mode of action of Nep1.

Biochemical and structural characterization of the first-discovered metazoan DNA cytosine-N4 methyltransferase from the bdelloid rotifer Adineta vaga

Much is known about the generation, removal, and roles of 5-methylcytosine (5mC) in eukaryote DNA, and there is a growing body of evidence regarding N6-methyladenine, but very little is known about N4-methylcytosine (4mC) in the DNA of eukaryotes. The gene for the first metazoan DNA methyltransferase generating 4mC (N4CMT) was reported and characterized recently by others, in tiny freshwater invertebrates called bdelloid rotifers. Bdelloid rotifers are ancient, apparently asexual animals, and lack canonical 5mC DNA methyltransferases. Here, we characterize the kinetic properties and structural features of the catalytic domain of the N4CMT protein from the bdelloid rotifer Adineta vaga. We find that N4CMT generates high-level methylation at preferred sites, (a/c)CG(t/c/a), and low-level methylation at disfavored sites, exemplified by ACGG. Like the mammalian de novo 5mC DNA methyltransferase 3A/3B (DNMT3A/3B), N4CMT methylates CpG dinucleotides on both DNA strands, generating hemimethylated intermediates and eventually fully methylated CpG sites, particularly in the context of favored symmetric sites. In addition, like DNMT3A/3B, N4CMT methylates non-CpG sites, mainly CpA/TpG, though at a lower rate. Both N4CMT and DNMT3A/3B even prefer similar CpG-flanking sequences. Structurally, the catalytic domain of N4CMT closely resembles the Caulobacter crescentus cell cycle-regulated DNA methyltransferase. The symmetric methylation of CpG, and similarity to a cell cycle-regulated DNA methyltransferase, together suggest that N4CMT might also carry out DNA synthesis-dependent methylation following DNA replication.

Structural and Biophysical properties of therapeutically important proteins Rv1509 and Rv2231A of Mycobacterium tuberculosis

Through comparative analyses using BLASTp and BLASTn of the 25 target sequences, our research identified two unique post-transcriptional modifiers, Rv1509 and Rv2231A, which serve as distinctive and characteristic proteins of M.tb - the Signature Proteins. Here, we have characterized these two signature proteins associated with pathophysiology of M.tb which may prove to be therapeutically important targets. Dynamic Light Scattering and Analytical Gel Filtration Chromatography exhibited that Rv1509 exists as a monomer while Rv2231A as a dimer in solution. Secondary structures were determined using Circular Dichroism and further validated through Fourier Transform Infrared spectroscopy. Both the proteins are capable of withstanding a wide range of temperature and pH variations. Fluorescence spectroscopy based binding affinity experiments showed that Rv1509 binds to iron and may promote organism growth by chelating iron. In the case of Rv2231A, a high affinity for its substrate RNA was observed, which is facilitated in presence of Mg2+ suggesting it might have RNAse activity, supporting the prediction through in-silico studies. This is the first study on biophysical characterization of these two therapeutically important proteins, Rv1509 and Rv2231A, providing important insights into their structure -function correlations which are crucial for development of new drugs/ early diagnostics tools targeting these proteins.

METTL7A (TMT1A) and METTL7B (TMT1B) Are Responsible for Alkyl S-Thiol Methyl Transferase Activity in Liver

S-methylation of drugs containing thiol-moieties often alters their activity and results in detoxification. Historically, scientists attributed methylation of exogenous aliphatic and phenolic thiols to a putative S-adenosyl-L-methionine (SAM)-dependent membrane-associated enzyme referred to as thiol methyltransferase (TMT). This putative TMT appeared to have a broad substrate specificity and methylated the thiol metabolite of spironolactone, mertansine, ziprasidone, captopril, and the active metabolites of the thienopyridine prodrugs, clopidogrel, and prasugrel. Despite TMT's role in the S-methylation of clinically relevant drugs, the enzyme(s) responsible for this activity remained unknown. We recently identified methyltransferase-like protein 7B (METTL7B) as an alkyl thiol methyltransferase. METTL7B is an endoplasmic reticulum-associated protein with similar biochemical properties and substrate specificity to the putative TMT. Yet, the historic TMT inhibitor 2,3-dichloro-α-methylbenzylamine (DCMB) did not inhibit METTL7B, indicating that multiple enzymes contribute to TMT activity. Here we report that methyltransferase-like protein 7A (METTL7A), an uncharacterized member of the METTL7 family, is also a SAM-dependent thiol methyltransferase. METTL7A exhibits similar biochemical properties to METTL7B and putative TMT, including inhibition by DCMB (IC50 = 1.17 μM). Applying quantitative proteomics to human liver microsomes and gene modulation experiments in HepG2 and HeLa cells, we determined that TMT activity correlates closely with METTL7A and METTL7B protein levels. Furthermore, purification of a novel His-GST-tagged recombinant protein and subsequent activity experiments prove that METTL7A can selectively methylate exogenous thiol-containing substrates, including 7α-thiospironolactone, dithiothreitol, 4-chlorothiophenol, and mertansine. We conclude that the METTL7 family encodes for two enzymes, METTL7A and METTL7B, which are now renamed thiol methyltransferase 1A (TMT1A) and thiol methyltransferase 1B (TMT1B), respectively, that are responsible for thiol methylation activity in human liver microsomes. SIGNIFICANCE STATEMENT: We identified methyltransferase-like protein 7A (thiol methyltransferase 1A) and methyltransferase-like protein 7B (thiol methyltransferase 1B) as the enzymes responsible for the microsomal alkyl thiol methyltransferase (TMT) activity. These are the first two enzymes directly associated with microsomal TMT activity. S-methylation of commonly prescribed thiol-containing drugs alters their pharmacological activity and/or toxicity, and identifying the enzymes responsible for this activity will improve our understanding of the drug metabolism and pharmacokinetic (DMPK) properties of alkyl- or phenolic thiol-containing therapeutics.

Structural basis for recognition and methylation of p97 by METTL21D, a valosin-containing protein lysine methyltransferase

p97 is a human AAA+ (ATPase associated with diverse cellular activities, also known as valosin-containing protein [VCP]) ATPase, which is involved in diverse cellular processes such as membrane fusion and proteolysis. Lysine-specific methyltransferase of p97 (METTL21D) was identified as a class I methyltransferase that catalyzes the trimethylation of Lys315 of p97, a so-called VCP lysine methyltransferase (VCPKMT). Interestingly, VCPKMT disassembles a single hexamer ring consisting of p97-D1 domain and methylates Lys315 residue. Herein, the structures of S-adenosyl-L-methionine-bound VCPKMT and S-adenosyl-L-homocysteine-bound VCPKMT in complex with p97 N/D1 (N21-Q458) were reported at a resolution of 1.8 Å and 2.8 Å, respectively. The structures revealed the molecular details for the recognition and methylation of monomeric p97 by VCPKMT. Using biochemical analysis, we also investigated whether the methylation of full-length p97 could be sufficiently enhanced through cooperation between VCPKMT and the C terminus of alveolar soft part sarcoma locus (ASPL). Our study provides the groundwork for future structural and mechanistic studies of p97 and inhibitors.

Defining the elusive oncogenic role of the methyltransferase TMT1B

Methyltransferases are enzymes fundamental to a wide range of normal biological activities that can become dysregulated during oncogenesis. For instance, the recent description of the methyltransferase-like (METTL) family of enzymes, has demonstrated the importance of the N⁶-adenosine-methyltransferase (m⁶A) modification in transcripts in the context of malignant transformation. Because of their importance, numerous METTL family members have been biochemically characterized to identify their cellular substrates, however some members such as METTL7B, recently renamed TMT1B and which is the subject of this review, remain enigmatic. First identified in the stacked Golgi, TMT1B is also localized to the endoplasmic reticulum as well as lipid droplets and has been reported as being upregulated in a wide range of cancer types including lung cancer, gliomas, and leukemia. Interestingly, despite evidence that TMT1B might act on protein substrates, it has also been shown to act on small molecule alkyl thiol substrates such as hydrogen sulfide, and its loss has been found to affect cellular proliferation and migration. Here we review the current evidence for TMT1B's activity, localization, and potential biological role in the context of both normal and cancerous cell types.