METTL13 Methylation of eEF1A Increases Translational Output to Promote Tumorigenesis.

Increased protein synthesis plays an etiologic role in diverse cancers. Here, we demonstrate that METTL13 (methyltransferase-like 13) dimethylation of eEF1A (eukaryotic elongation factor 1A) lysine 55 (eEF1AK55me2) is utilized by Ras-driven cancers to increase translational output and promote tumorigenesis in vivo. METTL13-catalyzed eEF1A methylation increases eEF1A's intrinsic GTPase activity in vitro and protein production in cells. METTL13 and eEF1AK55me2 levels are upregulated in cancer and negatively correlate with pancreatic and lung cancer patient survival. METTL13 deletion and eEF1AK55me2 loss dramatically reduce Ras-driven neoplastic growth in mouse models and in patient-derived xenografts (PDXs) from primary pancreatic and lung tumors. Finally, METTL13 depletion renders PDX tumors hypersensitive to drugs that target growth-signaling pathways. Together, our work uncovers a mechanism by which lethal cancers become dependent on the METTL13-eEF1AK55me2 axis to meet their elevated protein synthesis requirement and suggests that METTL13 inhibition may constitute a targetable vulnerability of tumors driven by aberrant Ras signaling.

Structural remodeling of AAA+ ATPase p97 by adaptor protein ASPL facilitates posttranslational methylation by METTL21D.

p97 is an essential AAA+ ATPase that extracts and unfolds substrate proteins from membranes and protein complexes. Through its mode of action, p97 contributes to various cellular processes, such as membrane fusion, ER-associated protein degradation, DNA repair, and many others. Diverse p97 functions and protein interactions are regulated by a large number of adaptor proteins. Alveolar soft part sarcoma locus (ASPL) is a unique adaptor protein that regulates p97 by disassembling functional p97 hexamers to smaller entities. An alternative mechanism to regulate the activity and interactions of p97 is by posttranslational modifications (PTMs). Although more than 140 PTMs have been identified in p97, only a handful of those have been described in detail. Here we present structural and biochemical data to explain how the p97-remodeling adaptor protein ASPL enables the metastasis promoting methyltransferase METTL21D to bind and trimethylate p97 at a single lysine side chain, which is deeply buried inside functional p97 hexamers. The crystal structure of a heterotrimeric p97:ASPL:METTL21D complex in the presence of cofactors ATP and S-adenosyl homocysteine reveals how structural remodeling by ASPL exposes the crucial lysine residue of p97 to facilitate its trimethylation by METTL21D. The structure also uncovers a role of the second region of homology (SRH) present in the first ATPase domain of p97 in binding of a modifying enzyme to the AAA+ ATPase. Investigation of this interaction in the human, fish, and plant reveals fine details on the mechanism and significance of p97 trimethylation by METTL21D across different organisms.

Methylation and phosphorylation of formin homology domain proteins (Fhod1 and Fhod3) by protein arginine methyltransferase 7 (PRMT7) and Rho Kinase (ROCK1).

Protein post translational modifications (PTMs) can regulate biological processes by altering an amino acid's bulkiness, charge, and hydrogen bonding interactions. Common modifications include phosphorylation, methylation, acetylation and ubiquitylation. Although a primary focus of studying PTMs is understanding the effects of a single amino acid modification, the possibility of additional modifications increases the complexity. For example, substrate recognition motifs for arginine methyltransferases and some serine/threonine kinases overlap, leading to potential enzymatic crosstalk. In this study we have shown that the human family of formin homology domain containing proteins (Fhods) contain a substrate recognition motif specific for human protein arginine methyltransferase 7 (PRMT7). In particular, PRMT7 methylates two arginine residues in the diaphanous autoinhibitory domain (DAD) of the family of Fhod proteins: R1588 and/or R1590 of Fhod3 isoform 4. Additionally, we confirmed that S1589 and S1595 in the DAD domain of Fhod3 can be phosphorylated by Rho/ROCK1 kinase. Significantly, we have determined that if S1589 is phosphorylated then PRMT7 cannot subsequently methylate R1588 or R1590. In contrast, if R1588 or R1590 of Fhod3 is methylated then ROCK1 phosphorylation activity is only slightly affected. Finally, we show that the interaction of the N-terminal DID domain can also inhibit the methylation of the DAD domain. Taken together these results suggest that the family of Fhod proteins, potential in vivo substrates for PRMT7, might be regulated by a combination of methylation and phosphorylation.

Methylation of elongation factor 1A by yeast Efm4 or human eEF1A-KMT2 involves a beta-hairpin recognition motif and crosstalks with phosphorylation.

Translation elongation factor 1A (eEF1A) is an essential and highly conserved protein required for protein synthesis in eukaryotes. In both Saccharomyces cerevisiae and human, five different methyltransferases methylate specific residues on eEF1A, making eEF1A the eukaryotic protein targeted by the highest number of dedicated methyltransferases after histone H3. eEF1A methyltransferases are highly selective enzymes, only targeting eEF1A and each targeting just one or two specific residues in eEF1A. However, the mechanism of this selectivity remains poorly understood. To reveal how S. cerevisiae elongation factor methyltransferase 4 (Efm4) specifically methylates eEF1A at K316, we have used AlphaFold-Multimer modeling in combination with crosslinking mass spectrometry (XL-MS) and enzyme mutagenesis. We find that a unique beta-hairpin motif, which extends out from the core methyltransferase fold, is important for the methylation of eEF1A K316 in vitro. An alanine mutation of a single residue on this beta-hairpin, F212, significantly reduces Efm4 activity in vitro and in yeast cells. We show that the equivalent residue in human eEF1A-KMT2 (METTL10), F220, is also important for its activity towards eEF1A in vitro. We further show that the eEF1A guanine nucleotide exchange factor, eEF1Bα, inhibits Efm4 methylation of eEF1A in vitro, likely due to competitive binding. Lastly, we find that phosphorylation of eEF1A at S314 negatively crosstalks with Efm4-mediated methylation of K316. Our findings demonstrate how protein methyltransferases can be highly selective towards a single residue on a single protein in the cell.

Structural characterization of methylation-independent PP2A assembly guides alphafold2Multimer prediction of family-wide PP2A complexes.

Dysregulation of phosphorylation-dependent signaling is a hallmark of tumorigenesis. Protein phosphatase 2 (PP2A) is an essential regulator of cell growth. One scaffold subunit (A) binds to a catalytic subunit (C) to form a core AC heterodimer, which together with one of many regulatory (B) subunits forms the active trimeric enzyme. The combinatorial number of distinct PP2A complexes is large, which results in diverse substrate specificity and subcellular localization. The detailed mechanism of PP2A assembly and regulation remains elusive and reports about an important role of methylation of the carboxy terminus of PP2A C are conflicting. A better understanding of the molecular underpinnings of PP2A assembly and regulation is critical to dissecting PP2A function in physiology and disease. Here, we combined biochemical reconstitution, mass spectrometry, X-ray crystallography, and functional assays to characterize the assembly of trimeric PP2A. In vitro studies demonstrated that methylation of the carboxy-terminus of PP2A C was dispensable for PP2A assembly in vitro. To corroborate these findings, we determined the X-ray crystal structure of the unmethylated PP2A Aα-B56ε-Cα trimer complex to 3.1 Å resolution. The experimental structure superimposed well with an Alphafold2Multimer prediction of the PP2A trimer. We then predicted models of all canonical PP2A complexes providing a framework for structural analysis of PP2A. In conclusion, methylation was dispensable for trimeric PP2A assembly and integrative structural biology studies of PP2A offered predictive models for all canonical PP2A complexes.

Chemoreceptors in Sinorhizobium meliloti require minimal pentapeptide tethers to provide adaptational assistance.

Sensory adaptation in bacterial chemotaxis is mediated by posttranslational modifications of methyl-accepting chemotaxis proteins (MCPs). In Escherichia coli, the adaptation proteins CheR and CheB tether to a conserved C-terminal receptor pentapeptide. Here,we investigated the function of the pentapeptide motif (N/D)WE(E/N)F in Sinorhizobium meliloti chemotaxis. Isothermal titration calorimetry revealed stronger affinity of the pentapeptides to CheR and activated CheB relative to unmodified CheB. Strains with mutations of the conserved tryptophan in one or all four MCP pentapeptides resulted in a significant decrease or loss of chemotaxis to glycine betaine, lysine, and acetate, chemoattractants sensed by pentapeptide-bearing McpX and pentapeptide-lacking McpU and McpV, respectively. Importantly, we discovered that the pentapeptide mediates chemotaxis when fused to the C-terminus of pentapeptide-lacking chemoreceptors via a flexible linker. We propose that adaptational assistance and a threshold number of available sites enable the efficient docking of adaptation proteins to the chemosensory array. Altogether, these results demonstrate that S. meliloti effectively utilizes a pentapeptide-dependent adaptation system with a minimal number of tethering units to assist pentapeptide-lacking chemoreceptors and hypothesize that the higher abundance of CheR and CheB in S. meliloti compared to E. coli allows for ample recruitment of adaptation proteins to the chemosensory array.

Nitric Oxide Regulates Protein Methylation during Stress Responses in Plants.

Methylation and nitric oxide (NO)-based S-nitrosylation are highly conserved protein posttranslational modifications that regulate diverse biological processes. In higher eukaryotes, PRMT5 catalyzes Arg symmetric dimethylation, including key components of the spliceosome. The Arabidopsis prmt5 mutant shows severe developmental defects and impaired stress responses. However, little is known about the mechanisms regulating the PRMT5 activity. Here, we report that NO positively regulates the PRMT5 activity through S-nitrosylation at Cys-125 during stress responses. In prmt5-1 plants, a PRMT5C125S transgene, carrying a non-nitrosylatable mutation at Cys-125, fully rescues the developmental defects, but not the stress hypersensitive phenotype and the responsiveness to NO during stress responses. Moreover, the salt-induced Arg symmetric dimethylation is abolished in PRMT5C125S/prmt5-1 plants, correlated to aberrant splicing of pre-mRNA derived from a stress-related gene. These findings define a mechanism by which plants transduce stress-triggered NO signal to protein methylation machinery through S-nitrosylation of PRMT5 in response to environmental alterations.

Polycomb Repressive Complex 2 Methylates Elongin A to Regulate Transcription.

Polycomb repressive complex 2 (PRC2-EZH2) methylates histone H3 at lysine 27 (H3K27) and is required to maintain gene repression during development. Misregulation of PRC2 is linked to a range of neoplastic malignancies, which is believed to involve methylation of H3K27. However, the full spectrum of non-histone substrates of PRC2 that might also contribute to PRC2 function is not known. We characterized the target recognition specificity of the PRC2 active site and used the resultant data to screen for uncharacterized potential targets. The RNA polymerase II (Pol II) transcription elongation factor, Elongin A (EloA), is methylated by PRC2 in vivo. Mutation of the methylated EloA residue decreased repression of a subset of PRC2 target genes as measured by both steady-state and nascent RNA levels and perturbed embryonic stem cell differentiation. We propose that PRC2 modulates transcription of a subset of low expression target genes in part via methylation of EloA.

Antibody toolkit to investigate eEF1A methylation dynamics in mRNA translation elongation.

Protein synthesis is a fundamental step in gene expression, with modulation of mRNA translation at the elongation step emerging as an important regulatory node in shaping cellular proteomes. In this context, five distinct lysine methylation events on eukaryotic elongation factor 1A (eEF1A), a fundamental nonribosomal elongation factor, are proposed to influence mRNA translation elongation dynamics. However, a lack of affinity tools has hindered progress in fully understanding how eEF1A lysine methylation impacts protein synthesis. Here we develop and characterize a suite of selective antibodies to investigate eEF1A methylation and provide evidence that methylation levels decline in aged tissue. Determination of the methyl state and stoichiometry on eEF1A in various cell lines by mass spectrometry shows modest cell-to-cell variability. We also find by Western blot analysis that knockdown of individual eEF1A-specific lysine methyltransferases leads to depletion of the cognate lysine methylation event and indicates active crosstalk between different sites. Further, we find that the antibodies are specific in immunohistochemistry applications. Finally, application of the antibody toolkit suggests that several eEF1A methylation events decrease in aged muscle tissue. Together, our study provides a roadmap for leveraging methyl state and sequence-selective antibody reagents to accelerate discovery of eEF1A methylation-related functions and suggests a role for eEF1A methylation, via protein synthesis regulation, in aging biology.

A snapshot of the Physcomitrella N-terminome reveals N-terminal methylation of organellar proteins.

KEY MESSAGE: Analysis of the N-terminome of Physcomitrella reveals N-terminal monomethylation of nuclear-encoded, mitochondria-localized proteins. Post- or co-translational N-terminal modifications of proteins influence their half-life as well as mediating protein sorting to organelles via cleavable N-terminal sequences that are recognized by the respective translocation machinery. Here, we provide an overview on the current modification state of the N-termini of over 4500 proteins from the model moss Physcomitrella (Physcomitrium patens) using a compilation of 24 N-terminomics datasets. Our data reveal distinct proteoforms and modification states and confirm predicted targeting peptide cleavage sites of 1,144 proteins localized to plastids and the thylakoid lumen, to mitochondria, and to the secretory pathway. In addition, we uncover extended N-terminal methylation of mitochondrial proteins. Moreover, we identified PpNTM1 (P. patens alpha N-terminal protein methyltransferase 1) as a candidate for protein methylation in plastids, mitochondria, and the cytosol. These data can now be used to optimize computational targeting predictors, for customized protein fusions and their targeted localization in biotechnology, and offer novel insights into potential dual targeting of proteins.

ProMetheusDB: An In-Depth Analysis of the High-Quality Human Methyl-proteome.

Protein arginine (R) methylation is a post-translational modification involved in various biological processes, such as RNA splicing, DNA repair, immune response, signal transduction, and tumor development. Although several advancements were made in the study of this modification by mass spectrometry, researchers still face the problem of a high false discovery rate. We present a dataset of high-quality methylations obtained from several different heavy methyl stable isotope labeling with amino acids in cell culture experiments analyzed with a machine learning-based tool and show that this model allows for improved high-confidence identification of real methyl-peptides. Overall, our results are consistent with the notion that protein R methylation modulates protein-RNA interactions and suggest a role in rewiring protein-protein interactions, for which we provide experimental evidence for a representative case (i.e., NONO [non-POU domain-containing octamer-binding protein]-paraspeckle component 1 [PSPC1]). Upon intersecting our R-methyl-sites dataset with the PhosphoSitePlus phosphorylation dataset, we observed that R methylation correlates differently with S/T-Y phosphorylation in response to various stimuli. Finally, we explored the application of heavy methyl stable isotope labeling with amino acids in cell culture to identify unconventional methylated residues and successfully identified novel histone methylation marks on serine 28 and threonine 32 of H3. The database generated, named ProMetheusDB, is freely accessible at https://bioserver.ieo.it/shiny/app/prometheusdb.

The Role of Protein Methyltransferases in Immunity.

The immune system protects our body from bacteria, viruses, and toxins and removes malignant cells. Activation of immune cells requires the onset of a network of important signaling proteins. Methylation of these proteins affects their structure and biological function. Under stimulation, T cells, B cells, and other immune cells undergo activation, development, proliferation, differentiation, and manufacture of cytokines and antibodies. Methyltransferases alter the above processes and lead to diverse outcomes depending on the degree and type of methylation. In the previous two decades, methyltransferases have been reported to mediate a great variety of immune stages. Elucidating the roles of methylation in immunity not only contributes to understanding the immune mechanism but is helpful in the development of new immunotherapeutic strategies. Hence, we review herein the studies on methylation in immunity, aiming to provide ideas for new approaches.

Non-histone protein methylation in Trypanosoma cruzi epimastigotes.

Post-translational methylation of proteins, which occurs in arginines and lysines, modulates several biological processes at different levels of cell signaling. Recently, methylation has been demonstrated in the regulation beyond histones, for example, in the dynamics of protein-protein and protein-nucleic acid interactions. However, the presence and role of non-histone methylation in Trypanosoma cruzi, the etiologic agent of Chagas disease, has not yet been elucidated. Here, we applied mass spectrometry-based-proteomics (LC-MS/MS) to profile the methylproteome of T. cruzi epimastigotes, describing a total of 1252 methyl sites in 824 proteins. Functional enrichment and protein-protein interaction analysis show that protein methylation impacts important biological processes of the parasite, such as translation, RNA and DNA binding, amino acid, and carbohydrate metabolism. In addition, 171 of the methylated proteins were previously reported to bear phosphorylation sites in T. cruzi, including flagellar proteins and RNA binding proteins, indicating that there may be an interplay between these different modifications in non-histone proteins. Our results show that a broad spectrum of functions is affected by methylation in T. cruzi, indicating its potential to impact important processes in the biology of the parasite and other trypanosomes.

Extracellular 5'-methylthioadenosine inhibits intracellular symmetric dimethylarginine protein methylation of FUSE-binding proteins.

Methylthioadenosine phosphorylase (MTAP) is a key enzyme in the methionine salvage pathway that converts the polyamine synthesis byproduct 5'-deoxy-5'-methylthioadenosine (MTA) into methionine. Inactivation of MTAP, often by homozygous deletion, is found in both solid and hematologic malignancies and is one of the most frequently observed genetic alterations in human cancer. Previous work established that MTAP-deleted cells accumulate MTA and contain decreased amounts of proteins with symmetric dimethylarginine (sDMA). These findings led to the hypothesis that accumulation of intracellular MTA inhibits the protein arginine methylase (PRMT5) responsible for bulk protein sDMAylation. Here, we confirm that MTAP-deleted cells have increased MTA accumulation and reduced protein sDMAylation. However, we also show that addition of extracellular MTA can cause a dramatic reduction of the steady-state levels of sDMA-containing proteins in MTAP+ cells, even though no sustained increase in intracellular MTA is found because of catabolism of MTA by MTAP. We determined that inhibition of protein sDMAylation by MTA occurs within 48 h, is reversible, and is specific. In addition, we have identified two enhancer-binding proteins, FUBP1 and FUBP3, that are differentially sDMAylated in response to MTAP and MTA. These proteins work via the far upstream element site located upstream of Myc and other promoters. Using a transcription reporter construct containing the far upstream element site, we demonstrate that MTA addition can reduce transcription, suggesting that the reduction in FUBP1 and FUBP3 sDMAylation has functional consequences. Overall, our findings show that extracellular MTA can inhibit protein sDMAylation and that this inhibition can affect FUBP function.

Protein methylation in mitochondria.

Many proteins are modified by posttranslational methylation, introduced by a number of methyltransferases (MTases). Protein methylation plays important roles in modulating protein function and thus in optimizing and regulating cellular and physiological processes. Research has mainly focused on nuclear and cytosolic protein methylation, but it has been known for many years that also mitochondrial proteins are methylated. During the last decade, significant progress has been made on identifying the MTases responsible for mitochondrial protein methylation and addressing its functional significance. In particular, several novel human MTases have been uncovered that methylate lysine, arginine, histidine, and glutamine residues in various mitochondrial substrates. Several of these substrates are key components of the bioenergetics machinery, e.g., respiratory Complex I, citrate synthase, and the ATP synthase. In the present review, we report the status of the field of mitochondrial protein methylation, with a particular emphasis on recently discovered human MTases. We also discuss evolutionary aspects and functional significance of mitochondrial protein methylation and present an outlook for this emergent research field.

Human protein arginine methyltransferases (PRMTs) can be optimally active under nonphysiological conditions.

Protein arginine methylation is involved in many biological processes and can be enhanced in cancer. In mammals, these reactions are catalyzed on multiple substrates by a family of nine protein arginine methyltransferases (PRMTs). However, conditions that may regulate the activity of each enzyme and that may help us understand the physiological role of PRMTs have not been fully established. Previous studies had suggested unexpected effects of temperature and ionic strength on PRMT7 activity. Here we examine in detail the effects of temperature, pH, and ionic strength on recombinant human PRMT1, PRMT5, and PRMT7. We confirmed the unusual temperature dependence of PRMT7, where optimal activity was observed at 15 °C. On the other hand, we found that PRMT1 and PRMT5 are most active near physiological temperatures of 37 °C. However, we showed all three enzymes still have significant activity at 0 °C. Furthermore, we determined that PRMT1 is most active at a pH of about 7.7, while PRMT5 activity is not dependent on pH in the range of 6.5 to 8.5. Significantly, PRMT7 is most active at an alkaline pH of 8.5 but shows little activity at the physiological intracellular pH of about 7.2. We also detected decreased activity at physiological salt conditions for PRMT1, PRMT5, and PRMT7. We demonstrate that the loss of activity is due to the increasing ionic strength. Taken together, these results open the possibility that PRMTs respond in cells undergoing temperature, salt, or pH stress and demonstrate the potential for in vivo regulation of protein arginine methylation.

Limited proteolysis of mycobacterial DNA-binding protein 1 with an extended, lysine-rich, intrinsically disordered region to unveil posttranslational modifications.

The basic, intrinsically disordered regions of eukaryotic histones and their bacterial counterparts are presumed to act as signaling hubs to regulate the compaction of chromosomes or nucleoids and various DNA processes such as gene expression, recombination, and DNA replication. Posttranslational modifications (PTMs) on these regions are pivotal in regulating chromosomal or nucleoid compaction and DNA processes. However, the low sequence complexity and the presence of short lysine-rich repeats in the regions have hindered the accurate determination of types and locations of PTMs using conventional proteomic procedures. We described a limited proteolysis protocol using trypsin to analyze PTMs on mycobacterial DNA-binding protein 1 (MDP1), a nucleoid-associated protein in mycobacterial species that possesses an extended, lysine-rich, intrinsically disordered region in its C-terminal domain. This limited proteolysis approach successfully revealed significant methylation on many lysine residues in the C-terminal domain of MDP1 purified from Mycobacterium tuberculosis, which was lacking in the corresponding region of recombinant MDP1 expressed in Escherichia coli.

Methylarginine efflux in nutrient-deprived yeast mitigates disruption of nitric oxide synthesis.

Protein arginine N-methyltransferases (PRMTs) have emerged as important actors in the eukaryotic stress response with implications in human disease, aging, and cell signaling. Intracellular free methylarginines contribute to cellular stress through their interaction with nitric oxide synthase (NOS). The arginine-dependent production of nitric oxide (NO), which is strongly inhibited by methylarginines, serves as a protective small molecule against oxidative stress in eukaryotic cells. NO signaling is highly conserved between higher and lower eukaryotes, although a canonical NOS homologue has yet to be identified in yeast. Since stress signaling pathways are well conserved among eukaryotes, yeast is an ideal model organism to study the implications of PRMTs and methylarginines during stress. We sought to explore the roles and fates of methylarginines in Saccharomyces cerevisiae. We starved methyltransferase-, autophagy-, and permease-related yeast knockouts by incubating them in water and monitored methylarginine production. We found that under starvation, methylarginines are expelled from yeast cells. We found that autophagy-deficient cells have an impaired ability to efflux methylarginines, which suggests that methylarginine-containing proteins are degraded via autophagy. For the first time, we determine that yeast take up methylarginines less readily than arginine, and we show that methylarginines impact yeast NO production. This study reveals that yeast circumvent a potential methylarginine toxicity by expelling them after autophagic degradation of arginine-modified proteins.

Structural basis for antigen recognition by methylated lysine-specific antibodies.

Proteins are modulated by a variety of posttranslational modifications including methylation. Despite its importance, the majority of protein methylation modifications discovered by mass spectrometric analyses are functionally uncharacterized, partly owing to the difficulty in obtaining reliable methylsite-specific antibodies. To elucidate how functional methylsite-specific antibodies recognize the antigens and lead to the development of a novel method to create such antibodies, we use an immunized library paired with phage display to create rabbit monoclonal antibodies recognizing trimethylated Lys260 of MAP3K2 as a representative substrate. We isolated several methylsite-specific antibodies that contained unique complementarity determining region sequence. We characterized the mode of antigen recognition by each of these antibodies using structural and biophysical analyses, revealing the molecular details, such as binding affinity toward methylated/nonmethylated antigens and structural motif that is responsible for recognition of the methylated lysine residue, by which each antibody recognized the target antigen. In addition, the comparison with the results of Western blotting analysis suggests a critical antigen recognition mode to generate cross-reactivity to protein and peptide antigen of the antibodies. Computational simulations effectively recapitulated our biophysical data, capturing the antibodies of differing affinity and specificity. Our exhaustive characterization provides molecular architectures of functional methylsite-specific antibodies and thus should contribute to the development of a general method to generate functional methylsite-specific antibodies by de novo design.

Enzymology and significance of protein histidine methylation.

Cells synthesize proteins using 20 standard amino acids and expand their biochemical repertoire through intricate enzyme-mediated post-translational modifications (PTMs). PTMs can either be static and represent protein editing events or be dynamically regulated as a part of a cellular response to specific stimuli. Protein histidine methylation (Hme) was an elusive PTM for over 5 decades and has only recently attracted considerable attention through discoveries concerning its enzymology, extent, and function. Here, we review the status of the Hme field and discuss the implications of Hme in physiological and cellular processes. We also review the experimental toolbox for analysis of Hme and discuss the strengths and weaknesses of different experimental approaches. The findings discussed in this review demonstrate that Hme is widespread across cells and tissues and functionally regulates key cellular processes such as cytoskeletal dynamics and protein translation. Collectively, the findings discussed here showcase Hme as a regulator of key cellular functions and highlight the regulation of this modification as an emerging field of biological research.