γ-enolase (ENO2) is methylated at the Nτ position of His-190 among enolase isozymes

Protein methylation is mainly observed in lysine, arginine and histidine residues. Histidine methylation occurs at one of two different nitrogen atoms of the imidazole ring, producing Nτ-methylhistidine and Nπ-methylhistidine, and it has recently attracted attention with the identification of SETD3, METTL18 and METTL9 as catalytic enzymes in mammals. Although accumulating evidence had suggested the presence of more than 100 proteins containing methylated histidine residues in cells, much less information has been known regarding histidine-methylated proteins than lysine- and arginine-methylated ones, because no method has been developed to identify substrates for histidine methylation. Here, we established a method to screen novel target proteins for histidine methylation, using biochemical protein fractionation combined with the quantification of methylhistidine by LC-MS/MS. Interestingly, the differential distribution pattern of Nτ-methylated proteins was found between the brain and skeletal muscle, and identified γ-enolase where the His-190 at the Nτ position is methylated in mouse brain. Finally, in silico structural prediction and biochemical analysis showed that the His-190 in γ-enolase is involved in the intermolecular homodimeric formation and enzymatic activity. In the present study, we provide a new methodology to find histidine-methylated proteins in vivo and suggest an insight into the importance of histidine methylation.

Differential Proteome and Interactome Analysis Reveal the Basis of Pleiotropy Associated With the Histidine Methyltransferase Hpm1p

The methylation of histidine is a post-translational modification whose function is poorly understood. Methyltransferase histidine protein methyltransferase 1 (Hpm1p) monomethylates H243 in the ribosomal protein Rpl3p and represents the only known histidine methyltransferase in Saccharomyces cerevisiae. Interestingly, the hpm1 deletion strain is highly pleiotropic, with many extraribosomal phenotypes including improved growth rates in alternative carbon sources. Here, we investigate how the loss of histidine methyltransferase Hpm1p results in diverse phenotypes, through use of targeted mass spectrometry (MS), growth assays, quantitative proteomics, and differential crosslinking MS. We confirmed the localization and stoichiometry of the H243 methylation site, found unreported sensitivities of Δhpm1 yeast to nonribosomal stressors, and identified differentially abundant proteins upon hpm1 knockout with clear links to the coordination of sugar metabolism. We adapted the emerging technique of quantitative large-scale stable isotope labeling of amino acids in cell culture crosslinking MS for yeast, which resulted in the identification of 1267 unique in vivo lysine-lysine crosslinks. By reproducibly monitoring over 350 of these in WT and Δhpm1, we detected changes to protein structure or protein-protein interactions in the ribosome, membrane proteins, chromatin, and mitochondria. Importantly, these occurred independently of changes in protein abundance and could explain a number of phenotypes of Δhpm1, not addressed by expression analysis. Further to this, some phenotypes were predicted solely from changes in protein structure or interactions and could be validated by orthogonal techniques. Taken together, these studies reveal a broad role for Hpm1p in yeast and illustrate how crosslinking MS will be an essential tool for understanding complex phenotypes.

siRNA screening identifies METTL9 as a histidine Nπ-methyltransferase that targets the proinflammatory protein S100A9

Protein methylation is one of the most common post-translational modifications observed in basic amino acid residues, including lysine, arginine, and histidine. Histidine methylation occurs on the distal or proximal nitrogen atom of its imidazole ring, producing two isomers: Nτ-methylhistidine or Nπ-methylhistidine. However, the biological significance of protein histidine methylation remains largely unclear owing in part to the very limited knowledge about its contributing enzymes. Here, we identified mammalian seven-β-strand methyltransferase METTL9 as a histidine Nπ-methyltransferase by siRNA screening coupled with methylhistidine analysis using LC–tandem MS. We demonstrated that METTL9 catalyzes Nπ-methylhistidine formation in the proinflammatory protein S100A9, but not that of myosin light chain kinase MYLK2, in vivo and in vitro. METTL9 does not affect the heterodimer formation of S100A9 and S100A8, although Nπ-methylation of S100A9 at His-107 overlaps with a zinc-binding site, attenuating its affinity for zinc. Given that S100A9 exerts an antimicrobial activity, probably by chelation of zinc essential for the growth of bacteria and fungi, METTL9-mediated S100A9 methylation might be involved in the innate immune response to bacterial and fungal infection. Thus, our findings suggest a functional consequence for protein histidine Nπ-methylation and may add a new layer of complexity to the regulatory mechanisms of post-translational methylation.

Protein Histidine Methylation

Protein histidine methylation is a rarely studied posttranslational modification in eukaryotes. Although the presence of N-methylhistidine was demonstrated in actin in the early 1960s, so far, only a limited number of proteins containing N-methylhistidine have been reported, including S100A9, myosin, skeletal muscle myosin light chain kinase (MLCK 2), and ribosomal protein Rpl3. Furthermore, the role of histidine methylation in the functioning of the protein and in cell physiology remains unclear due to a shortage of studies focusing on this topic. However, the molecular identification of the first two distinct histidine-specific protein methyltransferases has been established in yeast (Hpm1) and in metazoan species (actin-histidine N-methyltransferase), giving new insights into the phenomenon of protein methylation at histidine sites. As a result, we are now beginning to recognize protein histidine methylation as an important regulatory mechanism of protein functioning whose loss may have deleterious consequences in both cells and in organisms. In this review, we aim to summarize the recent advances in the understanding of the chemical, enzymological, and physiological aspects of protein histidine methylation.

An engineered variant of SETD3 methyltransferase alters target specificity from histidine to lysine methylation

Most characterized SET domain (SETD) proteins are protein lysine methyltransferases, but SETD3 was recently demonstrated to be a protein (i.e. actin) histidine-N₃ methyltransferase. Human SETD3 shares a high structural homology with two known protein lysine methyltransferases-human SETD6 and the plant LSMT-but differs in the residues constituting the active site. In the SETD3 active site, Asn²⁵⁵ engages in a unique hydrogen-bonding interaction with the target histidine of actin that likely contributes to its >1300-fold greater catalytic efficiency (k _cat/K_m ) on histidine than on lysine. Here, we engineered active-site variants to switch the SETD3 target specificity from histidine to lysine. Substitution of Asn²⁵⁵ with phenylalanine (N255F), together with substitution of Trp²⁷³ with alanine (W273A), generated an active site mimicking that of known lysine methyltransferases. The doubly substituted SETD3 variant exhibited a 13-fold preference for lysine over histidine. We show, by means of X-ray crystallography, that the two target nitrogen atoms-the N₃ atom of histidine and the terminal ϵ-amino nitrogen of lysine-occupy the same position and point toward and are within a short distance of the incoming methyl group of SAM for a direct methyl transfer during catalysis. In contrast, SETD3 and its Asn²⁵⁵ substituted derivatives did not methylate glutamine (another potentially methylated amino acid). However, the glutamine-containing peptide competed with the substrate peptide, and glutamine bound in the active site, but too far away from SAM to be methylated. Our results provide insight into the structural parameters defining the target amino acid specificity of SET enzymes.

Regulation of Ergothioneine Biosynthesis and Its Effect on Mycobacterium tuberculosis Growth and Infectivity

Ergothioneine (EGT) is synthesized in mycobacteria, but limited knowledge exists regarding its synthesis, physiological role, and regulation. We have identified Rv3701c from Mycobacterium tuberculosis to encode for EgtD, a required histidine methyltransferase that catalyzes first biosynthesis step in EGT biosynthesis. EgtD was found to be phosphorylated by the serine/threonine protein kinase PknD. PknD phosphorylates EgtD both in vitro and in a cell-based system on Thr(213). The phosphomimetic (T213E) but not the phosphoablative (T213A) mutant of EgtD failed to restore EGT synthesis in a ΔegtD mutant. The findings together with observed elevated levels of EGT in a pknD transposon mutant during in vitro growth suggests that EgtD phosphorylation by PknD negatively regulates EGT biosynthesis. We further showed that EGT is required in a nutrient-starved model of persistence and is needed for long term infection of murine macrophages.