PTMScan^® Methyl Histidine Motif (me-H) Kit #68811

Histidine methylation (me-H) is a naturally occurring post-translational modification (PTM) that occurs on the imidazole ring of the histidine side chain. There are two isomers of histidine methylation: 1-methyl- (1mH) and 3-methyl-histidine (3mH), differing in which of the two nitrogens on the imidazole ring is methylated. According to IUPAC nomenclature, the nitrogen adjacent to the side chain is labeled as the "π" or "3" position, while the nitrogen atom farthest from the side chain is labeled as the "τ" or "1" position (1). Thus, IUPAC and most chemical suppliers refer to methylation on those nitrogen atoms as 3mH and 1mH, respectively. However, many biochemical papers reverse the numerical assignment of the nitrogens.

Early studies identified actin and myosin, isolated from skeletal muscle, as protein substrates containing me-H (2). Due to the abundance of this PTM in muscle tissue, the measurement of free me-H excreted in urine has been proposed as a biomarker of muscle breakdown and turnover (3,4). Additional non-cytoskeletal substrates include histones (5) and the large ribosomal subunit protein (6,7). Both the human and Saccharomyces cerevisiae homologs of the large ribosomal subunit protein contain a conserved histidine residue (RPL3 H245 and Rpl3 H243, respectively) that is methylated by METTL18 and Hpm1, respectively. Methylation on the histidine residue blocks hydrogen bond formation with the 28S rRNA subunit, which may affect overall ribosomal structure and translation efficiency.

Despite the identification of me-H sites on some abundant proteins, such as actin, proteomics studies estimate that the number of histidine methylation sites is less than the number of lysine or arginine methylation sites (8). The elucidation of me-H sites, as well as the enzymes that regulate this PTM, remain an area of active research (9).