A recent study, conducted by Lu-Culligan et al., published in Nature, proposed Nε-acetyl-Nε -methyllysine (Kacme) that both methylation and acetylation occur on the same side chain of lysine as a cellular posttranslational modification (PTM) on histone H4.1 Kacme can be recognized and bound by the chromatin protein bromodomain-containing 2 (BRD2), associating with active chromatin marks and enhanced transcriptional initiation. This discovery offers a novel avenue for investigation of chromatin biology (Figure 1).
Histones play a crucial role in regulating gene expression and chromatin structure through PTMs such as acetylation (Kac) and methylation (Kme), impacting transcriptional activity. Acetylation neutralizes histone's positive charge, weakening the DNA–histone interaction for easier binding with transcription factors. Unlike acetylation, methylation affects reader protein binding and leads to changes in chromatin structure, resulting in transcription suppression or activation.2 Although it is commonly believed that acetylation and monomethylation are mutually exclusive modifications on a single residue, chemical principles permit a lysine residue to be stably acetylated and monomethylated to create a tertiary amide, Kacme.
To provide evidence for the existence of Kacme in cellular proteins, researchers synthesized Fmoc–Lys (Ac, Me)-OH as a building block to create a central Kacme residue peptide library and used them as an antigen to generate a specific antiserum against Kacme.1 Kacme antisera demonstrated high specificity toward Kacme peptides but not otherwise identical Kac, Kme1, and propionyllysine (Kpr). By utilizing this antiserum, researchers analyzed intracellular Kacme modifications in fruit fly, mouse, and human cell lines and identified histone H4 Lys5 and Lys12 as Kacme-modified sites in human cells. To confirm Kacme modification through an antisera-independent approach, the authors isotopically labeled the synthetic H4K5acme peptide to obtain distinct ion diagnostic peaks before conducting intracellular proteomic analysis, which further supported the presence of Kacme in histones.
Chromatin immunoprecipitation sequencing (ChIP-seq) is an extremely powerful tool for studying interactions between multiple transcription factors and other chromatin-associated proteins and DNA.3 By performing ChIP-seq with Kacme antisera in fruit flies and human cells, the authors found that Kacme was significantly enriched around gene promoters, especially in highly expressed genes, and its localization was strongly associated with active chromatin modifications. Subsequently, Lu-Culligan et al. conducted transient-transcriptome time-lapse sequencing to examine transcriptional activity, and start-time-lapse sequencing to investigate the kinetics of promoter–proximal pausing,1