Some couples seem unlikely, yet somehow, they work (a recent pop star-football player pair comes to mind). Now, an even more surprising couple—a methyl group and an acetyl group—hanging out together on the very same lysine residues on histone H4s has just been spotted across species and appears to be involved in active transcription. Mind. Blown.
The molecular matchmakers in Matthew Simon’s lab (Yale) realized that, theoretically, histone H4 could be both acetylated (Kac) and monomethylated (Kme) on the epsilon amine of lysine, resulting in a “Kacme” modification. To show that it exists in real cells, the team produced antibodies against a peptide library with a central Kacme. The antibodies recognized histone H4 in immunoblots of fruit fly, mouse, and human extracts. Using the antibodies and a construct with Lys-to-Arg mutations along the H4 tail, the team showed it’s likely that Kacme can form at Lys5 and Lys12. And with mass spectrometry and isotopically labeled standards, the group saw that histones from mammalian cells have H4K5acme.
But what could Kacme be doing? Spying on the new power couple with a proteomics approach revealed that H4K5acme is present at about the same levels in histones from human, mouse, and fly cells, and at similar levels to those of histone modifications involved in active transcription, which piqued the researchers’ interest. Here’s more info hinting at Kacme’s activities:
- ChIP-seq with fly and human cells located Kacme near promoters, especially near highly expressed genes
- Kacme also hangs out with modifications associated with active chromatin
- In human cells, more Kacme is associated with more transcription and transcription initiation
- Promoters with a higher Kacme to H4Kac ratio have motifs for E2F transcription factors
To figure out which comes first to the party, methylation or acetylation, the researchers performed ChIP-seq and ChIP-PCR and found that Kacme and the lysine acetyltransferase p300 are often at the same places. In vitro assays further showed that p300 can acetylate Kme1 on H4 peptides. Experiments in yeast with a methyltransferase knocked out reduced Kacme levels, but not Kac. So, it’s likely that lysine gets methylated first, then acetylated.
Finally, the team figured that Kacme might bind to bromodomain-containing proteins that are important drug-development targets. Sure enough, BRD2 and BRD3 bound to biotinylated Kacme peptides in extracts. In addition, the crystal structure of the BRD2 bromodomain1 with a H4K5acmeK12acme peptide showed that Kacme can bind in the same pocket that Kac can. Also, they propose that the histone modification could be a simple way of hiding and revealing lysine methylation when needed, but a pretty sure bet is that Kacme marks active transcription start sites.
No need to hide in the bushes like the paparazzi to learn more about Kacme—just click over to Nature, October 2023.
