Quantitative Proteomics Reveals Histone Modifications in Crosstalk with H3 Lysine 27 Methylation

Methylation at histone H3 lysine 27 (H3K27me) is an evolutionarily conserved epigenetic mark associated with transcriptional repression and replication elongation. We have previously shown that in Tetrahymena thermophila, a unicellular eukaryote, the histone methyltransferases (HMTs) TXR1 and EZL2 are primarily responsible for H3K27 mono-methylation (H3K27me1) and di-/tri-methylation (H3K27me2/3), respectively. Using N-15 metabolically labeled histones as the internal reference, we quantified global changes in histone post-translational modifications in TXR1 and EZL2 cells, to systematically identify potential crosstalk between H3K27 methylation and other PTMs across all four core histones as well as their variants. Most prominently, we observed hyper-acetylation of histones H2A, H2A.Z, and H4 in their N-terminal domains in response to decreased H3K27 methylation. We also provide additional evidence implicating hyper-acetylation in the DNA damage response pathway in replication-defective TXR1 cells, in apparent contrast to the transcriptional role of hyper-acetylation in EZL2 cells.