Histone H2A ubiquitination inhibits the enzymatic activity of H3 lysine 36 methyltransferases

Area of Science:

• Epigenetics
• Molecular Biology
• Biochemistry

Background:

• Histone H3 lysine 27 (H3K27) methylation and H2A monoubiquitination (ubH2A) are key regulators of Polycomb silencing.
• H3K27 trimethylation (H3K27me3) and H3K36 di/tri-methylation (H3K36me2/3) are generally mutually exclusive on histone H3 tails.
• Previous research indicated PRC2 inhibition by H3K36me2/3, but the mechanism preventing H3K36me2/3 during Polycomb silencing remained unclear.

Purpose of the Study:

• To investigate the mechanism by which Polycomb modifications negatively regulate H3K36-specific methyltransferases.
• To determine if ubH2A, a modification associated with Polycomb silencing, affects the activity of H3K36 methyltransferases.

Main Methods:

• Developed a protocol for purifying milligram quantities of ubH2A from mammalian cells.
• Reconstituted nucleosome substrates using purified ubH2A.
• Assayed the enzymatic activity of various histone methyltransferases, including H3K36-specific enzymes, on ubH2A-containing nucleosomes.

Main Results:

• Purified ubH2A effectively inhibited all tested H3K36-specific methyltransferases (ASH1L, HYPB, NSD1, NSD2).
• ubH2A did not affect the activity of other tested methyltransferases, including PRC2, G9a, and Pr-Set7.
• These findings provide a molecular explanation for the mutual exclusion of H3K36me2/3 and Polycomb modifications.

Conclusions:

• Histone H2A monoubiquitination acts as a direct inhibitor of H3K36-specific histone methyltransferases.
• This inhibition mechanism explains the observed reciprocal relationship between H3K36 methylation and Polycomb-mediated gene silencing.
• The study elucidates a critical aspect of epigenetic crosstalk in regulating chromatin states.