BRD4 methylation by the methyltransferase SETD6 regulates selective transcription to control mRNA translation.

The transcriptional coactivator BRD4 has a fundamental role in transcription regulation and thus became a promising epigenetic therapeutic candidate to target diverse pathologies. However, the regulation of BRD4 by posttranslational modifications has been largely unexplored. Here, we show that BRD4 is methylated on chromatin at lysine-99 by the protein lysine methyltransferase SETD6. BRD4 methylation negatively regulates the expression of genes that are involved in translation and inhibits total mRNA translation in cells. Mechanistically, we provide evidence that supports a model where BRD4 methylation by SETD6 does not have a direct role in the association with acetylated histone H4 at chromatin. However, this methylation specifically determines the recruitment of the transcription factor E2F1 to selected target genes that are involved in mRNA translation. Together, our findings reveal a previously unknown molecular mechanism for BRD4 methylation-dependent gene-specific targeting, which may serve as a new direction for the development of therapeutic applications.

Oligomerization and Auto-methylation of the Human Lysine Methyltransferase SETD6.

Signaling via lysine methylation by protein lysine methyltransferases (PKMTs), has been linked to diverse biological and disease processes. The mono-methyltransferase SETD6 (SET-domain-containing protein 6) is a member of the PKMT family and was previously shown to regulate essential cellular processes such as the NF-κB, WNT and the oxidative stress pathways. However, on the biochemical level, little is known about the enzymatic mode of action of SETD6. Here we provide evidence that SETD6 forms high-molecular-weight structures. Specifically, we demonstrate that SETD6 monomeric, dimeric and trimeric forms are stabilized by the methyl donor, S-adenosyl-l-methionine. We then show that SETD6 has auto-methylation activity at K39 and K179, which serves as the major auto-methylation sites with a moderate auto-methylation activity toward K372. A point mutation at K179 but not at K39 and K372, located at the SET domain of SETD6, impaired SETD6 ability to form a trimer, strongly implying a link between the auto-methylation and the oligomerization state. Finally, by radioactive in vitro methylation experiments and biochemical kinetics analysis, we show that the auto-methylation at K39 and K179 increases the catalytic rate of SETD6. Collectively, our data support a model by which SETD6 auto-methylation and self-interaction positively regulate its enzymatic activity in vitro and may suggest that other PKMTs are regulated in the same manner.