BRD4 is a histone acetyltransferase that evicts nucleosomes from chromatin

Authors

• DN Ballachanda
• C Case-Borden
• A Gegonne
• CH Hsu
• Q Chen
• D Meerzaman
• DS Singer

Abstract

Bromodomain protein 4 (BRD4) is a master transcriptional and epigenetic regulator which plays a pivotal role in cancer development and immune diseases. Inhibiting its interaction with chromatin has been a successful therapeutic strategy against many cancers including acute myeloid leukemia, Burkitt's lymphoma, breast, colon and lung cancer. While BRD4 association with chromatin is known to de-compact it and activate transcription of key proto-oncogenes, the mechanism involved is currently unknown. Here, we report that BRD4 has novel and intrinsic histone acetyltransferase (HAT) activity through which it acetylates histones H3 and H4 in nucleosomes. BRD4 HAT activity maps to two consensus acetyl CoA binding sites and a 40 aa catalytic site. BRD4 HAT activity is distinct from all other known HAT's with a unique lysine acetylation 'fingerprint' that includes acetylation of H3K4, H3K9, H3K18 and H3K27 but not H3K14. Most importantly, BRD4 acetylates H3K122, a key lysine residue critical for nucleosome stability located on the dyad axis of the nucleosome. BRD4 HAT activity and H3K122 acetylation is responsible for nucleosome eviction and opening of the chromatin, as evidenced by the ability of BRD4, but not BRD4 HAT mutants, to evict nucleosomes both in vitro and in vivo. Nucleosome clearance by BRD4 is selective and localized to the gene loci it regulates such as c-Myc, c-Fos and Aurora B kinase. Consistent with its role in localized chromatin de-compaction, BRD4 HAT activity regulates transcription at these gene loci as well. BRD4 HAT activity thus plays a key role in chromatin remodeling and transcription.

Scientific Focus Area: Molecular Biology and Biochemistry

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