The monocytic leukemic zinc-finger (MOZ) histone acetyltransferase (HAT) acetylates free histones H3, H4, H2A, and H2B in vitro and is associated with up-regulation of gene transcription. The MOZ HAT functions as a quaternary complex with the bromodomain-PHD finger protein 1 (BRPF1), inhibitor of growth 5 (ING5), and hEaf6 subunits. BRPF1 links the MOZ catalytic subunit to the ING5 and hEaf6 subunits, thereby promoting MOZ HAT activity. Human BRPF1 contains multiple effector domains with known roles in gene transcription, and chromatin binding and remodeling. However, the biological function of the BRPF1 bromodomain remains unknown. Our findings reveal novel interactions of the BRPF1 bromodomain with multiple acetyllysine residues on the N-terminus of histones, and show it preferentially selects for H2AK5ac, H4K12ac and H3K14ac. We used chemical shift perturbation data from NMR titration experiments to map the BRPF1 bromodomain ligand binding pocket and identified key residues responsible for coordination of the post-translationally modified histones. Extensive molecular dynamics simulations were used to generate structural models of bromodomain-histone ligand complexes, to analyze H-bonding and other interactions, and to calculate the binding free energies. Our results outline the molecular mechanism driving binding specificity of the BRPF1 bromodomain for discrete acetyllysine residues on the N-terminal histone tails. Together these data provide insights on how histone recognition by the bromodomain directs the biological function of BRPF1, ultimately targeting the MOZ HAT complex to chromatin substrates.

BRPF1 is part of the MOZ HAT complex and contains a unique combination of domains typically found in chromatin-associated factors, which include PHD fingers, a bromodomain and a PWWP domain. Bromodomains are conserved structural motifs generally known to recognize acetylated histones, and the BRPF1 bromodomain preferentially selects for H2AK5ac, H4K12ac and H3K14ac. We solved the X-ray crystal structures of the BRPF1 bromodomain in complex with the H2AK5ac and H4K12ac histone peptides. Site-directed mutagenesis on residues in the BRPF1 bromodomain-binding pocket was carried out to investigate the contribution of specific amino acids on ligand binding. Our results provide critical insights into the molecular mechanism of ligand binding by the BRPF1 bromodomain, and reveal that ordered water molecules are an essential component driving ligand recognition.

The monocytic leukemic zinc‐finger (MOZ) histone acetyltransferase (HAT) was first identified as a fusion partner with the CREB binding protein (CBP) in a chromosomal translocation found in acute myeloid leukemia (AML). Disruption of MOZ HAT activity leads to leukemogenic transformations and oncogenesis. Bromodomain‐PHD finger protein 1 (BRPF1) is essential for MOZ HAT activity and links the MOZ catalytic subunit to the inhibitor of growth 5 (ING5) and hEaf6 subunits in the complex. BRPF1 also contains multiple epigenetic reader domains including a unique double plant homeodomain (PHD), a bromodomain (BRD) and a PWWP domain. These chromatin reader domains recognize post‐translationally modified histones and are often found in nuclear proteins that control fundamental cellular processes including gene transcription, DNA replication and recombination. Retention of these domains in leukemic fusion proteins is speculated to direct aberrant HAT activity and may be responsible for oncogenic transformations. However, the histone target(s) of the BRPF1 BRD are unknown, and its role in modulating the activity of the MOZ HAT complex is not well understood. A unique combination of peptide array, nuclear magnetic resonance, isothermal titration calorimetry and molecular docking experiments were used to identify the BRPF1 BRD ligands and characterize the molecular determinants driving histone recognition.