Structural insights into acetylated histone ligand recognition by the BDP1 bromodomain of Plasmodium falciparum

The CECR2 bromodomain displays distinct binding modes to select for acetylated histone proteins versus non-histone ligands

The cat eye syndrome chromosome region candidate 2 (CECR2) protein is an epigenetic regulator involved in chromatin remodeling and transcriptional control. The CECR2 bromodomain (CECR2-BRD) plays a pivotal role in directing the activity of CECR2 through its capacity to recognize and bind acetylated lysine residues on histone proteins. This study elucidates the binding specificity and structural mechanisms of CECR2-BRD interactions with both histone and non-histone ligands, employing techniques such as isothermal titration calorimetry (ITC), nuclear magnetic resonance (NMR) spectroscopy, and a high-throughput peptide assay. The CECR2-BRD selectively binds acetylated histone H3 and H4 ligands, exhibiting a preference for multi-acetylated over mono-acetylated targets. The highest affinity was observed for tetra-acetylated histone H4. Neighboring post-translational modifications, including methylation and phosphorylation, modulate acetyllysine recognition, with significant effects observed for histone H3 ligands. Additionally, this study explored the interaction of the CECR2-BRD with the acetylated RelA subunit of NF-κB, a pivotal transcription factor in inflammatory signaling. Dysregulated NF-κB signaling is implicated in numerous pathologies, including cancer progression, with acetylation of RelA at lysine 310 (K310ac) being critical for its transcriptional activity. Recent evidence linking the CECR2-BRD to RelA suggests it plays a role in inflammatory and metastatic pathways, underscoring the need to understand the molecular basis of this interaction. We found the CECR2-BRD binds to acetylated RelA with micromolar affinity, and uses a distinctive binding mode to recognize this non-histone ligand. These results provide new insight on the role of CECR2 in regulating NF-κB-mediated inflammatory pathways. Functional mutagenesis of critical residues, such as Asn514 and Asp464, highlight their roles in ligand specificity and binding dynamics. Notably, the CECR2-BRD remained monomeric in solution and exhibited differential conformational responses upon ligand binding, suggesting adaptive recognition mechanisms. Furthermore, the CECR2-BRD exclusively interacts with nucleosome substrates containing multi-acetylated histones, emphasizing its role in transcriptional activation within euchromatic regions. These findings position the CECR2-BRD as a key chromatin reader and a promising therapeutic target for modulating transcriptional and inflammatory processes, particularly through the development of selective bromodomain inhibitors.

Revisiting the Plasmodium falciparum druggable genome using predicted structures and data mining

The identification of novel drug targets for the purpose of designing small molecule inhibitors is key component to modern drug discovery. In malaria parasites, discoveries of antimalarial targets have primarily occurred retroactively by investigating the mode of action of compounds found through phenotypic screens. Although this method has yielded many promising candidates, it is time- and resource-consuming and misses targets not captured by existing antimalarial compound libraries and phenotypic assay conditions. Leveraging recent advances in protein structure prediction and data mining, we systematically assessed the Plasmodium falciparum genome for proteins amenable to target-based drug discovery, identifying 867 candidate targets with evidence of small molecule binding and blood stage essentiality. Of these, 540 proteins showed strong essentiality evidence and lack inhibitors that have progressed to clinical trials. Expert review and rubric-based scoring of this subset based on additional criteria such as selectivity, structural information, and assay developability yielded 67 high priority candidates. This study also provides a genome-wide data resource and implements a generalizable framework for systematically evaluating and prioritizing novel pathogenic disease targets.

Histone code: a common language and multiple dialects to meet the different developmental requirements of apicomplexan parasites

Apicomplexan parasites have complex life cycles, often requiring transmission between two different hosts, facing periods of dormancy within the host or in the environment to maximize chances of transmission. To support survival in these different conditions, tightly regulated and correctly timed gene expression is critical. The modification of histones and nucleosome composition makes a significant contribution to this regulation, and as eukaryotes, the fundamental mechanisms underlying this process in apicomplexans are similar to those in model eukaryotic organisms. However, single-celled intracellular parasites face unique challenges, and regulation of gene expression at the epigenetic level provides tight control for responses that must often be rapid and robust. Here, we discuss the recent advances in understanding the dynamics of histone modifications across Apicomplexan life cycles and the molecular mechanisms that underlie epigenetic regulation of gene expression to promote parasite life cycle progression, dormancy, and transmission.

An apicomplexan bromodomain protein, TgBDP1, associates with diverse epigenetic factors to regulate essential transcriptional processes in Toxoplasma gondii

The protozoan pathogen Toxoplasma gondii relies on tight regulation of gene expression to invade and establish infection in its host. The divergent gene regulatory mechanisms of Toxoplasma and related apicomplexan pathogens rely heavily on regulators of chromatin structure and histone modifications. The important contribution of histone acetylation for Toxoplasma in both acute and chronic infection has been demonstrated, where histone acetylation increases at active gene loci. However, the direct consequences of specific histone acetylation marks and the chromatin pathway that influences transcriptional regulation in response to the modification are unclear. As a reader of lysine acetylation, the bromodomain serves as a mediator between the acetylated histone and transcriptional regulators. Here we show that the bromodomain protein, TgBDP1, which is conserved among Apicomplexa and within the Alveolata superphylum, is essential for Toxoplasma asexual proliferation. Using cleavage under targets and tagmentation, we demonstrate that TgBDP1 is recruited to transcriptional start sites of a large proportion of parasite genes. Transcriptional profiling during TgBDP1 knockdown revealed that loss of TgBDP1 leads to major dysregulation of gene expression, implying multiple roles for TgBDP1 in both gene activation and repression. This is supported by interactome analysis of TgBDP1 demonstrating that TgBDP1 forms a core complex with two other bromodomain proteins and an ApiAP2 factor. This core complex appears to interact with other epigenetic factors such as nucleosome remodeling complexes. We conclude that TgBDP1 interacts with diverse epigenetic regulators to exert opposing influences on gene expression in the Toxoplasma tachyzoite. IMPORTANCE Histone acetylation is critical for proper regulation of gene expression in the single-celled eukaryotic pathogen Toxoplasma gondii. Bromodomain proteins are "readers" of histone acetylation and may link the modified chromatin to transcription factors. Here, we show that the bromodomain protein TgBDP1 is essential for parasite survival and that loss of TgBDP1 results in global dysregulation of gene expression. TgBDP1 is recruited to the promoter region of a large proportion of parasite genes, forms a core complex with two other bromodomain proteins, and interacts with different transcriptional regulatory complexes. We conclude that TgBDP1 is a key factor for sensing specific histone modifications to influence multiple facets of transcriptional regulation in Toxoplasma gondii.