Parasite-specific essential bromodomain protein TgBDP4 is a key epigenetic reader and a potential drug target for the parasite Toxoplasma gondii.

Toxoplasma gondii, an apicomplexan parasite, relies heavily on epigenetic regulation of gene expression, which is controlled by chromatin-modifying enzymes and histone acetylation, to invade and establish infection in the host. Bromodomain proteins are important epigenetic regulators in parasites, acting as "readers" of histone lysine acetylation to control gene expression. While many bromodomain proteins are unique to parasites, only a few have been explored for therapeutic applications. In this study, we characterized the parasite-specific bromodomain protein TgBDP4 as a key epigenetic reader and a potential drug target for toxoplasmosis. Protein-peptide interaction and pull-down studies reveal that TgBDP4 interacts strongly with acetylated histone H3 as well as unphosphorylated and Ser5-phosphorylated forms of RNA polymerase II-CTD, which are necessary for gene activation. Using a conditional knockdown approach, we demonstrate that TgBDP4 is essential for parasite survival in both cell culture and a mouse host. When the therapeutic potential of TgBDP4 was evaluated using three bromodomain inhibitors, I-BRD9, (+)-JQ1, and I-BET151, we found that I-BRD9, a selective inhibitor of HsBRD9, effectively inhibits TgBDP4 activity at much lower concentrations than HsBRD9. This inhibition occurs through the binding of I-BRD9 to conserved key residues of the acetyl-lysine-binding pocket of TgBDP4, resulting in a complete arrest of parasite replication in culture and extending the survival time of infected mice. Overall, this study highlights the indispensable role of TgBDP4-mediated gene regulation for parasite survival, establishing TgBDP4 as a promising drug target for treating toxoplasmosis.