Discovery of the natural product 3',4',7,8-tetrahydroxyflavone as a novel and potent selective BRD4 bromodomain 2 inhibitor.

Bromodomain-containing protein 4 (BRD4) binds acetylated lysine residues on the N-terminal tails of histones through two bromodomains (BD1 and BD2) to regulate gene transcription. Inhibiting one or both of bromodomains resulted in different phenotypes, suggesting BD1 and BD2 may have different functions. Here we report the characterisation of a natural product 3',4',7,8-tetrahydroxyflavone as a novel and potent selective BRD4 inhibitor. The compound is 100-fold more selective for BRD4-BD2 (IC50 = 204 nM) than BRD4-BD1 (IC50=17.9 µM). Co-crystal structures show 3',4',7,8-tetrahydroxyflavone binds to the acetylated lysine binding pocket of BRD4-BD1 or BRD4-BD2, but establishes more interactions with BRD4-BD2 than BRD4-BD1. Our data suggest 3',4',7,8-tetrahydroxyflavone as a potent selective inhibitor of BRD4-BD2 with a novel chemical scaffold. Given its distinct chemical structure from current BRD4 inhibitors, this compound may open the door for a novel class of anti-BRD4 inhibitors by serving as a lead compound.

Structures of the CcmABCD heme release complex at multiple states.

Cytochromes c use heme as a cofactor to carry electrons in respiration and photosynthesis. The cytochrome c maturation system I, consisting of eight membrane proteins (CcmABCDEFGH), results in the attachment of heme to cysteine residues of cytochrome c proteins. Since all c-type cytochromes are periplasmic, heme is first transported to a periplasmic heme chaperone, CcmE. A large membrane complex, CcmABCD has been proposed to carry out this transport and linkage to CcmE, yet the structural basis and mechanisms underlying the process are unknown. We describe high resolution cryo-EM structures of CcmABCD in an unbound form, in complex with inhibitor AMP-PNP, and in complex with ATP and heme. We locate the ATP-binding site in CcmA and the heme-binding site in CcmC. Based on our structures combined with functional studies, we propose a hypothetic model of heme trafficking, heme transfer to CcmE, and ATP-dependent release of holoCcmE from CcmABCD. CcmABCD represents an ABC transporter complex using the energy of ATP hydrolysis for the transfer of heme from one binding partner (CcmC) to another (CcmE).