Photosynthetic complex I enables cyclic electron flow around photosystem I, a regulatory mechanism for photosynthetic energy conversion. We report a 3.3-angstrom-resolution cryo–electron microscopy structure of photosynthetic complex I from the cyanobacterium Thermosynechococcus elongatus. The model reveals structural adaptations that facilitate binding and electron transfer from the photosynthetic electron carrier ferredoxin. By mimicking cyclic electron flow with isolated components in vitro, we demonstrate that ferredoxin directly mediates electron transfer between photosystem I and complex I, instead of using intermediates such as NADPH (the reduced form of nicotinamide adenine dinucleotide phosphate). A large rate constant for association of ferredoxin to complex I indicates efficient recognition, with the protein subunit NdhS being the key component in this process.
The BET proteins are major transcriptional regulators and have emerged as new drug targets, but their functional distinction has remained elusive. In this study, we report that the BET family members Brd2 and Brd4 exert distinct genomic functions at genes whose transcription they co-regulate during mouse T-helper 17 (Th17) cell differentiation. Brd2 is associated with the chromatin insulator CTCF and the cohesin complex to support cis-regulatory enhancer assembly for gene transcriptional activation. In this context, Brd2 binds the transcription factor Stat3 in an acetylation-sensitive manner and facilitates Stat3 recruitment to active enhancers occupied with transcription factors Irf4 and Batf. In parallel, Brd4 temporally controls RNA polymerase II (Pol II) processivity during transcription elongation through cyclinT1/Cdk9 recruitment and Pol II Ser2 phosphorylation. Collectively, our study uncovers both separate and interdependent Brd2 and Brd4 functions in potentiating the genetic program required for Th17 cell development and adaptive immunity.
SUMMARY Histone lysine acylations play an important role in regulation of gene transcription in chromatin. Unlike histone acetyl-lysine, molecular recognition of recently identified crotonyl-lysine mark is much less understood. Here, we report that the YEATS domain of AF9 preferentially binds crotonyl-lysine over acetyl-lysine in histone H3. NMR structural analysis reveals that crotonyl-lysine of histone H3 lysine 18 is engulfed deep into an aromatic cage of the YEATS domain where carbonyl oxygen of crotonyl-lysine forms a hydrogen bond to backbone amide of protein residue Tyr78. The crotonyl-lysine through its unique electron-rich double bond side chain engages π–π aromatic stacking and extended hydrophobic/aromatic interactions with the YEATS domain as compared to acetyl-lysine. Our mutational analysis confirmed key protein residues Phe59 and Tyr78 for crotonyl-lysine recognition. Importantly, our findings present a new structural mechanism of protein-protein interactions mediated by histone lysine crotonylation, and show how the cells interpret acyl-lysine marks in different biological contexts.
SUMMARY The Bromodomains and Extra-Terminal domain (BET) proteins direct gene transcription in chromatin, and represent new drug targets for cancer and inflammation. Here we report that the ET domain of the BET protein BRD4 recognizes an amphipathic protein sequence motif through establishing a two-strand antiparallel β-sheet anchored on a hydrophobic cleft of the three-helix bundle. This structural mechanism likely explains BRD4 interactions with numerous cellular and viral proteins such as Kaposi’s sarcoma-associated herpesvirus (KSHV) latency–associated nuclear antigen (LANA), and NSD3 whose interaction with BRD4 via this ET domain mechanism is essential for acute myeloid leukemia (AML) maintenance.
Jumonji domain-containing protein 6 (JMJD6) is a member of the Jumonji C family of Fe(II) and 2-oxoglutarate (2OG) dependent oxygenases. It possesses unique bi-functional oxygenase activities, acting as both an arginine demethylase and a lysyl-hydroxylase. JMJD6 has been reported to be over-expressed in oral, breast, lung, and colon cancers and plays important roles in regulation of transcription through interactions with transcription regulator BRD4, histones, U2AF65, Luc7L3, and SRSF11. Here, we report a structural mechanism revealed by NMR of JMJD6 recognition by the extraterminal (ET) domain of BRD4 in that a JMJD6 peptide (Lys84-Asn96) adapts an α-helix when bound to the ET domain. This intermolecular recognition is established through JMJD6 interactions with the conserved hydrophobic core of the ET domain, and reinforced by electrostatic interactions of JMJD6 with residues in the inter-helical α1-α2 loop of the ET domain. Notably, this mode of ligand recognition is different from that of ET domain recognition of NSD3, LANA of herpesvirus, and integrase of MLV, which involves formation of an intermolecular amphipathic two- or three- strand antiparallel β sheet. Furthermore, we demonstrate that the association between the BRD4 ET domain and JMJD6 likely requires a protein conformational change induced by single-stranded RNA binding.
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