journal menu
abstracts
Interleukin (IL)-18 [1], a proinflammatory cytokine belonging to the IL-1 superfamily, plays important roles in both innate and adaptive immune system, which is involved in not only the function of host defense mechanism but also allergic reactions. IL-18 is secreted by various types of cells and strongly augments the production of interferon-γ. IL-18 is synthesized as a biologically inactive precursor (proIL-18), which is matured by the action of caspase-1 in the cell upon stimulation. The matured IL-18 is subsequently secreted extracellularly and binds to IL-18 receptor α (Rα) and IL-18 receptor β (Rβ) in a stepwise manner, forming the IL-18/Rα/Rβ ternary complex. The complex initiates the signaling that finally activates NF-κB via the MyD88 dependent pathway [2]. Here, we show the crystal structure of IL-18 (Fig a), the IL-18/Rα binary complex (Fig b) and the IL-18/Rα/Rβ signaling ternary complex (Fig c, d) at 2.33, 3.10 and 3.10 Å resolution, respectively. Overall, the recognition manner of IL-18 by the receptors was similar to that of IL-1β [3], although some remarkable differences such as the orientation of Ig-like domains of IL-18Rβ were revealed. We also demonstrate that carbohydrate chains on IL-18Rα contributes to the recognition of IL-18. Biochemical experiments based on the structure identify amino acid residues that are important in forming the ternary complex and the signal transduction. Our results not only show the common extracellular signaling architecture of IL-1 family cytokines but also unveil unique recognition mechanism of IL-18 with the detailed atomic structures. The structure of the signaling ternary complex of IL-18 would contribute to both understanding the pathogeneses of the IL-18 related diseases and designing new efficient therapeutic agents.
abstracts
Two major epigenetic traits, histone modifications and DNA methylation, regulate various chromatin-template processes in mammals. The pattern of these epigenetic traits is cooperatively established in early embryogenesis and cell development, and inherited during the cell cycle. UHRF1 (also known as Np95 or ICBP90) is believed to play an important role in linking the two major epigenetic traits. UHRF1 has five functional domains, UBL, Tandem Tudor (TTD), pland homeo domain (PHD), SET and RING-associated doain (SRA) and RING finger. To maintain DNA methylation pattern, UHRF1 recognizes hemi-methylated DNA generated during DNA replication through interactions with its SRA domain, and recruit maintenance of DNA methyltransferase Dnmt1 to the site [1], [2]. UHRF1 also recognizes histone H3 containing tri-methylated Lys9 (H3K9me3) via its TTD-PHD moiety. [3]. To obtain the structural basis for recognition of epigenetic marks by UHRF1, we determined the crystal structure of the SRA domain in complex with hemi-methylated DNA. The structure showed that the DNA binding caused a loop and an N-terminal tail of the SRA domain. Interestingly, the methyl-cytosine base at the hemi-methylation site was flipped out from the DNA helix, which has not observed in other DNA binding proteins. These results suggest that the Base flip out mechanism is important event for maintenance of DNA methylation. We also determined the crystal structure of TTD-PHD region of UHRF1 in complex with H3K9me3 peptide. To our surprise, the linker region between the reader modules, which is predicted as an intrinsically disorder, was formed a stable structure with binding to the groove of TTD and plays an essential role in the formation of histone H3 binding hole between the reader modules. The structure revealed how multiple histone modifications were simultaneously decoded by the linked histone reader modules of UHRF1.
