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Structual-function relationships of recombinases and mediators
g}Homologous recombination (HR) is a reaction that nucleotide sequences are exchanged between two similar or identical molecules of DNA. HR plays a central role in the repair of normal DNA replication, DNA double-strand break and collapsed replication fork. HR also has an important role in generating genetic diversity from bacteria to humans
HR is promoted by recombinases, catalyzing strand exchage reaction. The reactions carry out the search for homology, strand invasion and strand exchange. These reactions are at the heart of HR. Recombinases interact with several proteins, called mediators, during homologous recombination reactions. These mediators regulate strand exchange reaction, interacting with recombinases and other mediators.
Several mediators were identified and have essential role in homologous recombination. However, the structure-function relationships of mediators are unknown. The research of our laboratory is focused on the regulation mechanism of mediators in HR. The experimental approach combines molecular biological, biochemical and biophysical methods with emphasis on X-ray crystallography for structure determination.

Structural biology of innate immunity

   Immune system is divided into two types, acquired and innate immune systems. Innate immunity is the first line of defense against microbial infections. Microbial components such as lipopeptide, lipopolysaccharide (LPS), and nucleic acids trigger the innate immune responses through a variety of pattern-recognition receptors including Toll-like receptors (TLR) and Nod-like receptors. Upon ligand recoginition by these receptors, various intracellular TIR domain-containing adaptor proteins such as MyD88 are recruited, and finally, inflammatory transcription factors are activated. In addition, the innate immune responses stimulate the acquired immune system. Recently, it is reported that the innate immune responses cause chronic inflammations which is possibly involved in the development of various diseases such as cancer, autoimmune disease, arteriosclerosis, obesitys, and diabetics, and is paid attention in the field of clinical and the drug development.

LPS in the outer membrane of Gram-negative bacteria potently stimulates the immune responses through a complex of TLR4 and MD-2 proteins. Also, RP105 and MD-1 proteins are supposed to mediate the LPS recognition. We have determined the crystal structures of MD-2 itself and of its complex with lipid IVa in which most of lipid IVa was confined in a hydrophobic cavity of MD-2 (Science, 2007). Moreover, we have shown by crystallography that endogenous phospholipid molecules bound to the hydrophobic cavity of MD-1 (J. Mol. Biol., 2010).

   Our research aims to analyze by use of structural biology techniques the recognition mechanism of pathogenic ligands by innate immunity proteins, to clarify the relations to the diseases, and to provide structural basis to the drug development.