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1.  Isolation and Characterization of Signermycin B, an Antibiotic That Targets the Dimerization Domain of Histidine Kinase WalK 
The WalK (histidine kinase)/WalR (response regulator) two-component signal transduction system is a master regulatory system for cell wall metabolism and growth. This system is conserved in low G+C Gram-positive bacteria, including Bacillus subtilis, Staphylococcus aureus, Enterococcus faecalis, and Streptococcus mutans. In this study, we found the first antibiotic that functions as a WalK inhibitor (signermycin B) by screening 10,000 Streptomyces extracts. The chemical structure (C23H35NO4; molecular weight, 389.5) comprises a tetramic acid moiety and a decalin ring. Signermycin B exhibited antimicrobial activity, with MIC values ranging from 3.13 μg/ml (8 μM) to 6.25 μg/ml (16 μM) against Gram-positive bacteria that possess the WalK/WalR two-component signal transduction system, including the drug-resistant bacteria methicillin-resistant Staphylococcus aureus and vancomycin-resistant Enterococcus faecalis. The half-maximal inhibitory concentrations of signermycin B against WalK in these organisms ranged from 37 to 61 μM. To determine the mechanism of action of signermycin B, surface plasmon resonance response analysis with the two WalK domains of Bacillus subtilis and competition assay with ATP were performed. The results showed that signermycin B binds to the dimerization domain but not the ATP-binding domain of WalK. In the presence of the cross-linker glutaraldehyde, signermycin B did not cause protein aggregation but interfered with the cross-linking of WalK dimers. These results suggest that signermycin B targets the conserved dimerization domain of WalK to inhibit autophosphorylation. In Bacillus subtilis and Staphylococcus aureus, signermycin B preferentially controlled the WalR regulon, thereby inhibiting cell division. These phenotypes are consistent with those of cells starved for the WalK/WalR system.
PMCID: PMC3393415  PMID: 22526318
2.  PKCε–PKD1 signaling complex at Z-discs plays a pivotal role in the cardiac hypertrophy induced by G-protein coupling receptor agonists 
Cardiac hypertrophy is triggered in response to mechanical stress and various neurohumoral factors, such as G-protein coupling receptor (GPCR) and gp130 cytokine receptor agonists. Recent studies have suggested cardiac Z-disc plays a pivotal role to regulate these cellular responses. Here, we demonstrate stimulations with GPCR agonists (norepinephrine, angiotensin II, and endothelin 1) and phorbol ester activated and translocated protein kinase D1 (PKD1) to the Z-discs in neonatal rat cardiomyocytes in a protein kinase C (PKC)-dependent manner, whereas gp130 agonist did not. Especially, upon the α-adrenergic receptor agonist stimulations, following the PKCε–PKD1 complex formation, PKCε-dependent activation of PKD1 was essential to induce hypertrophic responses. Constitutively active mutant of either PKD1 or PKCε also induced cardiac hypertrophy ex vivo. Taken together, the PKCε–PKD1 complex at Z-discs could play a pivotal role in the cardiac hypertrophy induced by GPCR agonists, at least α-adrenergic receptor agonist.
PMCID: PMC3224855  PMID: 15652511
PKD1; PKC; Hypertrophy; Cardiomyocyte; Z-disc; G-protein coupling receptor; Gp130; α-Adrenergic receptor; Phorbol ester
3.  Detection of the reaction intermediates catalyzed by a copper amine oxidase 
Journal of Synchrotron Radiation  2010;18(Pt 1):58-61.
Consecutive temporal analyses of enzyme structure have been performed during reactions in order to clarify the structure-based reaction mechanism. Four intermediate structures have been determined.
To reveal the chemical changes and geometry changes of active-site residues that cooperate with a reaction is important for understanding the functional mechanism of proteins. Consecutive temporal analyses of enzyme structures have been performed during reactions to clarify structure-based reaction mechanisms. Phenylethylamine oxidase from Arthrobacter globiformis (AGAO) contains a copper ion and topaquinone (TPQox). The catalytic reaction of AGAO catalyzes oxidative deaminations of phenylethylamine and consists of reductive and oxidative half-reactions. In the reduction step, TPQox reacts with a phenylethylamine (PEA) substrate giving rise to a topasemiquinone (TPQsq) formed Schiff-base and produces phenylacetaldehyde. To elucidate the mechanism of the reductive half-reaction, an attempt was made to trap the reaction intermediates in order to analyze their structures. The reaction proceeded within the crystals when AGAO crystals were soaked in a PEA solution and freeze-trapped in liquid nitrogen. The reaction stage of each crystal was confirmed by single-crystal microspectrometry, before X-ray diffraction measurements were made of four reaction intermediates. The structure at 15 min after the onset of the reaction was analyzed at atomic resolution, and it was shown that TPQox and some residues in the substrate channel were alternated via catalytic reductive half-reactions.
PMCID: PMC3004256  PMID: 21169693
copper amine oxidase; AGAO; topaquinone; reaction intermediate; single-crystal microspectroscopy; X-ray crystal structure analysis
4.  Involvement of MAPK signaling molecules and Runx2 in the NELL1-induced osteoblastic differentiation 
FEBS letters  2007;582(2):365-371.
NELL1 is an extracellular protein inducing osteogenic differentiation and bone formation of osteoblastic cells. To elucidate the intracellular signaling cascade evoked by NELL1, we have shown that NELL1 protein transiently activates the MAPK signaling cascade, induces the phosphorylation of Runx2, and promotes the rapid intracellular accumulation of Tyr-phosphorylated proteins. Unlike BMP2, NELL1 protein does not activate the Smad signaling cascade. These findings suggest that upon binding to a specific receptor NELL1 transduces an osteogenic signal through activation of certain Tyr-kinases associated with the Ras-MAPK cascade, and finally leads to the osteogenic differentiation.
PMCID: PMC2959102  PMID: 18082140
MAP kinase; Osteoblast; Differentiation; Bone; Tyrosine phosphorylation
5.  Crystal structures of Lymnaea stagnalis AChBP in complex with neonicotinoid insecticides imidacloprid and clothianidin 
Invertebrate Neuroscience   2008;8(2):71-81.
Neonicotinoid insecticides, which act on nicotinic acetylcholine receptors (nAChRs) in a variety of ways, have extremely low mammalian toxicity, yet the molecular basis of such actions is poorly understood. To elucidate the molecular basis for nAChR–neonicotinoid interactions, a surrogate protein, acetylcholine binding protein from Lymnaea stagnalis (Ls-AChBP) was crystallized in complex with neonicotinoid insecticides imidacloprid (IMI) or clothianidin (CTD). The crystal structures suggested that the guanidine moiety of IMI and CTD stacks with Tyr185, while the nitro group of IMI but not of CTD makes a hydrogen bond with Gln55. IMI showed higher binding affinity for Ls-AChBP than that of CTD, consistent with weaker CH–π interactions in the Ls-AChBP–CTD complex than in the Ls-AChBP–IMI complex and the lack of the nitro group-Gln55 hydrogen bond in CTD. Yet, the NH at position 1 of CTD makes a hydrogen bond with the backbone carbonyl of Trp143, offering an explanation for the diverse actions of neonicotinoids on nAChRs.
PMCID: PMC2413115  PMID: 18338186
Acetylcholine binding protein (Lymnaea stagnalis); Crystal structures; Neonicotinoids; Nicotinic acetylcholine receptors; Ion channels

Results 1-5 (5)