N-terminal acetyltransferase 3 gene is essential for robust circadian rhythm of bioluminescence reporter in Chlamydomonas reinhardtii

Chlamydomonas reinhardtii is a model species of algae for studies on the circadian clock. Previously, we isolated a series of mutants showing defects in the circadian rhythm of a luciferase reporter introduced into the chloroplast genome, and identified the genes responsible for the defective circadian rhythm. However, we were unable to identify the gene responsible for the defective circadian rhythm of the rhythm of chloroplast 97 (roc97) mutant because of a large genomic deletion. Here, we identified the gene responsible for the roc97 mutation through a genetic complementation study. This gene encodes a protein that is homologous to the subunit of N-terminal acetyltransferase (NAT) which catalyzes N-terminal acetylation of proteins. Our results provide the first example of involvement of the protein N-terminal acetyltransferase in the circadian rhythm.     

Suppression of MIP-2 or IL-8 production by annexins A1 and A4 during coculturing of macrophages with late apoptotic human peripheral blood neutrophils

Annexin A1 (ANXA1) is a well-known anti-inflammatory protein that is expressed on the surface of apoptotic cells. Annexin A4 (ANXA4) is also recruited from the nucleus to the cytoplasm in apoptotic cells, although it is not known whether or not ANXA4 is expressed on the surface of apoptotic cells. In this study, we obtained rabbit anti-human ANXA1 and ANXA4 antibodies, and then examined whether or not ANXA1 and ANXA4 are expressed on the surface of early and late human apoptotic cells. ANXA1 and, to a lesser extent, ANXA4 were detected on late but not early apoptotic HeLa cells, whereas ANXA1 and a small amount of ANXA4 were detected on both early and late apoptotic human neutrophils. We then examined the effects of the anti-human ANXA1 and ANXA4 antibodies on the mouse or human macrophage response to human apoptotic cells. Upon coculturing of mouse or human macrophages with late apoptotic human neutrophils, anti-human ANXA1 antibodies and, to a lesser extent, anti-human ANXA4 antibodies increased MIP-2 or IL-8 production significantly, suggesting that ANXA1 and ANXA4 suppress MIP-2 or IL-8 production by macrophages in response to late apoptotic human neutrophils.     

Roles of SAM and DDHD domains in mammalian intracellular phospholipase A1 KIAA0725p

Members of the intracellular phospholipase A1 family of proteins have been implicated in organelle biogenesis and membrane trafficking. The mammalian family comprises three members: phosphatidic acid-preferring phospholipase A1 (PA-PIA1)/DDHD1, p125/Sec23ip and KIAA0725p/DDHD2, all of which have a DDHD domain. PA-PLAI is mostly cytosolic, while KIAA0725p and p125 are more stably associated with the Golgi/endoplasmic reticulum (ER)-Golgi intermediate compartment (ERGIC) and ER exit sites, respectively. Here we show that KIAAO725p and p125 are novel phosphoinositide-binding proteins. Deletion and mutational analyses of KIAAO725p suggested that a sterile alpha-motif (SAM), which is also present inp125, but not in cytosolic PA-PLAI, and the following DDHD domain comprise a minimal region for phosphatidylinositol 4-phosphate (Pl(4)P)-binding. A construct with mutations in the positively charged cluster of the SAM domain is defective in both phosphoinositide-binding and Golgi/ERGIC targeting. Consistent with the view that the Pl(4)P-binding is important for the membrane association of KIAA0725p, expression of phosphoinositide phosphatase Sacd reduces the association of expressed KIAAO725p with membranes. In addition, we show that deletion of the DDHD domain or introduction of point mutations at the conserved aspartate or histidine residues in the domain abolishes the phospholipase activity of KIAAO725p and PA-PLA1. Together, our results suggest that KIAAO725p is targeted to specific organelle membranes in a phosphoinositide-dependent manner, and that its SAM and DDHD domains are essential for its phosphoinositide-binding and phospholipase activity.     

Biochemical characterization of pantoate kinase, a novel enzyme necessary for coenzyme a biosynthesis in the archaea

Although bacteria and eukaryotes share a pathway for coenzyme A (CoA) biosynthesis, we previously clarified that most archaea utilize a distinct pathway for the conversion of pantoate to 4'-phosphopantothenate. Whereas bacteria/eukaryotes use pantothenate synthetase and pantothenate kinase (PanK), the hyperthermophilic archaeon Thermococcus kodakarensis utilizes two novel enzymes: pantoate kinase (PoK) and phosphopantothenate synthetase (PPS). Here, we report a detailed biochemical examination of PoK from T. kodakarensis. Kinetic analyses revealed that the PoK reaction displayed Michaelis-Menten kinetics toward ATP, whereas substrate inhibition was observed with pantoate. PoK activity was not affected by the addition of CoA/acetyl-CoA. Interestingly, PoK displayed broad nucleotide specificity and utilized ATP, GTP, UTP, and CTP with comparable k(cat)/K(m) values. Sequence alignment of 27 PoK homologs revealed seven conserved residues with reactive side chains, and variant proteins were constructed for each residue. Activity was not detected when mutations were introduced to Ser104, Glu134, and Asp143, suggesting that these residues play vital roles in PoK catalysis. Kinetic analysis of the other variant proteins, with mutations S28A, H131A, R155A, and T186A, indicated that all four residues are involved in pantoate recognition and that Arg155 and Thr186 play important roles in PoK catalysis. Gel filtration analyses of the variant proteins indicated that Thr186 is also involved in dimer assembly. A sequence comparison between PoK and other members of the GHMP kinase family suggests that Ser104 and Glu134 are involved in binding with phosphate and Mg(2+), respectively, while Asp143 is the base responsible for proton abstraction from the pantoate hydroxy group.     

Structural basis for inhibition of xyloglucan-specific endo-β-1,4-glucanase (XEG) by XEG-protein inhibitor

Microorganisms such as plant pathogens secrete glycoside hydrolases (GHs) to digest the polysaccharide chains of plant cell walls. The degradation of cell walls by these enzymes is a crucial step for nutrition and invasion. To protect the cell wall from these enzymes, plants secrete glycoside hydrolase inhibitor proteins (GHIPs). Xyloglucan-specific endo-β-1,4-glucanase (XEG), a member of GH family 12 (GH12), could be a great threat to many plants because xyloglucan is a major component of the cell wall in most plants. Understanding the inhibition mechanism of XEG by GHIP is therefore of great importance in the field of plant defense, but to date the mechanism and specificity of GHIPs remain unclear. We have determined the crystal structure of XEG in complex with extracellular dermal glycoprotein (EDGP), a carrot GHIP that inhibits XEG. The structure reveals that the conserved arginines of EDGP intrude into the active site of XEG and interact with the catalytic glutamates of the enzyme. We have also determined the crystal structure of the XEG-xyloglucan complex. These structures show that EDGP closely mimics the XEG-xyloglucan interaction. Although EDGP shares structural similarity to a wheat GHIP (Triticum aestivum xylanase inhibitor-IA (TAXI-IA)) that inhibits GH11 family xylanases, the arrangement of GH and GHIP in the XEG-EDGP complex is distinct from that in the xylanase-TAXI-IA complex. Our findings imply that plants have evolved structures of GHIPs to inhibit different GH family members that attack their cell walls.     

Organ-specific sulfation patterns of heparan sulfate generated by extracellular sulfatases Sulf1 and Sulf2 in mice

Heparan sulfate endosulfatases Sulf1 and Sulf2 hydrolyze 6-O-sulfate in heparan sulfate, thereby regulating cellular signaling. Previous studies have revealed that Sulfs act predominantly on UA2S-GlcNS6S disaccharides and weakly on UA-GlcNS6S disaccharides. However, the specificity of Sulfs and their role in sulfation patterning of heparan sulfate in vivo remained unknown. Here, we performed disaccharide analysis of heparan sulfate in Sulf1 and Sulf2 knock-out mice. Significant increases in ΔUA2S-GlcNS6S were observed in the brain, small intestine, lung, spleen, testis, and skeletal muscle of adult Sulf1(-/-) mice and in the brain, liver, kidney, spleen, and testis of adult Sulf2(-/-) mice. In addition, increases in ΔUA-GlcNS6S were seen in the Sulf1(-/-) lung and small intestine. In contrast, the disaccharide compositions of chondroitin sulfate were not primarily altered, indicating specificity of Sulfs for heparan sulfate. For Sulf1, but not for Sulf2, mRNA expression levels in eight organs of wild-type mice were highly correlated with increases in ΔUA2S-GlcNS6S in the corresponding organs of knock-out mice. Moreover, overall changes in heparan sulfate compositions were greater in Sulf1(-/-) mice than in Sulf2(-/-) mice despite lower levels of Sulf1 mRNA expression, suggesting predominant roles of Sulf1 in heparan sulfate desulfation and distinct regulation of Sulf activities in vivo. Sulf1 and Sulf2 mRNAs were differentially expressed in restricted types of cells in organs, and consequently, the sulfation patterns of heparan sulfate were locally and distinctly altered in Sulf1 and Sulf2 knock-out mice. These findings indicate that Sulf1 and Sulf2 differentially contribute to the generation of organ-specific sulfation patterns of heparan sulfate.     

Siglec-15 protein regulates formation of functional osteoclasts in concert with DNAX-activating protein of 12 kDa (DAP12)

Osteoclasts are multinucleated giant cells that reside in osseous tissues and resorb bone. Signaling mediated by receptor activator of nuclear factor (NF)-κB (RANK) and its ligand leads to the nuclear factor of activated T cells 2/c1 (NFAT2 or NFATc1) expression, a critical step in the formation of functional osteoclasts. In addition, adaptor proteins harboring immunoreceptor tyrosine-based activation motifs, such as DNAX-activating protein of 12 kDa (DAP12), play essential roles. In this study, we identified the gene encoding the lectin Siglec-15 as NFAT2-inducible, and we found that the protein product links RANK ligand-RANK-NFAT2 and DAP12 signaling in mouse osteoclasts. Both the recognition of sialylated glycans by the Siglec-15 V-set domain and the association with DAP12 through its Lys-272 are essential for its function. When Siglec-15 expression was knocked down, fewer multinucleated cells developed, and those that did were morphologically contracted with disordered actin-ring structures. These changes were accompanied by significantly reduced bone resorption. Siglec-15 formed complexes with Syk through DAP12 in response to vitronectin. Furthermore, chimeric molecules consisting of the extracellular and transmembrane regions of Siglec-15 with a K272A mutation and the cytoplasmic region of DAP12 significantly restored bone resorption in cells with knocked down Siglec-15 expression. Together, these results suggested that the Siglec-15-DAP12-Syk-signaling cascade plays a critical role in functional osteoclast formation.     

Antifungal activity of morinol B derivatives of tetrahydropyran sesquilignan

The relationship between the structure of naturally occurring (7R,7'R,8R,8'R)-morinol B and its antifungal activity was examined. 3-Demethoxy morinol B showed much higher activity than the natural compound. The activity of the 4-butoxy-3-demethoxy derivative was higher than that of 3-demethoxy morinol B.