X-linked inhibitor of apoptosis protein (XIAP) inhibits caspase-3 and -7 in distinct modes

The inhibitor of apoptosis proteins (IAP) regulates cell death by inhibiting caspases. The region of X-linked (X) IAP containing the second baculovirus IAP repeat domain (BIR2) is sufficient for inhibiting caspase-3 and -7. In this study, we found that the modes of inhibition of these two caspases were different: caspase-3 is inhibited in a competitive manner whereas caspase-7 inhibition occurs through a mixed competitive and noncompetitive mechanism. Binding assays revealed that the inhibition of caspase-3 by XIAP was totally dependent on the interaction between the active site of caspase-3 and the linker region between the BIR1 and BIR2 domains of XIAP. In contrast, the active site and the NH(2)-terminal region of caspase-7 bound to the linker region and the BIR2, respectively. Moreover the BIR2 with a mutated linker region, which inhibited caspase-3 very weakly, still bound to and inhibited caspase-7. Furthermore, a chimeric caspase-7/3 comprising the NH(2)-terminal portion of caspase-7 and COOH-terminal portion of caspase-3 was inhibited by XIAP by a mixed competitive and noncompetitive mechanism. Our results suggest that the linker region between BIR1 and BIR2 domains is responsible for active site-directed, competitive inhibition of both caspase-3 and -7, whereas the BIR2 itself is involved in noncompetitive inhibition of caspase-7.     

Human intelectin is a novel soluble lectin that recognizes galactofuranose in carbohydrate chains of bacterial cell wall

Galactofuranosyl residues are present in various microorganisms but not in mammals. In this study, we identified a human lectin binding to galactofuranosyl residues and named this protein human intelectin (hIntL). The mature hIntL was a secretory glycoprotein consisting of 295 amino acids and N-linked oligosaccharides, and its basic structural unit was a 120-kDa homotrimer in which 40-kDa polypeptides were bridged by disulfide bonds. The hIntL gene was split into 8 exons on chromosome 1q21.3, and hIntL mRNA was expressed in the heart, small intestine, colon, and thymus. hIntL showed high levels of homology with mouse intelectin, Xenopus laevis cortical granule lectin/oocyte lectin, lamprey serum lectin, and ascidian galactose-specific lectin. These homologues commonly contained no carbohydrate recognition domain, which is a characteristic of C-type lectins, although some of them have been reported as Ca(2+)-dependent lectins. Recombinant hIntL revealed affinities to d-pentoses and a d-galactofuranosyl residue in the presence of Ca(2+), and recognized the bacterial arabinogalactan of Nocardia containing d-galactofuranosyl residues. These results suggested that hIntL is a new type lectin recognizing galactofuranose, and that hIntL plays a role in the recognition of bacteria-specific components in the host.     

O-helix mutant T664P of Thermus aquaticus DNA polymerase I: altered catalytic properties for incorporation of incorrect nucleotides but not correct nucleotides

Previous studies indicate that the O-helix of Thermus aquaticus (Taq) DNA polymerase I (pol I) plays an important role in the replication fidelity of the enzyme. This study examines the role of Thr-664, which lies in the middle of the O-helix of Taq pol I. A mutant of Taq Pol I with a proline substitution of Thr-664 (T664P) exhibits much lower replication fidelity than the wild type enzyme in a forward mutation assay. T664P produces base substitution, single-base deletion, and single-base insertion errors at 20-, 5, and 50-fold higher rates than wild type, respectively. In specific activity and steady-state kinetic experiments, T664P was catalytically robust for insertion of correct nucleotides. In contrast, it incorporated incorrect nucleotides 6.1- to 10-fold more efficiently than wild type at a template dC. Mismatched primer termini were extended by T664P 4.2- to 9.5-fold more efficiently than wild type. These data imply that the O-helix with a proline at position 664 functions like wild type Taq pol I for correct nucleotide incorporations, but bends and enlarges the catalytic pocket of the enzyme and increases the rate of nucleotide misincorporation.     

Inhibition of macrophage invasion by monoclonal antibodies specific to Leishmania (Viannia) braziliensis promastigotes and characterisation of their antigens

Monoclonal antibodies that specifically recognise Leishmania (Viannia) braziliensis promastigotes were produced and termed SST-2, SST-3 and SST-4. SST-2 recognises a conformational epitope present in a 24-28 kDa doublet and in a 72 kDa component, as verified by Western blotting. Indirect immunofluorescence showed that the antigen recognised by SST-2 is distributed homogeneously on the parasite surface. SST-3 recognises a flagellar glycoprotein of approximately 180 kDa. The reactivity of this mAb was abolished by sodium m-periodate treatment, indicating that SST-3 reacts with a carbohydrate epitope of the 180 kDa antigen. SST-4 recognises a conformational epitope of a 98 kDa antigen. SST-2, SST-3 and SST-4 were specific to L. (V.) braziliensis promastigote forms. Indirect immunofluorescence did not show reactivity of SST-2 or SST-3 with amastigotes of L. (V.) braziliensis, or with promastigotes of Leishmania (Viannia) panamensis, Leishmania (Viannia) guyanensis, Leishmania (Viannia) naiffi, Leishmania (Viannia) lainsoni, Leishmania (Leishmania) amazonensis, Leishmania (Leishmania) major, or Leishmania (Leishmania) chagasi. We also evaluated the involvement of SST-2, SST-3 and SST-4 antigens in parasite-macrophage interaction. Fab fragments of SST-3 and SST-4 significantly inhibited the infectivity of L. (V.) braziliensis promastigotes to mouse peritoneal macrophages.     

Ancestral residues stabilizing 3-isopropylmalate dehydrogenase of an extreme thermophile: experimental evidence supporting the thermophilic common ancestor hypothesis

Ancestral amino acid residues were inferred for 3-isopropylmalate dehydrogenase (IPMDH), and were introduced into the enzyme of an extreme thermophile, Sulfolobus sp. strain 7. The thermostability of the mutant enzymes was compared with that of the wild type enzyme. At least five of the seven mutants tested showed higher thermal stability than the wild type IPMDH. The results are compatible with the hyperthermophilic universal ancestor hypothesis. The results also provide a new method for designing thermostable enzymes. The method only relies on the first dimensional structures of homologous enzymes that can be obtained from genetic databases.     

A unique phosphatidylinositol bearing a novel branched-chain fatty acid from Rhodococcus equi binds to influenza virus hemagglutinin and inhibits the infection of cells

From the aquatic bacterium Rhodococcus equi strain S(420), we isolated a substance that strongly binds to influenza viruses. Structural analyses revealed that it is a unique type of phosphatidylinositol (PtdIns) bearing a branched-chain fatty acid (14-methyloctadecanoic acid). In a TLC/virus-binding immunostaining assay, this PtdIns bound to all subtypes of hemagglutinin (HA) of influenza A viruses tested, isolated from humans, ducks and swine, and also to human influenza B viruses. Furthermore, the PtdIns significantly prevented the infection of MDCK cells by influenza viruses, and also inhibited the virus-mediated hemagglutination and low pH-induced hemolysis of human erythrocytes, which represents the fusogenic activities of the viral HA. We also used purified hemagglutinin instead of virions to examine the interaction between viral HA and PtdIns, showing that the PtdIns binds to hemagglutinin. These findings indicate that the inhibitory mechanism of PtdIns on the influenza virus infection may be through its binding to viral HA spikes and host cell endosomal/lysosomal membranes, which are mediated by the function of viral HA.     

Mapping the ribonucleolytic active site of eosinophil-derived neurotoxin (EDN). High resolution crystal structures of EDN complexes with adenylic nucleotide inhibitors

Eosinophil-derived neurotoxin (EDN), a basic ribonuclease found in the large specific granules of eosinophils, belongs to the pancreatic RNase A family. Although its physiological function is still unclear, it has been shown that EDN is a neurotoxin capable of inducing the Gordon phenomenon in rabbits. EDN is also a potent helminthotoxin and can mediate antiviral activity of eosinophils against isolated virions of the respiratory syncytial virus. EDN is a catalytically efficient RNase sharing similar substrate specificity with pancreatic RNase A with its ribonucleolytic activity being absolutely essential for its neurotoxic, helminthotoxic, and antiviral activities. The crystal structure of recombinant human EDN in the unliganded form has been determined previously (Mosimann, S. C., Newton, D. L., Youle, R. J., and James, M. N. G. (1996) J. Mol. Biol. 260, 540-552). We have now determined high resolution (1.8 A) crystal structures for EDN in complex with adenosine-3',5'-diphosphate (3',5'-ADP), adenosine-2',5'-di-phosphate (2',5'-ADP), adenosine-5'-diphosphate (5'-ADP) as well as for a native structure in the presence of sulfate refined at 1.6 A. The inhibition constant of these mononucleotides for EDN has been determined. The structures present the first detailed picture of differences between EDN and RNase A in substrate recognition at the ribonucleolytic active site. They also provide a starting point for the design of tight-binding inhibitors, which may be used to restrain the RNase activity of EDN.