European journal of biochemistry最新文献

The Hfq (Host factor 1) polypeptide is a nucleic acid binding protein involved in the synthesis of many polypeptides. Hfq particularly affects the translation and the stability of several RNAs. In an earlier study, the use of fold recognition methods allowed us to detect a relationship between Escherichia coli Hfq and the Sm topology. This topology was further validated by a series of biophysical studies and the Hfq structure was modelled on an Sm protein. Hfq forms a beta-sheet ring-shaped hexamer. As our previous study predicted a large number of alternative conformations for the C-terminal region, we have determined whether the last 19 C-terminal residues are necessary for protein function. We find that the C-terminal truncated protein is fully capable of binding a polyadenylated RNA (K(d) of 120 pm vs. 50 pm for full-length Hfq). This result shows that the functional core of E. coli Hfq resides in residues 1-70 and confirms previous genetic studies. Using equilibrium unfolding studies, however, we find that full-length Hfq is 1.8 kcal x mol(-1) more stable than its truncated variant. Electron microscopy analysis of both truncated and full-length proteins indicates a structural rearrangement between the subunits upon truncation. This conformational change is coupled to a reduction in beta-strand content, as determined by Fourier transform infra-red. On the basis of these results, we propose that the C-terminal domain could protect the interface between the subunits and stabilize the hexameric Hfq structure. The origin of this C-terminal domain is also discussed.

In order to estimate the size of the cavity remaining around the heme of the 3A3-microperoxidase 8 (MP8) hemoabzyme, the formation of 3A3-MP8-Fe(II)-nitrosoalkane complexes upon oxidation of N-monosubstituted hydroxylamines was examined. This constituted a new reaction for hemoabzymes and is the first example of fully characterized Fe(II)-metabolite complexes of antibody-porphyrin. Also, via a comparison of the reactions with N-substituted hydroxylamines of various size and hydrophobicity, antibody 3A3 was confirmed to bring about a partial steric hindrance on the distal face of MP8. Subsequently, the influence of the antibody on the stereoselectivity of the S-oxidation of sulfides was examined. Our results showed that MP8 alone and the antibody-MP8 complex catalyze the oxidation of thioanisole by H(2)O(2) and tert-butyl hydroperoxide, following a peroxidase-like two-step oxygen-transfer mechanism involving a radical-cation intermediate. The best system, associating H(2)O(2) as oxidant and 3A3-MP8 as a catalyst, in the presence of 5% tert-butyl alcohol, led to the stereoselective S-oxidation of thioanisole with a 45% enantiomeric excess in favour of the R isomer. This constitutes the highest enantiomeric excess reported to date for the oxidation of sulfides catalyzed by hemoabzymes.

Observations of thioredoxin inhibition by cadmium and of a positive role for thioredoxin in protection from Cd(2+) led us to investigate the thioredoxin-cadmium interaction properties. We used calorimetric and spectroscopic methods at different pH values to explore the relative contribution of putative binding residues (Cys32, Cys35, Trp28, Trp31 and Asp26) within or near the active site. At pH 8 or 7.5 two binding sites were identified by isothermal titration calorimetry with affinity constants of 10 x 10(6) m(-1) and 1 x 10(6) m(-1). For both sites, a proton was released upon Cd(2+) binding. One mole of Cd(2+) per mole of reduced thioredoxin was measured by mass spectrometry at these pH values, demonstrating that the two binding sites were partially occupied and mutually exclusive. Cd(2+) binding at either site totally inhibited the thiol-disulfide transferase activity of Trx. The absence of Cd(2+) interaction detected for oxidized or alkylated Trx and the inhibition of the enzymatic activity of thioredoxin by Cd(2+) supported the role of Cys32 at the first site. The fluorescence profile of Cd(2+)-bound thioredoxin differed, however, from that of oxidized thioredoxin, indicating that Cd(2+) was not coordinated with Cys32 and Cys35. From FTIR spectroscopy, we inferred that the second site might involve Asp26, a buried residue that deprotonates at a rather high and unusual pK(a) for a carboxylate (7.5/9.2). The pK(a) of the two residues Cys32 and Asp26 have been shown to be interdependent [Chivers, T. P. (1997) Biochemistry36, 14985-14991]. A mechanism is proposed in which Cd(2+) binding at the solvent-accessible thiolate group of Cys32 induces a decrease of the pK(a) of Asp26 and its deprotonation. Conversely, interaction between the carboxylate group of Asp26 and Cd(2+) at a second binding site induces Cys32 deprotonation and thioredoxin inhibition, so that Cd(2+) inhibits thioredoxin activity not only by binding at the Cys32 but also by interacting with Asp26.

Three point mutations R335S, L336V and V476L, distinguish the sequence of a cytochrome P450 CYP6A2 variant assumed to be responsible for 1,1,1-trichloro-2,2-bis-(4'-chlorophenyl)ethane (DDT) resistance in the RDDT(R) strain of Drosophila melanogaster. To determine the impact of each mutation on the function of CYP6A2, the wild-type enzyme (CYP6A2wt) of Cyp6a2 was expressed in Escherichia coli as well as three variants carrying a single mutation, the double mutant CYP6A2vSV and the triple mutant CYP6A2vSVL. All CYP6A2 variants were less stable than the CYP6A2wt protein. Two activities enhanced in the RDDT(R) strain were measured with all recombinant proteins, namely testosterone hydroxylation and DDT metabolism. Testosterone was hydroxylated at the 2beta position with little quantitative variation among the variants. In contrast, metabolism of DDT was strongly affected by the mutations. The CYP6A2vSVL enzyme had an enhanced metabolism of DDT, producing dicofol, dichlorodiphenyldichloroethane and dichlorodiphenyl acetic acid. The apparent affinity of the enzymes CYP6A2wt and CYP6A2vSVL for DDT and testosterone was not significantly different as revealed by the type I difference spectra. Sequence alignments with CYP102A1 provided clues to the positions of the amino acids mutated in CYP6A2. These mutations were found spatially clustered in the vicinity of the distal end of helix I relative to the substrate recognition valley. Thus this area, including helix J, is important for the structure and activity of CYP6A2. Furthermore, we show here that point mutations in a cytochrome P450 can have a prominent role in insecticide resistance.

A gene encoding thermostable Lon protease from Brevibacillus thermoruber WR-249 was cloned and characterized. The Br. thermoruber Lon gene (Bt-lon) encodes an 88 kDa protein characterized by an N-terminal domain, a central ATPase domain which includes an SSD (sensor- and substrate-discrimination) domain, and a C-terminal protease domain. The Bt-lon is a heat-inducible gene and may be controlled under a putative Bacillus subtilis sigmaA-dependent promoter, but in the absence of CIRCE (controlling inverted repeat of chaperone expression). Bt-lon was expressed in Escherichia coli, and its protein product was purified. The native recombinant Br. thermoruber Lon protease (Bt-Lon) displayed a hexameric structure. The optimal temperature of ATPase activity for Bt-Lon was 70 degrees C, and the optimal temperature of peptidase and DNA-binding activities was 50 degrees C. This implies that the functions of Lon protease in thermophilic bacteria may be switched, depending on temperature, to regulate their physiological needs. The peptidase activity of Bt-Lon increases substantially in the presence of ATP. Furthermore, the substrate specificity of Bt-Lon is different from that of E. coli Lon in using fluorogenic peptides as substrates. Notably, the Bt-Lon protein shows chaperone-like activity by preventing aggregation of denatured insulin B-chain in a dose-dependent and ATP-independent manner. In thermal denaturation experiments, Bt-Lon was found to display an indicator of thermostability value, Tm of 71.5 degrees C. Sequence comparison with mesophilic Lon proteases shows differences in the rigidity, electrostatic interactions, and hydrogen bonding of Bt-Lon relevant to thermostability.

Acharan sulfate, a recently discovered glycosaminoglycan isolated from Achatina fulica, has a major disaccharide repeating unit of -->4)-2-acetyl,2-deoxy-alpha-d-glucopyranose(1-->4)-2-sulfo-alpha-l-idopyranosyluronic acid (1-->, making it structurally related to both heparin and heparan sulfate. It has been suggested that this glycosaminoglycan is polydisperse, with an average molecular mass of 29 kDa and known minor disaccharide sequence variants containing unsulfated iduronic acid. Acharan sulfate was found to be located in the body of this species using alcian blue staining and it was suggested to be the main constituent of the mucus. In the present work, we provide further information on the structure and compartmental distribution of acharan sulfate in the snail body. Different populations of acharan sulfate presenting charge and/or molecular mass heterogeneities were isolated from the whole body, as well as from mucus and from the organic shell matrix. A minor glycosaminoglycan fraction susceptible to degradation by nitrous acid was also purified from the snail body, suggesting the presence of N-sulfated glycosaminoglycan molecules. In addition, we demonstrate the in vivo metabolic labeling of acharan sulfate in the snail body after a meal supplemented with [35S]free sulfate. This simple approach might be applied to the study of acharan sulfate biosynthesis. Finally, we developed histochemical assays to localize acharan sulfate in the snail body by metachromatic staining and by histoautoradiography following metabolic radiolabeling with [35S]sulfate. Our results show that acharan sulfate is widely distributed among several organs.

In order to determine the amino-terminal sequence requirements for protein N-myristoylation, site-directed mutagenesis of the N-terminal region was performed using tumor necrosis factor (TNF) mutants as model substrate proteins. Subsequently, the susceptibility of these mutants to protein N-myristoylation was evaluated by metabolic labeling in an in vitro translation system using rabbit reticulocyte lysate. A TNF mutant having the sequence MGAAAAAAAA at its N-terminus was used as the starting sequence to identify elements critical for protein N-myristoylation. Sequential vertical-scanning mutagenesis of amino acids at a distinct position in this model N-terminal sequence revealed the major sequence requirements for protein N-myristoylation: the combination of amino acids at position 3 and 6 constitutes a major determinant for the susceptibility to protein N-myristoylation. When Ser was located at position 6, 11 amino acids (Gly, Ala, Ser, Cys, Thr, Val, Asn, Leu, Ile, Gln, His) were permitted at position 3 to direct efficient protein N-myristoylation. In this case, the presence of Lys at position 7 was found to affect the amino acid requirement at position 3 and Lys became permitted at this position. When Ser was not located at position 6, only 3 amino acids (Ala, Asn, Gln) were permitted at position 3 to direct efficient protein N-myristoylation. The amino acid requirements found in this study were fully consistent with the N-terminal sequence of 78 N-myristoylated proteins in which N-myristoylation was experimentally verified. These observations strongly indicate that the combination of amino acids at position 3, 6 and 7 is a major determinant for protein N-myristoylation.

The platelet integrin receptor alphaIIbbeta3 plays a critical role in thrombosis and haemostasis by mediating interactions between platelets and several ligands but primarily fibrinogen. It has been shown previously that the YMESRADR KLAEVGRVYLFL (313-332) sequence of the alphaIIb subunit plays an important role in platelet activation, fibrinogen binding and alphaIIbbeta3-mediated outside-in signalling. Furthermore, we recently showed that the 20-residue peptide (20-mer) alphaIIb 313-332, is a potent inhibitor of platelet aggregation and fibrinogen binding to alphaIIbbeta3, interacting with fibrinogen rather than the receptor. In an effort to determine the sequence and the minimum length required for the biological activity of the above 20-mer, we synthesized seven octapeptides, each overlapping by six residues, covering the entire sequence and studied their effect on platelet activation as well as fibrinogen binding to activated platelets. We show for the first time that octapeptides containing the RAD sequence are capable of inhibiting platelet aggregation and secretion as well as fibrinogen binding to the activated alphaIIbbeta3, possibly interacting with the ligand rather than the receptor. This suggests that the RAD sequence, common to all the inhibitory peptides, is critical for their biological activity. However, the presence of the YMES sequence, adjacent to RAD, significantly increases the peptide's biological potency. The development of such inhibitors derived from the 313-332 region of the alphaIIb subunit may be advantageous against the RGD-like antagonists as they could inhibit platelet activation without interacting with alphaIIbbeta3, thus failing to further induce alphaIIbbeta3-mediated outside-in signalling.

The information required for successful assembly of an icosahedral virus is encoded in the native conformation of the capsid protein and in its interaction with the nucleic acid. Here we investigated how the packing and stability of virus capsids are sensitive to single amino acid substitutions in the coat protein. Tryptophan fluorescence, bis-8-anilinonaphthalene-1-sulfonate fluorescence, CD and light scattering were employed to measure urea- and pressure-induced effects on MS2 bacteriophage and temperature sensitive mutants. M88V and T45S particles were less stable than the wild-type forms and completely dissociated at 3.0 kbar of pressure. M88V and T45S mutants also had lower stability in the presence of urea. We propose that the lower stability of M88V particles is related to an increase in the cavity of the hydrophobic core. Bis-8-anilinonaphthalene-1-sulfonate fluorescence increased for the pressure-dissociated mutants but not for the urea-denatured samples, indicating that the final products were different. To verify reassembly of the particles, gel filtration chromatography and infectivity assays were performed. The phage titer was reduced dramatically when particles were treated with a high concentration of urea. In contrast, the phage titer recovered after high-pressure treatment. Thus, after pressure-induced dissociation of the virus, information for correct reassembly was preserved. In contrast to M88V and T45S, the D11N mutant virus particle was more stable than the wild-type virus, in spite of it also possessing a temperature sensitive growth phenotype. Overall, our data show how point substitutions in the capsid protein, which affect either the packing or the interaction at the protein-RNA interface, result in changes in virus stability.

Calpain 1 behaviour toward cytoskeletal targets was investigated using two alpha-actinin isoforms from smooth and skeletal muscles. These two isoforms which are, respectively, sensitive and resistant to calpain cleavage, interact with the protease when using in vitro binding assays. The stability of the complexes in EGTA [Kd(-Ca2+) = 0.5 +/- 0.1 microM] was improved in the presence of 1 mm calcium ions [Kd(+Ca2+) = 0.05 +/- 0.01 microM]. Location of the binding structures shows that the C-terminal domain of alpha-actinin and each calpain subunit, 28 and 80 kDa, participates in the interaction. In particular, the autolysed calpain form (76/18) affords a similar binding compared to the 80/28 intact enzyme, with an identified binding site in the catalytic subunit, located in the C-terminal region of the chain (domain III-IV). The in vivo colocalization of calpain 1 and alpha-actinin was shown to be likely in the presence of calcium, when permeabilized muscle fibres were supplemented by exogenous calpain 1 and the presence of calpain 1 in Z-line cores was shown by gold-labelled antibodies. The demonstration of such a colocalization was brought by coimmunoprecipitation experiments of calpain 1 and alpha-actinin from C2.7 myogenic cells. We propose that calpain 1 interacts in a resting state with cytoskeletal targets, and that this binding is strengthened in pathological conditions, such as ischaemia and dystrophies, associated with high calcium concentrations.