Binding of chimeric metal-binding green fluorescent protein to lipid monolayer

Membrane-based bioanalytical devices for metal determination using green fluorescent protein as the sensor molecule may be a useful future biomimetic material. However, in order to develop such a device, it is necessary first to understand the interaction of the protein with lipid membranes. Thus we have investigated the interaction between chimeric cadmium-binding green fluorescent proteins (CdBPGFPs) and lipid monolayers, using a film-balance technique complemented with epifluorescence microscopy. The binding avidity was monitored from the surface pressure vs. area isotherms or from the measured increase in the lateral pressure upon injection of the chimeric CdBPGFPs beneath the lipid monolayer. Increased fluidization as well as expansion of the surface area were shown to depend on the concentration of the CdBPGFPs. The kinetics of the protein-induced increase in lateral pressure was found to be biphasic. The chimeric CdBPGFPs possessed high affinity to the 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) monolayer with a dissociation constant of K_d=10^–8M. Epifluorescence measurements showed that this affinity is due to the presence of the Cd-binding peptide, which caused the GFP to incorporate preferentially to the liquid phase and defect part of the rigid domain at low interfacial pressure. At high compression, the Cd-binding peptide could neither incorporate nor remain in the lipid core. However, specific orientation of the chimeric CdBPGFPs underneath the air–water interface was achieved, even under high surface pressure, when the proteins were applied to the metal-chelating lipid-containing surfaces. This specific binding could be controlled reversibly by the addition of metal ions or metal chelator. The reversible binding of the chimeric CdBPGFPs to metal-chelating lipids provided a potential approach for immobilization, orientation and lateral organization of a protein at the membrane interface. Furthermore, the feasibility of applying the chelator lipids for the codetermination of metal ions with specific ligands was also revealed. Our finding clearly demonstrates that a strong interaction, particularly with fluid lipid domains, could potentially be used for sensor development in the future.Abbreviations GFP green fluorescent protein - CdBPGFPs cadmium-binding green fluorescent protein - DPPC 1,2-dipalmitoyl-sn-glycero-3-phosphocholine - AAS atomic absorption spectrometry - Cd²⁺ cadmium (II) - Zn²⁺ zinc (II) - Cu²⁺ copper (II) - Ni²⁺ nickel (II) - E. coli Escherichia coli - NTA-DOGS 1,2-dioleoyl-sn-glycero-3-(N-(5-amino-1-carboxypentyl iminodiacetic acid) succinyl) - His6GFP hexahistidine green fluorescent protein - CdBP4GFP four-repeat cadmium-binding peptide green fluorescent protein - His6CdBP4GFP hexahistidine four-repeat cadmium-binding peptide green fluorescent protein - IMAC immobilized-metal-affinity chromatography - PBS phosphate-buffered saline - mN/m millinewton per metre - le liquid expanded - lc liquid condensed - PE phosphatidyl ethanolamine - PI phosphatidyl inositol - NTA nitrilotriacetic acid - EDTA ethylenediamine tetraacetic acid - RESA ring-infected erythrocyte surface antigen - CdBP cadmium-binding peptide     

Interference of the galactose-dependent binding of lectins by novel pentapeptide ligands

A library of pentapeptides containing the sequence -Y-X-Y- based on rational design was screened with six different lectins. Sequences were identified that modulate galectin binding to its natural carbohydrate ligand. SPR showed inhibition values 2-3 times stronger than galactose and NMR studies suggested real carbohydrate mimicry.     

Repair of hydrolytic DNA deamination damage in thermophilic bacteria: cloning and characterization of a Vsr endonuclease homolog from Bacillus stearothermophilus

Hydrolytic deamination of 5-methyl cytosine in double stranded DNA results in formation of a T/G mismatch that—if left unrepaired—leads to a C→T transition mutation in half of the progeny. In addition to several mismatch-specific glycosylases that have been found in both pro- and eukaryotes to channel this lesion into base excision repair by removing the T from the mismatch, Vsr endonuclease from Escherichia coli has been described which initiates repair by an endonucleolytic strand incision 5′ to the mismatched T. We have isolated a gene coding for a homolog of E.coli Vsr endonuclease from the thermophilic bacterium Bacillus stearothermophilus H3 (Vsr.Bst) using a method that allows PCR amplification with degenerated primers of gene segments which code for only one highly conserved amino acid region. Vsr.Bst was produced heterologously in E.coli and purified to apparent homogeneity. Vsr.Bst specifically incises heteroduplex DNA with a preference for T/G mismatches. The selectivity of Vsr.Bst for the sequence context of the T/G mismatch appears less pronounced than for Vsr.Eco.     

Induction of antigen-specific regulatory T cells following overexpression of a Notch ligand by human B lymphocytes

In mice, activation of the Notch pathway in T cells by antigen-presenting cells overexpressing Notch ligands favors differentiation of regulatory T lymphocytes responsible for antigen-specific tolerance. To determine whether this mechanism operates in human T cells, we used Epstein-Barr virus-positive lymphoblastoid cell lines (EBV-LCL) as our (viral) antigen-presenting cells and overexpressed the Notch ligand Jagged-1 (EBV-LCL J1) by adenoviral transduction. The EBV-LCL J1s were cocultured with autologous T cells, and the proliferative and cytotoxic responses to EBV antigens were measured. Transduction had no effect on EBV-LCL expression of major histocompatibility complex (MHC) antigens or of costimulatory molecules CD80, CD86, and CD40. However, we observed a 35% inhibition of proliferation and a >65% reduction in cytotoxic-T-cell activity, and interleukin 10 production was increased ninefold. These EBV-LCL J1-stimulated T lymphocytes act as antigen-specific regulatory cells, since their addition to fresh autologous T cells cultured with autologous nontransduced EBV-LCL cells significantly inhibited both proliferation and cytotoxic effector function. Within the inhibitory population, CD4(+)CD25(+) and CD8(+)CD25(-) T cells had the greatest activity. This inhibition appears to be antigen-specific, since responses to Candida and cytomegalovirus antigens were unaffected. Hence, transgenic expression of Jagged-1 by antigen-presenting cells can induce antigen-specific regulatory T cells in humans and modify immune responses to viral antigens.     

Unique conformer selection of human growth-regulatory lectin galectin-1 for ganglioside GM1 versus bacterial toxins

Endogenous lectins induce effects on cell growth by binding to antennae of natural glycoconjugates. These complex carbohydrates often present more than one potential lectin-binding site in a single chain. Using the growth-regulatory interaction of the pentasaccharide of ganglioside GM(1) with homodimeric galectin-1 on neuroblastoma cell surfaces as a model, we present a suitable strategy for addressing this issue. The approach combines NMR spectroscopic and computational methods and does not require isotope-labeled glycans. It involves conformational analysis of the two building blocks of the GM(1) glycan, i.e., the disaccharide Galbeta1-3GalNAc and the trisaccharide Neu5Acalpha2-3Galbeta1-4Glc. Their bound-state conformations were determined by transferred nuclear Overhauser enhancement spectroscopy. Next, measurements on the lectin-pentasaccharide complex revealed differential conformer selection regarding the sialylgalactose linkage in the tri- versus pentasaccharide (Phi and Psi value of -70 degrees and 15 degrees vs 70 degrees and 15 degrees, respectively). To proceed in the structural analysis, the characteristic experimentally detected spatial vicinity of a galactose unit and Trp68 in the galectin's binding site offered a means, exploiting saturation transfer from protein to carbohydrate protons. Indeed, we detected two signals unambiguously assigned to the terminal Gal and the GalNAc residues. Computational docking and interaction energy analyses of the entire set of ligands supported and added to experimental results. The finding that the ganglioside's carbohydrate chain is subject to differential conformer selection at the sialylgalactose linkage by galectin-1 and GM(1)-binding cholera toxin (Phi and Psi values of -172 degrees and -26 degrees, respectively) is relevant for toxin-directed drug design. In principle, our methodology can be applied in studies aimed at blocking galectin functionality in malignancy and beyond glycosciences.     

D-Peptides as inhibitors of the DnaK/DnaJ/GrpE chaperone system

DnaK, a Hsp70 homolog of Escherichia coli, together with its co-chaperones DnaJ and GrpE protects denatured proteins from aggregation and promotes their refolding by an ATP-consuming mechanism. DnaJ not only stimulates the gamma-phosphate cleavage of DnaK-bound ATP but also binds polypeptide substrates on its own. Unfolded polypeptides, such as denatured luciferase, thus form ternary complexes with DnaJ and DnaK. A previous study has shown that d-peptides compete with l-peptides for the same binding site in DnaJ but do not bind to DnaK (Feifel, B., Sch?nfeld, H.-J., and Christen, P. (1998) J. Biol. Chem. 273, 11999-12002). Here we report that d-peptides efficiently inhibit the refolding of denatured luciferase by the DnaK/DnaJ/GrpE chaperone system (EC50 = 1-2 microM). The inhibition of the chaperone action is due to the binding of d-peptide to DnaJ (Kd = 1-2 microM), which seems to preclude DnaJ from forming ternary (ATP.DnaK)m.substrate.DnaJn complexes. Apparently, simultaneous binding of DnaJ and DnaK to one and the same target polypeptide is essential for effective chaperone action.     

Mechanism of elongation factor (EF)-Ts-catalyzed nucleotide exchange in EF-Tu. Contribution of contacts at the guanine base.

Nucleotide exchange in elongation factor Tu (EF-Tu) is catalyzed by elongation factor Ts (EF-Ts). Similarly to other GTP-binding proteins, the structural changes in the P loop and the Mg(2+) binding site are known to be important for nucleotide release from EF-Tu. In the present paper, we determine the contribution of the contacts between helix D of EF-Tu at the base side of the nucleotide and the N-terminal domain of EF-Ts to the catalysis. The rate constants of the multistep reaction between Escherichia coli EF-Tu, EF-Ts, and GDP were determined by stopped-flow kinetic analysis monitoring the fluorescence of either Trp-184 in EF-Tu or mant-GDP. Mutational analysis shows that contacts between helix D of EF-Tu and the N-terminal domain of EF-Ts are important for both complex formation and the acceleration of GDP dissociation. The kinetic results suggest that the initial contact of EF-Ts with helix D of EF-Tu weakens binding interactions around the guanine base, whereas contacts of EF-Ts with the phosphate binding side that promotes the release of the phosphate moiety of GDP appear to take place later. This "base-side-first" mechanism of guanine nucleotide release resembles that found for Ran x RCC1 and differs from mechanisms described for other GTPase x GEF complexes where interactions at the phosphate side of the nucleotide are released first.     

Molecular cloning and characterization of the gene coding for the aerobic azoreductase from Xenophilus azovorans KF46F

The gene coding for an aerobic azoreductase was cloned from Xenophilus azovorans KF46F (formerly Pseudomonas sp. strain KF46F), which was previously shown to grow with the carboxylated azo compound 1-(4'-carboxyphenylazo)-2-naphthol (carboxy-Orange II) as the sole source of carbon and energy. The deduced amino acid sequence encoded a protein with a molecular weight of 30,278 and showed no significant homology to amino acid sequences currently deposited at the relevant data bases. A presumed NAD(P)H-binding site was identified in the amino-terminal region of the azoreductase. The enzyme was heterologously expressed in Escherichia coli and the azoreductase activities of resting cells and cell extracts were compared. The results suggested that whole cells of the recombinant E. coli strains were unable to take up sulfonated azo dyes and therefore did not show in vivo azoreductase activity. The turnover of several industrially relevant azo dyes by cell extracts from the recombinant E. coli strain was demonstrated.     

Inactivation of the elongation factor Tu by mosquitocidal toxin-catalyzed mono-ADP-ribosylation

The mosquitocidal toxin (MTX) produced by Bacillus sphaericus strain SSII-1 is an approximately 97-kDa single-chain toxin which contains a 27-kDa enzyme domain harboring ADP-ribosyltransferase activity and a 70-kDa putative binding domain. Due to cytotoxicity toward bacterial cells, the 27-kDa enzyme fragment cannot be produced in Escherichia coli expression systems. However, a nontoxic 32-kDa N-terminal truncation of MTX can be expressed in E. coli and subsequently cleaved to an active 27-kDa enzyme fragment. In vitro the 27-kDa enzyme fragment of MTX ADP-ribosylated numerous proteins in E. coli lysates, with dominant labeling of an approximately 45-kDa protein. Matrix-assisted laser desorption ionization-time-of-flight mass spectrometry combined with peptide mapping identified this protein as the E. coli elongation factor Tu (EF-Tu). ADP ribosylation of purified EF-Tu prevented the formation of the stable ternary EF-Tuaminoacyl-tRNAGTP complex, whereas the binding of GTP to EF-Tu was not altered. The inactivation of EF-Tu by MTX-mediated ADP-ribosylation and the resulting inhibition of bacterial protein synthesis are likely to play important roles in the cytotoxicity of the 27-kDa enzyme fragment of MTX toward E. coli.