Zinc as an agent for the prevention of biofilm formation by pathogenic bacteria.

AIMS: Biofilm formation is important for the persistence of bacteria in hostile environments. Bacteria in a biofilm are usually more resistant to antibiotics and disinfectants than planktonic bacteria. Our laboratory previously reported that low concentrations of zinc inhibit biofilm formation of Actinobacillus pleuropneumoniae. The aim of this study is to evaluate the effect of zinc on growth and biofilm formation of other bacterial swine pathogens. METHODS AND RESULTS: To determine the effect of zinc on biofilm formation, biofilms were grown with or without zinc in 96-well plates and stained with crystal violet. At micromolar concentrations (0-250 µ mol l(-1)), zinc weakly inhibited bacterial growth and it effectively blocked biofilm formation by A. pleuropneumoniae, Salmonella Typhymurium and Haemophilus parasuis in a dose-dependent manner. Additionally, biofilm formation of Escherichia coli, Staphylococcus aureus and Streptococcus suis was slightly inhibited by zinc. However, zinc did not disperse preformed biofilms. To determine whether zinc inhibits biofilm formation when poly-N-acetylglucosamine (PGA) is present, PGA was detected with the lectin wheat germ agglutinin. Only A. pleuropneumoniae and Staph. aureus biofilms were found to contain PGA. CONCLUSION: Zinc used at nonbactericidal concentrations can inhibit biofilm formation by several Gram-negative and Gram-positive bacterial swine pathogens. SIGNIFICANCE AND IMPACT OF STUDY: The antibiofilm activity of zinc could provide a tool to fight biofilms, and the nonspecific inhibitory effect may well extend to other important human and animal bacterial pathogens.

Anthrax toxins.

Bacillus anthracis, the etiological agent of anthrax, secretes three polypeptides that assemble into toxic complexes on the cell surfaces of the host it infects. One of these polypeptides, protective antigen (PA), binds to the integrin-like domains of ubiquitously expressed membrane proteins of mammalian cells. PA is then cleaved by membrane endoproteases of the furin family. Cleaved PA molecules assemble into heptamers, which can then associate with the two other secreted polypeptides: edema factor (EF) and/or lethal factor (LF). The heptamers of PA are relocalized to lipid rafts where they are quickly endocytosed and routed to an acidic compartment. The low pH triggers a conformational change in the heptamers, resulting in the formation of cation-specific channels and the translocation of EF/LF. EF is a calcium- and calmodulin-dependent adenylate cyclase that dramatically raises the intracellular concentration of cyclic adenosine monophosphate (cAMP). LF is a zinc-dependent endoprotease that cleaves the amino terminus of mitogen-activated protein kinase kinases (Meks). Cleaved Meks cannot bind to their substrates and have reduced kinase activity, resulting in alterations of the signaling pathways they govern. The structures of PA, PA heptamer, EF, and LF have been solved and much is now known about the molecular details of the intoxication mechanism. The in vivo action of the toxins, on the other hand, is still poorly understood and hotly debated. A better understanding of the toxins will help in the design of much-needed anti-toxin drugs and the development of new toxin-based medical applications.

Identification of the cellular receptor for anthrax toxin.

The tripartite toxin secreted by Bacillus anthracis, the causative agent of anthrax, helps the bacterium evade the immune system and can kill the host during a systemic infection. Two components of the toxin enzymatically modify substrates within the cytosol of mammalian cells: oedema factor (OF) is an adenylate cyclase that impairs host defences through a variety of mechanisms including inhibiting phagocytosis; lethal factor (LF) is a zinc-dependent protease that cleaves mitogen-activated protein kinase kinase and causes lysis of macrophages. Protective antigen (PA), the third component, binds to a cellular receptor and mediates delivery of the enzymatic components to the cytosol. Here we describe the cloning of the human PA receptor using a genetic complementation approach. The receptor, termed ATR (anthrax toxin receptor), is a type I membrane protein with an extracellular von Willebrand factor A domain that binds directly to PA. In addition, a soluble version of this domain can protect cells from the action of the toxin.

Designing a polyvalent inhibitor of anthrax toxin.

Screening peptide libraries is a proven strategy for identifying inhibitors of protein-ligand interactions. Compounds identified in these screens often bind to their targets with low affinities. When the target protein is present at a high density on the surface of cells or other biological surfaces, it is sometimes possible to increase the biological activity of a weakly binding ligand by presenting multiple copies of it on the same molecule. We isolated a peptide from a phage display library that binds weakly to the heptameric cell-binding subunit of anthrax toxin and prevents the interaction between cell-binding and enzymatic moieties. A molecule consisting of multiple copies of this nonnatural peptide, covalently linked to a flexible backbone, prevented assembly of the toxin complex in vitro and blocked toxin action in an animal model. This result demonstrates that protein-protein interactions can be inhibited by a synthetic, polymeric, polyvalent inhibitor in vivo.

Dominant-negative mutants of a toxin subunit: an approach to therapy of anthrax.

The protective antigen moiety of anthrax toxin translocates the toxin's enzymic moieties to the cytosol of mammalian cells by a mechanism that depends on its ability to heptamerize and insert into membranes. We identified dominant-negative mutants of protective antigen that co-assemble with the wild-type protein and block its ability to translocate the enzymic moieties across membranes. These mutants strongly inhibited toxin action in cell culture and in an animal intoxication model, suggesting that they could be useful in therapy of anthrax.

Involvement of domain 3 in oligomerization by the protective antigen moiety of anthrax toxin.

Protective antigen (PA), a component of anthrax toxin, binds receptors on mammalian cells and is activated by a cell surface protease. The resulting active fragment, PA(63), forms ring-shaped heptamers, binds the enzymic moieties of the toxin, and translocates them to the cytosol. Of the four crystallographic domains of PA, domain 1 has been implicated in binding the enzymic moieties; domain 2 is involved in membrane insertion and oligomerization; and domain 4 binds receptor. To determine the function of domain 3, we developed a screen that allowed us to isolate random mutations that cause defects in the activity of PA. We identified several mutations in domain 3 that affect monomer-monomer interactions in the PA(63) heptamer, indicating that this may be the primary function of this domain.

ATP modulates subunit-subunit interactions in an ATP-binding cassette transporter (MalFGK2) determined by site-directed chemical cross-linking.

The binding protein-dependent maltose transport system of enterobacteria (MalFGK(2)), a member of the ATP-binding cassette (ABC) transporter superfamily, is composed of two integral membrane proteins, MalF and MalG, and of two copies of an ATPase subunit, MalK, which hydrolyze ATP, thus energizing the translocation process. In addition, an extracellular (periplasmic) substrate-binding protein (MalE) is required for activity. Ligand translocation and ATP hydrolysis are dependent on a signaling mechanism originating from the binding protein and traveling through MalF/MalG. Thus, subunit-subunit interactions in the complex are crucial to the transport process but the chemical nature of residues involved is poorly understood. We have investigated the proximity of residues in a conserved sequence ("EAA" loop) of MalF and MalG to residues in a helical segment of the MalK subunits by means of site-directed chemical cross-linking. To this end, single cysteine residues were introduced into each subunit at several positions and the respective malF and malG alleles were individually co-expressed with each of the malK alleles. Membrane vesicles were prepared from those double mutants that contained a functional transporter in vivo and treated with Cu(1,10-phenanthroline)(2)SO(4) or bifunctional cross-linkers. The results suggest that residues Ala-85, Lys-106, Val-114, and Val-117 in the helical segment of MalK, to different extents, participate in constitution of asymmetric interaction sites with the EAA loops of MalF and MalG. Furthermore, both MalK monomers in the complex are in close contact to each other through Ala-85 and Lys-106. These interactions are strongly modulated by MgATP, indicating a structural rearrangement of the subunits during the transport cycle. These data are discussed with respect to current transport models.

Structure-function study of MalF protein by random mutagenesis.

MalF is one of the two integral inner membrane proteins of the maltose-maltodextrin transport system. To identify functional regions in this protein, we characterized a collection of malF mutants obtained by random mutagenesis. We analyzed their growth on maltose and maltodextrins, the steady-state levels and subcellular localization of the mutant proteins, and the subcellular localization of MalK. Only 2 of the 21 MalF mutant proteins allowed growth on maltose and maltodextrins. Most mutations resulting in immunodetectable proteins mapped to hydrophilic domains, indicating that insertions affecting transmembrane segments gave rise to unstable or lethal proteins. All MalF mutant proteins, even those C-terminally truncated or with large N-terminal deletions, were inserted into the cytoplasmic membrane. Having identified mutations leading to reduced steady-state level, to partial mislocation, and/or to misfolding, we were able to assign to some regions of MalF a role in the assembly of the MalFGK2 complex and/or in the transport mechanism.

In vitro interaction between components of the inner membrane complex of the maltose ABC transporter of Escherichia coli: modulation by ATP.

Interactions between domains of ATP-binding cassette (ABC) transporters are of great functional importance and yet are poorly understood. To gain further knowledge of these protein-protein interactions, we studied the inner membrane complex of the maltose transporter of Escherichia coli. We focused on interactions between the nucleotide-binding protein, MalK, and the transmembrane proteins, MalF and MalG. We incubated purified MalK with inverted membrane vesicles containing MalF and MalG. MalK bound specifically to MalF and MalG and reconstituted a functional complex. We used this approach and limited proteolysis with trypsin to show that binding and hydrolysis of ATP, inducing conformational changes in MalK, modulate its interaction with MalF and MalG. MalK in the reconstituted complex was less sensitive to protease added from the cytoplasmic side of the membrane, and one proteolytic cleavage site located in the middle of a putative helical domain of MalK was protected. These results suggest that the putative helical domain of the nucleotide-binding domains is involved, through its conformational changes, in the coupling between the transmembrane domains and ATP binding/hydrolysis at the nucleotide-binding domains.

Subunit interactions in ABC transporters: a conserved sequence in hydrophobic membrane proteins of periplasmic permeases defines an important site of interaction with the ATPase subunits.

The cytoplasmic membrane proteins of bacterial binding protein-dependent transporters belong to the superfamily of ABC transporters. The hydrophobic proteins display a conserved, at least 20 amino acid EAA---G---------I-LP region exposed in the cytosol, the EAA region. We mutagenized the EAA regions of MalF and MalG proteins of the Escherichia coli maltose transport system. Substitutions at the same positions in MalF and MalG have different phenotypes, indicating that EAA regions do not act symmetrically. Mutations in malG or malF that slightly affect or do not affect transport, determine a completely defective phenotype when present together. This suggests that EAA regions of MalF and MalG may interact during transport. Maltose-negative mutants fall into two categories with respect to the cellular localization of the MalK ATPase: in the first, MalK is membrane-bound, as in wild-type strains, while in the second, it is cytosolic, as in strains deleted in the malF and malG genes. From maltose-negative mutants of the two categories, we isolated suppressor mutations within malK that restore transport. They map mainly in the putative helical domain of MalK, suggesting that EAA regions may constitute a recognition site for the ABC ATPase helical domain.

Heat shock induction by a misassembled cytoplasmic membrane protein complex in Escherichia coli.

We analysed the effects of the overproduction of parts or all of a multisubunit ATP-binding cassette (ABC) transporter, the MalFGK2 complex, involved in the uptake of maltose and maltodextrins in Escherichia coli. We found that production of the MalF protein alone was inducing the phtrA promoter, which is under the control of a recently discovered sigma factor, sigma24, involved in the response to extracytoplasmic stresses. The production level, stability and localization of MalF were not altered when produced without its partners, suggesting that the protein was correctly inserted in the membrane. Our results indicate that a large periplasmic loop located between the third and fourth transmembrane segment of MalF, the L3 loop, is responsible for phtrA induction: (i) deleted MalF proteins with no L3 loop or with a L3 loop lacking 120 amino acids do not induce the phtrA promoter; (ii) the export to the periplasm of the L3 loop alone or fused to MalE induces the phtrA promoter. Moreover, the proteolytic sensitivity of MalF is different when it is produced alone and when MalF and MalG are produced together, suggesting a change in the conformation and/or accessibility of MalF. These results suggest that some inner membrane proteins can be sensed outside the cytoplasm by a quality control apparatus or by the export machinery. Moreover, the observation of the phtrA induction by MalF could be a useful new tool for studying the insertion and assembly of the MalFGK2 complex.