Site-specific conjugation of single domain antibodies to liposomes enhances photosensitizer uptake and photodynamic therapy efficacy.

Photodynamic therapy for therapy-resistant cancers will greatly benefit from targeted delivery of tumor photosensitizing agents. In this study, a strategy for the site-specific conjugation of single domain antibodies onto liposomes containing the photosensitizer zinc phthalocyanine was developed and tested.

Reduction of Clostridium sporogenes spore outgrowth in natural sausage casings using nisin.

Preservation of natural sausage casings using dry salt or saturated brine is regarded as sufficient to inactivate vegetative pathogenic non-spore-forming bacteria present on the casings. Although the outgrowth of bacterial spores is prevented by salt or saturated brine preservation, these spores will remain present and develop into vegetative cells when conditions are more favourable. To prevent subsequent outgrowth additional preservation measures should be implemented. In the experiments described the use of nisin was evaluated to reduce outgrowth of spores in desalinated casings. The bacteriocin nisin was chosen because of its known efficacy against spore-forming bacteria and their spores in various foodstuffs. Clostridium spore suspensions (Clostridium sporogenes, ATCC 3584) were used in two concentrations to inoculate three nisin concentrations (10, 50, 100 µg/mL) in water containing gamma-irradiated casings. Additionally, the binding of nisin to casings, using (14)C-labeled nisin Z and subsequent availability of nisin were evaluated. Results demonstrate that nisin is partly reversibly bound to casings and can reduce the outgrowth of Clostridium spores in the model used by approximately 1 log(10) (90%). However, the biological relevance of these results needs to be determined further by conducting industrial trials before any recommendation can be made on the practical implementation of nisin in the preservation of natural sausage casings.

Size and orientation of the lipid II headgroup as revealed by AFM imaging.

In this study, we investigated the size and orientation of the bacterial Lipid II (L II) headgroup when the L II molecule is present in liquid-crystalline domains of DOPC in a supported DPPC bilayer. Using atomic force microscopy, we detected that L II causes the appearance of a 1.9 nm thick layer, situated over the DOPC headgroup region. With an increased scanning force, this layer can be penetrated by the AFM tip down to the level of the DOPC bilayer. Using different L II precursor molecules, we demonstrated that the detected layer consists of the headgroups of L II and that the MurNAc-pentapeptide unit of the headgroup is responsible for the measured 1.9 nm height of that layer. Monolayer experiments provided information about the in-plane dimensions of the L II headgroup. On the basis of these results and considerations of the molecular dimensions of L II headgroup constituents, we propose a model for the orientation of the L II headgroup in the membrane. In this model, the pentapeptide of the L II headgroup is rather extended and points away from the bilayer surface, which could be important for biological processes, in which L II is involved.

Modification and inhibition of vancomycin group antibiotics by formaldehyde and acetaldehyde.

It is shown that several vancomycin group antibiotics (vancomycin, eremomycin, and avoparcin) undergo spontaneous chemical modifications when kept at room temperature at neutral pH in aqueous solutions containing traces of formaldehyde or acetaldehyde. This chemical modification predominantly results in a mass increase of 12 Da in the reaction with formaldehyde and 26 Da in the case of acetaldehyde. By using tandem mass spectrometry the modification can unambiguously be identified as originating from the formation of a ring-closed 4-imidazolidinone moiety at the N-terminus of the glycopeptide antibiotics, that is, near the receptor binding pocket of the glycopeptide antibiotics. Bioaffinity mass spectrometry shows that this ring-closure results in a dramatically decreased affinity for the peptidoglycan-mimicking D-alanyl-D-alanine receptor. Additionally, in vitro inhibition measurements on two different strains of bacteria have revealed that the modified antibiotics display reduced antibacterial activity. The ring-closure is also shown to have a dissociative effect on the dimerization of the vancomycin-analogue eremomycin. The spontaneous reaction of vancomycin with formaldehyde or acetaldehyde may have implications not only for the clinical use of this class of antibiotics, but also for the effectiveness of these antibiotics when they are used in chiral separation chromatography or capillary electrophoresis.

Specific binding of nisin to the peptidoglycan precursor lipid II combines pore formation and inhibition of cell wall biosynthesis for potent antibiotic activity.

Unlike numerous pore-forming amphiphilic peptide antibiotics, the lantibiotic nisin is active in nanomolar concentrations, which results from its ability to use the lipid-bound cell wall precursor lipid II as a docking molecule for subsequent pore formation. Here we use genetically engineered nisin variants to identify the structural requirements for the interaction of the peptide with lipid II. Mutations affecting the conformation of the N-terminal part of nisin comprising rings A through C, e.g. [S3T]nisin, led to reduced binding and increased the peptide concentration necessary for pore formation. The binding constant for the S3T mutant was 0.043 x 10(7) m(-1) compared with 2 x 10(7) m(-1) for the wild-type peptide, and the minimum concentration for pore formation increased from the 1 nm to the 50 nm range. In contrast, peptides mutated in the flexible hinge region, e.g. [DeltaN20/DeltaM21]nisin, were completely inactive in the pore formation assay, but were reduced to some extent in their in vivo activity. We found the remaining in vivo activity to result from the unaltered capacity of the mutated peptide to bind to lipid II and thus to inhibit its incorporation into the peptidoglycan network. Therefore, through interaction with the membrane-bound cell wall precursor lipid II, nisin inhibits peptidoglycan synthesis and forms highly specific pores. The combination of two killing mechanisms in one molecule potentiates antibiotic activity and results in nanomolar MIC values, a strategy that may well be worth considering for the construction of novel antibiotics.

Binding of Nisin Z to bilayer vesicles as determined with isothermal titration calorimetry.

Nisin Z, a 34-residue lantibiotic, is secreted by some lactic acid bacteria and exerts its antibacterial activity against various Gram-positive bacteria by permeabilizing the cell membrane. It is a cationic amphiphilic peptide with several unusual dehydro residues and thioether-bridged lanthionines. Isothermal titration calorimetry was used to provide a quantitative thermodynamic description for nisin Z adsorption to and penetration into negatively charged and neutral lipid bilayers. The binding of the cationic peptide (electric charge z approximately 3.8) to anionic membranes was found to be dominated by electrostatic forces which could be described with the Gouy-Chapman theory. For biologically relevant conditions with a membrane surface potential of -40 mV, the peptide concentration near the membrane surface increases by about 2-3 orders of magnitude compared to the bulk concentration. The binding step proper, i.e., the transition from the lipid-water interface into the membrane, is almost exclusively driven by the high surface concentration. Binding can be described by a partition equilibrium of the form X(b) = KC(M) = KC(p,f) exp(-z(p)psi(0)F(0)/RT), where C(M) is the peptide surface concentration, C(p,f) the bulk concentration, and psi(0) the membrane surface potential. The intrinsic partition coefficient (K = 1.8 M(-)(1)) is remarkably small, indicating a correspondingly small hydrophobic energy contribution to the binding process. The electrostatic model was confirmed with nisin Z mutants in which valine-32 was replaced with either lysine (V32K) or glutamate (V32E), increasing or decreasing the electric charge by 1 unit. The extent of peptide binding increased for V32K and decreased for V32E as predicted by the electrostatic theory. In contrast, electrostatic effects were almost negligible for the binding of nisin Z to neutral membranes. However, the binding isotherms were characterized by a distinctly larger intrinsic binding constant K(0) of approximately 540 M(-)(1) and an enhanced hydrophobic free energy of binding. The binding of nisin Z to sonicated lipid vesicles is exothermic with a DeltaH degrees of ca. -9 and -3.4 kcal/mol for charged and neutral membranes, respectively.

Engineering a disulfide bond and free thiols in the lantibiotic nisin Z.

The antimicrobial peptide nisin contains the uncommon amino acid residues lanthionine and methyl-lanthionine, which are post-translationally formed from Ser, Thr and Cys residues. To investigate the importance of these uncommon residues for nisin activity, a mutant was designed in which Thr13 was replaced by a Cys residue, which prevents the formation of the thioether bond of ring C. Instead, Cys13 couples with Cys19 via an intramolecular disulfide bridge, a bond that is very unusual in lantibiotics. NMR analysis of this mutant showed a structure very similar to that of wild-type nisin, except for the configuration of ring C. The modification was accompanied by a dramatic reduction in antimicrobial activity to less than 1% of wild-type activity, indicating that the lanthionine of ring C is very important for this activity. The nisin Z mutants S5C and M17C were also isolated and characterized; they are the first lantibiotics known that contain an additional Cys residue that is not involved in bridge formation but is present as a free thiol. Secretion of these peptides by the lactococcal producer cells, as well as their antimicrobial activity, was found to be strongly dependent on a reducing environment. Their ability to permeabilize lipid vesicles was not thiol-dependent. Labeling of M17C nisin Z with iodoacetamide abolished the thiol-dependence of the peptide. These results show that the presence of a reactive Cys residue in nisin has a strong effect on the antimicrobial properties of the peptide, which is probably the result of interaction of these residues with thiol groups on the outside of bacterial cells.

The lantibiotic nisin, a special case or not?

Nisin is a 34-residue-long peptide belonging to the group A lantibiotics with antimicrobial activity against Gram-positive bacteria. The presence of dehydrated residues and lanthionine rings (thioether bonds) in nisin, imposing structural restrains on the peptide, make it an interesting case for studying the mode of action. In addition, the relatively high activity (nM range) of nisin against Gram-positive bacteria indicates that nisin may be a special case in the large family of pore-forming peptides antibiotics. In this review, we attempted to dissect the mode of action of nisin concentrating on studies that used model membranes or biological membranes. The picture that emerges suggests that in model membrane systems, composed of only phospholipids, nisin behaves similar to the antimicrobial peptide magainin, albeit with an activity that is much lower as compared to its activity towards biological membranes. This difference can be contributed to a missing factor which nisin needs for its high activity. Novel results have identified the factor as Lipid II, a precursor in the bacterial cell wall synthesis. The special high affinity interaction of nisin with Lipid II resulting in high activity and the active role of Lipid II in the pore-formation process make nisin a special case.

Use of the cell wall precursor lipid II by a pore-forming peptide antibiotic.

Resistance to antibiotics is increasing in some groups of clinically important pathogens. For instance, high vancomycin resistance has emerged in enterococci. Promising alternative antibiotics are the peptide antibiotics, abundant in host defense systems, which kill their targets by permeabilizing the plasma membrane. These peptides generally do not act via specific receptors and are active in the micromolar range. Here it is shown that vancomycin and the antibacterial peptide nisin Z use the same target: the membrane-anchored cell wall precursor Lipid II. Nisin combines high affinity for Lipid II with its pore-forming ability, thus causing the peptide to be highly active (in the nanomolar range).

Computational study of nisin interaction with model membrane.

Nisin is a 34-residue lantibiotic widely used as food preservative. Its mode of action on the bacterial cytoplasmic membrane is unclear. It should form ion channels but a molecular description of the interaction between nisin and phospholipids is lacking. The interactions between nisin and a membrane and the influence of phospholipids are here analysed by molecular modelling. The NMR structures of nisin in a micellar environment were previously determined (Van den Hooven et al., Eur. J. Biochem. 235 (1996) 382-393) Those structures were used to start with. They were refined by running a Monte Carlo procedure at a model lipid/water interface. It was shown that nisin is adsorbing onto the interface, with its N-terminal moiety more deeply inserted in lipids than the C-end, indicating distinct hydrophobic properties of the N- and C-domains. Therefore, we suggest that the N-terminal part is implied in the insertion of nisin in lipids, while the C-terminal moiety could be involved in the initial interaction with the membrane surface. Modelling the interaction of nisin with different neutral or anionic phospholipids shows that it disturbs the lipid organisation. The disturbance is maximal with phosphatidylglycerol. In this system, nisin curves the surface of phosphatidylglycerol layer round suggesting it could induce micelle formation. This could be a preliminary step to pore formation. It suggests that phosphatidylglycerol could have a direct action on nisin insertion and on ion channel formation. Appearance of a curvature also agrees with the 'wedge model' proposed in the literature for the nisin pore formation.

Pore formation by nisin involves translocation of its C-terminal part across the membrane.

Nisin is an amphiphilic peptide with a strong antimicrobial activity against various Gram-positive bacteria. Its activity results from permeabilization of bacterial membranes, causing efflux of cytoplasmic compounds. To get information on the molecular mechanism of membrane permeabilization, a mutant of nisin Z containing the C-terminal extension Asp-(His)6 was produced. The biological and anionic lipid-dependent membrane activity of this peptide was very similar to that of nisin Z. Analysis of the pH dependence of model membrane interactions with the elongated peptide indicated the importance of electrostatic interactions of the C-terminus with the target membrane for membrane permeabilization. Most importantly, the membrane topology of the C-terminus of the molecule could be determined by trypsin digestion experiments, in which trypsin was encapsulated in the lumen of large unilamellar vesicles. The results show that the C-terminal part of the peptide translocates across model membranes. The pH and anionic lipid dependence of translocation closely paralleled the results of membrane permeabilization studies. Binding of nickel ions to the histidines blocked translocation of the C-terminus and concomitantly resulted in a 4-fold reduced capacity to induce K+ leakage. The results demonstrate for the first time that pore formation of nisin involves translocation of the C-terminal region of the molecule across the membrane.

The orientation of nisin in membranes.

Nisin is a 34 residue long peptide belonging to the group A lantibiotics with antimicrobial activity against Gram-positive bacteria. The antimicrobial activity is based on pore formation in the cytoplasmic membrane of target organisms. The mechanism which leads to pore formation remains to be clarified. We studied the orientation of nisin via site-directed tryptophan fluorescence spectroscopy. Therefore, we engineered three nisin Z variants with unique tryptophan residues at positions 1, 17, and 32, respectively. The activity of the tryptophan mutants against Gram-positive bacteria and in model membrane systems composed of DOPC or DOPG was established to be similar to that of wild type nisin Z. The tryptophan fluorescence emission maximum showed an increasing blue-shift upon interaction with vesicles containing increased amounts of DOPG, with the largest effect for the 1W peptide. Studies with the aqueous quencher acrylamide showed that all tryptophans became inaccessible from the aqueous phase in the presence of negatively charged lipids in the vesicles. From these results it is concluded that anionic lipids mediate insertion of the tryptophan residues in at least three positions of the molecule into the lipid bilayer. The depth of insertion of the tryptophan residues was determined via quenching of the tryptophan fluorescence by spin-labeled lipids. The results showed that the depth of insertion was dependent on the amount of negatively charged lipids. In membranes containing 50% DOPG, the distances from the bilayer center were determined to be 15.7, 15.0, and 18.4 A for the tryptophan at position 1, 17, and 32, respectively. In membranes containing 90% DOPG, these distances were calculated to be 10.8, 11.5, and 13.1 A, respectively. These results suggest an overall parallel average orientation of nisin in the membrane, with respect to the membrane surface, with the N-terminus more deeply inserted than the C-terminus. These data were used to model the orientation of nisin in the membrane.

Influence of charge differences in the C-terminal part of nisin on antimicrobial activity and signaling capacity.

Three mutants of the antibiotic nisin Z, in which the Val32 residue was replaced by a Glu, Lys or Trp residue, were produced and characterized for the purpose of establishing the role of charge differences in the C-terminal part of nisin on antimicrobial activity and signaling properties. 1H-NMR analyses showed that all three mutants harbor an unmodified serine residue at position 33, instead of the usual dehydroalanine. Apparently, the nature of the residue preceding the serine to be dehydrated, strongly affects the efficiency of modification. Cleavage of [Glu32,Ser33]nisin Z by endoproteinase Glu-C yielded [Glu32]nisin Z(1-32)-peptide, which has a net charge difference of -2 relative to wild-type nisin Z. The activity of [Lys32,Ser33]nisin Z against Micrococcus flavus was similar to that of wild-type nisin, while [Trp32,Ser33]nisin Z, [Glu32,Ser33]nisin Z and [Glu32]nisin Z(1-32)-peptide exhibited 3-5-fold reduced activity, indicating that negative charges in the C-terminal part of nisin Z are detrimental for activity. All variants showed significant loss of activity against Streptococcus thermophilus. The potency of the nisin variants to act as signaling molecules for auto-induction of biosynthesis was significantly reduced. To obtain mutant production, extracellular addition of (mutant) nisin Z to the lactococcal expression strains was essential.

Inhibition of preprotein translocation and reversion of the membrane inserted state of SecA by a carboxyl terminus binding mAb.

SecA is the peripheral subunit of the preprotein translocase of Escherichia coli. SecA consists of two independently folding domains, i.e., the N-domain bearing the high-affinity nucleotide binding site (NBS-I) and the C-domain that harbors the low-affinity NBS-II. ATP induces SecA insertion into the membrane during preprotein translocation. Domain-specific monoclonal antibodies (mAbs) were developed to analyze the functions of the SecA domains in preprotein translocation. The antigen binding sites of the obtained mAbs were confined to five epitopes. One of the mAbs, i.e., mAb 300-1K5, recognizes an epitope in the C-domain in a region that has been implicated in membrane insertion. This mAb, either as IgG or as Fab, completely inhibits in vitro proOmpA translocation and SecA translocation ATPase activity. It prevents SecA membrane insertion and, more strikingly, reverses membrane insertion and promotes the release of SecA from the membrane. Surface plasmon resonance measurements demonstrate that the mAb recognizes the ADP- and the AMP-PNP-bound state of SecA either free in solution or bound at the membrane at the SecYEG protein. It is concluded that the mAb actively reverses a conformation essential for membrane insertion of SecA. The other mAbs directed to various epitopes in the N-domain were found to be without effect, although all bind the native SecA. These results demonstrate that the C-domain plays an important role in the SecA membrane insertion, providing further evidence that this process is needed for preprotein translocation.

The C-terminal region of nisin is responsible for the initial interaction of nisin with the target membrane.

The interaction of nisin Z and a nisin Z mutant carrying a negative charge in the C-terminus ([Glu-32]-nisin Z) with anionic lipids was characterized in model membrane systems, and bacterial membrane systems. We focused on three possible steps in the mode of action of nisin, i.e., binding, insertion, and pore formation of nisin Z. Increasing amounts of anionic lipids in both model and natural membranes were found to strongly enhance the interaction of nisin Z with the membranes at all stages. The results reveal a good correlation between the anionic lipid dependency of the three stages of interaction, of which the increased binding is probably the major determinant for antimicrobial activity. Maximal nisin Z activity could be observed for negatively charged lipid concentrations exceeding 50-60%, both in model membrane systems as well as in bacterial membrane systems. We propose that the amount of negatively charged lipids of the bacterial target membrane is a major determinant for the sensitivity of the organism for nisin. Nisin Z induced leakage of the anionic carboxyfluorescein was more efficient as compared to the leakage of the potassium cation. This lead to the conclusion that an anion-selective pore is formed. In contrast to the results obtained for nisin Z, the binding of [Glu-32]-nisin Z to vesicles remained low even in the presence of high amounts of negatively charged lipids. The insertion and pore-forming ability of [Glu-32]-nisin Z were also decreased. These results demonstrate that the C-terminus of nisin is responsible for the initial interaction of nisin, i.e., binding to the target membrane.

Influence of the signal sequence and chaperone SecB on the interaction between precursor protein prePhoE and phospholipids.

To investigate in a direct way the interaction between a precursor protein and phospholipids, monolayer studies were performed using the purified precursor of Escherichia coli outer-membrane protein PhoE. It was demonstrated that prePhoE can insert efficiently into monolayers of dioleoylglycerophosphoglycerol (Ole2GroPGro) and dioleoylglycerophosphoethanolamine (Ole2GroPEtn), this insertion was mainly driven by hydrophobic forces. Compared with previous results obtained with PhoE signal peptide, the full-length precursor protein does not show the specific interaction with acidic lipids. PrePhoE inserted into a Ole2GroPGro monolayer occupies an area of 28 +/- 3 [corrected] nm2/molecule, which is approximately 10-fold larger than the area occupied by the PhoE signal peptide. The purified mature PhoE protein has a lower capacity to insert into Ole2GroPGro and Ole2GroPEtn monolayers and is, in contrast to prePhoE, fully accessible to proteinase K after interacting with a Ole2GroPGro monolayer. The results demonstrate that in the context of the precursor protein both the signal sequence and mature domain of prePhoE insert into lipid monolayers. It was found that PhoE, like prePhoE, can form in vitro a complex with the cytosolic chaperone SecB. Complexation with SecB increases the insertion of (pre)PhoE into acidic lipid monolayers. The high lipid affinity of prePhoE was also demonstrated by vesicle-binding experiments which showed that SecB dissociates from the SecB-prePhoE complex upon binding of the precursor to the bilayer. The implications of these findings for preprotein translocation are discussed and in addition some extrapolations to the insertion of PhoE into the outer membrane are made.

The C terminus of SecA is involved in both lipid binding and SecB binding.

Using C-terminal deletion mutations in secA, we localized the previously proposed (Breukink, E., Keller, R.C. A., and de Kruijff, B. (1993), FEBS Lett. 331, 19-24) second lipid binding site on SecA. Since removal of these residues completely abolished the property of SecA to cause aggregation of negatively charged phosphatidyl-glycerol vesicles, we conclude that the C-terminal 70 amino acid residues of SecA are involved in lipid-binding. The C-terminal 70 amino acid residues of SecA are important for efficient in vitro translocation of the SecB-dependent precursor of PhoE across inverted inner membrane vesicles. Moreover, in vivo studies showed that this region is essential for growth. SecB and a SecB-precursor complex were shown to inhibit the SecA-mediated lipid vesicle aggregation, suggesting that the overall acidic SecB protein binds at or near the second lipid binding site on SecA. This together with the observation that the SecA mutant protein lacking the C-terminal 70 residues had a strongly reduced ability to mediate binding of SecB-precursor complexes to inverted inner membrane vesicles demonstrates that the C terminus of SecA is also involved in SecB binding.

A dual role for phosphatidylglycerol in protein translocation across the Escherichia coli inner membrane.

The involvement of phosphatidylglycerol in the SecA-independent translocation of M13 procoat in Escherichia coli was demonstrated. Processing of procoat to mature coat protein was retarded when the level of phosphatidylglycerol was reduced. In vitro translocation experiments using inner membrane vesicles isolated from a strain with inducible synthesis of phosphatidylglycerol, showed that translocation of procoat and of a SecA-dependent procoat analog was proportional to the content of phosphatidylglycerol. Moreover, introduction of phosphatidylglycerol by means of a lipid transfer method into phosphatidylglycerol-depleted inner membrane vesicles, efficiently restored procoat translocation. The phosphatidylglycerol dependence in both the SecA-dependent and -independent translocation pathway indicates that phosphatidylglycerol plays a dual role in translocation. We suggest that besides membrane binding of SecA this lipid has a direct interaction with the M13 procoat in translocation across the inner membrane.

Nucleotide and negatively charged lipid-dependent vesicle aggregation caused by SecA. Evidence that SecA contains two lipid-binding sites.

SecA which is an overall acidic protein was found to induce an increase in the turbidity of a solution of vesicles consisting of negatively charged phospholipids. This increase was found to be due to an aggregation of the vesicles mediated by SecA. The SecA-mediated vesicle aggregation was not found for zwitterionic 1,2-dioleoyl-sn-glycero-3-phosphocholine and showed a large dependence on both temperature and ionic strength. Furthermore it was shown that ATP and to a lesser extent ADP+Pi were able to reduce the SecA-mediated vesicle aggregation, while no effect could be seen for a non-hydrolysable ATP analog AMP-PNP. Using the steady state fluorescence anisotropy of 1,6-diphenyl-1,3,5-hexatriene present in 1,2-dioleoyl-sn-glycero-3-phosphoglycerol vesicles we could show that SecA inserts in the bilayer. Monolayer studies confirmed that SecA is able to cause close contact between two membranes and gave a direct insight into the different types of lipid-protein interactions involved. From our results we propose that the SecA monomer possesses two lipid-binding sites which in the functional dimer conformation are responsible for the SecA-mediated vesicle aggregation.