Interaction of poly(ethylene-glycols) with air-water interfaces and lipid monolayers: investigations on surface pressure and surface potential

We have characterized the surface activity of different-sized poly(ethylene-glycols) (PEG; M(r) 200-100,000 Da) in the presence or absence of lipid monolayers and over a wide range of bulk PEG concentrations (10(-8)-10% w/v). Measurements of the surface potential and surface pressure demonstrate that PEGs interact with the air-water and lipid-water interfaces. Without lipid, PEG added either to the subphase or to the air-water interface forms relatively stable monolayers. Except for very low molecular weight polymers (PEGs < 1000 Da), low concentrations of PEG in the subphase (between 10(-5) and 10(-4)% w/v) increase the surface potential from zero (with respect to the potential of a pure air-water interface) to a plateau value of approximately 440 mV. At much higher polymer concentrations, > 10(-1)% (w/v), depending on the molecular weight of the PEG and corresponding to the concentration at which the polymers in solution are likely to overlap, the surface potential decreases. High concentrations of PEG in the subphase cause a similar decrease in the surface potential of densely packed lipid monolayers spread from either diphytanoyl phosphatidylcholine (DPhPC), dipalmitoyl phosphatidylcholine (DPPC), or dioleoyl phosphatidylserine (DOPS). Adding PEG as a monolayer at the air-water interface also affects the surface activity of DPhPC or DPPC monolayers. At low lipid concentration, the surface pressure and potential are determined by the polymer. For intermediate lipid concentrations, the surface pressure-area and surface potential-area isotherms show that the effects due to lipid and PEG are not always additive and that the polymer's effect is distinct for the two lipids. When PEG-lipid-mixed monolayers are compressed to surface pressures greater than the collapse pressure for a PEG monolayer, the surface pressure-area and surface potential-area isotherms approach that of the lipid alone, suggesting that for this experimental condition PEG is expelled from the interface.

The Fusobacterium nucleatum major outer-membrane protein (FomA) forms trimeric, water-filled channels in lipid bilayer membranes

The pore-forming activity of the major outer-membrane protein FomA of the anaerobic Fusobacterium nucleatum was studied in artificial lipid bilayer membranes. FomA was isolated from F. nucleatum strains Fev1, ATCC 10953, and ATCC 25586 by extraction with lithium dodecyl sulfate and lithium chloride and had an apparent molecular mass of about 40 kDa. When solubilized at low temperatures, the protein ran with an apparent molecular mass of about 62 kDa on SDS/PAGE. Cross-linking experiments and two-dimensional SDS/PAGE gave evidence that the 62-kDa protein band represented the trimeric form of FomA. The protein trimers were susceptible to SDS and temperature. The stability of the porin trimers varied among the strains. The properties of the FomA channels were studied in reconstitution experiments with black lipid bilayer membranes. The F. nucleatum porins formed channels with single-channel conductances in the range 0.66-1.30 nS in M KCl. The single-channel conductance was a function of the mobilities of the ions present in the aqueous solution bathing the bilayer membrane. This means that FomA forms general diffusion channels since (a) the conductance showed a linear dependence on the salt concentration, (b) the ion selectivity was small and varied for the three strains, and (c) the channels did not exhibit any binding site for maltotriose or triglycine. The water-filled channel was voltage dependent, and conductance decrements were observed at transmembrane potentials of +/- 50 mV. The conductance decrement steps were about one-third of the total conductance of a functional unit in its fully 'open' state. This strongly suggests that the trimer is the functional unit of the porin.

Thyroid testing using the Cobas Core immunoassay system. A multicentre study

The random access immunoanalyzer Cobas Core and the Cobas Core Thyroid Assays were assessed as to their clinical usefulness in the analysis of different thyroid diseases. Four centres participated in this study measuring the following five thyroid tests per sample on the instrument: thyrotropin, free thyroxine, thyroxine, triiodothyronine and free triiodothyronine. The assessment was based on studies of precision and clinical samples. Within- and between-series precisions showed a mean CV over all assays of 4.3 and 6.1%, respectively. Comparison of the test results with clinical data demonstrated that the Cobas Core results are in accordance with diagnosed thyroid diseases. A good discrimination between normal and disease status and between untreated and treated status was found. Furthermore, 370 sera measured with Technicon Immuno-1 TSH and FT4 assays were compared to the respective Cobas Core EIAs). A good correlation between the assays was demonstrated.

Carmeda surface heparinization in neonatal ECMO systems: long-term experiments in a sheep model

The thromboresistance in three Carmeda (Stockholm, Sweden) heparin-coated neonatal ECMO systems with a runtime of 45, 56 and 96 hours, respectively, and three noncoated systems with a runtime of 12, 42 and 66 hours, respectively, were compared using a sheep model. The flow rate was 200 ml/min and the activated clotting time (ACT) was kept at approximately 120 seconds. At the end of the experiment, the heparin-coated systems only contained minimal clotting while the controls showed major clotting in the entire system. Fibrin monomers were not detected until after 24 hours in the heparin-coated group, but demonstrated within 60 minutes in the noncoated group. It is concluded that the Carmeda heparin coating has a thromboresistant effect, and may be used to reduce the need for systemic heparinization in ECMO treatment of neonates.

The deletion of 70 amino acids near the N-terminal end of the sucrose-specific porin ScrY causes its functional similarity to LamB in vivo and in vitro

A deletion mutant ScrY delta 3-73 of the sucrose-specific porin ScrY was constructed in which 70 amino acids of the mature protein were deleted near the N-terminal end. ScrY delta 3-72 was still able to oligomerize and inserted properly into the outer membrane of an Escherichia coli strain. The protein was isolated and purified by standard procedures. The mutant protein showed, in contrast to wild-type ScrY, a tight association with the murein. Reconstitution experiments with artificial lipid bilayer membranes demonstrated that ScrY delta 3-72 produced defined cation-selective channels in planar lipid bilayers. Its single-channel conductance was reduced to about half of the value of wild-type ScrY. The deletion had a relatively small influence on the stability constants for carbohydrate binding. However, in contrast to wild-type ScrY, [14C]-maltopentaose was efficiently taken up into whole E. coli cells containing ScrY delta 3-72. The sequence of the N-terminus of mature ScrY was identified as starting with glutamine 23. The possible structure of ScrY and ScrY delta 3-72 in the outer membrane is discussed.

The membrane of leaf peroxisomes contains a porin-like channel

Spinach leaf peroxisomes were purified by Percoll density gradient centrifugation. After several freeze-thaw cycles, the peroxisomal membranes were separated from the matrix enzymes by sucrose density gradient centrifugation. The purity of the peroxisomal membranes was checked by measuring the activities of marker enzymes and by using antibodies. Lipid bilayer membrane experiments with the purified peroxisomal membranes, solubilized with a detergent, demonstrated that the membranes contain a channel-forming component, which may represent the major permeability pathway of these membranes. Control experiments with membranes of other cell organelles showed that the peroxisomal channel was not caused by the contamination of the peroxisomes with mitochondria or chloroplasts. The peroxisomal channel had a comparatively small single channel conductance of 350 pS in 1 M KCl as compared with channels from other cell organelles. The channel is slightly anion selective, which is in accordance with its physiological function. The single channel conductance was found to be only moderately dependent on the salt concentration in the aqueous phase. This may be explained by the presence of positive point net charges in or near the channel or by the presence of a saturable binding site inside the channel. The possible role of the channel in peroxisomal metabolism is discussed.

Shape of the potential energy barrier of the iodine-mediated halide transport

Voltage-clamp experiments were performed on lipid bilayer membranes to study the voltage dependence of the iodine-mediated halide transport. Under all experimental conditions only one exponential current relaxation, apart from the capacitive spike, could be resolved up to a clamp voltage of 200 mV. The current relaxation could be described by an initial conductance, G0, the relaxation time constant, tau, and the relaxation amplitude, alpha, that is the difference between the initial current, I0, and the steady state current, I chi, divided by the steady state current. The occurrence of one single exponential relaxation suggested that one of the different transport steps involved in the carrier-mediated ion transport according to the Lüger-model is always in equilibrium. This is most probably the transport of the free carriers across the membrane. The voltage dependence of G0, tau, and of alpha were used to determine the voltage dependence of the translocation rate constants of the complexed carriers, kAS. In the case of the iodine-mediated iodide transport, G0, tau and alpha were only mediate voltage-dependent, which means the voltage dependent translocation of the complex encounters a trapezoidal barrier shape. For the iodine-mediated bromide translocation G0, tau and alpha exhibited no dependence on the applied clamp-voltage, which suggested that a square Nernst-Planck barrier limits the transport of the corresponding complex.

Role of sterols in the functional reconstitution of water-soluble mitochondrial porins from different organisms

Experiments were performed on lipid bilayer membranes with water-soluble mitochondrial porins from different eukaryotic organisms, such as Dictyostelium discoideum, Paramecium, and rat liver, to study the requirements of functional reconstitution of the porins. The water-soluble porins lost their associated lipids and sterols and are unable to form channels in lipid bilayer membranes. We demonstrate that the water-soluble porins regain their channel-forming ability after preincubation of the polypeptides with sterols in the presence of detergents. Mitochondrial porin from Dictyostelium discoideum maintained after this procedure its original properties, in particular the voltage dependence. Water-soluble mitochondrial porins from Paramecium tetraurelia and from rat liver were also activated upon preincubation with different sterols in detergent but showed voltage-dependences that were different from those of detergent-purified porins. Furthermore, the voltage dependence depended on the sterol used for preincubation. Interestingly, the preincubation with sterols can likewise be used to activate detergent-purified mitochondrial porins that may have lost associated sterol during isolation and purification procedures.

Evaluation of the rate constants of sugar transport through maltoporin (LamB) of Escherichia coli from the sugar-induced current noise

LamB (maltoporin) of Escherichia coli outer membrane was reconstituted into artificial lipid bilayer membranes. The channel contains a binding site for sugars and is blocked for ions when the site is occupied by a sugar. The on and off reactions of sugar binding cause an increase of the noise of the current through the channel. The sugar-induced current noise of maltoporin was used for the evaluation of the sugar-binding kinetics for different sugars of the maltooligosaccharide series and for sucrose. The on rate constant for sugar binding was between 10(6) and 10(7) M-1.s-1 for the maltooligosaccharides and corresponds to the movement of the sugars from the aqueous phase to the central binding site. The off rate (corresponding to the release of the sugars from the channel) decreased with increasing number of glucose residues in the maltooligosaccharides from approximately 2,000 s-1 for maltotriose to 180 s-1 for maltoheptaose. The kinetics for sucrose movement was considerably slower. The activation energies of the stability constant and of the rate constants for sugar binding were evaluated from noise experiments at different temperatures. The role of LamB in the transport of maltooligosaccharides across the outer membrane is discussed.

Effects of light and inhibitors of ATP-synthesis on the chloride carrier of the alga Valonia utricularis: is the carrier a chloride pump?

The effect of metabolic inhibitors, such as cyanide, antimycin A and azide was studied on the chloride transport system of the giant marine alga Valonia utricularis by using the charge pulse relaxation method. Two clearly defined voltage relaxations were resolved. The addition of 10-30 microM cyanide to the artificial sea water (ASW) bathing the algal cells increased the time constants of the slow voltage relaxation, tau 2, significantly when the algal cells were kept in the dark. The cyanide-effect reached a plateau value at 100-300 microM and was fully reversible when cyanide was removed from the ASW. Analysis of the charge pulse data in terms of the Läuger-model demonstrated that the translocation rates of the free, kS, and the charged carrier, kAS, decreased. The decrease of kS was more pronounced than that of kAS. 10 microM antimycin A and 3 mM azide had similar effects on the rate constants when the light was switched off. Upon illumination the cyanide- and antimycin A-, but not the azide-mediated effects disappeared. At concentrations higher than 1 mM cyanide caused a further, dramatic decrease of kS and kAS, while the surface concentration of the carrier molecules, N0, was not affected. This cyanide-effect was also reversible, but not light-dependent. Measurements of the ATP level showed that 3 mM cyanide reduced the ATP level by about 70% both under light and dark conditions. In the presence of 30 microM cyanide (or 10 microM antimycin A) the ATP level decreased by about 50%, but only in the dark. These results suggest two different effects of cyanide on the Cl(-)-carrier system: in the micromolar concentration range cyanide (and antimycin A) reduced predominantly the translocation of the free carrier by inhibition of ATP synthesis by oxidative phosphorylation, whereas in the millimolar concentration range cyanide apparently inhibits the translocation rates of both the free and charged carriers by its binding to the carrier. The results provide some evidence that the chloride transport of V. utricularis could be coupled to metabolic energy but it is an open question whether it is a pump or not.

Adenylate cyclase toxin (CyaA) of Bordetella pertussis. Evidence for the formation of small ion-permeable channels and comparison with HlyA of Escherichia coli

The interaction between the adenylate cyclase toxin (CyaA) of Bordetella pertussis and lipid was studied using the lipid bilayer assay. The addition of CyaA to the aqueous phase bathing lipid bilayer membranes composed of different lipids resulted in the increase of the membrane conductance. This increase was rather small for membranes formed of pure lipids as compared with lipid mixtures such as asolectin. The toxin formed in asolectin membranes small transient ion-permeable channels with a single-channel conductance of 27 pS in 1 M KCl, which is considerably smaller than that of the alpha-hemolysin (HlyA) of Escherichia coli (1500 pS). Experiments with different salts suggested that the CyaA-induced channels were exclusively cation-selective because of negative charges localized at the channel mouth. The single-channel conductance of channels initiated by CyaA was independent of whether the toxin was purified from B. pertussis or from recombinant E. coli. However, the channel-forming activity of the CyaA expressed in B. pertussis was substantially higher than that of the recombinant toxin. Experiments with mutant forms of CyaA suggested that both the activation of CyaA by CyaC and the hemeolytic part of the toxin, but not the repeats and the cyclase activity, are required for channel formation in lipid bilayer membranes.

Biochemical, molecular, and functional characterization of porin isoforms from potato mitochondria

The mitochondrial outer membrane of eukaryotic cells contains a voltage-dependent anion channel termed porin. In the organisms studied so far only one type of porin has been identified at the protein level. Here we present a biochemical and molecular genetic analysis of two different porin polypeptides of M(r) 34,000 and 36,000 from the outer membranes of potato mitochondria (termed POM 34 and POM 36, respectively). N-terminal sequencing and the use of labeled oligonucleotide mixtures derived from these amino acid sequences allowed the isolation of cDNA clones encoding the 34- and 36-kDa proteins. They have similar steady state protein levels and share about 75% identical amino acids suggesting that they represent isoforms. In addition, a third cDNA clone coding for a slightly different isoform of the 36-kDa protein was characterized. The polypeptides encoded by the three cDNA clones share the highest degree of sequence identity with mitochondrial porins from fungi and mammals. Tentative models of the secondary structure of the 34- and 36-kDa proteins suggest the occurrence of a 16-stranded beta-barrel typical for bacterial and mitochondrial porins. Purification of the 34-kDa protein by hydroxyapatite chromatography allowed conductance measurements in artificial bilayers. The 34-kDa protein is a voltage-dependent, channel-forming component with single channel conductances of 3.5 and 2.0 nanosiemens in 1 M KCl. In spite of the striking functional similarities to mitochondrial porins from other organisms neither the 34- nor the 36-kDa proteins are able to complement the respiratory defect of a yeast por- mutant.

Porins from plants. Molecular cloning and functional characterization of two new members of the porin family

Porins are voltage-gated diffusion pores found in all eukaryotic kingdoms. Here we describe, for the first time, the identification and characterization of two cDNAs encoding porins from plants. Peptide sequences obtained from a 30-kDa protein of envelope membranes from pea root plastids allowed the isolation of two cDNA clones from pea and maize. On the protein level, both proteins are homologous by 58%. Sequence comparison against the Swiss-Prot sequence data base revealed a homology of about 25% to mitochondrial porins from fungi and human. Computer-aided predictions of the secondary structure of the plant porins revealed the presence of 16 antiparallel beta-strands that are also found in mitochondrial porins. Porins from non-green plastids and from the outer mitochondrial membrane were reconstituted into planar lipid bilayers. The proteins showed high pore-forming activities and similar single-channel conductances. In vitro translated porin was preferentially imported only into non-green plastids but not into chloroplasts. To our knowledge, this is the first example of selective import of a plastid protein into different types of plastids. This finding is in line with the observation that an immunoreactive 30-kDa band was only found in non-green plastids and mitochondria but not in chloroplasts. We conclude that mitochondria and non-green plastids possess homologous porin proteins, whereas chloroplasts are characterized by a different type of porin.

Permeability of the cell wall of Mycobacterium smegmatis

The cell wall of Mycobacterium smegmatis mc2155 was shown to be an effective permeability barrier to hydrophilic compounds. Permeability coefficients to beta-lactams ranged from 10 x 10(-7) to 0.5 x 10(-7) cms-1. Cell wall proteins were solubilized with EDTA and Genapol and were tested for channel-forming activity by reconstitution into lipid bilayers. Proteins were able to induce a voltage-gated cation-selective channel. The mycobacterial porin channel appeared to be water-filled since the single-channel conductance followed the mobility sequence of hydrated ions in the aqueous phase. On the basis of the Renkin equation and the single-channel conductance, the channel diameter was estimated to be around 3 nm. Model calculations showed that cation selectivity may be caused by four negative point-charges at the channel mouth. The permeability properties of the cell wall of intact cells were in good agreement with those of the reconstituted channel. Negatively charged cephalosporins, cefamandole and cephalothin, diffused at a 10- to 20-fold lower rate than the zwitterionic cephaloridine. The mycobacterial porin represents a major hydrophilic pathway of the cell wall of M. smegmatis.

Contribution of electrogenic ion transport to impedance of the algae Valonia utricularis and artificial membranes

The cell membrane of Valonia utricularis contains an electrogenic carrier system for chloride (Wang et al., Biophys J. 59:235-248 (1991)). The electrical impedance of V. utricularis was measured in the frequency range between 1 Hz and 50 kHz. The analysis of the impedance spectra from V. utricularis and its comparison with equivalent circuit models showed that the transport system created a characteristic contribution to the impedance in the frequency range between 10 Hz and 5 kHz. The fit of the impedance spectra with the formalism derived from the theory of carrier-mediated transport allowed the determination of the kinetic parameters of chloride transport through the cell membrane of V. utricularis, and its passive electrical properties. Simultaneous measurements of the kinetic parameters with the charge pulse method demonstrated the equivalence of both experimental approaches with respect to the evaluation of the translocation rate constants of the free and the charged carriers and the total density of carriers within the membrane. Moreover, the impedance spectra of the protonophor-mediated proton transport by FCCP (carbonylcyanide p-trifluoromethoxyphenyl-hydrazone) were measured in model membranes. The carrier system made a substantial contribution to the impedance of the artificial membranes. The analysis of the spectra in terms of a simple carrier system (Benz and McLaughlin, 1983, Biophys. J. 41:381-398) allowed the evaluation of the kinetic and equilibrium parameters of the FCCP-mediated proton transport. The possible application of the measurement of impedance spectra for the study of biological transport systems is discussed.

Role of androgens in the regulation of the human menstrual cycle

Although previous investigations have examined the importance of androgens in the regulation of the human menstrual cycle, no consensus has been reached, due to conflicting results. We have therefore used the non-steroidal anti-androgen flutamide as a pharmacological probe to evaluate the role of androgens in the control of gonadotropin secretion in normally cycling women. Eight women were studied during control and treatment cycles, during which either placebo (as control) or flutamide (750 mg orally) was given daily. Blood was sampled every other day during the follicular and luteal phases and daily around the expected midcycles for determination of luteinizing hormone (LH), follicle stimulating hormone (FSH), estradiol, progesterone and androgens (testosterone, androstenedione, dehydroepiandrosterone sulfate) by radioimmunoassay and immunoradiometric assay. To establish unstimulated and gonadotropin releasing hormone (GnRH)-stimulated gonadotropin profiles, blood samples were frequently collected (every 10 min for 8 h, GnRH 25 micrograms i.v. after 7 h on day 10 in both the control and treatment cycles. Compared to control conditions, the durations of both the follicular and luteal phases did not change considerably during flutamide treatments. Serum androgen levels (testosterone, androstenedione, dehydroepiandrosterone sulfate) were significantly (p < 0.01) reduced during androgen antagonism. Daily gonadotropin and estradiol levels did not differ between control and flutamide cycles, while progesterone secretion tended to be attenuated (p = 0.2) during the luteal phases of the flutamide cycles. The LH and FSH secretory profiles and the GnRH-stimulated gonadotropin responses remained virtually unchanged during androgen antagonism.(ABSTRACT TRUNCATED AT 250 WORDS)

Interaction of Serratia marcescens hemolysin (ShlA) with artificial and erythrocyte membranes. Demonstration of the formation of aqueous multistate channels

Pore formation by hemolysin (ShlA) of Serratia marcescens was studied in erythrocytes and in artificial lipid bilayer membranes. The results with erythrocytes demonstrated that hemolysin pores varied in size. In erythrocyte membranes with reduced fluidity (0 degrees C), the toxin formed small pores with diameter 1-1.5 nm. In fluid membranes (above 20 degrees C), hemolysin pores with larger diameters (approximately 2.5-3.0 nm) were observed, which may be caused by association of ShlA monomers into oligomers. Comparison of the channels formed by Staphylococcus aureus alpha-toxin with channels formed by ShlA indicated a slightly smaller pore diameter of ShlA pores. Analysis of ShlA in artificial lipid bilayers showed the formation of pores with a broad distribution of single channel conductances, suggesting variable sizes of the ShlA pore. The lower limit for the pore diameter was approximately 1.0 nm. The ShlA pores did not exhibit pronounced ion selectivity nor voltage dependence, supporting the presence of a large water-filled pore.

Interaction of Clostridium botulinum C2 toxin with lipid bilayer membranes. Formation of cation-selective channels and inhibition of channel function by chloroquine

Lipid bilayer experiments were performed with the C2-II binding component of the ADP-ribosylating C2 toxin from Clostridium botulinum. The trypsin-activated but not the nonactivated form of the protein was able to increase the specific conductance of artificial lipid bilayer membranes by the formation of ion-permeable channels. The channels had on average a single-channel conductance of 55 pS in 0.1 M KCl and were found to be cation-selective and voltage-dependent. The single-channel conductance was only moderately dependent on the bulk aqueous KCl concentration, which indicated point charge effects on the channel properties. Incubation of the activated C2-II binding component with antibodies against C2-II or with C2-I toxin inhibited channel formation to a large extent. Addition of chloroquine, a known inhibitor of endocytosis in cells, led to a dose-dependent decrease of the C2-II-induced membrane conductance. This result suggested that the activated C2-II component contains a binding site for chloroquine inside the channel. It is discussed that the channels formed by C2-II component are involved in the translocation of C2-I toxin across the target cell membrane.

Energy-coupled transport through the outer membrane of Escherichia coli small deletions in the gating loop convert the FhuA transport protein into a diffusion channel

Active transport of Fe3+ as ferrichrome complex through the outer membrane of Escherichia coli is mediated by the FhuA outer membrane protein and the TonB-ExbB-ExbD protein complex in the cytoplasmic membrane. The required energy is provided by the electrochemical potential of the cytoplasmic membrane which is assumed to induce a conformation of the TonB protein that causes a conformational change in FhuA so that bound ferrichrome is released into the periplasmic space located between the outer and the cytoplasmic membrane. Excision of segments as small as 12 amino acids in the largest surface loop of FhuA converted FhuA into an open channel through which ferrichrome and antibiotics diffused independent of TonB-ExbB-ExbD. It is proposed that FhuA forms a closed channel which is opened by movement of the gating loop through a kind of allosteric interaction with TonB. The gating loop is also involved in binding of all FhuA ligands which in addition to ferrichrome are the phages T1, T5, phi 80, colicin M and the antibiotic albomycin.

Noise analysis of ion current through the open and the sugar-induced closed state of the LamB channel of Escherichia coli outer membrane: evaluation of the sugar binding kinetics to the channel interior

LamB, a sugar-specific channel of Escherichia coli outer membrane was reconstituted into lipid bilayer membranes and the current noise was investigated using fast Fourier transformation. The current noise through the open channels had a rather small spectral density, which was a function of the inverse frequency up to about 100 Hz. The spectral density of the noise of the open LamB channels was a quadratic function of the applied voltage. Its magnitude was not correlated to the number of channels in the lipid bilayer membrane. Upon addition of sugars to the aqueous phase the current decreased in a dose-dependent manner. Simultaneously, the spectral density of the current noise increased drastically, which indicated interaction of the sugars with the binding site inside the channel. The frequency dependence of the spectral density was of Lorentzian type, although the power of its frequency dependence was not identical to -2. Analysis of the power density spectra using a previously proposed simple model (Benz, R., A. Schmid, and G. H. Vos-Scheperkeuter. 1987. J. Membr. Biol. 100: 12-29), allowed the evaluation of the on- and the off-rate constants for the maltopentaose binding to the binding site inside the LamB channels. This means also that the maltopentaose flux through the LamB channel could be estimated by assuming a simple one-site, two-barrier model for the sugar transport from the results of the noise analysis.

Pore formation in artificial membranes by the secreted hemolysins of Proteus vulgaris and Morganella morganii

Lipid-bilayer experiments were performed with the related hemolysins from Proteus vulgaris and Morganella morganii (HlyA). The addition of the toxins to the aqueous phase bathing lipid-bilayer membranes composed of different lipids resulted in the formation of transient ion-permeable channels. Membranes formed of pure lipids were rather inactive targets for the hemolysins as compared with lipid mixtures such as asolectin. The channels had several different substrates. The major open state had single-channel conductances of 500 pS in 0.15 M KCl at small transmembrane voltages. Experiments with different salts suggested that the hemolysin-induced channels of P. vulgaris and M. morganii were exclusively cation selective at neutral pH, caused by negative charges localized at the channel mouth. The mobility sequence of the cations within the channels was similar if not identical to their mobility sequence in the aqueous phase. The single-channel data were consistent with wide, water-filled channels with estimated minimal diameters of about 1 nm since the large organic cation Tris+ can permeate the channels without any detectable interaction with its interior. Pore-forming properties of these hemolysins were compared with those of HlyA of Escherichia coli. All these toxins share common features, oligomerize probably to form pores in lipid-bilayer membranes and form channels with similar properties which suggests that their structures are more or less identical.

The mode of action of SA-FF22, a lantibiotic isolated from Streptococcus pyogenes strain FF22

SA-FF22 is a lanthionine-containing bacteriocidal peptide isolated from Streptococcus pyogenes strain FF22. The peptide interacts closely with non-energised artificial phospholipid vesicles, as evidenced by a 'blue shift' in the fluorescent emissions associated with a tryptophan residue within the peptide sequence. Furthermore, SA-FF22 induced efflux of radiolabelled amino acids from artificially energised cytoplasmic membrane vesicles and arrested uptake of amino acids by intact cells. By measuring the decrease in membrane potential of both starved and energised SA-FF22-treated cells, and through the use of artificial planar membranes, a potential of approximately 100 mV was deduced as the minimum required to induce pore formation by SA-FF22. This threshold potential is independent of the orientation of the applied voltage (i.e. trans or cis orientations are equally effective). Single channel conductance measurements suggested that the pores formed by SA-FF22 are relatively unstable, short-lived and approximately 0.5-0.6 nm in diameter. This is somewhat smaller than those of the previously described, pore-forming lantibiotics and should not allow significant efflux of large molecules such as ATP. Thus, death of affected cells seems to result from membrane-potential disruption and subsequent exhaustion of the cells.

Kinetics of the iodine- and bromine-mediated transport of halide ions: demonstration of an interfacial complexation mechanism

Stationary and kinetic experiments were performed on lipid bilayer membranes to study the mechanism of iodine- and bromine-mediated halide transport in detail. The stationary conductance data suggested that four different 1:1 complexes between I2 and Br2 and the halides I- and Br- were responsible for the observed conductance increase by iodine and bromine (I3-, I2Br-, Br2I-, and Br3-). Charge pulse experiments allowed the further elucidation of the transport mechanism. Only two of three exponential voltage relaxations predicted by the Läuger model could be resolved under all experimental conditions. This means that either the heterogeneous complexation reactions kR (association) and kD (dissociation) were too fast to be resolved or that the neutral carriers were always in equilibrium within the membrane. Experiments at different carrier and halide concentrations suggested that the translocation of the neutral carrier is much faster than the other processes involved in carrier-mediated ion transport. The model was modified accordingly. From the charge pulse data at different halide concentrations, the translocation rate constant of the complexed carriers, kAS, the dissociation constant, kD, and the total surface concentration of charged carriers, NAS, could be evaluated from one single charge pulse experiment. The association rate of the complex, kR, could be obtained in some cases from the plot of the stationary conductance data as a function of the halide concentration in the aqueous phase. The translocation rate constant, kAS, of the different complexes is a function of the image force and of the Born charging energy. It increases 5000-fold from Br3- to I3- because of an enlarged ion radius.