Interaction of phloretin with lipid monolayers: relationship between structural changes and dipole potential change

Phloretin is known to adsorb to lipid surfaces and alters the dipole potential of lipid monolayers and bilayers. Its adsorption to biological and artificial membranes results in a change of the membrane permeability for a variety of charged and neutral compounds. In this respect phloretin represents a model substance to study the effect of dipole potentials on membrane permeability. In this investigation we studied the interaction of phloretin with monolayers formed of different lipids in the liquid-expanded and the condensed state. Phloretin integrated into the monolayers as a function of the aqueous concentration of its neutral form, indicated by an increase of the surface pressure in the presence of phloretin. Simultaneous recording of the surface potential of the monolayers allowed us to correlate the degree of phloretin integration and the phloretin-induced dipole potential change. Increasing the surface pressure decreased the phloretin-induced shift of the isotherms, but did not influence the phloretin-induced surface potential change. This means that phloretin adsorption to the lipid surface can occur without affecting the lipid packing. The surface potential effect of phloretin is accompanied by a change of the lipid dipole moment vector dependent on the lipid packing. This means that the relation between the surface potential change and the lipid packing cannot be described by a static model alone. Taking into account the deviations of the surface potential change versus molecular area isotherms of the experimental data to the theoretically predicted course, we propose a model that relates the area change to the dipole moment in a dynamic manner. By using this model the experimental data can be described much better than with a static model.

Cloning of the mspA gene encoding a porin from Mycobacterium smegmatis

Porins form channels in the mycolic acid layer of mycobacteria and thereby control access of hydrophilic molecules to the cell. We purified a 100 kDa protein from Mycobacterium smegmatis and demonstrated its channel-forming activity by reconstitution in planar lipid bilayers. The mspA gene encodes a mature protein of 184 amino acids and an N-terminal signal sequence. MALDI mass spectrometry of the purified porin revealed a mass of 19 406 Da, in agreement with the predicted mass of mature MspA. Dissociation of the porin by boiling in 80% dimethyl sulphoxide yielded the MspA monomer, which did not form channels any more. Escherichia coli cells expressing the mspA gene produced the MspA monomer and a 100 kDa protein, which had the same channel-forming activity as whole-cell extracts of M. smegmatis with organic solvents. These proteins were specifically detected by a polyclonal antiserum that was raised to purified MspA of M. smegmatis. These results demonstrate that the mspA gene encodes a protein of M. smegmatis, which assembles to an extremely stable oligomer with high channel-forming activity. Database searches did not reveal significant similarities to any other known protein. Southern blots showed that the chromosomes of fast-growing mycobacterial species contain homologous sequences to mspA, whereas no hybridization could be detected with DNA from slow growing mycobacteria. These results suggest that MspA is the prototype of a new class of channel-forming proteins.

Properties of a cyclodextrin-specific, unusual porin from Klebsiella oxytoca

The function of CymA, 1 of the 10 gene products involved in cyclodextrin uptake and metabolism by Klebsiella oxytoca, was characterized. CymA is essential for growth on cyclodextrins, but it can also complement the deficiency of a lamB (maltoporin) mutant of Escherichia coli for growth on linear maltodextrins, indicating that both cyclic and linear oligosaccharides are accepted as substrates. CymA was overproduced in E. coli and purified to apparent homogeneity. CymA is a component of the outer membrane, is processed from a signal peptide-containing precursor, and possesses a high content of antiparallel beta-sheet. Incorporation of CymA into lipid bilayers and conductance measurements revealed that it forms ion-permeable channels, which exhibit a substantial current noise. CymA-induced membrane conductance decreased considerably upon addition of alpha-cyclodextrin. Titration experiments allowed the calculation of a half-saturation constant, K(S), of 28 microM for its binding to CymA. CymA assembled in vitro to two-dimensionally crystalline tubular membranes, which, on electron microscopy, are characterized by a p1-related two-sided plane group. The crystallographic unit cell contains four monomeric CymA molecules showing a central pore. The lattice parameters are a = 16.1 nm, b = 3.8 nm, gamma = 93 degrees. CymA does not form trimeric complexes in lipid membranes and shows no tendency to trimerize in solution. CymA thus is an atypical porin with novel properties specialized to transfer cyclodextrins across the outer membrane.

Evidence for a small anion-selective channel in the cell wall of Mycobacterium bovis BCG besides a wide cation-selective pore

Two channels were observed in extracts of whole Mycobacterium bovis BCG cells using organic solvents and detergents. The channels derived from organic solvent treatment had a single-channel conductance of about 4.0 nS in 1 M KCl in lipid bilayer membranes with properties similar to those of the channels discovered previously in Mycobacterium smegmatis and Mycobacterium chelonae. The channel was in its open configuration only at low transmembrane potentials. At higher voltages it switched to closed states that were almost impermeable for ions. Lipid bilayer experiments in the presence of detergent extracts of whole cells revealed another channel with a single-channel conductance of only 780 pS in 1 M KCl. Our results indicate that the mycolic acid layer of M. bovis BCG contains two channels, one is cation-selective and its permeability properties can be finely controlled by cell wall asymmetry or potentials. The other one is anion-selective, has a rather small single-channel conductance and is voltage-insensitive. The concentration of channel-forming proteins in the cell wall seems to be small, which is in agreement with the low cell wall permeability for hydrophilic solutes.

Influence of proline residues on the antibacterial and synergistic activities of alpha-helical peptides

To investigate the influence of proline residues on the activity of alpha-helical peptides, variants were synthesized with insertions of proline residues to create peptides without proline, or with one or two prolines. The influence of the proline-induced bends was assessed by circular dichroism in the presence of liposomes, and the ability of the peptides to kill microorganisms, to permeabilize the outer and cytoplasmic membranes of Escherichia coli, to bind to liposomes, to form channels in planar lipid bilayers, and to synergize with conventional antibiotics. Representative peptides adopted alpha-helical conformations in phosphatidylcholine/phosphatidylglycerol (POPC/POPG, 7:3) liposomes as well as in 60% trifluoroethanol solution, as revealed by circular dichroism (CD) spectroscopy. However, the percent of helicity decreased as the number of proline residues increased. Tryptophan fluorescence spectroscopy showed that all of these peptides inserted into the membranes of liposomes as indicated by a blue shift in the emission maximum and an increase in the fluorescence intensity of the single tryptophan at residue 2. Quenching experiments further prove that the tryptophan residue was no longer accessible to the aqueous quencher KI. The peptide that lacked proline exhibited the highest activity [minimal inhibitory concentrations (MICs) of 0.5-4 microg/mL] against all tested Gram-negative and Gram-positive bacteria, but was hemolytic at 8 microg/mL. The single-proline peptides exhibited intermediate antibacterial activity. Peptides with two proline residues were even less active with moderate MICs only against E. coli. With only one exception from each group, the peptides were nonhemolytic. The ability of the peptides to demonstrate synergy in combination with conventional antibiotics increased as the antibacterial effectiveness decreased. All peptides bound to bacterial lipopolysaccharide and permeabilized the outer membrane of E. coli to similar extents. However, their ability to permeabilize the cytoplasmic membrane of E. coli as assessed by the unmasking of cytoplasmic beta-galactosidase decreased substantially as the number of proline residues increased. Correspondingly, increasing the number of proline residues caused a decreased ability to form channels in planar lipid bilayers, and the hemolytic, proline-free peptide tended to cause rapid breakage of planar membranes. Thus, the number of bends created by insertion of proline residues is an important determinant of antimicrobial, hemolytic, and synergistic activity.

Mechanism of interaction of different classes of cationic antimicrobial peptides with planar bilayers and with the cytoplasmic membrane of Escherichia coli

Antimicrobial cationic peptides are prevalent throughout nature as part of the intrinsic defenses of most organisms, and have been proposed as a blueprint for the design of novel antimicrobial agents. They are known to interact with membranes, and it has been frequently proposed that this represents their antibacterial target. To see if this was a general mechanism of action, we studied the interaction, with model membranes and the cytoplasmic membrane of Escherichia coli, of 12 peptides representing all 4 structural classes of antimicrobial peptides. Planar lipid bilayer studies indicated that there was considerable variance in the interactions of the peptides with model phospholipid membranes, but generally both high concentrations of peptide and high transmembrane voltages (usually -180 mV) were required to observe conductance events (channels). The channels observed for most peptides varied widely in magnitude and duration. An assay was developed to measure the interaction with the Escherichia coli cytoplasmic membrane employing the membrane potential sensitive dye 3,5-dipropylthiacarbocyanine in the outer membrane barrier-defective E. coli strain DC2. It was demonstrated that individual peptides varied widely in their ability to depolarize the cytoplasmic membrane potential of E. coli, with certain peptides such as the loop peptide bactenecin and the alpha-helical peptide CP26 being unable to cause depolarization at the minimal inhibitory concentration (MIC), and others like gramicidin S causing maximal depolarization below the MIC. We discuss the mechanism of interaction with the cytoplasmic membrane in terms of the model of Matsuzaki et al. [(1998) Biochemistry 37, 15144-15153] and the possibility that the cytoplasmic membrane is not the target for some or even most cationic antimicrobial peptides.

The haemolysin-secreting ShlB protein of the outer membrane of Serratia marcescens: determination of surface-exposed residues and formation of ion-permeable pores by ShlB mutants in artificial lipid bilayer membranes

The ShlB protein in the outer membrane of Serratia marcescens is the only protein known to be involved in secretion of the ShlA protein across the outer membrane. At the same time, ShlB converts ShlA into a haemolytic and a cytolytic toxin. Surface-exposed residues of ShlB were determined by reaction of an M2 monoclonal antibody with the M2 epitope DYKDDDDK inserted at 25 sites along the entire ShlB polypeptide. The antibody bound to the M2 epitope at 17 sites in intact cells, which indicated surface exposure of the epitope, and to 23 sites in isolated outer membranes. Two insertion mutants contained no ShlB(M2) protein in the outer membrane. The ShlB derivatives activated and/or secreted ShlA. To gain insights into the secretion mechanism, we studied whether highly purified ShlB and ShlB deletion derivatives formed pores in artificial lipid bilayer membranes. Wild-type ShlB formed channels with very low single channel conductance that rarely assumed an open channel configuration. In contrast, open channels with a considerably higher single channel conductance were observed with the deletion mutants ShlB(Delta65-186), ShlB(Delta87-153), and ShlB(Delta126-200). ShlB(Delta126-200) frequently formed permanently open channels, whereas the conductance caused by ShlB(Delta65-186) and ShlB(Delta87-153) did not assume a stationary value, but fluctuated rapidly between open and closed configurations. The results demonstrate the orientation of large portions of ShlB in the outer membrane and suggest that ShlB may function as a specialized pore through which ShlA is secreted.

In vivo and in vitro studies of major surface loop deletion mutants of the Escherichia coli K-12 maltoporin: contribution to maltose and maltooligosaccharide transport and binding

The trimeric protein LamB of Escherichia coli K-12 (maltoporin) specifically facilitates the diffusion of maltose and maltooligosaccharides through the outer membrane. Each monomer consists of an 18-stranded antiparallel beta-barrel with nine surface loops (L1 to L9). The effects on transport and binding of the deletion of some of the surface loops or of combinations of several of them were studied in vivo and in vitro. In vivo, single-, DeltaL4, DeltaL5, DeltaL6, and double-loop deletions, DeltaL4 + DeltaL5 and DeltaL5 + DeltaL6, abolished maltoporin functions, but not the double deletion DeltaL4 + DeltaL6 and the triple deletion DeltaL4 + DeltaL5 + DeltaL6. While deletion of the central variable portion of loop L9 (DeltaL9v) affected maltoporin function only moderately, the combination of DeltaL9v with the double deletion of loops L4 and L6 (triple deletion DeltaL4 + DeltaL6 + DeltaL9v) strongly impaired maltoporin function and resulted in sensitivity to large hydrophilic antibiotics without change in channel size as measured in vitro. In vitro, the carbohydrate-binding properties of the different loop mutants were studied in titration experiments using the asymmetric and symmetric addition of the mutant porins and of the carbohydrates to one or both sides of the lipid bilayer membranes. The deletion of loop L9v alone (LamBDeltaL9v), of two loops L4 and L6 (LamBDeltaL4 + DeltaL6), of three loops L4, L5 and L6 (LamBDeltaL4 + DeltaL5 + DeltaL6) or of L4, L6 and L9v (LamBDeltaL4 + DeltaL6 + DeltaL9v) had relatively little influence on the carbohydrate-binding properties of the mutant channels, and they had approximately similar binding properties for carbohydrate addition to both sides compared with only one side. The deletion of one of the loops L4 (LamBDeltaL4) or L6 (LamBDeltaL6) resulted in an asymmetric carbohydrate binding. The in vivo and in vitro results, together with those of the purification across the starch column, suggest that maltooligosaccharides enter the LamB channel from the cell surface side with the non-reducing end in advance. The absence of some of the loops leads to obstruction of the channel from the outside, which results in a considerable difference in the on-rate of carbohydrate binding from the extracellular side compared with that from the periplasmic side.

Collective membrane motions in the mesoscopic range and their modulation by the binding of a monomolecular protein layer of streptavidin studied by dynamic light scattering

Using a dedicated dynamic light scattering setup, we have studied the angstrom-scale amplitude undulations of freely suspended planar lipid bilayers, so-called black lipid membranes (BLM's), over a previously not accessible spread of frequencies (relaxation times ranging from 10(-2) to 10(-6) s) and wave vectors (250 cm(-1)<q<35 000 cm(-1)). This allowed a critical test of a simple hydrodynamic theory of collective membrane modes, and the results obtained for a synthetic lecithin BLM are found to be in excellent agreement with the theoretical predictions. In particular, the transition of the transverse shear mode of a BLM between an oscillatory or propagating regime and an overdamped regime by passing through a bifurcation point was clearly observed. It is shown that the collective motions in the time- and wave-vector regime covered are dominated by the membrane tension, while membrane curvature does not contribute. The binding of the protein streptavidin to the BLM via membrane anchored specific binders (receptors) causes a drastic change in frequency and amplitude of the collective motions, resulting in a drastic increase of the membrane tension by a factor of 3. This effect is probably caused by a steric hindrance of the transverse shear motions of the lipid by the tightly bound proteins.

Identification of positively charged residues of FomA porin of Fusobacterium nucleatum which are important for pore function

FomA porin is the major outer-membrane protein of Fusobacterium nucleatum. It exhibits the functional properties of a general diffusion porin, but has no sequence similarity to other porins. According to the proposed topology model, each monomer of this trimeric protein is a beta-barrel consisting of 16 transmembrane segments with eight surface-exposed loops. Several conserved charged residues are proposed to extend from the beta-barrel wall into the aqueous channel lumen, and may contribute to a transverse electric field similar to that at the pore constriction of porins with known structure. The goal of our study was to identify particular basic residues contributing to such an electric field in FomA. Several arginines and lysines were replaced by negatively charged glutamates or uncharged alanines. The mutated FomA porins were expressed in Escherichia coli, and the effects on pore function were studied in vivo, by assaying the uptake rate of beta-lactam antibiotics, and in vitro after reconstitution of the purified proteins in lipid bilayer membranes. Some of the point mutations had a significant impact on the channel properties. The substitution R92A produced a 130% increased permeability of the zwitterionic beta-lactam cephaloridine, and the cation selectivity of R92E increased by 70%. The effects of the R90E substitution on channel properties were similar. Most of the point mutations had a minor effect on the voltage gating of the FomA channel, resulting in an increased sensitivity, except for K78E, which showed a decreased sensitivity. The latter mutation had no effect on cation selectivity, but the K78A substitution improved the uptake rate of cephaloridine. The results presented here indicate that arginines 90 and 92 are probably part of the constriction zone of the FomA porin, and lysine 78 and arginines 115 and 117 are probably in close proximity to this region as well.

Ion selectivity reversal and induction of voltage-gating by site-directed mutations in the Paracoccus denitrificans porin

The porin from Paracoccus denitrificans, a slightly anion specific outer membrane pore protein, was expressed in Escherichia coli, isolated from inclusion bodies, and refolded in the presence of urea and detergents. The purified recombinant protein was reconstituted into black lipid bilayer membranes and showed no difference in its functional properties in comparison to the native porin isolated from P.denitrificans membranes. To investigate the molecular basis of its ion selectivity and voltage-gating, a series of site-directed mutants was constructed, comprising acidic residues located on the third extracellular loop (L3), which forms the constriction zone of the channel, and basic residues along the opposing barrel wall. Measurements using zero-current membrane potentials indicated that the selectivity changed drastically from a slight anion to a distinct cation selectivity with the exchange of residues R29 and R31 by glutamate, whereas replacements on the L3 loop went largely unaffected. However, when assaying the voltage-dependent closure of channels, only mutations located on the L3 loop showed an effect, in contrast to the voltage-independent recombinant and native Paracoccus porin.

The cell wall porin of the gram-positive bacterium Nocardia asteroides forms cation-selective channels that exhibit asymmetric voltage dependence

Detergent-solubilized cell wall extracts of the gram-positive, strictly aerobic bacterium Nocardia asteroides contain channel-forming activity as judged from reconstitution experiments using lipid bilayer membranes. The cell wall porin was identified as a protein with an apparent molecular mass of about 84 kDa based on SDS-PAGE. The porin was purified to homogeneity using preparative SDS-PAGE. The 84-kDa protein was no longer observed after heating in SDS buffer. The presumed dissociation products were not observed on SDS-polyacrylamide gels. The cell wall porin increased the specific conductance of artificial lipid bilayer membranes from phosphatidylcholine/phosphatidylserine mixtures by the formation of cation-selective channels, which had an average single-channel conductance of 3.0 nS in 1 M KCl. The single-channel conductance was only moderately dependent on the bulk aqueous KCl concentration, which indicated negative point charge effects on the channel properties. The analysis of the concentration dependence of the single-channel conductance using the effect of negative charges on channel conductance suggested that the diameter of the cell wall channel is about 1.4 nm. Asymmetric addition of the cell wall porin to lipid bilayer membranes resulted in an asymmetric voltage dependence. The cell wall channel switched into substates, when the cis side of the membrane, the side of the addition of the protein, had negative polarity. Positive potentials at the cis side had no influence on the conductance of the cell wall channel.

Analysis of the SlyA-controlled expression, subcellular localization and pore-forming activity of a 34 kDa haemolysin (ClyA) from Escherichia coli K-12

Escherichia coli K-12 harbours a chromosomal gene, clyA (sheA, hlyE), that encodes a haemolytic 34 kDa protein. Recombinant E. coli overexpressing the cloned clyA gene accumulated this haemolysin in the periplasm and released only very small amounts of it into the external medium. The secretion of ClyA was confined to the log phase and paralleled by the partial release of several other periplasmic proteins. Sequencing of ClyA revealed the translational start point of the clyA gene and demonstrated that the clyA gene product is not N-terminally processed during transport. The transcription of clyA from its native promoter region was positively controlled by SlyA, a regulatory protein found in E. coli, Salmonella typhimurium and other Enterobacteriaceae. SlyA-controlled transcription started predominantly 72 bp upstream from clyA, as shown by primer extension. The corresponding putative promoter contains an unusual -10 sequence (TATGAAT) that is separated from a conventional -35 sequence by a GC-rich spacer. Site-directed deletion of the G in the -10 sequence abrogated the SlyA requirement for strong ClyA production, whereas a reduction in the G + C content of the spacer diminished the capability of SlyA to activate the clyA expression. Osmotic protection assays and lipid bilayer experiments suggested that ClyA forms stable, moderately cation-selective transmembrane pores that have a diameter of about 2.5-3 nm.

The gene bglH present in the bgl operon of Escherichia coli, responsible for uptake and fermentation of beta-glucosides encodes for a carbohydrate-specific outer membrane porin

The cryptic gene bglH from the Escherichia coli chromosome was cloned into a tacOP-driven expression vector. The resulting plasmid was transferred into the porin-deficient E. coli strain KS26 and the protein was expressed by addition of IPTG. The BglH protein was localized in the outer membrane. It was purified to homogeneity using standard methods. Reconstitution experiments with lipid bilayer membranes defined BglH as a channel-forming component, i.e. it is an outer membrane porin. The single-channel conductance of BglH (560 pS in 1 M KCl) was only one-third of that of the general diffusion porins of E. coli outer membrane. The presence of carbohydrates in the aqueous phase led to a dose-dependent block of ion transport through the channel, similar to that found for LamB (maltoporin) of E. coli and Salmonella typhimurium, which means that BglH is a porin specific for the uptake of carbohydrates. The binding constants of a variety of different carbohydrates were calculated from titration experiments of the BglH-induced membrane conductance. The tightest binding was observed with the aromatic beta-D-glucosides arbutin and salicin, and with gentibiose and cellobiose. Binding of maltooligosaccharides to BglH was in contrast to their binding to LamB in that it was much weaker, indicating that the binding site of BglH for carbohydrates is different from that of LamB (maltoporin). The kinetics of cellopentaose binding to BglH was investigated using the carbohydrate-induced current noise and was compared with that of cellopentaose binding to LamB (maltoporin) and ScrY (sucroseporin).

Biochemical and biophysical characterization of the cell wall porin of Corynebacterium glutamicum: the channel is formed by a low molecular mass polypeptide

The cell wall of the Gram-positive bacterium Corynebacterium glutamicum contains a channel (porin) for the passage of hydrophilic solutes. The channel-forming protein was identified, by lipid bilayer experiments, in the cell envelope fractions isolated by sucrose-density centrifugations and in organic solvent of whole cells. It was purified to homogeneity by fast-protein liquid chromatography across a Mono-Q column. The pure protein had a rather low molecular mass of about 5 kDa as judged by SDS-PAGE, which suggested that the cell wall channel is formed by a protein oligomer. The monomer has according to partial sequencing no significant homology to known protein sequences. The purified protein formed large ion-permeable channels in lipid bilayer membranes from phosphatidylcholine/phosphatidylserine mixtures with a single-channel conductance of 5.5 nS in 1 M KCl. Experiments with different salts suggested that the cell wall channel of C. glutamicum was highly cation-selective caused by negative charges localized at the channel mouth. The analysis of the single-channel conductance data using the Renkin correction factor suggested that the diameter of the cell wall channel is about 2.2 nm. Channel-forming properties of the cell wall channel of C. glutamicum were compared with those of mycobacteria. These channels share common features because they form large and water-filled channels that contain point net charges.

Effects of chronic opioid antagonism on gonadotrophin and ovarian sex steroid secretion during the luteal phase

OBJECTIVES

Since short-term opioid antagonism increases LH pulsatility during the luteal phase in women, we postulated that prolonged opioid antagonism may also accelerate the LH secretory episodes at this time. If so, the functional and temporal links between secretory episodes of pituitary LH and oestradiol (E2) and progesterone (P) release from the mature human corpus luteum may be disrupted.

STUDY DESIGN

Prolonged opioid blockade with the oral antagonist naltrexone (100 mg daily) was effected in eight women during the entire luteal phase of their cycles. Following documented ovulation in both placebo (control) and naltrexone cycles, blood samples were obtained daily and frequently (every 10 minutes for 10 h) on days 6-8 after ovulation.

MEASUREMENTS

In all blood samples, LH, E2 and P were determined by IRMA.

RESULTS

Compared to control cycles, the temporal organization and the endocrine characteristics of the luteal phase remained virtually unchanged during chronic opioid blockade. Periodic fluctuations were detected (by cluster analysis) in LH, E2 and P data series established by frequent sample collections in both the control and naltrexone cycles. LH secretory profiles were remarkably similar during control and naltrexone cycles, and the E2 and P secretory episodes tended to be coupled to LH pulses during both cycles. As determined by time-series analysis, the cross-correlations between the LH/E2 and LH/P data series remained unaltered by opioid blockade.

CONCLUSIONS

Chronic opioid antagonism with naltrexone did not disrupt the temporal organization or endocrine characteristics of the luteal phase. In particular, prolonged opioid blockade did not change LH secretory patterns. The functional and temporal links between LH inputs and sex steroid release from the mature corpus luteum remained unaffected by prolonged opioid antagonism. In contrast to the effects of short-term opioid blockade on LH pulsatile release during the luteal phase, the effects of chronic opioid antagonism on LH release may be transient and may not persist throughout the entire luteal phase, suggesting desensitization of the opiate receptors.

Study of sugar binding to the sucrose-specific ScrY channel of enteric bacteria using current noise analysis

ScrY, an outer membrane channel of enteric Gram-negative bacteria, which confers to the bacteria the rapid uptake of sucrose through the outer membrane was reconstituted into lipid bilayer membranes and the current noise was investigated in the open and in the carbohydrate-induced closed state of the channel. The open state of the channel exhibited up to about 200 Hz 1/f-noise with a rather small spectral density. Upon addition of carbohydrates to the aqueous phase the current through the ScrY channels decreased in a dose-dependent manner. Simultaneously, the spectral density of the current noise increased drastically, which indicated interaction of the carbohydrates with the binding site inside the channel and its reversible block. The frequency dependence of the spectral density was of the Lorentzian type but very often two Lorentzians were observed, from which the slow one may not be related to carbohydrate binding. Analysis of the power density spectra of the second Lorentzian using a previously proposed simple model of carbohydrate binding allowed the evaluation of the on- and the off-rate constants for the carbohydrate association with the binding site inside the ScrY channel and of a mutant (ScrYDelta3-72), in which 70 amino acids at the N-terminus are deleted. The binding of carbohydrates to ScrY was compared to those of the closely related maltoporin channels of Escherichia coli and Salmonella typhimurium by assuming that only the time constant and spectral density of the high frequency Lorentzian is related to carbohydrate transport.

The cell wall porin of Nocardia farcinica: biochemical identification of the channel-forming protein and biophysical characterization of the channel properties

A channel-forming protein was identified in cell wall extracts of the Gram-positive, strictly aerobic bacterium Nocardia farcinica. The cell wall porin was purified to homogeneity and had an apparent molecular mass of about 87 kDa on tricine-containing SDS-PAGE. When the 87 kDa protein was boiled for a longer time in sodium dodecylsulphate (SDS) it dissociated into two subunits with molecular masses of about 19 and 23 kDa. The 87 kDa form of the protein was able to increase the specific conductance of artificial lipid bilayer membranes from phosphatidylcholine (PC) phosphatidylserine (PS) mixtures by the formation of ion-permeable channels. The channels had on average a single-channel conductance of 3.0 nS in 1M KCl, 10mM Tris-HCl, pH8, and were found to be cation selective. Asymmetric addition of the cell wall porin to lipid bilayer membranes resulted in an asymmetric voltage dependence. The single-channel conductance was only moderately dependent on the bulk aqueous KCl concentration, which indicated point charge effects on the channel properties. The analysis of the single-channel conductance data in different salt solutions using the Renkin correction factor, and the effect of negative charges on channel conductance suggested that the diameter of the cell wall porin is about 1.4-1.6nm. Channel-forming properties of the cell wall porin of N. farcinica were compared with those of mycobacteria and corynebacteria. The cell wall porins of these members of the order Actinomycetales share common features because they form large and water-filled channels that contain negative point charges.

The porin RafY encoded by the raffinose plasmid pRSD2 of Escherichia coli forms a general diffusion pore and not a carbohydrate-specific porin

The gene rafY from the plasmid pRSD2, which enables Escherichia coli to grow on raffinose, was transferred into expression plasmid pUSL77. The protein was expressed in the porin-deficient Escherichia coli strain KS26 and was isolated and purified to homogeneity. The pure protein was reconstituted into lipid bilayer membranes. It formed an ion-permeable channel with a single-channel conductance of 2.9 nS of the open state in 1 M KCl, which is approximately twice of that of the general diffusion pores OmpF and OmpC of E. coli outer membrane. At lower pH the channel exhibited rapid flickering between three substates of the open channel. The RafY channel appears to be wide and water filled and has a small selectivity for cations over anions. Although RafY is part of an uptake and fermentation system for raffinose it does not contain a binding site for carbohydrates. Our results suggest that RafY is a general diffusion pore with a diameter, larger than that of the general diffusion porins OmpF and OmpC, that allows the diffusion of high-molecular-mass carbohydrates through the outer membrane.

The preprotein translocation channel of the outer membrane of mitochondria

The preprotein translocase of the outer membrane of mitochondria (TOM complex) facilitates the recognition, insertion, and translocation of nuclear-encoded mitochondrial preproteins. We have purified the TOM complex from Neurospora crassa and analyzed its composition and functional properties. The TOM complex contains a cation-selective high-conductance channel. Upon reconstitution into liposomes, it mediates integration of proteins into and translocation across the lipid bilayer. TOM complex particles have a diameter of about 138 A, as revealed by electron microscopy and image analysis; they contain two or three centers of stain-filled openings, which we interpret as pores with an apparent diameter of about 20 A. We conclude that the structure reported here represents the protein-conducting channel of the mitochondrial outer membrane.

Interaction of lipophilic ions with the plasma membrane of mammalian cells studies by electrorotation

The electrical properties of biological and artificial membranes were studied in the presence of a number of negatively charged tungsten carbonyl complexes, such as [W(CO)5(CN)]- , [W(CO)5(NCS)]-, [W2(CO)10(CN)]-, and [W(CO)5(SCH2C6H5)]-, using the single-cell electrorotation and the charge-pulse relaxation techniques. Most of the negatively charged tungsten complexes were able to introduce mobile charges into the membranes, as judged from electrorotation spectra and relaxation experiments. This means that the tungsten derivatives act as lipophilic anions. They greatly contributed to the polarizability of the membranes and led to a marked dielectric dispersion (frequency dependence of the membrane capacitance and conductance). The increment and characteristic frequency of the dispersion reflect the structure, environment, and mobility of the charged probe molecule in electrorotation experiments with biological membranes. The partition coefficients and the translocation rate constants derived from the electrorotation spectra of cells agreed well with the corresponding data obtained from charge-pulse experiments on artificial lipid bilayers.

The adsorption of phloretin to lipid monolayers and bilayers cannot be explained by langmuir adsorption isotherms alone

Phloretin and its analogs adsorb to the surfaces of lipid monolayers and bilayers and decrease the dipole potential. This reduces the conductance for anions and increases that for cations on artificial and biological membranes. The relationship between the change in the dipole potential and the aqueous concentration of phloretin has been explained previously by a Langmuir adsorption isotherm and a weak and therefore negligible contribution of the dipole-dipole interactions in the lipid surface. We demonstrate here that the Langmuir adsorption isotherm alone is not able to properly describe the effects of dipole molecule binding to lipid surfaces--we found significant deviations between experimental data and the fit with the Langmuir adsorption isotherm. We present here an alternative theoretical treatment that takes into account the strong interaction between membrane (monolayer) dipole field and the dipole moment of the adsorbed molecule. This treatment provides a much better fit of the experimental results derived from the measurements of surface potentials of lipid monolayers in the presence of phloretin. Similarly, the theory provides a much better fit of the phloretin-induced changes in the dipole potential of lipid bilayers, as assessed by the transport kinetics of the lipophilic ion dipicrylamine.

Permeability properties of the porin of spinach leaf peroxisomes

The membrane of spinach leaf peroxisomes contains an anion-selective channel. Reconstitution experiments were performed with lipid bilayer membranes to study its permeability properties. A variety of different monovalent inorganic and organic anions were found to be permeable through the porin channel. Its single-channel conductance for these different ions suggested that the channel has a minimum diameter of about 0.6 nm. From selectivity measurement in KCl solution a ratio of the anion permeability to cation permeability of less than 0.04 was determined, indicating an almost ideal selectivity of the peroxisomal channel for chloride. The permeation of chloride through the peroxisomal channel could be blocked efficiently by the addition of increasing concentrations of organic anions to the aqueous phase. The results are consistent with a binding site for dicarboxylic anions inside the peroxisomal channel. A particular high stability constant for the binding was obtained for peroxisomal metabolites such as malate, oxaloacetate, succinate, and 2-oxoglutarate, which have to cross the membrane of plant peroxisomes in vivo. Among these solutes maximal binding affinity was determined for C4 dicarboxylic anions. The results indicate that the peroxisomal channel does not form a general diffusion pore similarly to known eukaryotic porins, but has specific properties comparable to specific and inducible porins, which have been characterized in some gram-negative bacteria.

Evidence for the Presence of a Porin in the Membrane of Glyoxysomes of Castor Bean

Glyoxysomes of endosperm tissue of castor bean (Ricinus communis L.) seedlings were solubilized in a detergent and added to a lipid bilayer. Conductivity measurements revealed that the glyoxysomal preparation contained a porin-like channel. Using an electrophysiological method, which we established for semiquantitative determination of porin activity, we were able to demonstrate that glyoxysomal membranes purified by sucrose density gradient centrifugation contain an integral membrane protein with porin activity. The porin of glyoxysomes was shown to have a relatively small single-channel conductance of about 330 picosiemens in 1 M KCl and to be strongly anion selective. Thus, the glyoxysomal porin differs from the other previously characterized porins in the outer membrane of mitochondria or plastids, but is similar to the porin of spinach (Spinacia oleracea L.) leaf peroxisomes. Our results suggest that, in analogy to the porin of leaf peroxisomes, the glyoxysomal porin facilitates the passage of small metabolites, such as succinate, citrate, malate, and aspartate, through the membrane.

Interferon-alpha and -gamma differentially reduce rapid immature T-cell death by contact with HIV-1 carrier cell clones in vitro

The non-antigen specific rapid cytotoxic (CT) death of immature TdT+CD4+CD8+ T cells due to contact with HIV-1 carrier T-cell clones we have found recently is a novel phenomenon. The effects of interferons (IFN) on this CT reaction were studied in vitro. Treatment of the HIV-1 carrier clones, referred to as "effectors," with IFN-alpha but not IFN-gamma, or of the susceptible immature TdT+CD4+CD8+ T cells, referred to as "targets," with IFN-gamma but not IFN-alpha, for 24 hr prior to CT testing was found to reduce the CT reaction. Simultaneously, a down-regulated CD8 expression and an up-regulated antigen expression of both major histocompatibility antigen complex class I (MHC-I) and HIV-1 gp120/gp160 in the IFN-alpha treated effector (gp120+CD8+ HPB-ALL/HIV), and/or simultaneously up-regulated antigen expression of both CD8 and MHC-I in the IFN-gamma treated target (CD4+CD8+ HPB-ALL) were found to be associated with reduced CT reaction. However, altered antigen expression in the IFN-gamma treated effectors or IFN-alpha treated targets did not affect the ultimate degree of CT reaction. This study thus suggests a possible therapeutic efficacy of IFN by reducing the direct elimination of the T-cell precursors in HIV-1 infection.