The role of proton motive force in expression of the Staphylococcus aureus cid and lrg operons

The Staphylococcus aureus cidABC and lrgAB operons have been shown to play a key role in the regulation of murein hydrolase activity and cell death in a manner thought to be analogous to bacteriophage-encoded holins and anti-holins respectively. Because of these functions, it has been proposed that the regulation of these operons is tightly controlled and responsive to key metabolic signals. The current study revealed the presence of two overlapping regulatory pathways controlling cidABC and lrgAB expression, one dependent on acetic acid and the other dependent on proton motive force (PMF). The latter pathway was analysed using agents that affect various aspects of the PMF. Gramicidin and carbonyl cyanide m-chlorophenylhydrazone (CCCP), antimicrobial agents that dissipate the DeltapH and membrane potential (DeltaPsi), both enhanced lrgAB expression. Restoration of the PMF by incubation of the bacteria in the presence of glucose restored lrgAB expression back to the uninduced state. In addition, valinomycin, which specifically collapses the DeltaPsi, also induced lrgAB expression. In contrast, nigericin, which dissipates the DeltapH component of the PMF, was found to have a minimal effect on DeltaPsi and lrgAB transcription. Finally, the DeltaPsi-inducible expression of lrgAB was shown to be dependent on the previously characterized LytSR two-component regulatory system that is involved in the regulation of autolysis. The results of this study support a model in which the LytSR regulatory system responds to a collapse in DeltaPsi by inducing the transcription of the lrgAB operon.

[Research development of chemistry and bioactive activity of plant peptides]

As the technologies of separation, purification and determination develop rapidly, more and more peptide compounds, which have special bioactive and medical value, have been separated from natural plants, such as oligopeptides and cyclopeptides. The chemical structures and function of these plant peptides have been researched profoundly. This paper mainly reviews the composition, structure, bioactive function and medicine value of representative plant peptides in recent years, and can give some references about research and application of plant bioactive peptides.

Requirements for Bacillus subtilis bacteriophage phi29 DNA ejection

Phage phi29 infects Bacillus subtilis and ejects its linear DNA with a right to left polarity in a two-step, "push-pull" mechanism. In the first step 65% of the DNA is pushed inside the cell, presumably by the pressure built inside the capsid. In the second step, the remaining DNA is pulled by a hypothetical motor that comprises at least viral protein p17, encoded by the right early operon, in an energy-dependent process. We have further studied phi29 DNA ejection by using energy poisons and DNA replication and transcription inhibitors. The first step is passive, as it does not require an external energy source. The second step is transcription-independent and is completely abolished by novobiocin, suggesting a requirement for negatively supercoiled DNA. Viral DNA pulling also requires an electrochemical proton gradient, as the process is highly impaired by specific energy poisons such as gramicidin and CCCP (carbonyl cyanide m-chlorophenylhydrazone). The fact that azide has no effect in the absence of p17 suggests that this protein is essential for energy transduction.

Gramicidin D and dithiothreitol effects on erythrocyte exovesiculation

The use of either diphenylhexatriene, trimethylamino-diphenylhexatriene, or heptadecyl-hydroxycoumarin (C17-HC) allows, simultaneously and with the same molecule, the induction of erythrocyte exovesiculation and labeling of the released vesicles with the fluorescent probe. This method was used to evaluate gramicidin D (a channel-forming peptide) and dithiothreitol (a reducing agent) effects on the human erythrocytes vesiculation process. The release of cholesterol and phospholipids in exovesicles at longer incubation times was only detectable in the presence of gramicidin or dithiothreitol. When C17-HC was used to induce the vesiculation, the presence of gramicidin or dithiothreitol lead to a drastic decrease on the [phospholipids]/[cholesterol] ratio. However, in the samples with dithiothreitol, this variation did not result in the expectable decrease of membrane fluidity. These effects can be related with the presence of lipid rafts, the transbilayer lipids reorientation induced by gramicidin or dithiothreitol, and the cholesterol-dependent gramicidin channels inactivation.

Membrane on a chip: a functional tethered lipid bilayer membrane on silicon oxide surfaces

Tethered membranes have been proven during recent years to be a powerful and flexible biomimetic platform. We reported in a previous article on the design of a new architecture based on the self-assembly of a thiolipid on ultrasmooth gold substrates, which shows extremely good electrical sealing properties as well as functionality of a bilayer membrane. Here, we describe the synthesis of lipids for a more modular design and the adaptation of the linker part to silane chemistry. We were able to form a functional tethered bilayer lipid membrane with good electrical sealing properties covering a silicon oxide surface. We demonstrate the functional incorporation of the ion carrier valinomycin and of the ion channel gramicidin.

Brain-type creatine kinase activates neuron-specific K+-Cl- co-transporter KCC2

GABA, a major inhibitory neurotransmitter in the adult CNS, is excitatory at early developmental stages as a result of the elevated intracellular Cl- concentration ([Cl-]i). This functional switch is primarily attributable to the K+-Cl- co-transporter KCC2, the expression of which is developmentally regulated in neurons. Previously, we reported that KCC2 interacts with brain-type creatine kinase (CKB). To elucidate the functional significance of this interaction, HEK293 cells were transfected with KCC2 and glycine receptor alpha2 subunit, and gramicidin-perforated patch-clamp recordings were performed to measure the glycine reversal potential (Egly), giving an estimate of [Cl-]i. KCC2-expressing cells displayed the expected changes in Egly following alterations in the extracellular K+ concentration ([K+]o) or administration of an inhibitor of KCCs, suggesting that the KCC2 function was being properly assessed. When added into KCC2-expressing cells, dominant-negative CKB induced a depolarizing shift in Egly and reduced the hyperpolarizing shift in Egly seen in response to a lowering of [K+]o compared with wild-type CKB. Moreover, 2,4-dinitrofluorobenzene (DNFB), an inhibitor of CKs, shifted Egly in the depolarizing direction. In primary cortical neurons expressing CKB, the GABA reversal potential was also shifted in the depolarizing direction by DNFB. Our findings suggest that, in the cellular micro-environment, CKB activates the KCC2 function.

Ion selectivity of scorpion toxin-induced pores in cardiac myocytes

The lytic activity of parabutoporin (PP) and opistoporin 1 (OP1) on mammalian and bacterial membranes have been described. We investigated pore-formation and ion selectivity in cardiac myocytes by measuring the whole cell leak current by means of the patch clamp technique. Pore formation was observed as the induction of leak currents. Ion selectivity of the pores was indicated by the shift of the reversal potential (E(rev)) upon substitution of intra- and extra-cellular ions. Results were compared with the effect of gramicidin A (gramA). PP and OP1 induced a fluctuating leak current and indicate non-selectivity of PP-induced pores. PP- and OP1-induced pores are between 1.38 and 1.78 nm in diameter.

Depressed responses to applied and synaptically-released GABA in CA1 pyramidal cells, but not in CA1 interneurons, after transient forebrain ischemia

Transient cerebral ischemia kills CA1 pyramidal cells of the hippocampus, whereas most CA1 interneurons survive. It has been proposed that calcium-binding proteins, neurotrophins, and/or inhibitory neuropeptides protect interneurons from ischemia. However, different synaptic responses early after reperfusion could also underlie the relative vulnerabilities to ischemia of pyramidal cells and interneurons. In this study, we used gramicidin perforated patch recording in ex vivo slices to investigate gamma-aminobutyric acid (GABA) synaptic function in CA1 pyramidal cells and interneurons 4 h after a bilateral carotid occlusion accompanied by hypovolemic hypotension. At this survival time, the amplitudes of both miniature inhibitory postsynaptic currents (mIPSCs) and GABA-evoked currents were reduced in CA1 pyramidal cells, but not in CA1 interneurons. In addition, the mean rise time of mIPSCs was reduced in pyramidal cells. The reversal potential for the GABA current (E(GABA)) did not shift toward depolarizing values in either cell type, indicating that the driving force for chloride was unchanged at this survival time. We conclude that early during reperfusion GABAergic neurotransmission is attenuated exclusively in pyramidal neurons. This is likely explained by reduced GABAA receptor sensitivity or clustering and possibly also reduced GABA release, rather than by an elevation of intracellular chloride. Impaired GABA function may contribute to ischemic neuronal death by enhancing the excitability of CA1 pyramidal cells and facilitating N-methyl-D-aspartic acid channel opening. Therefore, normalizing GABAergic function might be a useful pharmacological approach to counter excessive, and potentially excitotoxic, glutamatergic activity during the postischemic period.

Differences in interactions with the dopamine transporter as revealed by diminishment of Na+ gradient and membrane potential: Dopamine versus other substrates

In heterologous cells expressing the dopamine transporter (DAT), simultaneous elevation of intracellular Na(+) and depolarization of the membrane with gramicidin reduced the potency of various DAT substrates, including dopamine, d-amphetamine, beta-phenethylamine, p-tyramine, and MPP(+), in inhibiting binding of the cocaine analog [(3)H]CFT, with the greatest reduction observed for d-amphetamine. In rat striatal synaptosomes, gramicidin exerted similar effects; in addition, the potency of d-amphetamine was reduced by the Na(+)-channel activator veratridine. The latter effect was counteracted by the Na(+)-channel blocker tetrodotoxin. In broken membranes, where, as the situation with gramicidin, both sides of the non-polarized membrane were exposed to 130 mM Na(+), gramicidin was ineffective. Dopamine had a potency for membrane preparations that was not significantly different from that for control cells or synaptosomes, while other substrates had potencies for membrane preparations that were reduced to a level similar to those observed in gramicidin-treated cells or synaptosomes. These results suggest that diminishing Na(+) gradient and membrane potential may convert DAT to a conformational state that dopamine could easily bind to when gaining free access to its intracellular portion. In contrast, non-dopamine substrates may not be able to readily interact with this state from either side of the membrane.

Fluorescence measurement of intracellular sodium concentration in single Escherichia coli cells

The energy-transducing cytoplasmic membrane of bacteria contains pumps and antiports maintaining the membrane potential and ion gradients. We have developed a method for rapid, single-cell measurement of the internal sodium concentration ([Na(+)](in)) in Escherichia coli using the sodium ion fluorescence indicator, Sodium Green. The bacterial flagellar motor is a molecular machine that couples the transmembrane flow of ions, either protons (H(+)) or sodium ions (Na(+)), to flagellar rotation. We used an E. coli strain containing a chimeric flagellar motor with H(+)- and Na(+)-driven components that functions as a sodium motor. Changing external sodium concentration ([Na(+)](ex)) in the range 1-85 mM resulted in changes in [Na(+)](in) between 5-14 mM, indicating a partial homeostasis of internal sodium concentration. There were significant intercell variations in the relationship between [Na(+)](in) and [Na(+)](ex), and the internal sodium concentration in cells not expressing chimeric flagellar motors was 2-3 times lower, indicating that the sodium flux through these motors is a significant fraction of the total sodium flux into the cell.

Rapid collapse of mitochondrial transmembrane potential in HL-60 cells and isolated mitochondria treated with anti-tumor 1,4-anthracenediones

Since synthetic analogs of 1,4-anthraquinone (AQ code number), such as AQ8, AQ9 and AQ10, can trigger cytochrome c release without caspase activation and retain their ability to induce apoptosis in multidrug-resistant (MDR) tumor cells, fluorescent probes of transmembrane potential have been used to determine whether these anti-tumor compounds might directly target mitochondria in cell and cell-free systems to cause the collapse of mitochondrial membrane potential (/Deltapsim) that is linked to permeability transition pore (PTP) opening. Using JC-1 dye, the abilities of various AQ analogs to induce the /Deltapsim in wild-type and MDR HL-60 cells are rapid (within 2.5-10 min), irreversible after drug removal, concentration dependent in the 0.256-10 micromol/l range and generally related to their anti-tumor activities in vitro. The /Deltapsim caused by AQ9 and AQ10, which are more potent than mitoxantrone, staurosporine and the reference depolarizing agent carbonyl cyanide m-chlorophenylhydrazone (CCCP) in HL-60 cells, are not prevented by caspase-2 or -8 inhibitors, suggesting that activations of these apical caspases upstream of mitochondria are not involved in this process. Antitumor AQ analogs (0.256-10 micromol/l) also mimic the abilities of the known depolarizing agents CCCP, alamethicin, gramicidin A and 100 micromol/l CaCl2 to directly induce within 15 min the /Deltapsim in isolated mitochondria prepared from mouse liver and loaded with rhodamine 123 dye. The fact that 20 micromol/l Ca2+, which is insufficient to trigger depolarization on its own, is required to reveal the depolarizing effect of AQ9 in isolated mitochondria suggests that anti-tumor AQ analogs might interact with the PTP to alter its conformation and increase its Ca2+ sensitivity. Indeed, such Ca2+-dependent /Deltapsim of isolated mitochondria treated with 1.6 micromol/l AQ9 or 100 micromol/l Ca2+ are blocked by ruthenium red. Daunorubicin (DAU) is unable to mimic the rapid /Deltapsim caused by anti-tumor AQ analogs within 2.5-40 min of treatment in HL-60 cells or isolated mitochondria. Moreover, the /Deltapsim caused by 1.6 micromol/l AQ9 or 100 micromol/l Ca2+ in isolated mitochondria are similarly blocked by cyclosporin A (CsA), bongkrekic acid and decylubiquinone, which prevent PTP opening, suggesting that, in contrast to DAU, anti-tumor AQ analogs that directly target mitochondria to trigger the Ca2+-dependent and CsA-sensitive /Deltapsim, might induce PTP opening and the mitochondrial pathway of apoptosis even in the absence of nuclear signals.

Resistance to antimicrobial peptides and stress response in Mycoplasma pulmonis

Antimicrobial peptides are widely distributed in nature, and in vertebrates, they play a key function in the innate immune defense system. It is generally agreed that these molecules may provide new antibiotics with therapeutic value. However, there are still many unsolved questions regarding the mechanisms underlying their antimicrobial activity as well as the mechanisms of resistance evolved by microorganisms against these molecules. The second point was addressed in this study. After determining the activity of 10 antimicrobial peptides against Mycoplasma pulmonis, a murine respiratory pathogen, the development of resistance was investigated. Following in vitro selection using subinhibitory concentrations of peptides, clones of this bacterium showing increased resistance to melittin or gramicidin D were obtained. For some of the clones, a cross-resistance was observed between these two peptides, in spite of their deep structural differences, and also with tetracycline. A proteomic analysis suggested that the stress response in these clones was constitutively activated, and this was confirmed by finding mutations in the hrcA gene; in mycoplasmas, bacteria which lack alternative sigma factors, the HrcA protein is supposed to play a key role as a negative regulator of heat shock proteins. By complementation of the hrcA mutants with the wild-type gene, the initial MICs of melittin and gramicidin D decreased to values close to the initial ones. This indicates that the resistance of M. pulmonis to these two antimicrobial peptides could result from a stress response involving HrcA-regulated genes.

Functional interrelationships between cell membrane and cell wall in antimicrobial peptide-mediated killing of Staphylococcus aureus

Perturbation of the Staphylococcus aureus cytoplasmic membrane (CM) is felt to play a key role in the microbicidal mechanism of many antimicrobial peptides (APs). However, it is not established whether membrane permeabilization (MP) alone is sufficient to kill susceptible staphylococci or if the cell wall (CW) and/or intracellular targets contribute to AP-induced lethality. We hypothesized that the relationships between MP and killing may differ for distinct APs. In this study, we investigated the association between AP-induced MP and lethality in S. aureus whole cells versus CW-free protoplasts, and in comparison to the MP of liposomes modeled after whole CMs in terms of phospholipid composition, fluidity and charge. Four APs with different structure-activity relationships were examined: thrombin-induced platelet microbicidal protein 1 (tPMP-1), human neutrophil protein 1 (hNP-1), gramicidin D, and polymyxin B. MP was quantified fluorometrically by calcein release. All APs tested, except polymyxin B, caused concentration-dependent MP and killing of whole cells, but not of protoplasts. The reduced AP susceptibility of protoplasts was associated with increased cardiolipin and lysyl-phosphatidylglycerol content and reduced fluidity of their CMs. However, liposomal MP induced by tPMP-1, hNP-1, and gramicidin D paralleled that of whole cells. Collectively, these results indicate that (i) structurally distinct APs likely exert their staphylocidal effects by differing mechanisms, (ii) MP is not the sole event leading to AP-induced staphylocidal activity, (iii) a complex interrelationship exists between the CM and CW in AP-induced killing, and (iv) liposomes modeled upon whole cell or protoplast CMs can recapitulate the respective susceptibilities to killing by distinct APs.

Orientation behavior of retinal photoreceptors in alternating electric fields

In alternating electric (AC) fields, particles experience polarizing effects that induce dipoles that orient elongated specimens either parallel or perpendicular to the field lines. In this work we studied the behavior of photoreceptor cells' rod outer segments (ROS) in AC fields of different frequencies. We showed that at low frequencies, ROS orient parallel to the field, whereas at higher frequencies they orient perpendicular to the field lines (in the frequency range from 100 Hz to 10 MHz). We found this behavior to be dependent on the physiological state of cells (due to modifications in their electrical properties). To simulate cell damage, the membrane conductivity was changed by treating the cell with gramicidin A, which resulted in a decrease of cytosol conductivity and, consequently, in a change of the orientation behavior of the treated cells. The change of cell orientation with cytosol conductivity is rather sharp, suggesting the potential of the method for accurate evaluation of the cell physiological status. We modeled the interaction between ROS and AC fields approximating the rod cell by a prolate spheroid with a very long axis. The internal compartment of the ellipsoid was considered to be filled with an inhomogeneous medium consisting of alternating layers of membrane and cytoplasm as media modeling the disks. This theoretical model proved to be in good agreement with the experimental results and enabled the derivation (by fitting with the experimental results) of the membrane and cytosol parameters for normal and damaged cells.

Glucose-induced electrical activity in rat pancreatic beta-cells: dependence on intracellular chloride concentration

A rise in glucose concentration depolarizes the beta-cell membrane potential leading to electrical activity and insulin release. It is generally believed that closure of KATP channels underlies the depolarizing action of glucose, though work from several laboratories has indicated the existence of an additional anionic mechanism. It has been proposed that glucose activates a volume-regulated anion channel, generating an inward current due to Cl- efflux. This mechanism requires that intracellular [Cl-] is maintained above its electrochemical equilibrium. This hypothesis was tested in rat beta-cells by varying [Cl-] in the patch pipette solution using the Cl--permeable antibiotic amphotericin B to allow Cl- equilibration with the cell interior. Under such conditions, a depolarization and electrical activity could be evoked by 16 mM glucose with pipette solutions containing 80 or 150 mM Cl-. At 40 or 20 mM Cl-, a subthreshold depolarization was usually observed, whilst further reduction to 12 or 6 mM abolished depolarization, in some cases leading to a glucose-induced hyperpolarization. With a pipette solution containing gramicidin, which forms Cl--impermeable pores, glucose induced a depolarization and electrical activity irrespective of [Cl-] in the pipette solution. Under the latter conditions, glucose-induced electrical activity was prevented by bumetanide, an inhibitor of the Na+-K+-2Cl- co-transporter. This inhibition could be overcome by the use of amphotericin B with a high [Cl-] pipette solution. These findings suggest that the maintenance of high intracellular [Cl-] in the beta-cell is an important determinant in glucose-induced depolarization, and support the hypothesis that beta-cell stimulation by glucose involves activation of the volume-regulated anion channel and generation of an inward Cl- current.

Capsaicin regulates voltage-dependent sodium channels by altering lipid bilayer elasticity

At submicromolar concentrations, capsaicin specifically activates the TRPV1 receptor involved in nociception. At micro- to millimolar concentrations, commonly used in clinical and in vitro studies, capsaicin also modulates the function of a large number of seemingly unrelated membrane proteins, many of which are similarly modulated by the capsaicin antagonist capsazepine. The mechanism(s) underlying this widespread regulation of protein function are not understood. We investigated whether capsaicin could regulate membrane protein function by changing the elasticity of the host lipid bilayer. This was done by studying capsaicin's effects on lipid bilayer stiffness, measured using gramicidin A (gA) channels as molecular force-transducers, and on voltage-dependent sodium channels (VDSC) known to be regulated by bilayer elasticity. Capsaicin and capsazepine (10-100 microM) increase gA channel appearance rate and lifetime without measurably altering bilayer thickness or channel conductance, meaning that the changes in bilayer elasticity are sufficient to alter the conformation of an embedded protein. Capsaicin and capsazepine promote VDSC inactivation, similar to other amphiphiles that decrease bilayer stiffness, producing use-dependent current inhibition. For capsaicin, the quantitative relation between the decrease in bilayer stiffness and the hyperpolarizing shift in inactivation conforms to that previously found for other amphiphiles. Capsaicin's effects on gA channels and VDSC are similar to those of Triton X-100, although these amphiphiles promote opposite lipid monolayer curvature. We conclude that capsaicin can regulate VDSC function by altering bilayer elasticity. This mechanism may underlie the promiscuous regulation of membrane protein function by capsaicin and capsazepine-and by amphiphilic drugs generally.

Differences between cation-chloride co-transporter functions in the visual cortex of pigmented and albino rats

Albinism in mammals is accompanied by specific morphological and functional alterations of the visual system. To understand their cellular basis we studied the physiological characteristics and transmembrane currents of pyramidal neurons in 350-microm-thick slices of visual cortex from pigmented and albino rats using whole-cell and gramicidin perforated patch-clamp recordings. The resting membrane potential was significantly more positive and the rheobase was significantly lower in neurons of layers II/III and V in albinos as compared with pigmented rats. No significant differences were found in the input resistance, time constant and chronaxy. Whereas the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor-mediated currents were not significantly different, the maximum gamma-aminobutyric acid (GABA)(A) receptor (GABA(A)R)-mediated currents and miniature inhibitory postsynaptic currents showed significantly lower amplitudes in neurons of layer V in visual cortex of albinos as compared with pigmented rats. The reversal potential of the GABA(A)R-mediated currents (E(GABA)) was significantly shifted to more positive values in albinos. Pharmacological experiments showed that this shift could be caused by an increased action of the inward chloride co-transporter NKCC1 and reduced action of the outward chloride co-transporter KCC2 in albino rats. This difference seems to be restricted to the visual cortex because in pyramidal neurons from frontal cortex E(GABA) was not significantly different in albinos as compared with pigmented rats. These results are discussed in relation to functional alterations in the albino visual system.

Differential expression of KCC2 accounts for the differential GABA responses between relay and intrinsic neurons in the early postnatal rat olfactory bulb

The rat olfactory bulb is anatomically immature at birth, and considerable neurogenesis and synaptogenesis are known to take place postnatally. In addition, significant physiological changes have also been reported in this period. For example, granule cell-mediated inhibition following electrical stimulations to the lateral olfactory tract is robust during the first postnatal week, and then decreases abruptly after the second week. However, the mechanism underlying this enhanced inhibition remains to be elucidated. To know the cause of this phenomenon, we investigated the expression patterns of cation-Cl(-) co-transporters (KCC1, KCC2 and NKCC1) mRNAs, which are responsible for the regulation of [Cl(-)](i). In addition, responses to gamma-aminobutyric acid (GABA) were measured by gramicidin-perforated patch-clamp recordings and Ca(2+) imaging using fura-2. We found that in the early postnatal period, mitral cells expressing KCC2 mRNA were inhibited by GABA, while granule cells lacking KCC2 mRNA expression were depolarized or excited by GABA. These results indicate that transient GABA-mediated excitation on granule cells might be the main cause of the enhanced inhibition on mitral cells, and suggest that these differential GABA responses between relay and intrinsic neurons play pivotal roles in the early postnatal rat olfactory bulb.

Test of molecular dynamics force fields in gramicidin A

The force fields commonly used in molecular dynamics simulations of proteins are optimized under bulk conditions. Whether the same force fields can be used in simulations of membrane proteins is not well established, although they are increasingly being used for such purposes. Here we consider ion permeation in the gramicidin A channel as a test of the AMBER force field in a membrane environment. The potentials of mean force for potassium ions are calculated along the channel axis and compared with the one deduced from the experimental conductance data. The calculated result indicates a rather large central barrier similar to those obtained from other force fields, which are incompatible with the conductance data. We suggest that lack of polarizability is the most likely cause of this problem, and, therefore, urge development of polarizable force fields for simulations of membrane proteins.

The effect of interaction between K+ ions and gramicidin D on the lecithin membrane interfacial tension

The effect of the presence of gramicidin D in a lecithin membrane on its interfacial tension has been studied. The studies have been carried out at various forming solution compositions and at various potassium ion concentrations in the electrolyte solution. Potassium chloride was used as the electrolyte. The complex was formed between the gramicidin molecule and K(+) ion. The following parameters describing the complex were determined: the surface area occupied by GK(+) complex (A(GK(+))), the interfacial tension of the GK(+) membrane complex (gamma(GK+)), and the stability constant of the gramicidin-K(+) complex (K). These values are 156 A(2), 1.89 mN m(-1) and 0.033 m(3) mol(-1), respectively.

Potassium homeostasis influences the locomotion and encystment of zoospores of plant pathogenic oomycetes

Zoospores of plant pathogenic oomycetes exhibit distinct swimming speeds and patterns under natural conditions. Zoospore swimming is influenced by ion homeostasis and changes in the ionic composition of media. Therefore, we used video microscopy to investigate swimming patterns of five oomycete species in response to changes in potassium homeostasis. In general, zoospore speed tended to be negatively correlated with zoospore size. Three Phytophthora species (Phytophthora palmivora, Phytophthora megakarya, and Phytophthora infestans) swam in straight patterns with speeds ranging from 50 to 250 microm/s whereas two Pythium species (Pythium aphanidermatum and Pythium dissotocum) swam at similar speeds ranging from 180 to 225 microm/s with a pronounced helical trajectory and varying amplitudes. High external concentrations of potassium salts reduced the swimming speed of Ph. palmivora and induced encystment. This was not observed for Py. aphanidermatum. Application of the potassium ionophores gramicidin, nigericin and valinomycin resulted in reduced swimming speeds and changes in the swimming patterns of the Phytophthora species. Therefore, potassium ions play a key role in regulating zoospore behavior.

Gramicidin-perforated patch recording revealed the oscillatory nature of secretory Cl- movements in salivary acinar cells

Elevations of cytoplasmic free calcium concentrations ([Ca²⁺]_i) evoked by cholinergic agonists stimulate isotonic fluid secretion in salivary acinar cells. This process is driven by the apical exit of Cl⁻ through Ca²⁺-activated Cl⁻ channels, while Cl⁻ enters the cytoplasm against its electrochemical gradient via a loop diuretic-sensitive Na⁺-K⁺-2Cl⁻ cotransporter (NKCC) and/or parallel operations of Cl⁻-HCO₃⁻ and Na⁺-H⁺ exchangers, located in the basolateral membrane. To characterize the contributions of those activities to net Cl⁻ secretion, we analyzed carbachol (CCh)-activated Cl⁻ currents in submandibular acinar cells using the “gramicidin-perforated patch recording configuration.” Since the linear polypeptide antibiotic gramicidin creates monovalent cation-selective pores, CCh-activated Cl⁻ currents in the gramicidin-perforated patch recording were carried by Cl⁻ efflux via Cl⁻ channels, dependent upon Cl⁻ entry through Cl⁻ transporters expressed in the acinar cells. CCh-evoked oscillatory Cl⁻ currents were associated with oscillations of membrane potential. Bumetanide, a loop diuretic, decreased the CCh-activated Cl⁻ currents and hyperpolarized the membrane potential. In contrast, neither methazolamide, a carbonic anhydrase inhibitor, nor elimination of external HCO₃⁻ had significant effects, suggesting that the cotransporter rather than parallel operations of Cl⁻-HCO₃⁻ and Na⁺-H⁺ exchangers is the primary Cl⁻ uptake pathway. Pharmacological manipulation of the activities of the Ca²⁺-activated Cl⁻ channel and the NKCC revealed that the NKCC plays a substantial role in determining the amplitude of oscillatory Cl⁻ currents, while adjusting to the rate imposed by the Ca²⁺-activated Cl⁻ channel, in the gramicidin-perforated patch configuration. By concerting with and being controlled by the cation steps, the oscillatory form of secretory Cl⁻ movements may effectively provide a driving force for fluid secretion in intact acinar cells.

The effect of temperature and lipid on the conformational transition of gramicidin A in lipid vesicles

The present study investigated the effect of temperature and lipid/peptide molar ratio on the conformational changes of the membrane peptide gramicidin A from a double-stranded helix to a single-stranded helical dimmer in 1,2-dimyristoyl-glycerol-3-phosphochloine (DMPC) vesicles. Tryptophan fluorescence spectroscopy results suggested that the conformational transition fitted a three-state (two-step) "folding" model. Rate constants, k(1) and k(2), were determined for each of the two steps. Since k(1) and k(2) increased with an increase in temperature, we hypothesized that the process corresponded to the breakage and formation of the backbone hydrogen bonds. The k(1) was from 10 to 45 folds faster than k(2), except for lipid/peptide molar ratios above 89.21, where k(2) increased rapidly. At molar ratios below 89.21, k(2) was insensitive to changes in lipid concentration. To account for this phenomenon, we proposed that while the driving interaction at high molar ratios is between the indole rings of the tryptophan residues and the lipid head groups, at low molar ratios there may be an intermolecular interaction between the tryptophan residues that causes gramicidin A to form an organized aggregated network. This aggregated network, caused by the tryptophan-tryptophan interaction, may be the main effect responsible for the slow down of the conformation change.

[The influence of plasma membrane ion permeability modulators on superoxide formation and bioelectrical characteristics of wheat root cells]

Changes in superoxide radical formation and bioelectrical characteristics of excised wheat root cells under modification of plasma membrane ion permeability were studied. It was shown that a 2 h treatment of excised roots with valinomycin (Val, 20 microM), N, N'-dicyclohexylcarbodimide (DCCD, 100 microM), gramicidin S (Gr, 20 microM), chlorpromazine (CPZ, 100 microM) caused an increased loss of potassium by cells, lowering of membrane potential (MP) and electrical input resistance (Rin) of the cells. The superoxide formation by excised root cells diminished (under DCCD) or remained at the control level (under Val), which was accompanied by a minor decrease of MP and Rin of the cells, a small increase in potassium loss by excised roots, and in no change of pH of incubation medium. Significant depolarization of plasma membrane, dropping of Rin and essential loss of potassium ions by the cells correlated with a rise in the medium alkalinization and superoxide formation by excised roots (in the presence of Gr, CPZ). Ion channel blocker gadolinium (Gd3+, 200 microM) caused an increase of MP and Rin reduction of potassium loss by cells, and a decrease of pH of the incubation medium, and also enhancement of superoxide formation by excised root cells. It is suggested that upon plasma membrane ion permeability modification the activity of superoxide generating systems depends on the specificity and mechanisms of action of modulators, and is determined by their influence on redox state of plasma membrane as well as by peculiarities of ion transport disturbance.

Synthesis and biological evaluation of gramicidin S dimers

The design and synthesis of analogues of the cyclic beta-sheet gramicidin S (GS), having additional functionalities in their turn regions, is reported. The monomeric GS analogues were transformed into dimers and their activities towards biological membranes, through antimicriobial and hemolytic assays, were evaluated. Finally, conductivity measurements have been performed to elucidate ion channel forming properties.