Effect of uncouplers on "downhill" beta-galactoside transport in energy-depleted cells of Escherichia coli

Interaction of daptomycin with lipid bilayers: a lipid extracting effect

Daptomycin is the first approved member of a new structural class of antibiotics, the cyclic lipopeptides. The peptide interacts with the lipid matrix of cell membranes, inducing permeability of the membrane to ions, but its molecular mechanism has been a puzzle. Unlike the ubiquitous membrane-acting host-defense antimicrobial peptides, daptomycin does not induce pores in the cell membranes. Thus, how it affects the permeability of a membrane to ions is not clear. We studied its interaction with giant unilamellar vesicles (GUVs) and discovered a lipid-extracting phenomenon that correlates with the direct action of daptomycin on bacterial membranes observed in a recent fluorescence microscopy study. Lipid extraction occurred only when the GUV lipid composition included phosphatidylglycerol and in the presence of Ca(2+) ions, the same condition found to be necessary for daptomycin to be effective against bacteria. Furthermore, it occurred only when the peptide/lipid ratio exceeded a threshold value, which could be the basis of the minimal inhibitory concentration of daptomycin. In this first publication on the lipid extracting effect, we characterize its dependence on ions and lipid compositions. We also discuss possibilities for connecting the lipid extracting effect to the antibacterial activity of daptomycin.

Distinguishing on-target versus off-target activity in early antibacterial drug discovery using a macromolecular synthesis assay

The macromolecular synthesis assay was optimized in both S. aureus and E. coli imp and used to define patterns of inhibition of DNA, RNA, protein, and cell wall biosynthesis of several drug classes. The concentration of drug required to elicit pathway inhibition differed among the antimicrobial agents tested, with inhibition detected at concentrations significantly below the minimum inhibitory concentration (MIC) for tedizolid; within 4-fold of the MIC for ciprofloxacin, cefepime, vancomycin, tetracycline, and chloramphenicol; and significantly above the MIC for rifampicin and kanamycin. In a DNA gyrase/topoisomerase IV structure-based drug design optimization program, the assay rapidly identified undesirable off-target activity within certain chemotypes, altering the course of the program to focus on the series that maintained on-target activity.

Transport of glycerol by Pediococcus pentosaceus isolated from wine

Pediococcus pentosaceus N(5)p is a strain isolated from wine that uses glycerol as its sole carbon source, mainly via the glycerol kinase pathway. The transport of glycerol was investigated in resting cells of this strain. Glycerol uptake followed a Michaelis-Menten relationship with an observed apparent K(m) of 33 microM and a V(max) of 2.5 nmol/min/mg of cell protein. The transport system was specific for glycerol, which was present in the cells grown either on glycerol or glucose suggesting its constitutive nature. The presence of uptake when resting cells were treated with HgCl(2) and the absence of counterflow indicate that facilitated diffusion is not involved in glycerol transport. On the other hand, glycerol uptake was inhibited by the metabolic poisons that affect ATP availability by acting on either electron transport or ATPase activity, and by the proton-conducting uncouplers without any effect on glycerol kinase activity. The restoration of glycerol uptake in de-energized cells by the addition of glucose and low concentration of cyanide-m-chlorophenyl hydrazone was achieved. These results, the first in the genus Pediococcus, provide evidence for an energy-dependent uptake of glycerol that involves the proton motive force directly or coupled with ATP synthesis.

How does a Bacillus split its septum right down the middle?

This is a speculative paper which considers the possible ways that Gram-positive cells might employ to achieve an even thickness of the two daughter poles resulting from the fission of the cross-wall. The first is that the protonmotive force generated by the extrusion of protons at the cytoplasmic membrane acts to inhibit autolysins to a distance of about 25 nm. The second has to do with the stresses that develop as the poles form. On the tacit assumption that the autolysins will function faster when their substrates are under tension, it is shown how this, too, can lead to even bisection of the cross-wall. These possibilities are not alternative, both probably function.

Ammonium (methylammonium) transport by Klebsiella pneumoniae

Klebsiella pneumoniae can accumulate methylammonium up to 80-fold by means of a transport system as indicated by the energy requirement, saturation kinetics and a narrow pH profile around pH 6.8. Methylammonium transport (apparent Km = 100 microM, V = 40 mumol/min per g dry weight at 15 degrees C) is competitively inhibited by ammonium (apparent Ki = 7 microM). The low Ki value and the finding that methylammonium cannot serve as a nitrogen source indicate that ammonium rather than methylammonium is the natural substrate. Uphill transport is driven by a component of the protonmotive force, probably the membrane potential. The transport system is under genetic control; it is partially repressed by amino acids and completely by ammonium. Analysis of mutants suggest that the synthesis of the ammonium transport system is subject to the same 'nitrogen control' as nitrogenase and glutamine synthetase.

Lactose-H+(-OH) transport system of Escherichia coli. Multistate gated pore model based on half-sites stoichiometry for high-affinity substrate binding in a symmetrical dimer

A model is proposed for the D-galactoside-H+(-OH) transporter of Escherichia coli that accounts for essentially all the experimental observations established for this system to date. In this model, the functional unit is postulated to be a dimer (consisting of two copies of lac Y-specified polypeptide) which spans the membrane with a 2-fold symmetry axis in the membrane plane (Lancaster, J.R. (1978) J. Theor. Biol. 75, 35-50). The functional dimer is assumed to possess a single pore flanked by an inner gate (gi) and an outer gate (go) and encompassing two oppositely oriented galactoside binding sites, designated m and mu. When go is open and gi is closed under non-energized conditions, binding site m adopts a configuration defined as State A (i.e., moA) exhibiting high affinity toward Class Ga galactosides (thiodigalactoside, melibiose, alpha-p-nitrophenygalactoside) but low affinity for Class Gb galactosides (lactose, beta-o-nitrophenylgalactoside, beta-isopropylthiogalactoside), whereas binding site mu adopts State B (i.e., muoB) displaying relatively high affinity toward Class Gb galactosides but comparatively low affinity for Class Ga galactosides; further, each moA : muoB dimer contains one thiol group whose reaction with N-ethylmaleimide inactivates the transporter unless blocked by galactoside binding at site moA, while the second homologous thiol of the dimer is unreactive toward thiol reagents. Translocation of the moA : muoB dimer involves closing of go followed by opening of gi, and causes the two thiols (as well as sites m and mu) to interchange roles in a symmetrical fashion: moA : muoB in equilibrium miB : muiA. In the presence of a substantial (negative) transmembrane delta potential of muH+, the m : mu dimer is postulated to undergo an electrogenic protein conformation change to a second form, *(m : mu), in which both sites m and mu possess low affinity toward internal Class Gb substrates; galactoside transport in both m : mu and *(m : mu) is assumed to be coupled to H+-symport (-OH-antiport) with a stoichiometry of approximately 1 : 1. Finally, five characteristic predictions of the half-sites model are outlined for further tests of its validity.

Energization of the sugar transport mechanism in the plasmalemma of isolated mesophyll protoplasts

The mechanism of 3-O-methyl-d-glucose transport through the plasmalemma has been investigated in protoplasts isolated from the mesophyll of Pisum sativum L. var. Dan.Analysis of the fluxes after 50 minutes of uptake showed that the gradual decrease in slope of the net uptake curve with time was not due to any decline in uptake capacity; it represented the approach to flux equilibrium of a small compartment of the protoplast, probably the cytoplasm.The energy of activation for initial flux into this compartment was 20 kilocalories per mole between 17 and 27 C. Very high discrimination was shown with regard to sugar isomers. Light strongly promoted flux (by a factor of 2.5 in the case of methyl glucose). Initial flux showed sharply contrasting inhibitor sensitivity in the light and the dark. Light uptake was sensitive to the proton conductor carbonyl cyanide m-chlorophenylhydrazone (CCCP), but stable for at least the first 10 minutes to the ATPase inhibitors quercetin, rutin, and diethylstilbestrol, as well as to arsenate. Dark uptake, on the other hand, was stable to CCCP but was immediately depressed by quercetin, rutin, diethylstilbestrol, and arsenate.Protoplasts which received a light pretreatment before incubation in the dark took up methyl glucose at the accelerated light rate for the first 7 minutes. Moreover, the light pretreatment sensitized subsequent initial dark uptake to CCCP, and conferred on it the stability to ATPase inhibitors and arsenate characteristic of light uptake. After about 7 minutes the characteristic inhibitor responses of dark uptake were resumed.It is proposed that more than one mode of energy-coupling for sugar transport may operate in these protoplasts.

Na+-dependent methyl beta-thiogalactoside transport in Salmonella typhimurium

We have studied the role of sodium ions in methyl beta-thiogalactoside (TMG) transport via the melibiose permease (TMG II) in Salmonella typhimurium. TMG uptake via TMG II in anaerobic, straved and metabolically poisoned cells is dependent on an inward-directed Na+ gradient. Cells which have been partially depleted of endogenous substrates show H+ extrusion upon sodium-stimulated TMG influx. Measurements of the electrochemical H+ gradient in cells, starved in different ways for endogenous substrates, suggest that this proton extrusion is probably not linked to the actual translocation mechanism but is the result of metabolism induced by TMG plug Na+ uptake.

Transport by the lactose permease of Escherichia coli as the basis of lactose killing

Lactose killing is a peculiar phenomenon in which 80 to 98% of the Escherichia coli cells taken from a lactose-limited chemostat die when plated on standard lactose minimal media. This unique form of suicide is caused by the action of the lactose permease. Since uptake of either lactose or galactose by the lactose permease caused death, the action of rapid transport across the membrane must be the cause of the phenomenon. Alternative causes of lactose killing, such as accumulation of toxic metabolic intermediates or action of the beta-galactosidase, have been eliminated. It is proposed that rapid uptake of sugars by the lactose permease disrupts membrane function, perhaps causing collapse of the membrane potential.

Metabolic control of lactose entry in Escherichia coli

A general method has been developed for determining the rate of entry of lactose into cells of Escherichia coli that contain beta-galactosidase. Lactose entry is measured by either the glucose or galactose released after lactose hydrolysis. Since lactose is hydrolyzed by beta-galactosidase as soon as it enters the cell, this assay measures the activity of the lactose transport system with respect to the translocation step. Using assays of glucose release, lactose entry was studied in strain GN2, which does not phosphorylate glucose. Lactose entry was stimulated 3-fold when cells were also presented with readily metabolizable substrates. Entry of omicron-nitrophenyl-beta-D-galactopyranoside (ONPG) was only slightly elevated (1.5-fold) under the same conditions. The effects of arsenate treatment and anaerobiosis suggest that lactose entry may be limited by the need for reextrusion of protons which enter during H+/sugar cotransport. Entry of omicron-nitrophenyl-beta-D-galactopyranoside is less dependent on the need for proton reextrusion, probably because the stoichiometry of H+/substrate cotransport is greater for lactose than for ONPG.

Reduction of membrane potential, an immediate effect of colicin K

Colicin K causes a rapid and drastic reduction of the membrane potential of Escherichia coli cells, as measured by the uptake of the lipophilic cation triphenylmethylphosphonium. The colicin causes no major changes in the pH gradient, as measured by the uptake of butyric acid. The decrease in membrane potential following addition of colicin K to the cells is a prompt response that parallels or precedes known physiological effects such as efflux of accumulated substrates. Hence the loss of membrane potential qualifies as the primary action by which the colicin uncouples membrane-associated function from respiration. Certain peculiarities of bacterial cells pretreated with EDTA in their response to uncoupling agents and lipophilic ions are described.

Amplification of the lactose carrier protein in Escherichia coli using a plasmid vector

The isolation and properties of a hybrid plasmid carrying the Y gene of the lac operon of Escherichia coli are described. The lactose carrier protein, coded for by the Y gene, is readily identified upon lac operon induction in strains carrying the plasmid. The protein comprises about 15% of the cytoplasmic membrane protein synthesized in the first generation after induction, compared with a wild type strain induced under the same conditions where lactose carrier protein comprises 1.4% of the cytoplasmic membrane protein.

Active transport of L-sorbose and 2-deoxy-D-galactose in Saccharomyces fragilis

Sorbose and 2-deoxy-D-galactose are taken up in Saccharomyces fragilis by an active transport mechanism, as indicated by the energy requirement of the process and the accumulation of free sugar against the concentration gradient. There are no indications for transport-associated phosphorylation as mechanism of energy coupling with these two sugars. The measured sugar-proton cotransport and the influx inhibition by uncouplers suggest a chemiosmotic coupling mechanism. Thus there are at least two different active transport mechanisms operative in Saccharomyces fragilis: transport-associated phosphorylation in the case of 2-deoxy-D-glucose and chemiosmotic coupling in the case of sorbose and 2-deoxy-D-galactose. The differences between the two mechanisms are discussed. Uncouplers do not stimulate downhill sorbose transport in energy-depleted cells and evoke an almost complete inhibition of efflux and of exchange transport. The differences between this sugar-proton cotransport system and similar systems in bacteria and Chlorella are discussed.

Functional symmetry of the beta-galactoside carrier in Escherichia coli

Cytoplasmic membrane vesicles with either normal or inverted orientation were prepared from Escherichia coli. The lactose transport activity of these vesicle preparations was compared. The parameters measured were net efflux, counterflux, and K+/valinomycin-induced active uptake of lactose. With membrane vesicles derived from both wild-type and cytochrome-deficient strains the right-side-out and inverted membrane preparations showed similar rates of lactose flux in all assays. According to these criteria, the activity of the beta-galactoside transport protein is inherently symmetrical. One major difference was observed between the native and inverted vesicle preparations: the inverted vesicles had approximately twice the specific activity of native vesicles in the counterflux and K+/valinomycin-induced uptake assays. This difference can be largely ascribed to the presence in the normal vesicle preparation of vesicles with a high passive permeability to lactose. Such vesicles are apparently absent from the inverted vesicle preparations.

The effects of N-ethylmaleimide on active amino acid transport in Escherichia coli

N-Ethylmaleimide (MalNEt) binds covalently and without specificity to accessible sulfhydryl residues in proteins. In some cases specificity has been imposed on this reaction by manipulating reaction conditions, yielding information concerning both enzyme mechanism and the identity of specific proteins (for example C.F. Fox and E.P. Kennedy (1965) Proc. Natl. Acad. Sci. u.s. 54, 891-899) and R.E. McCarty and J. Fagan (1973) Biochemistry 12, 1503-1507). We have examined the effects of MalNEt on the active accumulation of nine amino acids by Escherichia coli strains ML 308-225 and DL 54. Whole cells have been used in order that transport systems both dependent on and independent of periplasmic binding proteins could be studied under various conditions of energy supply for transport. Our results suggest that the systems transporting ornithine, phenylalanine and proline are those most likely to undergo inactivation by direct reaction of MalNEt with the transport apparatus, rather than merely via side effects such as interruption of their energy supply. The inhibition of proline transport is specifically enhanced by the presence of proline, competitive inhibitors of proline transport, or carbonylcyanide p-trifluoromethyoxyphenylhydrazone during MalNEt treatment. The other eight systems tested showed no analogous effects.

Transport of cyclitols by a proton symport in Klebsiella aerogenes

The respiration and the ATP content of Klebsiella aerogenes in the presence of various inhibitors were compared to the transport of scyllo-inositol. The ATPase was found to be inhibited by dicyclohexyl carbodiimide. The transport has been tested in anaerobiosis and aerobiosis. From the results obtained it is concluded that either ATP or respiration can sustain the transport activity in independent manner. 2. The energy derived from the respiratory chain reactions or the ATP hydrolysis results in electrogenic extrusion of protons. The electrochemical potential created drives the accumulation of scyllo-inositol, as shown by an increase of pH of the medium on addition of the substrate to cells in anaerobiosis. With non-induced cells no change in pH occurs, which demonstrates that proton flow is really linked to the transport. No H+/Na+ or K+ exchange is observed and the proton conductor carbonylcyanide m-chlorophenylhydrazone abolishes the pH shift caused by substrate addition. The stoichiometry of the symport H+/cyclitol is 1 and the half-maximum value of the pH variation as a function of the amount of scyllo-inositol added corresponds to a concentration of scyllo-inositol very close to the KT of influx.

Effect of uncoupler on "downhill" substrate efflux of Escherichia coli is dependent on (Mg2+, Ca2+). Adenosine triphosphatase

Previous studies have shown that mutations in the unc gene of Escherichia coli K12 cause defects in energy transduction as well as a membrane-bound (Mg2+, Ca2+)-adenosine triphosphatase. We studied the effect of this mutation on the "downhill" efflux of methyl-beta-D-galactopyranoside, a suboli K12 did not show significant differences in substrate influx of efflux, a differential effect of an uncoupler, 2,4-dinitrophenol was demonstrated. In contrast to the unc+, dinitrophenol failed to inhibit significantly the rate coefficient of efflux in the unc- strain. Analysis of spontaneous unc+ revertants of the unc- mutant provided additional evidence that a functional unc gene is necessary for dinitrophenol inhibition of efflux. Other uncouplers tested in the unc+ strain showed different effects on efflux. While arsenate, azide and carbonyl cyanide p-trifluoromethoxyphenulhydrazone caused little or no effect, 2,4-dibromophenol and pentachlorophenol increased efflux by a considerable factor.

Energy cost of galactoside transport to Escherichia coli

Energy reserves of Escherichia coli can be depleted by our previously reported procedure to a level such that even the "downhill" transport of o-nitrophenyl-beta-D-galactopyranoside (ONPG) is completely dependent upon the exogenous energy supply. The ONPG concentration is high externally to the cells and is low intracellular because of the action of cytoplasmic beta-galactosidase. In the present work, depleted cell suspensions have been infused at low, steady rates with glucose and other energy sources while measurements of transport were being made. Comparing the rate of ONPG transport with the rate of introduction of glucose under conditions where the chosen glucose infusion rate limits transport, we find that 89 molecules of ONPG are transported per molecule of fully oxidized glucose. This transport yield is constant over a 6.5-fold range in rate of glucose addition. This constancy over a range of infusion rates implies that transport is the major cellular function under these special conditions. The yield value if 89 is in the agreement with the predicitions of 76 from Mitchell's chemiosmotic theory and constitutes an independent proff of its validity, since all the other proposed mechanisms of engery coupling predict much smaller yields. The lag from the start of glucose infusion into the reaction cuvette, to the extrapolated time at which a steady rate of transport and concomitant hydrolysis are achieved, is short (approximately 1 min). Similarly, the time after the infusion is stopped until the rate of transport returns to the background rate is also short. The latter implies that the energy metabolism is directed almost entirely to transport and/or other ongoing cellular processes and not to repair or renewal of an energy-independent, facilitated diffusion system.