A density label for membranes

Unsaturated fatty acid synthesis in bacteria: Mechanisms and regulation of canonical and remarkably noncanonical pathways

Unsaturated phospholipid acyl chains are required for membrane function in most bacteria. The double bonds of the cis monoenoic chains arise by two distinct pathways depending on whether oxygen is required. The oxygen-independent pathway (traditionally called the anaerobic pathway) introduces the cis double bond by isomerization of the trans double bond intermediate of the fatty acid elongation cycle. Double bond isomerization occurs at an intermediate chain length (e.g., C10) and the isomerization product is elongated to the C16-C18 chains that become phospholipid monoenoic acyl chains. This pathway was first delineated in Escherichia coli and became the paradigm pathway. However, studies of other bacteria show deviations from this paradigm, the most exceptional being reversal of the fatty acid elongation cycle by a reaction paralleling the initial step in the β-oxidative degradation of fatty acids. In the oxygen-dependent pathway diiron enzymes called desaturases introduce a double bond into a saturated acyl chain by regioselective cis dehydrogenation through activation of molecular oxygen with an active-site diiron cluster. This difficult hydrogen abstraction from a methylene group often occurs at the midpoint of a saturated fatty acyl chain. In bacteria the acyl chain is a phospholipid acyl chain, and the desaturase is membrane bound. Both the oxygen-independent oxygen-dependent pathways are transcriptionally regulated by repressor and activator proteins that respond to small molecule ligands such as acyl-CoAs. However, in Bacillus subtilis the desaturase is synthesized only at low growth temperatures, a process controlled by a signal transduction regulatory pathway dependent on membrane lipid properties.

The Antimicrobial Peptide 1018-K6 Interacts Distinctly with Eukaryotic and Bacterial Membranes, the Basis of Its Specificity and Bactericidal Activity

Since penicillin was discovered, antibiotics have been critical in the fight against infections. However, antibiotic misuse has led to drug resistance, which now constitutes a serious health problem. In this context, antimicrobial peptides (AMPs) constitute a natural group of short proteins, varying in structure and length, that act against certain types of bacterial pathogens. The antimicrobial peptide 1018-K6 (VRLIVKVRIWRR- NH2) has significant bactericidal and antibiofilm activity against Listeria monocytogenes isolates, and against different strains and serotypes of Salmonella. Here, the mechanism of action of 1018-K6 was explored further to understand the peptide-membrane interactions relevant to its activity, and to define their determinants. We combined studies with model synthetic membranes (liposomes) and model biological membranes, assessing the absorption maximum and the quenching of 1018-K6 fluorescence in aqueous and lipid environments, the self-quenching of carboxyfluorescein, as well as performing lipid sedimentation assays. The data obtained reflect the differential interactions of the 1018-K6 peptide with eukaryotic and prokaryotic membranes, and the specific interactions and mechanisms of action in the three prokaryotic species studied: Salmonella Typhimurium^2GN, Escherichia coli^3GN, and Staphylococcus aureus^3GP. The AMP 1018-K6 is a candidate to prevent (food preservation) or treat (antibiotic use) infections caused by certain pathogenic bacteria, especially some that are resistant to current antibiotics.

Genetic and biochemical characterization of a mutation (fatA) that allows trans unsaturated fatty acids to replace the essential cis unsaturated fatty acids of Escherichia coli

Unsaturated fatty acid auxotrophs of Escherichia coli are able to use only unsaturated fatty acids of the cis configuration as the required growth supplement. A mutation in the fatA gene allows such auxotrophs to utilize unsaturated fatty acids with a trans double bond as well as fatty acids having a cis double bond. The fatA gene was mapped to min 69 near argG, and the allele studied (fatA1) was found to be dominant over the wild-type gene. fatA1 mutant strains grew at similar rates when supplemented with elaidate (trans-9-octadecenoate) or oleate (cis-9-octadecenoate). The fat+ strain, however, lysed when supplemented with the trans fatty acid. Physiological characterization of the fatA mutant strain was undertaken. The mutation appeared not to be involved with long-chain fatty acid transport. Introduction of lesions in known fatty acid transport genes abolished trans fatty acid utilization in the fatA mutant strain. Also, growth characteristics of the fat+ and the fatA1 mutant strains on elaidate as the sole carbon source were identical, which indicated comparables rate of fatty acid accumulation. The mutation appeared to be involved with recognition of the trans configuration after uptake into the cell. The levels of trans fatty acid incorporation into the phospholipids of the fat+ and the fatA strains differed considerably, with the mutant incorporating much higher levels. No significant accumulation of elaidate into nonphospholipid cellular components was observed. The fatA mutation did not appear to be involved with the cellular metabolic state, as cyclic AMP had no effect on the ability of the strains to utilize trans fatty acids.

Native surface association of a recombinant 38-kilodalton Treponema pallidum antigen isolated from the Escherichia coli outer membrane

A recombinant plasmid designated pAW305, containing a 6-kilobase insert of Treponema pallidum DNA, directed the expression of a 38-kilodalton (kDa) treponemal antigen in Escherichia coli. The 38-kDa antigen copurified with the outer membrane fraction of the E. coli cell envelope after treatment with nonionic detergents or sucrose density gradient centrifugation. Rabbits immunized with the recombinant 38-kDa antigen developed antibodies which reacted specifically with a 38-kDa T. pallidum antigen on immunoblots, and 38-kDa antisera specifically immobilized T. pallidum in a complement-dependent manner in the T. pallidum immobilization test. Antisera to the 38-kDa recombinant antigen were also used to demonstrate its native surface association on T. pallidum by immunoelectron microscopy.

Dependence of Escherichia coli hyperbaric oxygen toxicity on the lipid acyl chain composition

This study examines certain membrane-related aspects of oxygen poisoning in Escherichia coli K1060 (fabB fadE lacI) and its parent strain, K-12 Ymel. Cells were grown to exponential or stationary phase in a minimal medium and exposed to air plus 300 lb/in2 of O2 as a suspension in minimal salts. After an initial lag, both strains lost viability with apparent first-order kinetics. Hypebaric oxygen was more toxic to cells harvested during the exponential phase of growth than to cells harvested from the stationary phase of growth for both strains K-12 Ymel and K1060. Control suspensions exposed to air plus 300 lb/in2 of N2 did not lose viability during a 96-h exposure. The sensitivity of the unsaturated fatty acid auxotroph, strain K1060, to hyperbaric oxygen increased as the degree of unsaturation of the fatty acid supplement increased. Cells grown with a cyclopropane fatty acid (9,10=methylenoctadecanoate) were the most resistant; cells grown with a monounsaturated fatty acid (oleate) were intermediate; and those grown with polyunsaturated fatty acids (linoleate and linolenate) were most sensitive to hyperbaric oxygen. The parent strain, K-12 Ymel, lost viability in hyperbaric oxygen most similarly to strain K1060 supplemented with oleate. To determine the relative effect of hyperbaric oxygen on the survival of E. coli with saturated membranes, substrains of K1060 were selected for growth on 12-methyltetrade-canoate or on 9 or 10-monobromostearate. Substrains grown with a saturated fatty acid supplement were equally or more sensitive to hyperbaric oxygen than when the same substrains were grown with a cyclopropane fatty acid supplement. The lipid acyl chain composition was determined in E. coli K1060 before and after exposure to hyperbaric oxygen or hyperbaric nitrogen. The proportion of nonsaturated acyl chain lipid of either the oleate- or the 9,10-methyleneoctade-canoate-supplemented K1060 remained unchanged after hyperbaric gas exposure. In strain K1060 supplemented with linoleate and grown to stationary phase, however, the relative unsaturated acyl chain content after hyperbaric exposure decreased in both gases. This finding prompted an investigation of the role of lipid oxidation in hyperbaric oxygen toxicity. Assays of potential lipid oxidation products were performed with linoleate-grown cells. The lipid hydroperoxide and peroxide content of the lipid extract increased by 6.9 times after 48 h of air plus 300 lb/in2 of O2; malondialdehyde and fluorescent complex lipid oxidation products showed much smaller or no changes. Lipid extracts from hyperbaric oxygen-exposed cells were not toxic to viable E. coli K1060, nor did they increase the rate of loss of viability in cells simultaneously exposed to hyperbaric oxygen. Linoleic acid hydroperoxide at 1.0 mM had no effect on the viability of E. coli K-12 Ymel and only marginally decreased the viability of E. coli K1060 supplemented with linoleate. We conclude that the kinetics of oxygen toxicity in E...

Membrane lipid biosynthesis in Acholeplasma laidlawii B: incorporation of exogenous fatty acids into membrane glyco- and phospholipids by growing cells

The extent of incorporation of a wide variety of exogenous saturated, unsaturated, branched-chain, and cyclopropane fatty acids into the membrane lipids of Acholeplasma laidlawii B was systematically studied. Within each fatty acid class the extent of incorporation generally increased markedly with increasing chain length, reached a maximum, and then declined progressively but less sharply with further increases above that chain length giving maximal direct incorporation. Certain shorter-chain members of each fatty acid class underwent complete or partial conversion to longer-chain homologues before utilization for complex lipid biosynthesis. The degree and extent of chain elongation and direct incorporation and the characteristic dependence of each of these processes on fatty acid chain length and structure correlated well with the physical properties (melting temperatures) of the exogenous fatty acids. The in vivo specificity of the enzyme systems responsible for the incorporation of exogenous fatty acids was such that the fluidity and physical state of the membrane lipids were maintained within a definite, albeit a relatively wide, range. We also observed that the neutral glycolipids typically have similar fatty acid compositions, which are somewhat different from those of the major phosphatides, which also exhibit similar fatty acid spectra. The phosphorylated glycolipid glycerophosphoryldiglucosyl diglyceride, however, always maintained a unique fatty acid composition quite different from that of the diglucosyl diglyceride from which it is presumably derived. These characteristic differences in fatty acid composition appear to function to minimize differences in phase transition temperatures, thus producing a more physicochemically homogeneous mixture of membrane lipids than would result from a nonspecific incorporation of fatty acids.

A confirmation of the phase behavior of Escherichia coli cytoplasmic membrane lipids by X-ray diffraction

The lipid fatty acid composition of the cytoplasmic membranes of Escherichia coli can be varied by growing an unsaturated fatty acid auxotroph in the presence of different fatty acid supplements. Electron spin resonance (ESR) studies of spin-label partitioning into the cytoplasmic membranes of different lipid fatty acid compositions as a function of temperature have been interpreted as indicating a broad order-to-disorder transition in the membrane lipids, the end points of the transition depending upon the fatty acid composition. We have utilized x-ray diffraction to confirm the ESR studies for three different fatty acid supplements (oleic, elaidic, and bromostearic). We found that the characteristic end-point temperatures detected by ESR were indeed the end-point temperatures of a broad order-to-disorder transition of the cytoplasmic membrane lipids. In addition, Patterson functions calculated from lamellar x-ray diffraction from partially oriented cytoplasmic membranes indicate a decrease in average membrane thickness upon fatty acid chain melting.

Genetic and physiological regulation of intrinsic proteins of the outer membrane of Salmonella typhimurium

Four major outer membrane polypeptides, accounting for approximately 20% of the total protein of the outer membrane of Salmonella typhimurium, were induced by growth in minimal medium. The polypeptides were tightly bound membrane components. Physiological and genetic evidence indicates that the four polypeptides fall in two separate regulation groups. Synthesis of one of these groups was coordinately regulated by the concentration of iron in the medium, and a mutant strain has been identified in which there is constitutive synthesis of this group of major outer membrane proteins.

Proton-enhanced 13C nuclear magnetic resonance of lipids and biomembranes

A recently developed nuclear double resonance technique which permits sensitive detection, together with high resolution, of rare spins in solids or other dipolar-coupled nuclear systems [Pines, Gibby, and Waugh (1973) J. Chem. Phys. 59, 569] has been applied to the study of natural abundance (13)C-nuclear magnetic resonance in lipid mesophases and of selectively labeled carbon sites in bacterial membranes.Detailed microscopic information on the molecular organization and phase transitions of the lipid phases and their interaction with ions and other molecules can be obtained from the study of the chemical shift anisotropies and dynamical aspects of the (13)C NMR spectra of unsonicated lipid dispersions (liposomes). Experiments are reported which demonstrated the feasibility of quantitatively observing the (13)C-nuclear magnetic resonance of specifically labeled sites in unperturbed Escherichia coli membrane vesicles for the study of the physical state of the lipids with the aim of relating it to the known lipid-dependent functional properties of the membranes.

Attractant-directed motility in Escherichia coli: requirement for a fluid lipid phase

Attractant-directed motility (chemotaxis) in Escherichia coli has an absolute requirement for a fluid membrane. No such requirement was detected for motility per se.

Lateral phase separations in membrane lipids and the mechanism of sugar transport in Escherichia coli

Changes in slope of Arrhenius plots for transport can, in some instances, be detected at two different temperatures for cells that have a relatively simple fatty-acid composition in the membrane lipids. These characteristic temperatures correlate with the characteristic temperatures that define changes of state in membrane phospholipids as revealed by the paramagnetic resonance of the spin label TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl). The higher of these characteristic temperatures is that at which the formation of solid patches of membrane lipids is first detected. The lower is the end point of the course of lateral phase separations, at which all the membrane lipids are in a solid phase. For cells enriched for elaidic acid, the rate of transport increase by as much as 2-fold as the temperature is decreased by less than 1 degrees , at the higher characteristic temperature. At this characteristic temperature, lateral phase separations begin in the membrane phospholipids. This is also the temperature where one predicts a striking increase in the lateral compressibility of the membrane lipids. These data are thus interpreted to indicate that a component of the transport system vertically penetrates one or both monolayer faces of the membrane during transport, or that some other event involving the lateral compression of the phospholipids is important for transport.

Turnover of phospholipids in an unsaturated fatty acid auxotroph of Escherichia coli

The membrane phospholipids of an unsaturated fatty acid auxotroph of Escherichia coli were found to undergo turnover. These phospholipids were excreted into the culture medium, and were replaced in the cell with newly synthesized phospholipids. Phospholipids of growing cells supplemented with elaidic acid underwent rapid turnover, while those of cells supplemented with oleate, or cis-vaccenate plus palmitoleate, underwent slow turnover. Starvation for required amino acids stimulated this turnover in the latter two cases. Protein was also lost from growing cells. However, after amino acid starvation this loss ceased while phospholipid turnover continued. Electron micrographs of growing cells indicated that large pieces of membrane-like material were separating from the cell surface.

Rapid lateral diffusion of phospholipids in rabbit sarcoplasmic reticulum

Phospholipid spin labels incorporated in the sarcoplasmic reticulum from rabbit-skeletal muscle undergo rapid lateral diffusion within the plane of the membrane. The diffusion constant, D, is 6x10(-8) cm(2)/sec at 37 degrees . With this diffusion constant, a phospholipid molecule can diffuse a distance of the order of 5000 nm in 1 sec.