Lipids of Bacteria Living in Extreme Environments

Toward Detecting Biosignatures of DNA, Lipids, and Metabolic Intermediates from Bacteria in Ice Grains Emitted by Enceladus and Europa

The reliable identification of biosignatures is key to the search for life elsewhere. On ocean worlds like Enceladus or Europa, this can be achieved by impact ionization mass spectrometers, such as the SUrface Dust Analyzer (SUDA) on board NASA's upcoming Europa Clipper mission. During spacecraft flybys, these instruments can sample ice grains formed from subsurface water and emitted by these moons. Previous laboratory analog experiments have demonstrated that SUDA-type instruments could identify amino acids, fatty acids, and peptides in ice grains and discriminate between their abiotic and biotic origins. Here, we report experiments simulating impact ionization mass spectra of ice grains containing DNA, lipids, and metabolic intermediates extracted from two bacterial cultures: Escherichia coli and Sphingopyxis alaskensis. Salty Enceladan or Europan ocean waters were simulated using matrices with different NaCl concentrations. Characteristic mass spectral signals, such as DNA nucleobases, are clearly identifiable at part-per-million-level concentrations. Mass spectra of all substances exhibit unambiguous biogenic patterns, which in some cases show significant differences between the two bacterial species. Sensitivity to the biosignatures decreases with increasing matrix salinity. The experimental parameters indicate that future impact ionization mass spectrometers will be most sensitive to the investigated biosignatures for ice grain encounter speeds of 4-6 km/s.

Lipid-dependent gating of a voltage-gated potassium channel

Recent studies hypothesized that phospholipids stabilize two voltage-sensing arginine residues of certain voltage-gated potassium channels in activated conformations. It remains unclear how lipids directly affect these channels. Here, by examining the conformations of the KvAP in different lipids, we showed that without voltage change, the voltage-sensor domains switched from the activated to the resting state when their surrounding lipids were changed from phospholipids to nonphospholipids. Such lipid-determined conformational change was coupled to the ion-conducting pore, suggesting that parallel to voltage gating, the channel is gated by its annular lipids. Our measurements recognized that the energetic cost of lipid-dependent gating approaches that of voltage gating, but kinetically it appears much slower. Our data support that a channel and its surrounding lipids together constitute a functional unit, and natural nonphospholipids such as cholesterol should exert strong effects on voltage-gated channels. Our first observation of lipid-dependent gating may have general implications to other membrane proteins.

Lipid phosphodiesters affect the conformation of certain potassium channels, but the details of the lipid-channel interactions are unclear. Here, the KvAP channel is found to switch from an active to a resting state when the channels are transferred from a phospholipid membrane to a bilayer lacking phosphodiesters.

Long-chain diols: a new class of membrane lipids from a thermophilic bacterium

Glycerol-derived membrane lipids are essentially absent in the thermophilic bacterium Thermomicrobium roseum. A series of straight chain and internally methyl-branched 1,2-diols of carbon numbers C(18) to C(23) were found to replace glycerolipids in this bacterium. Fatty acids were present but were ester-linked to the diols or amide-linked to polar heads groups and not to glycerol. This thermophile has evolved the integration of diols as a novel approach for the construction of its cytoplasmic membrane.

Morphological manipulation of bolaamphiphilic polydiacetylene assemblies by controlled lipid doping

Morphological transformations of bolaamphiphilic polydiacetylene (L-Glu-Bis-3) lipid assemblies from helical ribbons to vesicles and flat sheets through controlled doping are described, and the role of specific lipid dopants in these processes is discussed. Upon doping with cell surface receptor G(M1) ganglioside, fluid vesicular structures start to emerge, coexisting with the micro-crystalline helical ribbons. The vesicle formation is further facilitated and stabilized by the introduction of cholesterol into the system, presumably through surface curvature variation induced by inhomogeneous distribution and dynamic clustering of G(M1) and cholesterol within the doped assemblies. Extended helical ribbons are "truncated" into patches of flat sheets when a sufficient amount of Bis-1, a structurally compatible symmetric bolaamphiphilic diacetylene lipid, is doped. The results reaffirm the important roles of packing geometry and headgroup chirality in the formation of extended helical ribbon structures. The doped assemblies of bolaamphiphiles allow for capture of intermediate structures of morphological transformation using transmission electron microscopy (TEM). A vesicle-to-ribbon transformation mechanism via lateral reorganization within relatively fluid vesicular microstructures has been suggested. Understanding of the doping-induced transformation process provides useful information for the design of advanced materials where the microscopic morphology of material is crucial to its function.

Biomass measurement of methane forming bacteria in environmental samples

Methane-forming bacteria contain unusual phytanylglycerol ether phospholipids which can be extracted from the bacteria in sediments and assayed quantitatively by high performance liquid chromatography (HPLC). In this procedure the lipids were extracted, the phospholipids recovered, hydrolyzed, purified by thin layer chromatography, derivatized and assayed by HPLC. Ether lipids were recovered quantitatively from Methanobacterium thermoautotrophicum and sediments at levels as low as 8 x 10(-14) moles. In freshwater and marine sediments the flux of methane to the atmosphere and the methane levels in the pore water reflects the recovery of the phytanyl glycerol ether lipid 'signature'. The proportion of the ether phospholipid to the total recoverable phospholipid was highest in anaerobic digester sewage sludge and deeper subsurface freshwater sediment horizons.

Amino acid transport in the thermophilic anaerobe Clostridium fervidus is driven by an electrochemical sodium gradient

Amino acid transport was studied in membranes of the peptidolytic, thermophilic, anaerobic bacterium Clostridium fervidus. Uptake of the negatively charged amino acid L-glutamate, the neutral amino acid L-serine, and the positively charged amino acid L-arginine was examined in membrane vesicles fused with cytochrome c-containing liposomes. Artificial ion diffusion gradients were also applied to establish the specific driving forces for the individual amino acid transport systems. Each amino acid was driven by the delta psi and delta mu Na+/F and not by the Z delta pH. The Na+ stoichiometry was estimated from the amino acid-dependent 22Na+ efflux and Na(+)-dependent 3H-amino acid efflux. Serine and arginine were symported with 1 Na+ and glutamate with 2 Na+. C. fervidus membranes contain Na+/Na+ exchange activity, but Na+/H+ exchange activity could not be demonstrated.

Cardiolipins and biomembrane function

Evidence is discussed for roles of cardiolipins in oxidative phosphorylation mechanisms that regulate State 4 respiration by returning ejected protons across and over bacterial and mitochondrial membrane phospholipids, and that regulate State 3 respiration through the relative contributions of proteins that transport protons, electrons and/or metabolites. The barrier properties of phospholipid bilayers support and regulate the slow proton leak that is the basis for State 4 respiration. Proton permeability is in the range 10(-3)-10(-4) cm s-1 in mitochondria and in protein-free membranes formed from extracted mitochondrial phospholipids or from stable synthetic phosphatidylcholines or phosphatidylethanolamines. The roles of cardiolipins in proton conductance in model phospholipid membrane systems need to be assessed in view of new findings by Hübner et al. [313]: saturated cardiolipins form bilayers whilst natural highly unsaturated cardiolipins form nonlamellar phases. Mitochondrial cardiolipins apparently participate in bilayers formed by phosphatidylcholines and phosphatidylethanolamines. It is not yet clear if cardiolipins themselves conduct protons back across the membrane according to their degree of fatty acyl saturation, and/or modulate proton conductance by phosphatidylcholines and phosphatidylethanolamines. Mitochondrial cardiolipins, especially those with high 18:2 acyl contents, strongly bind many carrier and enzyme proteins that are involved in oxidative phosphorylation, some of which contribute to regulation of State 3 respiration. The role of cardiolipins in biomembrane protein function has been examined by measuring retained phospholipids and phospholipid binding in purified proteins, and by reconstituting delipidated proteins. The reconstitution criterion for the significance of cardiolipin-protein interactions has been catalytical activity; proton-pumping and multiprotein interactions have yet to be correlated. Some proteins, e.g., cytochrome c oxidase are catalytically active when dimyristoylphosphatidylcholine replaces retained cardiolipins. Cardiolipin-protein interactions orient membrane proteins, matrix proteins, and on the outerface receptors, enzymes, and some leader peptides for import; activate enzymes or keep them inactive unless the inner membrane is disrupted; and modulate formation of nonbilayer HII-phases. The capacity of the proton-exchanging uncoupling protein to accelerate thermogenic respiration in brown adipose tissue mitochondria of cold-adapted animals is not apparently affected by the increased cardiolipin unsaturation; this protein seems to take over the protonophoric role of cardiolipins in other mitochondria. Many in vivo influences that affect proton leakage and carrier rates selectively alter cardiolipins in amount per mitochondrial phospholipids, in fatty acyl composition and perhaps in sidedness; other mitochondrial membrane phospholipids respond less or not at all.(ABSTRACT TRUNCATED AT 400 WORDS)

Thermal analysis of bacteria by differential scanning calorimetry: relationship of protein denaturation in situ to maximum growth temperature

Differential scanning calorimetry (DSC) was used to analyze thermal transitions in two strains of the thermophile Bacillus stearothermophilus (ATCC 12016 and WAT), the mesophile Bacillus megaterium and the psychrotroph Bacillus psychrophilus. The observed transitions, representing lipid melting and DNA and protein unfolding, are compared to the maximum growth temperature (Tmax) in each species as a means of identifying critical, thermolabile targets responsible for heat-induced inhibition of growth. A low temperature, lipid transition was detected in B. stearothermophilus and B. megaterium which varied slightly with Tmax but whose high temperature end is always 22-33 degrees C below Tmax. The transition temperature (Tm) of the main melting of DNA varies from 88 to 92 degrees C, 23-32 degrees C above Tmax. The main part of the profile representing irreversible transitions is resolvable into at least three distinct peaks and is identified primarily with protein denaturation. The onset temperature for denaturation (Tl), i.e., minimum temperature of detectable denaturation, is somewhat dependent on growth temperature (Tg). Tmax for B. stearothermophilus ATCC and WAT is 69 and 56 degrees C, respectively. For cells grown between 4 and 20 degrees C below Tmax, Tl is 2-4 degrees C lower than Tmax, demonstrating that some denaturation can be tolerated before complete inhibition of growth and suggesting that inhibition of growth is due to the denaturation of a critical protein with a Tm a few degrees above Tl or to the accumulation of denatured protein to a critical level. A similar pattern holds for B. megaterium and B. psychrophilus, except that Tmax is 48 and 32.5 degrees C (Tl = 45-46 degrees C and 30 degrees C), respectively. Thus, there is an excellent correlation between the onset of protein denaturation and maximum growth temperature for these three species of the same genus. This study also demonstrates the applicability of DSC for resolving transitions in intact cells on the basis of thermostability of cellular constituents and for obtaining an overall view of macromolecular stability.

Stereostructure of the archaebacterial C40 diol

The stereostructure of the archaebacterial C40 diol has been established as (3R,7R,11R,15S,18S,22R,26R,30R)-3,7,11,15,18,22,26,30- octamethyldotriacontane-1,32-diol by stereorational total synthesis. This provides the final evidence necessary to establish the structure of an archaebacterial membrane substance that is a 72-membered-ring tetraether with 18 stereocenters.

Structure and polymorphism of bipolar isopranyl ether lipids from archaebacteria

We describe in this work the structure and polymorphism of a variety of lipids extracted from Sulfolobus solfataricus, an extreme thermoacidophilic archaebacterium growing at about 85 degrees C and pH 2. These lipids are quite different from the usual fatty acid lipids of eukaryotes and prokaryotes: each molecule consists of two C40 omega-omega' biphytanyl residues (with 0 to 4 cyclopentane groups per residue), ether linked at both ends to two (variably substituted) glycerol or nonitol groups. Four lipid preparations were studied; the total and the polar lipid extracts, and two hydrolytic fractions, the symmetric glycerol dialkyl glycerol tetraether and the asymmetric glycerol dialkyl nonitol tetraether, as a function of water content and temperature, using X-ray scattering techniques. The main conclusions from the study of the four lipid preparations can be summarized as follows. (1) As with other lipids, a remarkable number and variety of phases are observed over a temperature-concentration range close to "physiological" conditions. The possibility is discussed that this polymorphism reflects a fundamental property of lipids, closely related to their physiological rôle. (2) As in other lipids, two types of chain conformations are observed: a disordered one (type alpha) at high temperature; at lower temperature, a more ordered packing of stiff chains, all parallel to each other (type beta'). At temperatures and degrees of hydration approaching the conditions prevailing in the living cell, the conformation is of type alpha. (3) In all the phases with chains in the alpha conformation, the unsubstituted glycerol headgroups, whose concentration is high in these lipids, segregate in the hydrocarbon matrix, away from the other polar groups. This property may have interesting biological consequences: for example, the chains of a fraction of the bipolar lipid molecules can span hydrocarbon gaps as wide as 75 A. (4) Two cubic phases are observed in the total and the polar lipid extracts, which display a remarkable degree of metastability, most unusual in lipid phase transitions involving structures with chains in the alpha conformation. This phenomenon can be explained by the interplay of the physical structure of the cubic phases (the two contain two intertwined and unconnected three-dimensional networks of rods) and the chemical structure of the lipid molecules: the two headgroups of most molecules being anchored on each of the two networks of rods, the migration of the lipid molecules is hindered by the two independent diffusion processes and by the entanglement of the chains.(ABSTRACT TRUNCATED AT 400 WORDS)