Novel, acid-labile, hydroxydiether lipid cores in methanogenic bacteria

Simultaneous total analysis of core and polar membrane lipids in archaea by high-performance liquid chromatography/high-resolution mass spectrometry coupled with heated electrospray ionization

RATIONALE

Archaea have characteristic membrane lipids including diether and/or tetraether isoprenoidal core lipids with various polar head groups. Since the polar group is removed soon after the end of archaeal activity, the occurrences of core and polar lipids are regarded as dead and active signals, respectively. The core and polar lipids have generally been analyzed separately using atmospheric pressure chemical ionization (APCI) and electrospray ionization (ESI), respectively, coupled with mass spectrometry.

METHODS

In this study, simultaneous analyses of core and polar archaeal lipids have been examined using heated electrospray ionization (HESI) by high-performance liquid chromatography/high-resolution mass spectrometry (HPLC/HRMS).

RESULTS

Both core and intact polar lipids can be analyzed simultaneously by HESI with good sensitivity (sub ng to 100 ng) and separation using a semi-bore diol column by normal-phase chromatography. The core lipids eluted firstly to separate archeaol, then glycerol dibiphytanyl glycerol tetraethers (GDGTs), followed by the polar lipids with glycosides and glycophosphates. The relative GDGT composition is identical between HESI and APCI methods.

CONCLUSIONS

The simultaneous analysis has the benefit of minimizing sample amount and elution solvent as well as preparation work. The method can also be applied to a compound class fractionation for compound-specific carbon and hydrogen isotope analysis.

Homeoviscous Adaptation of Membranes in Archaea

Because membranes play a central role in regulating fluxes inward and outward from the cells, maintaining the appropriate structure of the membrane is crucial to maintain cellular integrity and functions. Microbes often face contrasted and fluctuating environmental conditions, to which they need to adapt or die. Membrane adaptation is achieved by a modification of the membrane lipid composition, a strategy termed homeoviscous adaptation. Homeoviscous adaptation in archaea involves strategies similar to that observed in bacteria and eucarya, such as the regulation of lipid chain length or saturation levels, as well as strategies specific to archaea, such as the regulation of the number of cycles along the isoprenoid chains or the regulation of the ratio between mono and bipolar lipids. Although not described yet described in hyperthermophilic bacteria, it is possible that these two strategies also apply to these latter organisms.

Deciphering archaeal glycolipids of an extremely halophilic archaeon of the genus Halobellus by MALDI-TOF/MS

Polar membrane lipids of an archaeal microorganism recently isolated from the natural salt lake Fuente de Piedra (Málaga, Spain) have been studied by means of TLC in combination with MALDI-TOF mass spectrometry. The major phospholipids are the ether lipids phosphatidylglycerophosphate methyl ester and phosphatidylglycerosulfate, while phosphatidylglycerol is barely detectable; in addition the bisphosphatidylglycerol (archaeal cardiolipin) has been detected for the first time in a representative of the genus Halobellus. The structures of glycolipids, including a glycosyl-cardiolipin, have been elucidated by post source decay (PSD) mass spectrometry analysis. Besides the monosulfated diglycosyl diphytanylglyceroldiether, two variants of a bis-sulfated diglycosyl diphytanylglyceroldiether have been identified; furthermore the glycosyl-cardiolipin is found to have the same structure of the analogue present in Halorubrum trapanicum and Haloferax volcanii. The role of the abundant sulfated glycolipids in facing high extracellular salinity is discussed.

Archaeal lipids and their biotechnological applications

The lipids of Archaea, based on glycerol isopranoid ethers, can be used taxonomically to distinguish between phenotypic subgroups of the domain to delineate them clearly from all other organisms. This review is a general survey of the structural features of archaeal lipids and how they relate to survival in the harsh environments in which the Archaea live. The molecular organization of archaeal lipids in monolayers, artificial black membranes and vesicles and the unique properties and possible biotechnological applications of liposomes of the lipids are presented. The results with these liposomes are compared with similar data obtained with synthetic compounds which mimic the structure of archaeal lipids. Studies on computer simulation are also reported.

Record of archaeal activity at the serpentinite-hosted Lost City Hydrothermal Field

Samples of young, outer surfaces of brucite-carbonate deposits from the ultramafic-hosted Lost City hydrothermal field were analyzed for DNA and lipid biomarker distributions and for carbon and hydrogen stable isotope compositions of the lipids. Methane-cycling archaeal communities, notably the Lost City Methanosarcinales (LCMS) phylotype, are specifically addressed. Lost City is unlike all other hydrothermal systems known to date and is characterized by metal- and CO2 -poor, high pH fluids with high H2 and CH4 contents resulting from serpentinization processes at depth. The archaeal fraction of the microbial community varies widely within the Lost City chimneys, from 1-81% and covaries with concentrations of hydrogen within the fluids. Archaeal lipids include isoprenoid glycerol di- and tetraethers and C25 and C30 isoprenoid hydrocarbons (pentamethylicosane derivatives - PMIs - and squalenoids). In particular, unsaturated PMIs and squalenoids, attributed to the LCMS archaea, were identified for the first time in the carbonate deposits at Lost City and probably record processes exclusively occurring at the surface of the chimneys. The carbon isotope compositions of PMIs and squalenoids are remarkably heterogeneous across samples and show highly (13) C-enriched signatures reaching δ(13) C values of up to +24.6‰. Unlike other environments in which similar structural and isotopic lipid heterogeneity has been observed and attributed to diversity in the archaeal assemblage, the lipids here appear to be synthesized solely by the LCMS. Some of the variations in lipid isotope signatures may, in part, be due to unusual isotopic fractionation during biosynthesis under extreme conditions. However, we argue that the diversity in archaeal abundances, lipid structure and carbon isotope composition rather reflects the ability of the LCMS archaeal biofilms to adapt to chemical gradients in the hydrothermal chimneys and possibly to perform either methanotrophy or methanogenesis using dissolved inorganic carbon, methane or formate as a function of the prevailing environmental conditions.

Adaptation of the membrane in Archaea

Microbes often face contrasted and fluctuating environmental conditions, to which they need to adapt or die. Because membranes play a central role in regulating fluxes inward and outward from the cells, maintaining the appropriate structure of the membrane is crucial to maintain cellular integrity and functions. This is achieved in bacteria and eucarya by a modification of the membrane lipid compositions, a strategy termed homeoviscous adaptation. We review here evidence for homeoviscous adaptation in Archaea, and discuss the limits of this strategy and our knowledge in this very peculiar domain of life.

Notes on the characterization of prokaryote strains for taxonomic purposes

Taxonomy relies on three key elements: characterization, classification and nomenclature. All three elements are dynamic fields, but each step depends on the one which precedes it. Thus, the nomenclature of a group of organisms depends on the way they are classified, and the classification (among other elements) depends on the information gathered as a result of characterization. While nomenclature is governed by the Bacteriological Code, the classification and characterization of prokaryotes is an area that is not formally regulated and one in which numerous changes have taken place in the last 50 years. The purpose of the present article is to outline the key elements in the way that prokaryotes are characterized, with a view to providing an overview of some of the pitfalls commonly encountered in taxonomic papers.

Archaeosomes as Self-adjuvanting Delivery Systems for Cancer Vaccines*

Archaeal ether glycerolipid vesicles (archaeosomes) efficiently deliver exogenous antigen for induction of humoral and cell-mediated immunity. Because induction of CD8 cytotoxic T cells is critical for protective vaccination against tumors, we compared the ability of various archaeosome lipid compositions to evoke a strong CD8 CTL response to entrapped antigen. Subcutaneous immunization of mice with ovalbumin (OVA) entrapped in all archaeosome lipid compositions evoked a primary (day 10) splenic CTL response indicating processing for MHC class I presentation. Interestingly, several polar lipid compositions from halophilic archaea were very potent to adjuvant this early CTL response. Despite this, the lytic units reduced substantially by weeks 6-7. More importantly, at >50 weeks, only Methanobrevibacter smithii and Thermoplasma acidophilum both rich in bipolar membrane-spanning caldarchaeols, demonstrated recall memory CTLs. Immunization of mice with OVA entrapped in M. smithii, Halobacterium salinarum or T. acidophilum vesicles provided prophylactic protection against challenge with OVA-expressing solid tumors at 6 weeks. Even a dose of 3 microg OVA in archaeosomes significantly delayed tumor growth. Tumor protection was also noted in a therapeutic design wherein OVA-archaeosomes were injected concurrent with the tumor challenge. Interestingly, antigen-free T. acidophilum but not antigen-free H. salinarum archaeosomes provided innate therapeutic protection. Vaccination with a CTL peptide epitope from the melanoma differentiation antigen, tyrosinase-related protein 2, in archaeosomes induced a protective CD8 response against B16OVA metastasis, indicating potential for targeting self, tumor antigens. Thus, lipid structural properties of archaea may differentially modulate primary, long-term and/or innate immunity, impacting adjuvant choice for vaccine design.

Lipid biomarker and phylogenetic analyses to reveal archaeal biodiversity and distribution in hypersaline microbial mat and underlying sediment

This study has utilized the tools of lipid biomarker chemistry and molecular phylogenetic analyses to assess the archaeal contribution to diversity and abundance within a microbial mat and underlying sediment from a hypersaline lagoon in Baja California. Based on abundance of ether-linked isoprenoids, archaea made up from 1 to 4% of the cell numbers throughout the upper 100 mm of mat and sediment core. Below this depth archaeal lipid was two times more abundant than bacterial. Archaeol was the primary archaeal lipid in all layers. Relatively small amounts of caldarchaeol (dibiphytanyl glyceroltetraether) were present at most depths with phytanyl to biphytanyl molar ratios lowest (approximately 10 : 1) in the 4-17 mm and 100-130 mm horizons, and highest (132 : 1) in the surface 0-2 mm. Lipids with cyclic biphytanyl cores were only detected below 100 mm. A novel polar lipid containing a C(30) isoprenoid (squalane) moiety was isolated from the upper anoxic portion of the core and partially characterized. Hydrocarbon biomarker lipids included pentamethylicosane (2-10 mm) and crocetane (primarily below 10 mm). Archaeal molecular diversity varied somewhat with depth. With the exception of samples at 0-2 mm and 35-65 mm, Thermoplasmatales of marine benthic group D dominated clone libraries. A significant number of phylotypes representing the Crenarchaeota from marine benthic group B were generally present below 17 mm and dominated the 35-65 mm sample. Halobacteriaceae family made up 80% of the clone library of the surface 2 mm, and consisted primarily of sequences affiliated with the haloalkaliphilic Natronomonas pharaonis.

Cold adaptation in the Antarctic Archaeon Methanococcoides burtonii involves membrane lipid unsaturation

Direct analysis of membrane lipids by liquid chromatography-electrospray mass spectrometry was used to demonstrate the role of unsaturation in ether lipids in the adaptation of Methanococcoides burtonii to low temperature. A proteomics approach using two-dimensional liquid chromatography-mass spectrometry was used to identify enzymes involved in lipid biosynthesis, and a pathway for lipid biosynthesis was reconstructed from the M. burtonii draft genome sequence. The major phospholipids were archaeol phosphatidylglycerol, archaeol phosphatidylinositol, hydroxyarchaeol phosphatidylglycerol, and hydroxyarchaeol phosphatidylinositol. All phospholipid classes contained a series of unsaturated analogues, with the degree of unsaturation dependent on phospholipid class. The proportion of unsaturated lipids from cells grown at 4 degrees C was significantly higher than for cells grown at 23 degrees C. 3-Hydroxy-3-methylglutaryl coenzyme A synthase, farnesyl diphosphate synthase, and geranylgeranyl diphosphate synthase were identified in the expressed proteome, and most genes involved in the mevalonate pathway and processes leading to the formation of phosphatidylinositol and phosphatidylglycerol were identified in the genome sequence. In addition, M. burtonii encodes CDP-inositol and CDP-glycerol transferases and a number of homologs of the plant geranylgeranyl reductase. It therefore appears that the unsaturation of lipids may be due to incomplete reduction of an archaeol precursor rather than to a desaturase mechanism. This study shows that cold adaptation in M. burtonii involves specific changes in membrane lipid unsaturation. It also demonstrates that global methods of analysis for lipids and proteomics linked to a draft genome sequence can be effectively combined to infer specific mechanisms of key biological processes.

Tetraether-linked membrane monolayers in Ferroplasma spp: a key to survival in acid

Ferroplasma acidarmanus thrives in hot, extremely low pH, metal-rich solutions associated with dissolving metal sulfide ore deposits. Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry and thin layer chromatography analyses of F. acidarmanus membranes indicate that tetraether lipids predominate, with at least three core lipid structures. NMR measurements indicate that the cytoplasmic pH of F. acidarmanus is approximately 5.6. The optimal growth pH is approximately 1.2, and the lowest growth pH is approximately 0.0. Thus, these organisms maintain pH gradients across their membranes that approach 5 pH units. Tetraether lipids were originally thought to be specifically associated with thermophiles but are now known to be widely distributed within the archaeal domain. Our data, in combination with recently published results for thermophilic and mesothermophilic acidophilic archaea, indicate that there may be a stronger association between tetraether lipids and tolerance to acid and/or large metal ion gradients.

Archaeobacterial ether lipid liposomes (archaeosomes) as novel vaccine and drug delivery systems

Liposomes are artificial, spherical, closed vesicles consisting of one or more lipid bilayer(s). Liposomes made from ester phospholipids have been studied extensively over the last 3 decades as artificial membrane models. Considerable interest has been generated for applications of liposomes in medicine, including their use as diagnostic reagents, as carrier vehicles in vaccine formulations, or as delivery systems for drugs, genes, or cancer imaging agents. The objective of this article is to review the properties and potential applications of novel liposomes made from the membrane lipids of Archaeobacteria (Archaea). These lipids are unique and distinct from those encountered in Eukarya and Bacteria. Polar glycerolipids make up the bulk of the membrane lipids, with the remaining neutral lipids being primarily squalenes and other hydrocarbons. The polar lipids consist of regularly branched, and usually fully saturated, phytanyl chains of 20, 25, or 40 carbon length, with the 20 and 40 being most common. The phytanyl chains are attached via ether bonds to the sn-2,3 carbons of the glycerol backbone(s). It has been shown only recently that total polar lipids of archaeobacteria, and purified lipid fractions therefrom, can form liposomes. We refer to liposomes made with any lipid composition that includes ether lipids characteristic of Archaeobacteria as archaeosomes to distinguish them from vesicles made from the conventional lipids obtained from eukaryotic or eubacterial sources or their synthetic analogs. In general, archaeosomes demonstrate relatively higher stabilities to oxidative stress, high temperature, alkaline pH, action of phospholipases, bile salts, and serum proteins. Some archaeosome formulations can be sterilized by autoclaving, without problems such as fusion or aggregation of the vesicles. The uptake of archaeosomes by phagocytic cells can be up to 50-fold greater than that of conventional liposome formulations. Studies in mice have indicated that systemic administration of several test antigens entrapped within certain archaeosome compositions give humoral immune responses that are comparable to those obtained with the potent but toxic Freund's adjuvant. Archaeosome compositions can be selected to give a prolonged, sustained immune response, and the generation of a memory response. Tissue distribution studies of archaeosomes administered via various systemic and peroral routes indicate potential for targeting to specific organs. All in vitro and in vivo studies performed to date indicate that archaeosomes are safe and do not invoke any noticeable toxicity in mice. The stability, tissue distribution profiles, and adjuvant activity of archaeosome formulations indicate that they may offer a superior alternative to the use of conventional liposomes, at least for some biotechnology applications.

A structural comparison of the total polar lipids from the human archaea Methanobrevibacter smithii and Methanosphaera stadtmanae and its relevance to the adjuvant activities of their liposomes

Mice were immunized with bovine serum albumin (BSA) entrapped within archaeosomes (i.e. liposomes) composed of the total polar lipids (TPL) from the two methanogenic archaea common to the human digestive tract. Methanobrevibacter smithii archaeosomes boosted serum anti-BSA antibody to titers comparable to those achieved with Freund's adjuvant, whereas Methanosphaera stadtmanae archaeosomes were relatively poor adjuvants. An explanation for this difference was sought by analysis of the polar lipid composition of each archaeobacterium. Fast atom bombardment mass spectrometry and NMR analyses of the purified lipids revealed a remarkable similarity in the ether lipid structures present in each TPL extract. However, the relative amounts of each lipid species varied dramatically. The phospholipid fraction in M. stadtmanae TPL was dominated by archaetidylinositol (50 mol% of TPL) and the glycolipid fraction by beta-Glcp-(1,6)-beta-Glcp-(1,1)-archaeol (36 mol%), whereas in M. smithii extracts, both caldarchaeol and archaeol lipids containing a phosphoserine head group were relatively abundant. Liposomes prepared from purified archaetidylinositol and from M. stadtmanae TPL supplemented with increasing amounts of phosphatidylserine elicited poor humoral responses to encapsulated BSA. A dramatic loss in the adjuvanticity of M. smithii archaeosomes was seen upon incorporation of 36 mol% of the uncharged lipid diglucosyl archaeol and, to a lesser extent, of 50 mol% of archaetidylinositol. Interestingly, the relative rates of uptake of M. smithii and M. stadtmanae archaeosomes by phagocytic cultures in vitro were similar. Thus, the lipid composition may influence archaeosome adjuvanticity, particularly a high diglucosyl archaeol and/or archaetidyl inositol content, resulting in a low adjuvant activity.

Dibiphytanyl ether lipids in nonthermophilic crenarchaeotes

The kingdom Crenarchaeota is now known to include archaea which inhabit a wide variety of low-temperature environments. We report here lipid analyses of nonthermophilic crenarchaeotes, which revealed the presence of cyclic and acyclic dibiphytanylglycerol tetraether lipids. Nonthermophilic crenarchaeotes appear to be a major biological source of tetraether lipids in marine planktonic environments.

A novel ether core lipid with H-shaped C80-isoprenoid hydrocarbon chain from the hyperthermophilic methanogen Methanothermus fervidus

A new ether lipid core (designated as FU) was found in Methanothermus fervidus total lipid. Comparison with caldarchaeol showed lower mobility of FU on TLC and smaller molecular weight (m/z 1298) by 2 mass units on FAB-MS. Treatment of FU with HI followed by displacement with silver acetate afforded long chain alcohol acetate (ROAc), which was further saponified with mild alkali to its free alcohol (ROH). ROH is the long chain alcohol prepared from FU. The molecular weights of ROAc and ROH were shown by MS to be 1354 and 1186, respectively. These results suggested that the molecular formula of ROH was C80H162O4, and ROH had four hydroxyl groups, and one molecule of ROH was bound with two molecules of glycerol by four ether linkages. Because FU was not oxidized by NaIO4 and specific rotation [alpha]D of FU coincided with that of caldarchaeol, it seems that the ether linkages of FU are formed with hydroxyl groups of the sn-2 and sn-3 positions of each glycerol moiety. The structure of FU was suggested to be a modified caldarchaeol in which two hydrocarbon chains are bridged with a covalent bond. Although a few points remain to be elucidated before the final conclusion can be reached on the structure of FU due to difficulty in complete structure determination done even with every approach currently available, the most possible position of the bridge in FU hydrocarbon was proposed from the data of EI-MS of ROAc and 1H-NMR of FU. The hydrocarbon chain looks like H-shaped C80 isoprenoid.

Ether lipids of planktonic archaea in the marine water column

Acyclic and cyclic biphytanes derived from the membrane ether lipids of archaea were found in water column particulate and sedimentary organic matter from several oxic and anoxic marine environments. Compound-specific isotope analyses of the carbon skeletons suggest that planktonic archaea utilize an isotopically heavy carbon source such as algal carbohydrates and proteins or dissolved bicarbonate. Due to their high preservation potential, these lipids provide a fossil record of planktonic archaea and suggest that they have thrived in marine environments for more than 50 million years.

Structural features of ether lipids in the archaeobacterial thermophilesPyrococcus furiosus,Methanopyrus kandleri,Methanothermus fervidus, andSulfolobus acidocaldarius

The ether lipids of several thermophilic archaea (archaeobacteria) were compared by negative-ion fast atom bombardment mass spectrometry. The major polar lipids in extracts of Pyrococcus furiosus were assigned as archaeol lipids (phosphatidylglycerol diether, m/z 805; phosphatidylinositol diether, m/z 893; and diglycosyl diether, m/z 975) and caldarchaeol lipids (diglycosyl phosphatidylglycerol tetraether, m/z 1778; and diglycosyl phosphatidylinositol tetraether, m/z 1866). The polar lipids of Methanopyrus kandleri were primarily glycolipids consisting of a series of archaeol lipids with one to six hexose units, composed primarily of mannose (mannose:glucose 9:1); phospholipids consisting of archaeol lipids (phosphatidylinositol diether; and a novel phosphatidylcholine diether, m/z 802.7), and phosphoglycolipids as minor caldarchaeol lipids (primarily diglycosyl phosphatidylglycerol tetraether). Methanothermus fervidus extracts contained archaeol lipids (phosphatidylinositol diether; diglycosyl diether; and acetyldiglycosyl diether, m/z 1016), and caldarchaeol lipids (glycosyl phosphatidylinositol tetraether, m/z 1704; diglycosyl phosphatidylinositol tetraether; and acetyldiglycosyl phosphatidylinositol tetraether, m/z 1907). Acetylation of a sugar residue occurred commonly in this thermophile and increased as cells entered the stationary growth phase. Lipid extracts of Sulfolobus acidocaldarius contained detectable amounts of archaeol and hydroxyarchaeol analogs of phosphatidylinositol, phosphatidylglycerol, and phosphatidylethanolamine lipids, in addition to the dominant caldarchaeol lipids already reported. All four thermophiles contained both archaeol and caldarchaeol lipids and phosphoinositol head groups, but no single structural entity uniquely separated their lipids from those found previously in mesophilic archaea. By contrast, extremely halophilic archaea appear to be distinguished from the thermophilic archaea by the presence of a major phosphatidylglyceromethylphosphate lipid.Key words: ether lipids, mass spectrometry, hyperthermophiles, extreme halophiles, Archaea.

Two new phospholipids, hydroxyarchaetidylglycerol and hydroxyarchaetidylethanolamine, from the Archaea Methanosarcina barkeri

The structures of two new ether phospholipids of the methanogenic Archaea, Methanosarcina barkeri, were determined as hydroxyarchaetidylglycerol and hydroxyarchaetidylethanolamine by means of chemical, chromatographic and enzymatic analyses, and fast atom bombardment-mass spectrometry. These lipids are hydroxy diether analogs of phosphatidylglycerol and phosphatidylethanolamine, respectively, with beta-hydroxyarachaeol (2-O-(3'-hydroxy)phytanyl-3-O-phytanyl-sn-glycerol) as a core lipid. In addition, two other ether phospholipids, usual archaetidylglycerol and archaetidylethanolamine, were also identified in the organism. The stereochemical structure of the unalkylated glycerophosphate of hydroxyarchaetidylglycerol and archaetidylglycerol was determined as sn-glycerol-3-phosphate by use of sn-glycerol-3-phosphate dehydrogenase. The stereochemical configuration of the glycerophosphoglycerol backbone of these lipids was a mirror image of that of diacylphosphatidylglycerol from the organisms of the domains Bacteria and Eucarya, and it was shared with extremely halophilic Archaea. These four phospholipids, in addition to five lipids that had already been reported, accounted for 88% of the total polar lipids of this organism.

Tetraether lipids of Methanospirillum hungatei with head groups consisting of phospho-N,N-dimethylaminopentanetetrol, phospho-N,N,N-trimethylaminopentanetetrol, and carbohydrates

Acyclic, standard tetraether and diether lipids each account for about 50% of the total ether lipids found in Methanospirillum hungatei. Sixteen ether lipids were purified and defined according to relative weight percentage and staining reactions on thin-layer plates. Structures were elucidated for six previously uncharacterized tetraether lipids. Four of these lipids had as one head group either alpha-glcp-(1-2)-beta-gal(f)-, or beta-gal(f)-(1-6)-beta-gal(f)-, in glycosidic linkage to the first glycerol of the lipid backbone, and either a N,N-dimethyl-aminopentanetetrol or a N,N,N-trimethylaminopentanetetrol moiety in phosphodiester linkage to the second glycerol of the backbone. A fifth lipid was a tetraether structure novel in having carbohydrate moieties at both head group positions; namely alpha-glcp-(1-2)-gal(f)- and beta-gal(f)-. Two other lipids, a diether and a tetraether, had a single head group consisting of alpha-glcp-(1-2)-beta-gal(f)- modified by O-acetylation of the gal(f) residue at C-6. In addition to the seven new lipids described above, diether and tetraether analogs of phosphatidylglycerol were found.

Hydroxydiether Lipid Structures in Methanosarcina spp. and Methanococcus voltae

Hydroxylated diether lipids are the most abundant lipids in Methanosarcina acetivorans, Methanosarcina thermophila , and Methanosarcina barkeri MS and Fusaro, regardless of the substrate used for growth. Structural analysis of the lipid moiety freed of polar head groups revealed that the hydroxydiether lipids of all the Methanosarcina strains were hydroxylated at position 3 of sn -2 phytanyl chains. The finding that Methanosarcina strains synthesize the same hydroxydiether structure suggests that this is a taxonomic characteristic of the genus. Methanococcus voltae produced minor amounts of the 3-hydroxydiether characteristic of Methanosarcina spp. and also the 3′-hydroxydiether described previously for Methanosaeta concilii .

Freeze-fracture planes of methanogen membranes correlate with the content of tetraether lipids

Methanospirillum hungatei GP1 contained 50% of its ether core lipids (polar lipids less head groups) as tetraether lipids, and its plasma membrane failed to fracture along its hydrophobic domain during freeze-etching. The membrane of Methanosaeta ("Methanothrix") concilii did not contain tetraether lipids and easily fractured to reveal typical intramembranous particles. Methanococcus jannaschii grown at 50 degrees C contained 20% tetraether core lipids, which increased to 45% when cells were grown at 70 degrees C. The frequency of membrane fracture was reduced as the membrane-spanning tetraether lipids approached 45%. As the tetraether lipid content increased, and while fracture was still possible, the particle density in the membrane increased; these added particles could be tetraether lipid complexes torn from the opposing membrane face. The diether membrane (no tetraether lipid) of Methanococcus voltae easily fractured, and the intramembranous particle density was low. Protein-free liposomes containing tetraether core lipids (ca. 45%) also did not fracture, whereas those made up exclusively of diether lipids did split, indicating that tetraether lipids add considerable vertical stability to the membrane. At tetraether lipid concentrations below 45%, liposome bilayers fractured to reveal small intramembranous particles which we interpret to be tetraether lipid complexes.

Structures of archaebacterial membrane lipids

Structural data on archaebacterial lipids is presented with emphasis on the ether lipids of the methanogens. These ether lipids normally account for 80-95% of the membrane lipids with the remaining 5-20% of neutral squalenes and other isoprenoids. Genus-specific combinations of various lipid core structures found in methanogens include diether-tetraether, dietherhydroxydiether, or diether-macrocyclic diether-tetraether lipid moieties. Some species have only the standard diether core lipid, but none are known with predominantly tetraether lipids as found in certain sulfur-dependent archaebacteria. The relative proportions of these lipid cores are known to vary in relation to growth conditions in Methanococcus jannaschii and Methanobacterium thermoautotrophicum. Polar headgroups in glycosidic or phosphodiester linkage to the sn-1 or sn-1' carbons of glycerol consist of polyols, carbohydrates, and amino compounds. The available structural data indicate a close similarity among the polar lipids synthesized within the species of the same genus. Detection of lipid molecular ions by mass spectrometry of total polar lipid extracts is a promising technique to provide valuable comparative data. Since these lipid structures are stable within the extreme environments that many archaebacteria inhabit, there may be specific applications for their use in biotechnology.

Formation of unilamellar liposomes from total polar lipid extracts of methanogens

Unilamellar liposomes were formed by controlled detergent dialysis of mixed micelles consisting of acetone-insoluble total polar lipids extracted from various methanogens and the detergent n-octyl-beta-D-glucopyranoside. The final liposome populations were studied by dynamic light scattering and electron microscopy. Unilamellar liposomes with mean diameters smaller than 100 nm were obtained with lipid extracts of Methanococcus voltae, Methanosarcina mazei, Methanosaeta concilii, and Methanococcus jannaschii (grown at 50 degrees C), whereas larger (greater than 100-nm) unilamellar liposomes were obtained with lipid extracts of M. jannaschii grown at 65 degrees C. These liposomes were shown to be closed intact vesicles capable of retaining entrapped [14C]sucrose for extended periods of time. With the exception of Methanospirillum hungatei liposomes, all size distributions of the different liposome populations were fairly homogeneous.

Proportions of diether, macrocyclic diether, and tetraether lipids in Methanococcus jannaschii grown at different temperatures

Growth of Methanococcus jannaschii over a wide temperature range (47 to 75 degrees C) is correlated with an ability to alter dramatically the proportions of three ether lipid cores. These lipids shifted from predominantly diether (2,3-di-O-phytanyl-sn-glycerol) at the lower growth temperatures to macrocyclic diether and tetraether at near optimal growth temperatures. Lipid head groups varied as well, especially with respect to an increase in phosphate at the higher temperatures.

Hydroxyarchaetidylserine and hydroxyarchaetidyl-myo-inositol in Methanosarcina barkeri: polar lipids with a new ether core portion

Lipids of the methanogenic archaebacterium, Methanosarcina barkeri were analyzed. The lipid content was 5.4% of dry cell and polar lipids comprised 87% of the total lipid. Polar lipids were separated into 14 spots by two-dimensional thin-layer chromatography. These were six phospholipids, seven aminophospholipids and one glycolipid, of which two phospholipids and two aminophospholipids were major constituents. After removal of polar head groups from total lipids, two kinds of glycerol diether core lipids were found. One was 2,3-di-O-phytanyl-sn-glycerol (archaeol) and the other 2-O-(3'-hydroxy-3', 7', 11', 15'-tetramethyl)hexadecyl-3-O-phytanyl-sn- glycerol (hydroxyarchaeol). Those structures were identified on the basis of chemical analysis, fast atom bombardment spectrometry, gas-liquid chromatography-mass spectrometry and 1H- and 13C-NMR spectrometry. The latter was a new core lipid which was different from hydroxyarchaeol of Methanothrix concilii. The hydroxyarchaeol core lipid comprised 60% of polar lipid in M. barkeri. The structures of core lipids are quite different from those previously reported by De Rosa et al. (Biochim, Biophys. Acta (1986) 875, 487-492) concerning M. barkeri lipids. The structures of two major polar lipids, both of which had hydroxyarchaeol as core proteins, were elucidated. These lipids were 2-O-(3'-hydroxy)phytanyl-3-O-phytanyl-sn-glycero-1-phosphoserine (hydroxyarchaetidylserine) and 2-O-(3'-hydroxy)phytanyl-3-O-phytanyl-sn- glycerol-phospho-myo-inositol (hydroxyarchaetidyl-myo-inositol). Archaetidylserine and archaetidylinositol, which had the usual archaeol core portion, were also present as minor polar lipids.