At mammalian body temperature, the plague bacillus Yersinia pestis synthesizes lipopolysaccharide (LPS)-lipid A with poor Toll-like receptor 4 (TLR4)-stimulating activity. To address the effect of weak TLR4 stimulation on virulence, we modified Y. pestis to produce a potent TLR4-stimulating LPS. Modified Y. pestis was completely avirulent after subcutaneous infection even at high challenge doses. Resistance to disease required TLR4, the adaptor protein MyD88 and coreceptor MD-2 and was considerably enhanced by CD14 and the adaptor Mal. Both innate and adaptive responses were required for sterilizing immunity against the modified strain, and convalescent mice were protected from both subcutaneous and respiratory challenge with wild-type Y. pestis. Despite the presence of other established immune evasion mechanisms, the modified Y. pestis was unable to cause systemic disease, demonstrating that the ability to evade the LPS-induced inflammatory response is critical for Y. pestis virulence. Evading TLR4 activation by lipid A alteration may contribute to the virulence of various Gram-negative bacteria.

Francisella novicida U112 phospholipids, extracted without hydrolysis, consist mainly of phosphatidylethanolamine, phosphatidylglycerol, phosphatidylcholine, and two lipid A species, designated A1 and A2. These lipid A species, present in a ratio of 7:1, comprise 15 % of the total phospholipids, as judged by 32 P i labeling. Although lipopolysaccharide is detectable in F. novicida U112, less than 5 % of the total lipid A is covalently linked to it. A1 and A2 were analyzed by electrospray ionization and matrix-assisted laser desorption ionization mass spectrometry, gas chromatography/mass spectrometry and NMR spectroscopy. Both compounds are disaccharides of glucosamine, acylated with primary 3-hydroxystearoyl chains at positions 2, 3, and 2′, and a secondary palmitoyl residue at position 2′. Minor isobaric species and some lipid A molecules containing a 3-hydroxypalmitoyl chain in place of 3-hydroxystearate are also present. The 4′-and 3′-positions of A1 and A2 are not derivatized, and Kdo is not detectable. The 1-phosphate groups of both A1 and A2 are modified with an α-linked galactosamine residue, as shown by NMR spectroscopy and gas chromatography/mass spectrometry. An α-linked glucose moiety is attached to the 6′-position of A2. The lipid A released by mild acid hydrolysis of F. novicida lipopolysaccharide consists solely of component A1. F. novicida mutants lacking the arnT gene do not contain a galactosamine residue on their lipid A. Formation of free lipid A in F. novicida might be initiated by an unusual Kdo hydrolase present in the membranes of this organism.Lipopolysaccharide (LPS) makes up the outer leaflet of the outer membranes of most Gramnegative bacteria (1-3). It consists of a hydrophobic moiety known as lipid A, a non-repeating core oligosaccharide, and a distal repeating oligosaccharide, termed the O-antigen (1-3). Lipid A of wild-type E. coli is a hexa-acylated disaccharide of glucosamine that is phosphorylated at the 1-and 4′-positions (1-3) (Fig. 1A). It is recognized by the TLR4/MD2 receptor of the innate immune system (4-8), which triggers an inflammatory response and helps to clear localized infections. However, a more generalized response to lipid A in the context of a systemic infection, accompanied by massive over-production of cytokines, can lead to Gramnegative septic shock and death (9,10).Contact: C. R. H. Raetz,; E-mail: raetz@biochem.duke.edu. SUPPORTING INFORMATION AVAILABLE The NOESY analysis of HA2 and the HMBC spectra of HA1 and HA2 are available free of charge via the Internet at http://pubs.acs.org. NIH Public Access Author ManuscriptBiochemistry. Author manuscript; available in PMC 2008 October 17. NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author ManuscriptWild-type E. coli cells contain ∼ 0.15 lipid A residues per glycerophospholipid molecule (11). All of the lipid A residues are covalently attached to LPS, whereas free lipid A and lipid A precursors are not usually detectable by 32 P i labeling (12) or mass spectrometry (MS) (13). Heptose-deficie...

The lipid A anchor of Francisella tularensis lipopolysaccharide (LPS) lacks both phosphate groups present in Escherichia coli lipid A. Membranes of Francisella novicida (an environmental strain related to F. tularensis) contain enzymes that dephosphorylate lipid A and its precursors at the 1-and 4-positions. We now report the cloning and characterization of a membrane-bound phosphatase of F. novicida that selectively dephosphorylates the 1-position. By transferring an F. novicida genomic DNA library into E. coli and selecting for low level polymyxin resistance, we isolated FnlpxE as the structural gene for the 1-phosphatase, an inner membrane enzyme of 239 amino acid residues. Expression of FnlpxE in a heptose-deficient mutant of E. coli caused massive accumulation of a previously uncharacterized LPS molecule, identified by mass spectrometry as 1-dephospho-Kdo 2 -lipid A. The predicted periplasmic orientation of the FnLpxE active site suggested that LPS export might be required for 1-dephosphorylation of lipid A. LPS and phospholipid export depend on the activity of MsbA, an essential inner membrane ABC transporter. Expression of FnlpxE in the msbA temperature-sensitive E. coli mutant WD2 resulted in 90% 1-dephosphorylation of lipid A at the permissive temperature (30°C). However, the 1-phosphate group of newly synthesized lipid A was not cleaved at the nonpermissive temperature (44°C). Our findings provide the first direct evidence that lipid A 1-dephosphorylation catalyzed by LpxE occurs on the periplasmic surface of the inner membrane.

Francisella tularensis and related intracellular pathogens synthesize lipid A molecules that differ from their Escherichia coli counterparts. Although a functional orthologue of lpxK, the gene encoding the lipid A 4-kinase, is present in Francisella, no 4-phosphate moiety is attached to Francisella lipid A. We now demonstrate that a membrane-bound phosphatase present in Francisella novicida U112 selectively removes the 4-phosphate residue from tetra-and pentaacylated lipid A molecules. A clone that expresses the F. novicida 4-phosphatase was identified by assaying lysates of E. coli colonies, harboring members of an F. novicida genomic DNA library, for 4-phosphatase activity. Sequencing of a 2.5-kb F. novicida DNA insert from an active clone located the structural gene for the 4-phosphatase, designated lpxF. It encodes a protein of 222 amino acid residues with six predicted membrane-spanning segments. Rhizobium leguminosarum and Rhizobium etli contain functional lpxF orthologues, consistent with their lipid A structures. When F. novicida LpxF is expressed in an E. coli LpxM mutant, a strain that synthesizes pentaacylated lipid A, over 90% of the lipid A molecules are dephosphorylated at the 4-position. Expression of LpxF in wildtype E. coli has no effect, because wild-type hexaacylated lipid A is not a substrate. However, newly synthesized lipid A is not dephosphorylated in LpxM mutants by LpxF when the MsbA flippase is inactivated, indicating that LpxF faces the outer surface of the inner membrane. The availability of the lpxF gene will facilitate re-engineering lipid A structures in diverse bacteria.Francisella tularensis is an intracellular Gram-negative pathogen that causes tularemia, a severe and often fatal pulmonary infection of humans and animals (1, 2). Francisella novicida U112 is an environmental isolate that is relatively easy to grow and is much less virulent (3). It is a model system for studying F. tularensis biochemistry. The lipopolysaccharide (LPS) 2 of F. tularensis and F. novicida shows low toxicity when compared with Escherichia coli LPS (4, 5). This phenomenon may be due to the unusual structure of the lipid A moiety of Francisella LPS (Fig. 1), which is tetraacylated and lacks the 4Ј-phosphate residue (6, 7). In contrast, E. coli lipid A (Fig. 1) is hexaacylated and is phosphorylated at both the 1-and 4Ј-positions (8, 9). E. coli lipid A is a potent activator of toll-like receptor 4 of the mammalian innate immune system (10, 11). The phosphate groups of lipid A are crucial for this bioactivity (12, 13). Lipid A variants lacking both phosphate groups are inactive. Monophosphorylated lipid A analogues retain some of the immunostimulatory properties of native lipid A, but they are not toxic and are often used as adjuvants (14 -17).Although they possess an orthologue of LpxK, the kinase that incorporates the lipid A 4Ј-phosphate group (18, 19), F. tularensis and related organisms synthesize lipid A molecules lacking the 4Ј-phosphate moiety (6, 7). Similar phosphate-deficient lipid A variants h...

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The Salmonella and related bacteria modify the structure of the lipid A portion of their lipopolysaccharide in response to environmental stimuli. Some lipid A modifications are required for virulence and resistance to cationic antimicrobial peptides. We now demonstrate that membranes of Salmonella typhimurium contain a novel hydrolase that removes the 3′-acyloxyacyl residue of lipid A in the presence of 5 mM Ca 2+ . We have identified the gene encoding the S. typhimurium lipid A 3′-O-deacylase, designated lpxR, by screening an ordered S. typhimurium genomic DNA library, harbored in Escherichia coli K-12, for expression of Ca 2+ -dependent 3′-O-deacylase activity in membranes. LpxR is synthesized with an N-terminal type I signal peptide and is localized to the outer membrane. Mass spectrometry was used to confirm the position of lipid A deacylation in vitro and the release of the intact 3′-acyloxyacyl group. Heterologous expression of lpxR in the E. coli K-12 W3110, which lacks lpxR, resulted in production of significant amounts of 3′-O-deacylated lipid A in growing cultures. Orthologues of LpxR are present in the genomes of E. coli 0157:H7, Yersinia enterocolitica, Helicobacter pylori, and Vibrio cholerae. The function of LpxR is unknown, but it could play a role in pathogenesis because it might modulate the cytokine response of an infected animal.Salmonella typhimurium and related organisms are enteric Gram-negative bacteria. S. typhimurium cause gastroenteritis in human hosts but in mice can produce a fatal, typhoid-like sepsis (1,2). These bacteria invade the epithelial cells and M cells of Peyer patches and then pass into the lymphatic system by colonizing phagocytic cells. Subsequently, the bacteria survive and multiply within modified vacuoles of macrophages that can ultimately produce macrophage apoptosis (3-6).* This research was funded by National Institutes of Health Grants AI-064184 (to M. S. T.), GM-51310 (to C. R. H. R.), and GM-64402 (to R. J. C.). Lipopolysaccharide (LPS) 3 is the principal component of the outer leaflet of the outer membrane of Gram-negative bacteria. Recognition of LPS by the mammalian innate immune system results in the production of cell adhesion proteins in endothelial cells and of proinflammatory molecules such as tumor necrosis factor-α and interleukin-1β in monocytes (7,8). Lipid A, the hydrophobic anchor of LPS, produces most of these responses (9,10) after its detection by Toll-like receptor 4 (TLR-4) (11-13). Lipid A of S. typhimurium and Escherichia coli is a β1′-6-linked disaccharide of glucosamine, phosphorylated at the 1 and 4′ positions and acylated at the 2, 3, 2′, and 3′ positions with R-3-hydroxymyristate (Fig. 1A) We now report a novel, Ca 2+ -dependent 3′-O-deacylase present in the membranes of S. typhimurium (Fig. 2). The structural gene (lpxR) encoding the 3′-O-deacylase was identified and expressed in E. coli K-12. LpxR, like PagP and PagL, is localized to the outer membrane. Expression of LpxR in the E. coli K-12 strain W3110 results in a signific...

Modification of the phosphate groups of lipid A with amine-containing substituents, such as phosphoethanolamine, reduces the overall net negative charge of gram-negative bacterial lipopolysaccharide, thereby lowering its affinity to cationic antimicrobial peptides. Modification of the 1 position of Helicobacter pylori lipid A is a two-step process involving the removal of the 1-phosphate group by a lipid A phosphatase, LpxE HP (Hp0021), followed by the addition of a phosphoethanolamine residue catalyzed by EptA HP (Hp0022). To demonstrate the importance of modifying the 1 position of H. pylori lipid A, we generated LpxE HP -deficient mutants in various H. pylori strains by insertion of a chloramphenicol resistance cassette into lpxE HP and examined the significance of LpxE with respect to cationic antimicrobial peptide resistance. Using both mass spectrometry analysis and an in vitro assay system, we showed that the loss of LpxE HP activity in various H. pylori strains resulted in the loss of modification of the 1 position of H. pylori lipid A, thus confirming the function of LpxE HP . Due to its unique lipid A structure, H. pylori is highly resistant to the antimicrobial peptide polymyxin (MIC > 250 g/ml). However, disruption of lpxE HP in H. pylori results in a dramatic decrease in polymyxin resistance (MIC, 10 g/ml). In conclusion, we have characterized the first gram-negative LpxEdeficient mutant and have shown the importance of modifying the 1 position of H. pylori lipid A for resistance to polymyxin.

The toxic plant protein ricin has gained notoriety due to wide availability and potential use as a bioterrorism agent, with particular concern for food supply contamination. We have developed a sensitive and selective mass spectrometry-based method to detect ricin in tap water, 2% milk, apple juice, and orange juice. Ricin added to beverage matrices was extracted using antibody-bound magnetic beads and digested with trypsin. Absolute quantification was performed using isotope dilution mass spectrometry with a linear ion trap operating in product-ion-monitoring mode. The method allows for identification of ricin A chain and B chain and for distinction of ricin from ricin agglutinin within a single analytical run. Ricin-bound beads were also tested for deadenylase activity by incubation with a synthetic ssDNA oligomer. Depurination of the substrate by ricin was confirmed by matrix-assisted laser desorption ionization-time-of-flight mass spectrometry (MALDI-TOFMS). This method was used successfully to extract ricin from each beverage matrix. The activity of recovered ricin was assessed, and quantification was achieved, with a limit of detection of 10 fmol/mL (0.64 ng/mL).