Expression cloning of a Pseudomonas gene encoding a hydroxydecanoyl-acyl carrier protein-dependent UDP-GlcNAc acyltransferase

Lipopolysaccharide as an antibiotic target

Gram-negative bacteria, including Escherichia coli, Pseudomonas aeruginosa and Acinetobacter baumannii are amongst the highest priority drug-resistant pathogens, for which new antibiotics are urgently needed. Whilst antibiotic drug development is inherently challenging, this is particularly true for Gram-negative bacteria due to the presence of the outer membrane, a highly selective permeability barrier that prevents the ingress of several classes of antibiotic. This selectivity is largely due to an outer leaflet composed of the glycolipid lipopolysaccharide (LPS), which is essential for the viability of almost all Gram-negative bacteria. This essentiality, coupled with the conservation of the synthetic pathway across species and recent breakthroughs in our understanding of transport and membrane homeostasis has made LPS an attractive target for novel antibiotic drug development. Several different targets have been explored and small molecules developed that show promising activity in vitro. However, these endeavours have met limited success in clinical testing and the polymyxins, discovered more than 70 years ago, remain the only LPS-targeting drugs to enter the clinic thus far. In this review, we will discuss efforts to develop therapeutic inhibitors of LPS synthesis and transport and the reasons for limited success, and explore new developments in understanding polymyxin mode of action and the identification of new analogues with reduced toxicity and enhanced activity.

Targeting LPS biosynthesis and transport in gram-negative bacteria in the era of multi-drug resistance

Gram-negative bacteria pose a major threat to human health in an era fraught with multi-drug resistant bacterial infections. Despite extensive drug discovery campaigns over the past decades, no new antibiotic target class effective against gram-negative bacteria has become available to patients since the advent of the carbapenems in 1985. Antibiotic discovery efforts against gram-negative bacteria have been hampered by limited intracellular accumulation of xenobiotics, in large part due to the impermeable cell envelope comprising lipopolysaccharide (LPS) in the outer leaflet of the outer membrane, as well as a panoply of efflux pumps. The biosynthesis and transport of LPS are essential to the viability and virulence of most gram-negative bacteria. Thus, both LPS biosynthesis and transport are attractive pathways to target therapeutically. In this review, we summarize the LPS biosynthesis and transport pathways and discuss efforts to find small molecule inhibitors against targets within these pathways.

Tunable Enzymatic Synthesis of the Immunomodulator Lipid IVA To Enable Structure-Activity Analysis

The Lipid A family of glycolipids, found in the outer membranes of all Gram-negative bacteria, exhibits considerable structural diversity in both lipid and glycan moieties. The lack of facile methods to prepare analogues of these natural products represents a major roadblock in understanding the relationship between their structure and immunomodulatory activities. Here we present a modular, cell-free multienzymatic platform to access these structure-activity relationships. By individually purifying 19 Escherichia coli proteins and reconstituting them in vitro in the presence of acetyl-CoA, UDP- N-acetylglucosamine, NADPH, and ATP, we have developed a system capable of synthesizing Lipid IVA, the first bioactive intermediate in the Lipid A pathway. Our reconstituted multienzyme system revealed considerable promiscuity for orthologs with distinct substrate specificity, as illustrated by swapping enzymes from distantly related cyanobacterial and Pseudomonas species. Analysis of the agonistic and antagonistic activities of the resulting products against the THP-1 human monocytic cell line revealed hitherto unrecognized trends, while opening the door to harnessing the potent biological activities of these complex glycolipid natural products.

Structure guided design of an antibacterial peptide that targets UDP-N-acetylglucosamine acyltransferase

UDP-N-acetylglucosamine (UDP-GlcNAc) acyltransferase (LpxA) catalyzes the first step of lipid A biosynthesis, the transfer of an R-3-hydroxyacyl chain from its acyl carrier protein (ACP) to the 3-OH group of UDP-GlcNAc. Essential in the growth of Gram-negative bacteria, LpxA is a logical target for antibiotics design. A pentadecapeptide (Peptide 920) with high affinity towards LpxA was previously identified in a phage display library. Here we created a small library of systematically designed peptides with the length of four to thirteen amino acids using Peptide 920 as a scaffold. The concentrations of these peptides at which 50% of LpxA is inhibited (IC₅₀) range from 50 nM to >100 μM. We determined the crystal structure of E. coli LpxA in a complex with a potent inhibitor. LpxA-inhibitor interaction, solvent model and all contributing factors to inhibitor efficacy were well resolved. The peptide primarily occludes the ACP binding site of LpxA. Interactions between LpxA and the inhibitor are different from those in the structure of Peptide 920. The inhibitory peptide library and the crystal structure of inhibitor-bound LpxA described here may further assist in the rational design of inhibitors with antimicrobial activity that target LpxA and potentially other acyltransferases.

Cleaning and disinfection programs against Campylobacter jejuni for broiler chickens: productive performance, microbiological assessment and characterization

Detailed cleaning and disinfection programs aims to reduce infection pressure from microorganisms from one flock to the next. However, studies evaluating the benefits to poultry performance, the sanitary status of the facilities, and the sanitary quality of the meat are rarely found. Thus, this study was designed to evaluate 2 cleaning and disinfecting programs regarding their influence on productive performance, elimination of Campylobacter, and characterization of Campylobacter jejuni strains when applied to broiler chickens’ facilities. Two subsequent flocks with 960 birds each were distributed into 32 pens containing 30 birds each. In the first, the whole flock was inoculated with a known strain of Campylobacter jejuni in order to contaminate the environment. In the second flock, performance and microbiological evaluations were done, characterizing an observational study between 2 cleaning and disinfection programs, regular and proposed. The regular program consisted of sweeping facilities, washing equipment and environment with water and neutral detergent. The proposed cleaning program consisted of dry and wet cleaning, application of 2 detergents (one acid and one basic) and 2 disinfectants (250 g/L glutaraldehyde and 185 g/L formaldehyde at 0.5% and 210 g/L para-chloro-meta-cresol at 4%). Total microorganism count in the environment and Campylobacter spp. identification were done for the microbiological assessment of the environment and carcasses. The positive samples were submitted to molecular identification of Campylobacter spp. and posterior genetic sequencing of the species identified as Campylobacter jejuni. The birds housed in the facilities and submitted to the proposed treatment had better performance when compared to the ones in the regular treatment, most likely because there was a smaller total microorganism count on the floor, walls, feeders and drinkers. The proposed program also resulted in a reduction of Campylobacter spp. on floors, drinkers and birds. Moreover, it was possible to identify 6 different Campylobacter jejuni strains in the facilities. The proposed treatment resulted in a positive influence on the birds’ performance and reduction of environment contamination for broiler chickens.

Biosynthesis and structure-activity relationships of the lipid a family of glycolipids

Lipopolysaccharide (LPS), a glycolipid found in the outer membrane of Gram-negative bacteria, is a potent elicitor of innate immune responses in mammals. A typical LPS molecule is composed of three different structural domains: a polysaccharide called the O-antigen, a core oligosaccharide, and Lipid A. Lipid A is the amphipathic glycolipid moiety of LPS. It stimulates the immune system by tightly binding to Toll-like receptor 4. More recently, Lipid A has also been shown to activate intracellular caspase-4 and caspase-5. An impressive diversity is observed in Lipid A structures from different Gram-negative bacteria, and it is well established that subtle changes in chemical structure can result in dramatically different immune activities. For example, Lipid A from Escherichia coli is highly toxic to humans, whereas a biosynthetic precursor called Lipid IV_A blocks this toxic activity, and monophosphoryl Lipid A from Salmonella minnesota is a vaccine adjuvant. Thus, an understanding of structure-activity relationships in this glycolipid family could be used to design useful immunomodulatory agents. Here we review the biosynthesis, modification, and structure-activity relationships of Lipid A.

An Eight-Residue Deletion in Escherichia coli FabG Causes Temperature-Sensitive Growth and Lipid Synthesis Plus Resistance to the Calmodulin Inhibitor Trifluoperazine

FabG performs the NADPH-dependent reduction of β-keto acyl-acyl carrier protein substrates in the elongation cycle of fatty acid synthesis. We report the characterization of a temperature-sensitive mutation (fabGΔ8) in Escherichia colifabG that results from an in-frame 8-amino-acid residue deletion in the α6/α7 subdomain. This region forms part of one of the two dimerization interfaces of this tetrameric enzyme and is reported to undergo significant conformational changes upon cofactor binding, which define the entrance to the active-site cleft. The activity of the mutant enzyme is extremely thermolabile and is deficient in forming homodimers at nonpermissive temperatures with a corresponding decrease in fatty acid synthesis both in vivo and in vitro Surprisingly, the fabGΔ8 strain reverts to temperature resistance at a rate reminiscent of that of a point mutant with intragenic pseudorevertants located either on the 2-fold axes of symmetry or at the mouth of the active-site cleft. The fabGΔ8 mutation also confers resistance to the calmodulin inhibitor trifluoperazine and renders the enzyme extremely sensitive to Ca²⁺in vitro We also observed a significant alteration in the lipid A fatty acid composition of fabGΔ8 strains but only in an lpxC background, probably due to alterations in the permeability of the outer membrane. These observations provide insights into the structural dynamics of FabG and hint at yet another point of regulation between fatty acid and lipid A biosynthesis.IMPORTANCE Membrane lipid homeostasis and its plasticity in a variety of environments are essential for bacterial survival. Since lipid biosynthesis in bacteria and plants is fundamentally distinct from that in animals, it is an ideal target for the development of antibacterial therapeutics. FabG, the subject of this study, catalyzes the first cofactor-dependent reduction in this pathway and is active only as a tetramer. This study examines the interactions responsible for tetramerization through the biochemical characterization of a novel temperature-sensitive mutation caused by a short deletion in an important helix-turn-helix motif. The mutant strain has altered phospholipid and lipid A compositions and is resistant to trifluoperazine, an inhibitor of mammalian calmodulin. Understanding its structural dynamics and its influence on lipid A synthesis also allows us to explore lipid homeostasis as a mechanism for antibiotic resistance.

Invited review: Diversity of endotoxin and its impact on pathogenesis

Lipopolysaccharide or LPS is localized to the outer leaflet of the outer membrane and serves as the major surface component of the bacterial cell envelope. This remarkable glycolipid is essential for virtually all Gram-negative organisms and represents one of the conserved microbial structures responsible for activation of the innate immune system. For these reasons, the structure, function, and biosynthesis of LPS has been an area of intense research. The LPS of a number of bacteria is composed of three distinct regions — lipid A, a short core oligosaccharide, and the O-antigen polysaccharide. The lipid A domain, also known as endotoxin, anchors the molecule in the outer membrane and is the bioactive component recognized by TLR4 during human infection. Overall, the biochemical synthesis of lipid A is a highly conserved process; however, investigation of the lipid A structures of various organisms shows an impressive amount of diversity. These differences can be attributed to the action of latent enzymes that modify the canonical lipid A molecule. Variation of the lipid A domain of LPS serves as one strategy utilized by Gram-negative bacteria to promote survival by providing resistance to components of the innate immune system and helping to evade recognition by TLR4. This review summarizes the biochemical machinery required for the production of diverse lipid A structures of human pathogens and how structural modification of endotoxin impacts pathogenesis.

Invited review: Lipopolysaccharide biosynthesis: which steps do bacteria need to survive?

A detailed knowledge of LPS biosynthesis is of the utmost importance in understanding the function of the outer membrane of Gram-negative bacteria. The regulation of LPS biosynthesis affects many more compartments of the bacterial cell than the outer membrane and thus contributes to the understanding of the physiology of Gram-negative bacteria in general, on the basis of which only mechanisms of virulence and antibiotic resistance can be studied to find new targets for antibacterial treatment. The study of LPS biosynthesis is also an excellent example to demonstrate the limitations of `genomics' and `proteomics', since secondary gene products can be studied only by the combined tools of molecular genetics, enzymology and analytical structural biochemistry. Thus, the door to the field of `glycomics' is opened.

Lipid A as a Drug Target and Therapeutic Molecule

In this review, lipid A, from its discovery to recent findings, is presented as a drug target and therapeutic molecule. First, the biosynthetic pathway for lipid A, the Raetz pathway, serves as a good drug target for antibiotic development. Several assay methods used to screen for inhibitors of lipid A synthesis will be presented, and some of the promising lead compounds will be described. Second, utilization of lipid A biosynthetic pathways by various bacterial species can generate modified lipid A molecules with therapeutic value.

Structures of Pseudomonas aeruginosa LpxA Reveal the Basis for Its Substrate Selectivity

In Gram-negative bacteria, the first step of lipid A biosynthesis is catalyzed by UDP-N-acetylglucosamine acyltransferase (LpxA) through the transfer of a R-3-hydroxyacyl chain from the acyl carrier protein (ACP) to the 3-hydroxyl group of UDP-GlcNAc. Previous studies suggest that LpxA is a critical determinant of the acyl chain length found in lipid A, which varies among species of bacteria. In Escherichia coli and Leptospira interrogans, LpxA prefers to incorporate longer R-3-hydroxyacyl chains (C14 and C12, respectively), whereas in Pseudomonas aeruginosa, the enzyme is selective for R-3-hydroxydecanoyl, a 10-hydrocarbon long acyl chain. We now report three P. aeruginosa LpxA crystal structures: apo protein, substrate complex with UDP-GlcNAc, and product complex with UDP-3-O-(R-3-hydroxydecanoyl)-GlcNAc. A comparison between the apo form and complexes identifies key residues that position UDP-GlcNAc appropriately for catalysis and supports the role of catalytic His121 in activating the UDP-GlcNAc 3-hydroxyl group for nucleophilic attack during the reaction. The product-complex structure, for the first time, offers structural insights into how Met169 serves to constrain the length of the acyl chain and thus functions as the so-called hydrocarbon ruler. Furthermore, compared with ortholog LpxA structures, the purported oxyanion hole, formed by the backbone amide group of Gly139, displays a different conformation in P. aeruginosa LpxA, which suggests flexibility of this structural feature important for catalysis and the potential need for substrate-induced conformational change in catalysis. Taken together, the three structures provide valuable insights into P. aeruginosa LpxA catalysis and substrate specificity as well as templates for future inhibitor discovery.

Lipopolysaccharides as Microbe-associated Molecular Patterns: A Structural Perspective

The lipopolysaccharide (LPS) macromolecule is the major constituent of the external leaflet of the Gram-negative outer membrane, exerting a plethora of biological activities in animals and plants. Among all, it represents a defensive barrier which helps bacteria to resist antimicrobial compounds and external stress factors and is involved in most aspects of host–bacterium interactions such as recognition, adhesion and colonization. One of the most interesting and studied LPS features is its key role in the pathogenesis of Gram-negative infections potentially causing fever or circulatory shock. On the other hand, the LPS acts as a beneficial factor for the host since it is recognized by specific receptors of the host innate immune system; this recognition activates the host defenses culminating, in most cases, in destruction of the pathogen. Most of the biological roles of the LPS are strictly related to its primary structure; thus knowledge of the structural architecture of such a macromolecule, which is different even among bacterial strains belonging to the same species, is a first step but is essential in order to understand the molecular bases of the wide variety of biological activities exerted by LPSs.

Biosynthesis and export of bacterial lipopolysaccharides

Lipopolysaccharide molecules represent a unique family of glycolipids based on a highly conserved lipid moiety known as lipid A. These molecules are produced by most gram-negative bacteria, in which they play important roles in the integrity of the outer-membrane permeability barrier and participate extensively in host-pathogen interplay. Few bacteria contain lipopolysaccharide molecules composed only of lipid A. In most forms, lipid A is glycosylated by addition of the core oligosaccharide that, in some bacteria, provides an attachment site for a long-chain O-antigenic polysaccharide. The complexity of lipopolysaccharide structures is reflected in the processes used for their biosynthesis and export. Rapid growth and cell division depend on the bacterial cell's capacity to synthesize and export lipopolysaccharide efficiently and in large amounts. We review recent advances in those processes, emphasizing the reactions that are essential for viability.

Acyltransferases in bacteria

Long-chain-length hydrophobic acyl residues play a vital role in a multitude of essential biological structures and processes. They build the inner hydrophobic layers of biological membranes, are converted to intracellular storage compounds, and are used to modify protein properties or function as membrane anchors, to name only a few functions. Acyl thioesters are transferred by acyltransferases or transacylases to a variety of different substrates or are polymerized to lipophilic storage compounds. Lipases represent another important enzyme class dealing with fatty acyl chains; however, they cannot be regarded as acyltransferases in the strict sense. This review provides a detailed survey of the wide spectrum of bacterial acyltransferases and compares different enzyme families in regard to their catalytic mechanisms. On the basis of their studied or assumed mechanisms, most of the acyl-transferring enzymes can be divided into two groups. The majority of enzymes discussed in this review employ a conserved acyltransferase motif with an invariant histidine residue, followed by an acidic amino acid residue, and their catalytic mechanism is characterized by a noncovalent transition state. In contrast to that, lipases rely on completely different mechanism which employs a catalytic triad and functions via the formation of covalent intermediates. This is, for example, similar to the mechanism which has been suggested for polyester synthases. Consequently, although the presented enzyme types neither share homology nor have a common three-dimensional structure, and although they deal with greatly varying molecule structures, this variety is not reflected in their mechanisms, all of which rely on a catalytically active histidine residue.

Mechanisms decreasing in vitro susceptibility to the LpxC inhibitor CHIR-090 in the gram-negative pathogen Pseudomonas aeruginosa

Testing P. aeruginosa efflux pump mutants showed that the LpxC inhibitor CHIR-090 is a substrate for MexAB-OprM, MexCD-OprJ, and MexEF-OprN. Utilizing P. aeruginosa PAO1 with a chromosomal mexC::luxCDABE fusion, luminescent mutants arose on medium containing 4 μg/ml CHIR-090, indicating upregulation of MexCD-OprJ. These mutants were less susceptible to CHIR-090 (MIC, 4 μg/ml) and had mutations in the mexCD-oprJ repressor gene nfxB. Nonluminescent mutants (MIC, 4 μg/ml) that had mutations in the mexAB-oprM regulator gene mexR were also observed. Plating the clinical isolate K2153 on 4 μg/ml CHIR-090 selected mutants with alterations in mexS (immediately upstream of mexT), which upregulates MexEF-OprN. A mutant altered in the putative1ribosomal binding site (RBS) upstream of lpxC and overexpressing LpxC was selected on a related LpxC inhibitor and exhibited reduced susceptibility to CHIR-090. Overexpression of LpxC from a plasmid reduced susceptibility to CHIR-090, and introduction of the altered RBS in this construct further increased expression of LpxC and decreased susceptibility to CHIR-090. Using a mutS (hypermutator) strain, a mutant with an altered lpxC target gene (LpxC L18V) was also selected. Purified LpxC L18V had activity similar to that of wild-type LpxC in an in vitro assay but had reduced inhibition by CHIR-090. Finally, an additional class of mutant, typified by an extreme growth defect, was identified. These mutants had mutations in fabG, indicating that alteration in fatty acid synthesis conferred resistance to LpxC inhibitors. Passaging experiments showed progressive decreases in susceptibility to CHIR-090. Therefore, P. aeruginosa can employ several strategies to reduce susceptibility to CHIR-090 in vitro.

Genetic and Functional Diversity of Pseudomonas aeruginosa Lipopolysaccharide

Lipopolysccharide (LPS) is an integral component of the Pseudomonas aeruginosa cell envelope, occupying the outer leaflet of the outer membrane in this Gram-negative opportunistic pathogen. It is important for bacterium-host interactions and has been shown to be a major virulence factor for this organism. Structurally, P. aeruginosa LPS is composed of three domains, namely, lipid A, core oligosaccharide, and the distal O antigen (O-Ag). Most P. aeruginosa strains produce two distinct forms of O-Ag, one a homopolymer of D-rhamnose that is a common polysaccharide antigen (CPA, formerly termed A band), and the other a heteropolymer of three to five distinct (and often unique dideoxy) sugars in its repeat units, known as O-specific antigen (OSA, formerly termed B band). Compositional differences in the O units among the OSA from different strains form the basis of the International Antigenic Typing Scheme for classification via serotyping of different strains of P. aeruginosa. The focus of this review is to provide state-of-the-art knowledge on the genetic and resultant functional diversity of LPS produced by P. aeruginosa. The underlying factors contributing to this diversity will be thoroughly discussed and presented in the context of its contributions to host-pathogen interactions and the control/prevention of infection.

Crystal structure and acyl chain selectivity of Escherichia coli LpxD, the N-acyltransferase of lipid A biosynthesis

LpxD catalyzes the third step of lipid A biosynthesis, the R-3-hydroxyacyl-ACP-dependent N-acylation of UDP-3-O-(acyl)-alpha-D-glucosamine, and is a target for new antibiotic development. Here we report the 2.6 A crystal structure of the Escherichia coli LpxD homotrimer (EcLpxD). As is the case in Chlamydia trachomatis LpxD (CtLxpD), each EcLpxD chain consists of an N-terminal uridine-binding region, a left-handed parallel beta-helix (LbetaH), and a C-terminal alpha-helical domain. The backbones of the LbetaH domains of the two enzymes are similar, as are the positions of key active site residues. The N-terminal nucleotide binding domains are oriented differently relative to the LbetaH regions, but are similar when overlaid on each other. The orientation of the EcLpxD tripeptide (residues 303-305), connecting the distal end of the LbetaH and the proximal end of the C-terminal helical domains, differs from its counterpart in CtLpxD (residues 311-312); this results in a 120 degrees rotation of the C-terminal domain relative to the LbetaH region in EcLpxD versus CtLpxD. M290 of EcLpxD appears to cap the distal end of a hydrophobic cleft that binds the acyl chain of the R-3-hydroxyacyl-ACP donor substrate. Under standard assay conditions, wild-type EcLpxD prefers R,S-3-hydroxymyristoyl-ACP over R,S-3-hydroxypalmitoyl-ACP by a factor of 3, whereas the M290A mutant has the opposite selectivity. Both wild-type and M290A EcLpxD rescue the conditional lethality of E. coli RL25, a temperature-sensitive strain harboring point mutations in lpxD. Complementation with wild-type EcLpxD restores normal lipid A containing only N-linked hydroxymyristate to RL25 at 42 degrees C, as judged by mass spectrometry, whereas the M290A mutant generates multiple lipid A species containing one or two longer hydroxy fatty acids in place of the usual R-3-hydroxymyristate at positions 2 and 2'.

Review: Lipopolysaccharide biosynthesis in Pseudomonas aeruginosa

Pseudomonas aeruginosa causes serious nosocomial infections, and an important virulence factor produced by this organism is lipopolysaccharide (LPS). This review summarizes knowledge about biosynthesis of all three structural domains of LPS - lipid A, core oligosaccharide, and O polysaccharides. In addition, based on similarities with other bacterial species, this review proposes new hypothetical pathways for unstudied steps in the biosynthesis of P. aeruginosa LPS. Lipid A biosynthesis is discussed in relation to Escherichia coli and Salmonella, and the biosyntheses of core sugar precursors and core oligosaccharide are summarised. Pseudomonas aeruginosa attaches a Common Polysaccharide Antigen and O-Specific Antigen polysaccharides to lipid A-core. Both forms of O polysaccharide are discussed with respect to their independent synthesis mechanisms. Recent advances in understanding O-polysaccharide biosynthesis since the last major review on this subject, published nearly a decade ago, are highlighted. Since P. aeruginosa O polysaccharides contain unusual sugars, sugar-nucleotide biosynthesis pathways are reviewed in detail. Knowledge derived from detailed studies in the O5, O6 and O11 serotypes is applied to predict biosynthesis pathways of sugars in poorly-studied serotypes, especially O1, O4, and O13/O14. Although further work is required, a full understanding of LPS biosynthesis in P. aeruginosa is almost within reach.

Steady-state kinetics and mechanism of LpxD, the N-acyltransferase of lipid A biosynthesis

LpxD catalyzes the third step of lipid A biosynthesis, the (R)-3-hydroxymyristoyl-acyl carrier protein ( R-3-OHC14-ACP)-dependent N-acylation of UDP-3-O-[(R)-3-hydroxymyristoyl]-alpha-D-glucosamine [UDP-3-O-(R-3-OHC14)-GlcN]. We have now overexpressed and purified Escherichia coli LpxD to homogeneity. Steady-state kinetics suggest a compulsory ordered mechanism in which R-3-OHC14-ACP binds prior to UDP-3-O-(R-3-OHC14)-GlcN. The product, UDP-2,3-diacylglucosamine, dissociates prior to ACP; the latter is a competitive inhibitor against R-3-OHC14-ACP and a noncompetitive inhibitor against UDP-3-O-(R-3-OHC14)-GlcN. UDP-2-N-[(R)-3-Hydroxymyristoyl]-alpha-D-glucosamine, obtained by mild base hydrolysis of UDP-2,3-diacylglucosamine, is a noncompetitive inhibitor against both substrates. Synthetic (R)-3-hydroxylauroyl-methylphosphopantetheine is an uncompetitive inhibitor against R-3-OHC14-ACP and a competitive inhibitor against UDP-3-O-(R-3-OHC14)-GlcN, but (R)-3-hydroxylauroyl-methylphosphopantetheine is also a very poor substrate. A compulsory ordered mechanism is consistent with the fact that R-3-OHC14-ACP has a high binding affinity for free LpxD whereas UDP-3-O-(R-3-OHC14)-GlcN does not. Divalent cations inhibit R-3-OHC14-ACP-dependent acylation but not (R)-3-hydroxylauroyl-methylphosphopantetheine-dependent acylation, indicating that the acidic recognition helix of R-3-OHC14-ACP contributes to binding. The F41A mutation increases the K(M) for UDP-3-O-(R-3-OHC14)-GlcN 30-fold, consistent with aromatic stacking of the corresponding F43 side chain against the uracil moiety of bound UDP-GlcNAc in the X-ray structure of Chlamydia trachomatis LpxD. Mutagenesis implicates E. coli H239 but excludes H276 as the catalytic base, and neither residue is likely to stabilize the oxyanion intermediate.

Virulence of plant pathogenic bacteria attenuated by degradation of fatty acid cell-to-cell signaling factors

Diffusible signal factor (DSF) is a fatty acid signal molecule involved in regulation of virulence in several Xanthomonas species as well as Xylella fastidiosa. In this study, we identified a variety of bacteria that could disrupt DSF-mediated induction of virulence factors in Xanthomonas campestris pv. campestris. While many bacteria had the ability to degrade DSF, several bacterial strains belonging to genera Bacillus, Paenibacillus, Microbacterium, Staphylococcus, and Pseudomonas were identified that were capable of particularly rapid degradation of DSF. The molecular determinants for rapid degradation of DSF in Pseudomonas spp. strain G were elucidated. Random transposon mutants of strain G lacking the ability to degrade DSF were isolated. Cloning and characterization of disrupted genes in these strains revealed that carAB, required for the synthesis of carbamoylphosphate, a precursor for pyrimidine and arginine biosynthesis is required for rapid degradation of DSF in strain G. Complementation of carAB mutants restored both pyrimidine prototrophy and DSF degradation ability of the strain G mutant. An Escherichia coli strain harboring carAB of Pseudomonas spp. strain G degrades DSF more rapidly than the parental strain, and overexpression of carAB in trans increased the ability of Pseudomonas spp. strain G to degrade as compared with the parental strain. Coinoculation of X. campestris pv. campestris with DSF-degrading bacteria into mustard and cabbage leaves reduced disease severity up to twofold compared with plants inoculated only with the pathogen. Likewise, disease incidence and severity in grape stems coinoculated with Xylella fastidiosa and DSF-degrading strains were significantly reduced compared with plants inoculated with the pathogen alone. Coinoculation of grape plants with a carAB mutant of Pseudomonas spp. strain G complemented with carAB in trans reduced disease severity as well or better than the parental strain. These results indicate that overexpression of carAB in other endophytes could be a useful strategy of biocontrol for the control of diseases caused by plant pathogens that produce DSF.

Structural basis for the acyl chain selectivity and mechanism of UDP-N-acetylglucosamine acyltransferase

UDP-N-acetylglucosamine (UDP-GlcNAc) acyltransferase (LpxA) catalyzes the first step of lipid A biosynthesis, the reversible transfer of the R-3-hydroxyacyl chain from R-3-hydroxyacyl acyl carrier protein to the glucosamine 3-OH group of UDP-GlcNAc. Escherichia coli LpxA is highly selective for R-3-hydroxymyristate. The crystal structure of the E. coli LpxA homotrimer, determined previously in the absence of lipid substrates or products, revealed that LpxA contains an unusual, left-handed parallel beta-helix fold. We have now solved the crystal structures of E. coli LpxA with the bound product UDP-3-O-(R-3-hydroxymyristoyl)-GlcNAc at a resolution of 1.74 A and with bound UDP-3-O-(R-3-hydroxydecanoyl)-GlcNAc at 1.85 A. The structures of these complexes are consistent with the catalytic mechanism deduced by mutagenesis and with a recent 3.0-A structure of LpxA with bound UDP-GlcNAc. Our structures show how LpxA selects for 14-carbon R-3-hydroxyacyl chains and reveal two modes of UDP binding.

Lipid A modification systems in gram-negative bacteria

The lipid A moiety of lipopolysaccharide forms the outer monolayer of the outer membrane of most gram-negative bacteria. Escherichia coli lipid A is synthesized on the cytoplasmic surface of the inner membrane by a conserved pathway of nine constitutive enzymes. Following attachment of the core oligosaccharide, nascent core-lipid A is flipped to the outer surface of the inner membrane by the ABC transporter MsbA, where the O-antigen polymer is attached. Diverse covalent modifications of the lipid A moiety may occur during its transit from the outer surface of the inner membrane to the outer membrane. Lipid A modification enzymes are reporters for lipopolysaccharide trafficking within the bacterial envelope. Modification systems are variable and often regulated by environmental conditions. Although not required for growth, the modification enzymes modulate virulence of some gram-negative pathogens. Heterologous expression of lipid A modification enzymes may enable the development of new vaccines.

Preparation of isotopically labeled spinach acyl-acyl carrier protein for NMR structural studies

Acyl carrier proteins (ACPs) are important protein cofactors in fatty acid biosynthesis, but their acylated forms have not been well-studied. To permit detailed nuclear magnetic resonance studies of acylated spinach ACP isoform I, we have developed a new expression plasmid for recombinant production of the apo-protein and modified protocols for purifying the protein product and acylating it to form acyl-ACP. To solve plasmid stability problems associated with growth in minimal media, the ampicillin resistance gene from pSACP-2a was replaced with the tetA(C) gene from pBR322. The resulting plasmid, pSACP-2t, supported overexpression of apo-ACP in Escherichia coli BL21(DE3) cells in M9 medium containing 15NH4Cl as the sole nitrogen source. Apo-ACP was purified to homogeneity by means of polyethylene glycol precipitation and anion exchange. Two in vitro synthetic routes were used to produce acyl-ACPs. In one route, apo-ACP was converted to the holo form and the acyl form by a published protocol that employs a discrete enzymatic reaction for each step. As an alternative route to produce decanoyl-ACP, apo-ACP was directly converted to the acyl form by using holo-ACP synthase along with the non-natural substrate decanoyl-CoA. Two-dimensional 1H-15N NMR spectroscopy of decanoyl-ACP and stearoyl-ACP revealed that changes in the length of the covalently attached fatty acid do not affect the secondary structure of the protein but do influence the local conformation and dynamics.

Biosynthesis, transport, and modification of lipid A

Lipopolysaccharide (LPS) is the major surface molecule of Gram-negative bacteria and consists of three distinct structural domains: O-antigen, core, and lipid A. The lipid A (endotoxin) domain of LPS is a unique, glucosamine-based phospholipid that serves as the hydrophobic anchor of LPS and is the bioactive component of the molecule that is associated with Gram-negative septic shock. The structural genes encoding the enzymes required for the biosynthesis of Escherchia coli lipid A have been identified and characterized. Lipid A is often viewed as a constitutively synthesized structural molecule. However, determination of the exact chemical structures of lipid A from diverse Gram-negative bacteria shows that the molecule can be further modified in response to environmental stimuli. These modifications have been implicated in virulence of pathogenic Gram-negative bacteria and represent one of the molecular mechanisms of microbial surface remodeling used by bacteria to help evade the innate immune response. The intent of this review is to discuss the enzymatic machinery involved in the biosynthesis of lipid A, transport of the molecule, and finally, those enzymes involved in the modification of its structure in response to environmental stimuli.

Enzymatic synthesis of lipid A molecules with four amide-linked acyl chains. LpxA acyltransferases selective for an analog of UDP-N-acetylglucosamine in which an amine replaces the 3"-hydroxyl group

LpxA of Escherichia coli catalyzes the acylation of the glucosamine 3-OH group of UDP-GlcNAc, using R-3-hydroxymyristoyl-acyl carrier protein (ACP) as the donor substrate. We now demonstrate that LpxA in cell extracts of Mesorhizobium loti and Leptospira interrogans, which synthesize lipid A molecules containing 2,3-diamino-2,3-dideoxy-d-glucopyranose (GlcN3N) units in place of glucosamine, do not acylate UDP-GlcNAc. Instead, these LpxA acyltransferases require a UDP-Glc-NAc derivative (designated UDP 2-acetamido-3-amino-2,3-dideoxy-alpha-d-glucopyranose or UDP-GlcNAc3N), characterized in the preceding paper, in which an amine replaces the glucosamine 3-OH group. L. interrogans LpxA furthermore displays absolute selectivity for 3-hydroxylauroyl-ACP as the donor, whereas M. loti LpxA functions almost equally well with 10-, 12-, and 14-carbon 3-hydroxyacyl-ACPs. The substrate selectivity of L. interrogans LpxA is consistent with the structure of L. interrogans lipid A. The mechanism of L. interrogans LpxA appears to be similar to that of E. coli LpxA, given that the essential His(125) residue of E. coli LpxA is conserved and is also required for acyltransferase activity in L. interrogans. Acidithiobacillus ferrooxidans (an organism that makes lipid A molecules containing both GlcN and GlcN3N) has an ortholog of LpxA that is selective for UDP-GlcNAc3N, but the enzyme also catalyzes the acylation of UDP-GlcNAc at a slow rate. E. coli LpxA acylates UDP-GlcNAc and UDP-GlcNAc3N at comparable rates in vitro. However, UDP-GlcNAc3N is not synthesized in vivo, because E. coli lacks gnnA and gnnB. When the latter are supplied together with A. ferrooxidans lpxA, E. coli incorporates a significant amount of GlcN3N into its lipid A.

Expression of foreign LpxA acyltransferases in Neisseria meningitidis results in modified lipid A with reduced toxicity and retained adjuvant activity

A major problem in the development of vaccines against Gram-negative bacteria is the endotoxic -activity of lipopolysaccharide (LPS), which is determined by its lipid A moiety. Nevertheless, LPS would be an interesting vaccine component because of its immune-stimulating properties. In the present study, we have changed the fatty acid composition of Neisseria meningitidis LPS by replacing the lpxA gene of strain H44/76 with the Escherichia coli or Pseudomonas aeruginosa homologue. The majority of the O-linked 3-OH C12 in N. meningitidis lipid A was replaced by 3-OH C14 (strain HA01E) and 3-OH C10 (strain HA25P) respectively. Both strains, but most notably strain HA01E, had reduced amounts of LPS compared with the wild-type strain. In addition, growth was severely impaired for HA01E. The major outer membrane proteins were expressed normally. Outer membrane complexes of both strains normalized on their LPS content showed a 10-fold reduction in their ability to induce tumour necrosis factor (TNF)-alpha. Immunogenicity studies in BALB/c mice revealed that the adjuvant activity of the LPS was not affected. Thus, the replacement of the O-linked fatty acids in meningococcal lipid A results in immunogenic outer membranes with reduced endotoxic activity, more suitable for use in outer membrane vesicle vaccines.

Lipopolysaccharide endotoxins

Bacterial lipopolysaccharides (LPS) typically consist of a hydrophobic domain known as lipid A (or endotoxin), a nonrepeating "core" oligosaccharide, and a distal polysaccharide (or O-antigen). Recent genomic data have facilitated study of LPS assembly in diverse Gram-negative bacteria, many of which are human or plant pathogens, and have established the importance of lateral gene transfer in generating structural diversity of O-antigens. Many enzymes of lipid A biosynthesis like LpxC have been validated as targets for development of new antibiotics. Key genes for lipid A biosynthesis have unexpectedly also been found in higher plants, indicating that eukaryotic lipid A-like molecules may exist. Most significant has been the identification of the plasma membrane protein TLR4 as the lipid A signaling receptor of animal cells. TLR4 belongs to a family of innate immunity receptors that possess a large extracellular domain of leucine-rich repeats, a single trans-membrane segment, and a smaller cytoplasmic signaling region that engages the adaptor protein MyD88. The expanding knowledge of TLR4 specificity and its downstream signaling pathways should provide new opportunities for blocking inflammation associated with infection.

A Chlamydia trachomatis UDP-N-acetylglucosamine acyltransferase selective for myristoyl-acyl carrier protein. Expression in Escherichia coli and formation of hybrid lipid A species

Chlamydia trachomatis lipid A is unusual in that it is acylated with myristoyl chains at the glucosamine 3 and 3' positions. We have cloned and expressed the gene encoding UDP-N-acetylglucosamine 3-O-acyltransferase of C. trachomatis (CtlpxA), the first enzyme of lipid A biosynthesis. C. trachomatis LpxA displays approximately 20-fold selectivity for myristoyl-ACP over R/S-3-hydroxymyristoyl-ACP under standard assay conditions, consistent with the proposed structure of C. trachomatis lipid A. CtLpxA is the first reported UDP-N-acetylglucosamine acyltransferase that prefers a non-hydroxylated acyl-ACP to a hydroxyacyl-ACP. When CtlpxA was expressed in RO138, a temperature-sensitive lpxA mutant of Escherichia coli, five new hybrid lipid A species were made in vivo after 2 h at 42 degrees C, in place of Escherichia coli lipid A. These compounds were purified and analyzed by matrix-assisted laser desorption ionization/time of flight mass spectrometry. In each case, a myristoyl chain replaced one or both of the ester linked 3-hydroxymyristoyl residues of E. coli lipid A. With prolonged growth at 42 degrees C, all the ester-linked 3-hydroxymyristoyl residues were replaced with myristate chains. Re-engineering the structure of E. coli lipid A should facilitate the microbiological production of novel agonists or antagonists of the innate immunity receptor TLR-4, with possible uses as adjuvants or anti-inflammatory agents.

Expression, purification, and characterization of the Mycobacterium tuberculosis acyl carrier protein, AcpM

Mycolic acids are generated in Mycobacterium tuberculosis as a result of the interaction of two fatty acid biosynthetic systems: the multifunctional polypeptide, FASI, in which the acyl carrier protein (ACP) domain forms an integral part of the polypeptide, and the dissociated FASII system, which is composed of monofunctional enzymes and a discrete ACP (AcpM). In order to characterize enzymes of the FASII system, large amounts of AcpM are required to generate substrates such as holo-AcpM, malonyl-AcpM and acyl-AcpM. The M. tuberculosis acpM gene was overexpressed in Escherichia coli and AcpM purified, yielding approximately 15-20 mg/l of culture. Analysis of AcpM by mass spectrometry, N-terminal sequencing, amino acid analysis, and gas chromatography indicated the presence of three species, apo-, holo-, and acyl-AcpM, the former comprising up to 65% of the total pool. The apo-AcpM was purified away from the in vivo generated holo- and acyl-forms, which were inseparable and heterogeneous with respect to acyl chain lengths. Once purified, we were able to convert apo-AcpM into holo- and acyl-forms. These procedures provide the means for the preparation of the large quantities of AcpM and derivatives needed for characterization of the purified enzymes of the mycobacterial FASII system.

A Haemophilus influenzae gene that encodes a membrane bound 3-deoxy-D-manno-octulosonic acid (Kdo) kinase. Possible involvement of kdo phosphorylation in bacterial virulence

The lipopolysaccharide of Haemophilus influenzae contains a single 3-deoxy-D-manno-octulosonic acid (Kdo) residue derivatized with either a phosphate or an ethanolamine pyrophosphate moiety at the 4-OH position. In previous studies, we identified a kinase unique to H. influenzae extracts that phosphorylates Kdo-lipid IV(A), a key precursor of lipopolysaccharide in this organism. We have now identified the gene encoding the Kdo kinase by using an expression cloning approach. A cosmid library containing random DNA fragments from H. influenzae strain Rd was constructed in Escherichia coli. Extracts of 472 colonies containing individual hybrid cosmids were assayed for Kdo kinase activity. A single hybrid cosmid directing expression of the kinase was found. The kinase gene was identified by activity assays, sub-cloning, and DNA sequencing. When the putative kinase gene was expressed in E. coli behind a T7 promoter, massive overproduction of kinase activity was achieved ( approximately 8000-fold higher than in H. influenzae membranes). The catalytic properties and the product generated by the overexpressed kinase, assayed with Kdo-lipid IV(A) as the substrate, were the same as observed with H. influenzae membranes. Unexpectedly, the kinase gene was identical to a previously characterized open reading frame (orfZ), which had been shown to be important for establishing bacteremia in an infant rat model (Hood, D. W., Deadman, M. E., Allen, T., Masoud, H., Martin, A., Brisson, J. R., Fleischmann, R., Venter, J. C., Richards, J. C., and Moxon, E. R. (1996) Mol. Microbiol. 22, 951-965). However, based solely on the genome sequence of H. influenzae Rd, no biochemical function had been assigned to the product of orfZ, which we now designate kdkA ("Kdo kinase A"). Although Kdo phosphorylation may be critical for bacterial virulence of H. influenzae, it does not appear to be required for growth.

Spinach holo-acyl carrier protein: overproduction and phosphopantetheinylation in Escherichia coli BL21(DE3), in vitro acylation, and enzymatic desaturation of histidine-tagged isoform I

Spinach ACP isoform I was overexpressed in Escherichia coli BL21(DE3) using a gene synthesized from codons associated with high-level expression in E. coli. The synthetic gene has extensive changes in codon usage (23 of 77 total codons) relative to that of the originally synthesized plant gene (P. D. Beremand et al., 1987, Arch. Biochem. Biophys. 256, 90-100). After expression of the new synthetic gene, purified ACP and ACP-His6 were obtained in yields of up to 70 mg L-1 of culture medium, compared to approximately 1-6 mg L-1 of purified ACP obtained from the gene composed of predicted spinach codons. In either shaken flask or fermentation culture, approximately 15% conversion to holo-ACP or holo-ACP-His6 was obtained regardless of the level of protein expression. However, coexpression of ACP-His6 with E. coli holo-ACP synthase in E. coli BL21(DE3) during pH- and dissolved O2-controlled fermentation routinely yielded greater than 95% conversion to holo-ACP-His6. Electrospray ionization mass spectrometric analysis of the purified recombinant ACPs revealed that the amino terminal Met was efficiently removed, but only if the bacterial cell lysates were prepared in the absence of EDTA. This observation is consistent with the inhibition of endogenous Met-aminopeptidase by removal of catalytically essential Co(II) and introduces the importance of considering the catalytic properties of host enzymes providing ad hoc posttranslational modification of recombinant proteins. Stearoyl-ACP-His6 was shown to be indistinguishable from stearoyl-ACP as a substrate for enzymatic acylation and desaturation. In combination, these studies provide a coordinated scheme to produce and characterize quantities of acyl-ACPs sufficient to support expanded biophysical and structural studies.

Signal transduction triggered by lipid A-like molecules in 70Z/3 pre-B lymphocyte tumor cells

The lipid A (endotoxin) moiety of lipopolysaccharide (LPS) elicits rapid cellular responses from many cell types, including macrophages, lymphocytes, and monocytes. In CD14 transfected 70Z/3 pre-B lymphocyte tumor cells, these responses include activation of the MAP kinase homolog, p38, activation of NF-kappaB, and transcription of kappa light chains, leading to the assembly of surface IgM. In this work, we explored the specificity of the response with regard to lipid structure, and the requirement for p38 kinase activity prior to NF-kappaB activation in control and CD14 transfected 70Z/3 (CD14-70Z/3) cells. A p38-specific inhibitor, SB203580, was used to block p38 kinase activity in cells. CD14-70Z/3 cells were incubated with 1-50 microM SB203580, and then stimulated with LPS. Nuclear extracts were prepared, and NF-kappaB activation was measured using an electrophoretic mobility shift assay. SB203580 did not inhibit LPS induced NF-kappaB activation. In addition, LPS failed to activate p38 tyrosine phosphorylation in 70Z/3 cells lacking CD14, in spite of rapid NF-kappaB activation and robust surface IgM production with appropriate higher doses of LPS. LPS stimulation of p38 phosphorylation, NF-kappaB activation, and surface IgM expression were all blocked completely by lipid A-like endotoxin antagonists whether or not CD14 was present. Acidic glycerophospholipids and ceramides did not mimic lipid A-like molecules either as agonists or antagonists in this system. Our data support the hypothesis that lipid A-mediated activation of cells requires stimulation of a putative lipid A sensor that is downstream of CD14, but upstream of p38 and NF-kappaB.

Hydrocarbon rulers in UDP-N-acetylglucosamine acyltransferases

UDP-GlcNAc acyltransferase (LpxA), the first enzyme of lipid A biosynthesis, catalyzes the transfer of an acyl chain activated on acyl carrier protein (ACP) to UDP-GlcNAc. LpxAs are very selective for the lengths of their acyl donor substrates. Escherichia coli LpxA prefers R-3-hydroxymyristoyl-ACP to R-3-hydroxydecanoyl-ACP by a factor of approximately 1000, whereas Pseudomonas aeruginosa LpxA prefers the opposite. E. coli G173M LpxA and the reciprocal P. aeruginosa M169G LpxA show reversed substrate selectivity in vitro and in vivo, demonstrating the existence of precise hydrocarbon rulers in LpxAs.

A new metabolic link between fatty acid de novo synthesis and polyhydroxyalkanoic acid synthesis. The PHAG gene from Pseudomonas putida KT2440 encodes a 3-hydroxyacyl-acyl carrier protein-coenzyme a transferase

To investigate the metabolic link between fatty acid de novo synthesis and polyhydroxyalkanoic acid (PHA) synthesis, we isolated mutants of Pseudomonas putida KT2440 deficient in this metabolic route. The gene phaG was cloned by phenotypic complementation of these mutants; it encoded a protein of 295 amino acids with a molecular mass of 33,876 Da, and the amino acid sequence exhibited 44% amino acid identity to the primary structure of the rhlA gene product, which is involved in the rhamnolipid biosynthesis in Pseudomonas aeruginosa PG201. S1 nuclease protection assay identified the transcriptional start site 239 base pairs upstream of the putative translational start codon. Transcriptional induction of phaG was observed when gluconate was provided, and PHA synthesis occurred from this carbon source. No complementation of the rhlA mutant P. aeruginosa UO299-harboring plasmid pBHR81, expressing phaG gene under lac promoter control, was obtained. Heterologous expression of phaG in Pseudomonas oleovorans, which is not capable of PHA synthesis from gluconate, enabled PHA synthesis on gluconate as the carbon source. Native recombinant PhaG was purified by native polyacrylamide gel electrophoresis from P. oleovorans-harboring plasmid pBHR81. It catalyzes the transfer of the acyl moiety from in vitro synthesized 3-hydroxydecanoyl-CoA to acyl carrier protein, indicating that PhaG exhibits a 3-hydroxyacyl-CoA-acyl carrier protein transferase activity.

The Pseudomonas aeruginosa rhlG gene encodes an NADPH-dependent beta-ketoacyl reductase which is specifically involved in rhamnolipid synthesis

A Pseudomonas aeruginosa gene homologous to the fabG gene, which encodes the NADPH-dependent beta-ketoacyl-acyl carrier protein (ACP) reductase required for fatty acid synthesis, was identified. The insertional mutation of this fabG homolog (herein called rhlG) produced no apparent effect on the growth rate and total lipid content of P. aeruginosa cells, but the production of rhamnolipids was completely abrogated. These results suggest that the synthetic pathway for the fatty acid moiety of rhamnolipids is separate from the general fatty acid synthetic pathway, starting with a specific ketoacyl reduction step catalyzed by the RhlG protein. In addition, the synthesis of poly-beta-hydroxyalkanoate (PHA) is delayed in this mutant, suggesting that RhlG participates in PHA synthesis, although it is not the only reductase involved in this pathway. Traits regulated by the quorum-sensing response, other than rhamnolipid production, including production of proteases, pyocyanine, and the autoinducer butanoyl-homoserine lactone (PAI-2), were not affected by the rhlG mutation. We conclude that the P. aeruginosa rhlG gene encodes an NADPH-dependent beta-ketoacyl reductase absolutely required for the synthesis of the beta-hydroxy acid moiety of rhamnolipids and that it has a minor role in PHA production. Expression of rhlG mRNA under different culture conditions is consistent with this conclusion.

Acylation of Escherichia coli hemolysin: a unique protein lipidation mechanism underlying toxin function

The pore-forming hemolysin (HlyA) of Escherichia coli represents a unique class of bacterial toxins that require a posttranslational modification for activity. The inactive protoxin pro-HlyA is activated intracellularly by amide linkage of fatty acids to two internal lysine residues 126 amino acids apart, directed by the cosynthesized HlyC protein with acyl carrier protein as the fatty acid donor. This action distinguishes HlyC from all bacterial acyltransferases such as the lipid A, lux-specific, and nodulation acyltransferases, and from eukaryotic transferases such as N-myristoyl transferases, prenyltransferases, and thioester palmitoyltransferases. Most lipids directly attached to proteins may be classed as N-terminal amide-linked and internal ester-linked acyl groups and C-terminal ether-linked isoprenoid groups. The acylation of HlyA and related toxins does not equate to these but does appear related to a small number of eukaryotic proteins that include inflammatory cytokines and mitogenic and cholinergic receptors. While the location and structure of lipid moieties on proteins vary, there are common effects on membrane affinity and/or protein-protein interactions. Despite being acylated at two residues, HlyA does not possess a "double-anchor" motif and does not have an electrostatic switch, although its dependence on calcium binding for activity suggests that the calcium-myristoyl switch may have relevance. The acyl chains on HlyA may provide anchorage points onto the surface of the host cell lipid bilayer. These could then enhance protein-protein interactions either between HlyA and components of a host signal transduction pathway to influence cytokine production or between HlyA monomers to bring about oligomerization during pore formation.