Activation of Escherichia coli prohaemolysin to the mature toxin by acyl carrier protein-dependent fatty acylation

X-ray crystallographic studies on butyryl-ACP reveal flexibility of the structure around a putative acyl chain binding site

Acyl carrier protein (ACP) is an essential cofactor in biosynthesis of fatty acids and many other reactions that require acyl transfer steps. We have determined the first crystal structures of an acylated form of ACP from E. coli, that of butyryl-ACP. Our analysis of the molecular surface of ACP reveals a plastic hydrophobic cavity in the vicinity of the phosphopantethylated Ser36 residue that is expanded and occupied by the butyryl and beta-mercaptoethylamine moieties of the acylated 4'-phosphopantetheine group in one of our crystal forms. In the other form, the cavity is contracted, and we propose that the protein has adopted the conformation after delivery of substrate into the active site of a partner enzyme.

Toxin-induced calcium oscillations: a novel strategy to affect gene regulation in target cells

Although the mucosal linings continuously are exposed to microbes, the microbes rarely induce disease. This is because mucosal surfaces are protected by a first line of host defence termed the innate immunity system. The innate immune response is an outcome of the complex interplay between microbes and host target cells, and leads to the activation of inflammatory processes. Although inflammation is essential for clearing out infectious agents, it can also be harmful to the host and is therefore subjected to control at multiple levels. We recently discovered that alpha-haemolysin, a toxin secreted by uropathogenic E. coli induces constant, low-frequency Ca2+ oscillations in renal epithelial cells (Uhlén et al., Nature 405, 694-696 (2000)). Ca2+ oscillation at a specific periodicity of 12 min was found to affect gene expression in target epithelial cells, as the proinflammatory cytokine IL-6 and chemokine IL-8 were specifically induced by alpha-haemolysin-induced Ca2+ oscillations. This demonstrates a novel feature of bacterial toxin effects on host target cells: as inducers of second messenger responses which fine-tune gene expression in target epithelial cells into pathways leading to e. g. a pro-inflammatory response.

Role of the Hha/YmoA family of proteins in the thermoregulation of the expression of virulence factors

Virulent bacteria are able to sense temperature changes and respond by modifying the expression of--among others--genes that code for virulence factors. The chromatin-associated protein H-NS has been shown to play a role in the thermomodulation of virulence factor expression. In addition to H-NS, proteins of the Hha/YmoA family have also been identified in different enterobacteria as participating in the thermoregulation of some virulence factors. For one of these proteins, the Hha protein, it has been shown that it interacts with H-NS, and both proteins form a nucleoid-protein complex responsible for the thermoregulation of, at least, E. coli hemolysin. The presence of genes coding for homologues of both proteins on some conjugative plasmids and their relation to thermoregulation suggests that this complex could also play a role in the regulation of plasmid transfer.

The E. coli alpha-hemolysin secretion system and its use in vaccine development

Many Gram-negative bacteria use a type I secretion system to translocate proteins, including pore-forming toxins, proteases, lipases and S-layer proteins, across both the inner and outer membranes into the extracellular surroundings. The Escherichia coli alpha-hemolysin (HlyA) secretion system is the prototypical and best characterized type I secretion system. The structure and function of the components of the HlyA secretion apparatus, HlyB, HlyD and TolC, have been studied in great detail. The functional characteristics of this secretion system enable it to be used in a variety of different applications, including the presentation of heterologous antigens in live-attenuated bacterial vaccines. Such vaccines can be an effective delivery system for heterologous antigens, and the use of a type I secretion system allows the antigens to be actively exported from the cytoplasm of the bacterial carrier rather than only becoming accessible to the host immune system after bacterial disintegration.

Purification and characterization of acyl-acyl carrier protein synthetase from oleaginous yeast and its role in triacylglycerol biosynthesis

Fatty acids are activated in an ATP-dependent manner before they are utilized. We describe here how the 10 S triacylglycerol biosynthetic multienzyme complex from Rhodotorula glutinis is capable of activating non-esterified fatty acids for the synthesis of triacylglycerol. The photolabelling of the complex with [(32)P]azido-ATP showed labelling of a 35 kDa polypeptide. The labelled polypeptide was identified as acyl-acyl carrier protein (ACP) synthetase, which catalyses the ATP-dependent ligation of fatty acid with ACP to form acyl-ACP. The enzyme was purified by successive PAGE separations to apparent homogeneity from the soluble fraction of oleaginous yeast and its apparent molecular mass was 35 kDa under denaturing and reducing conditions. Acyl-ACP synthetase was specific for ATP. The K(m) values for palmitic, stearic, oleic and linoleic acids were found to be 42.9, 30.4, 25.1 and 22.7 microM, respectively. The antibodies to acyl-ACP synthetase cross-reacted with Escherichia coli acyl-ACP synthetase. Anti-ACP antibodies showed no cross-reactivity with the purified acyl-ACP synthetase, indicating no bound ACP with the enzyme. Immunoprecipitations with antibodies to acyl-ACP synthetase revealed that this enzyme is a part of the 10 S triacylglycerol biosynthetic complex. These results demonstrate that the soluble acyl-ACP synthetase plays a novel role in activating fatty acids for triacylglycerol biosynthesis in oleaginous yeast.

Isolation and characterization of Escherichia coli tolC mutants defective in secreting enzymatically active alpha-hemolysin

This study describes the isolation and characterization of a unique class of TolC mutants that, under steady-state growth conditions, secreted normal levels of largely inactive alpha-hemolysin. Unlike the reduced activity in the culture supernatants, the cell-associated hemolytic activity in these mutants was identical to that in the parental strain, thus reflecting a normal intracellular toxin activation event. Treatment of the secreted toxin with guanidine hydrochloride significantly restored cytolytic activity, suggesting that the diminished activity may have been due to the aggregation or misfolding of the toxin molecules. Consistent with this notion, sedimentation and filtration analyses showed that alpha-hemolysin secreted from the mutant strain has a mass greater than that secreted from the parental strain. Experiments designed to monitor the time course of alpha-hemolysin release showed delayed appearance of toxin in the culture supernatant of the mutant strain, thus indicating a possible defect in alpha-hemolysin translocation or release. Eight different TolC substitutions displaying this toxin secretion defect were scattered throughout the protein, of which six localized in the periplasmically exposed alpha-helical domain, while the remaining two mapped within the outer membrane-embedded beta-barrel domain of TolC. A plausible model for the secretion of inactive alpha-hemolysin in these TolC mutants is discussed in the context of the recently determined three-dimensional structure of TolC.

Site-directed mutagenesis of acyl carrier protein (ACP) reveals amino acid residues involved in ACP structure and acyl-ACP synthetase activity

Acyl carrier protein (ACP) interacts with many different enzymes during the synthesis of fatty acids, phospholipids, and other specialized products in bacteria. To examine the structural and functional roles of amino acids previously implicated in interactions between the ACP polypeptide and fatty acids attached to the phosphopantetheine prosthetic group, recombinant Vibrio harveyi ACP and mutant derivatives of conserved residues Phe-50, Ile-54, Ala-59, and Tyr-71 were prepared from glutathione S-transferase fusion proteins. Circular dichroism revealed that, unlike Escherichia coli ACP, V. harveyi-derived ACPs are unfolded at neutral pH in the absence of divalent cations; all except F50A and I54A recovered native conformation upon addition of MgCl(2). Mutant I54A was not processed to the holo form by ACP synthase. Some mutations significantly decreased catalytic efficiency of ACP fatty acylation by V. harveyi acyl-ACP synthetase relative to recombinant ACP, e.g. F50A (4%), I54L (20%), and I54V (31%), whereas others (V12G, Y71A, and A59G) had less effect. By contrast, all myristoylated ACPs examined were effective substrates for the luminescence-specific V. harveyi myristoyl-ACP thioesterase. Conformationally sensitive gel electrophoresis at pH 9 indicated that fatty acid attachment stabilizes mutant ACPs in a chain length-dependent manner, although stabilization was decreased for mutants F50A and A59G. Our results indicate that (i) residues Ile-54 and Phe-50 are important in maintaining native ACP conformation, (ii) residue Ala-59 may be directly involved in stabilization of ACP structure by acyl chain binding, and (iii) acyl-ACP synthetase requires native ACP conformation and involves interaction with fatty acid binding pocket residues, whereas myristoyl-ACP thioesterase is insensitive to acyl donor structure.

Beyond the specific plaque hypothesis: are highly leukotoxic strains of Actinobacillus actinomycetemcomitans a paradigm for periodontal pathogenesis?

Actinobacillus actinomycetemcomitans is a facultative anaerobe implicated in a variety of periodontal diseases. Its presence is most closely associated with localized juvenile periodontitis (LIP), although the exact role of the organism in this and other periodontal diseases is not entirely clear. While A. actinomycetemcomitans produces several different putative virulence factors, the most widely studied is the leukotoxin. The leukotoxin selectively kills polymorphonuclear leukocytes and macrophages in vitro, constituting the host's first line of defense. Interestingly, even though all strains of A. actinomycetemcomitans have the genes encoding the leukotoxin, there is variability in leukotoxin expression. Differences in the structure of the promoter region of the leukotoxin gene operon were shown to correlate directly with levels of leukotoxin production. Highly leukotoxic forms appear to exhibit increased pathogenic potential, as evidenced by recent studies that have shown a significant association between the prevalence of such strains and the occurrence of LIP in several different populations. This represents the first demonstration of an association between a particular subset of a pathogenic species and a specific periodontal disease. Early identification of A. actinomycetemcomitans by microbial and genetic assays to evaluate leukotoxicity may enhance the efficacy of preventive and/or therapeutic techniques. Future investigations should continue to evaluate pathogenic variations of additional virulence factors expressed in vivo, not only of A. actinomycetemcomitans, but also of other periodontal bacteria and infectious disease pathogens.

Membrane interaction of Escherichia coli hemolysin: flotation and insertion-dependent labeling by phospholipid vesicles

The 1,024-amino-acid acylated hemolysin of Escherichia coli subverts host cell functions and causes cell lysis. Both activities require insertion of the toxin into target mammalian cell membranes. To identify directly the principal toxin sequences dictating membrane binding and insertion, we assayed the lipid bilayer interaction of native protoxin, stably active toxin, and recombinant peptides. Binding was assessed by flotation of protein-liposome mixtures through density gradients, and insertion was assessed by labeling with a photoactivatable probe incorporated into the target lipid bilayer. Both the active acylated hemolysin and the inactive unacylated protoxin were able to bind and also insert. Ca(2+) binding, which is required for toxin activity, did not influence the in vitro interaction with liposomes. Three overlapping large peptides were expressed separately. A C-terminal peptide including residues 601 to 1024 did not interact in either assay. An internal peptide spanning residues 496 to 831, including the two acylation sites, bound to phospholipid vesicles and showed a low level of insertion-dependent labeling. In vitro acylation had no effect on the bilayer interaction of either this peptide or the full-length protoxin. An N-terminal peptide comprising residues 1 to 520 also bound to phospholipid vesicles and showed strong insertion-dependent labeling, ca. 5- to 25-fold that of the internal peptide. Generation of five smaller peptides from the N-terminal region identified the principal determinant of lipid insertion as the hydrophobic sequence encompassing residues 177 to 411, which is conserved among hemolysin-related toxins.

Cloning and characterization of a Moraxella bovis cytotoxin gene

OBJECTIVE

To identify the Moraxella bovis cytotoxin gene.

PROCEDURE

Hemolytic and nonhemolytic strains of M. bovis were compared by use of western blotting to identify proteins unique to hemolytic strains. Oligonucleotide primers, designed on the basis of amino acid sequences of 2 tryptic peptides derived from 1 such protein and conserved regions of the C and B genes from members of the repeats in the structural toxin (RTX) family of bacterial toxins, were used to amplify cytotoxin-specific genes from M. bovis genomic DNA. Recombinant proteins were expressed, and antisera against these proteins were produced in rabbits.

RESULTS

Several proteins ranging in molecular mass from 55 to 75 kd were unique to the hemolytic strain. An open reading frame encoding a 927-amino acid protein with a predicted molecular mass of 98.8 kd was amplified from M. bovis genomic DNA. The deduced amino acid sequence encoded by this open reading frame was homologous to RTX toxins. Antisera against the recombinant carboxy terminus encoded by this open reading frame neutralized hemolytic and cytolytic activities of native M. bovis cytotoxin.

CONCLUSIONS AND CLINICAL RELEVANCE

A gene was identified in M bovis that encodes a protein with sequence homology to other RTX toxins. Results of cytotoxin neutralization assays support the hypothesis that M. bovis cytotoxin is encoded by this gene and belongs in the RTX family of bacterial exoproteins. Identification of this gene and expression of recombinant cytotoxin could facilitate the development of improved vaccines against infectious bovine keratoconjunctivitis.

RTX toxin structure and function: a story of numerous anomalies and few analogies in toxin biology

It can be agreed that RTX toxins contribute to the pathogenesis of different diseases by causing dysfunction of the general cellular reactions of the immune response. The suggestion that RTX toxins induce cytokine production in nonimmune cells that would ultimately cause tissue damage is an expansion of their role in disease pathogenesis (Uhlen et al. 2000). Investigators in the RTX toxin field may not agree with me, but precise and satisfactory answers to the following questions are not yet available. How do RTX toxins mechanistically damage a cell? Do RTX toxins have receptors in the classic sense, in which there is a reversible ligand and receptor complex? What is responsible for the common Ca2+ ion influx in affected cells? The recent observation that an RTX toxin stimulates host-cell-mediated Ca2+ ion oscillation in part challenges the long held concept that these toxins damage cells by the direct formation of pores. Are the Ca2+ ion fluxes truly the noxious cellular insult? What is the final molecular structure of RTX toxins at the time they cause cellular death? How does the common requirement for acyl modification among RTX toxins fit into the toxin structure and mechanism of cellular killing, particularly when mixtures of unusual fatty acids are used by some toxins? There are a number of outstanding laboratories throughout the world that are seeking answers to these questions. We can reasonably expect that during the next decade research on the structure and function of RTX toxins will lead to new chemotherapeutic targets and reagents for basic cell biology and biotechnology.

Cloning and characterization of a phosphopantetheinyl transferase from Streptomyces verticillus ATCC15003, the producer of the hybrid peptide-polyketide antitumor drug bleomycin

BACKGROUND

Phosphopantetheinyl transferases (PPTases) catalyze the posttranslational modification of carrier proteins by the covalent attachment of the 4'-phosphopantetheine (P-pant) moiety of coenzyme A to a conserved serine residue, a reaction absolutely required for the biosynthesis of natural products including fatty acids, polyketides, and nonribosomal peptides. PPTases have been classified according to their carrier protein specificity. In organisms containing multiple P-pant-requiring pathways, each pathway has been suggested to have its own PPTase activity. However, sequence analysis of the bleomycin biosynthetic gene cluster in Streptomyces verticillus ATCC15003 failed to reveal an associated PPTase gene.

RESULTS

A general approach for cloning PPTase genes by PCR was developed and applied to the cloning of the svp gene from S. verticillus. The svp gene is mapped to an independent locus not clustered with any of the known NRPS or PKS clusters. The Svp protein was overproduced in Escherichia coli, purified to homogeneity, and shown to be a monomer in solution. Svp is a PPTase capable of modifying both type I and type II acyl carrier proteins (ACPs) and peptidyl carrier proteins (PCPs) from either S. verticillus or other Streptomyces species. As compared to Sfp, the only 'promiscuous' PPTase known previously, Svp displays a similar catalytic efficiency (k(cat)/K(m)) for the BlmI PCP but a 346-fold increase in catalytic efficiency for the TcmM ACP.

CONCLUSIONS

PPTases have recently been re-classified on a structural basis into two subfamilies: ACPS-type and Sfp-type. The development of a PCR method for cloning Sfp-type PPTases from actinomycetes, the recognition of the Sfp-type PPTases to be associated with secondary metabolism with a relaxed carrier protein specificity, and the availability of Svp, in addition to Sfp, should facilitate future endeavors in engineered biosynthesis of peptide, polyketide, and, in particular, hybrid peptide-polyketide natural products.

In vivo proteolytic degradation of the Escherichia coli acyltransferase HlyC

Escherichia coli hemolysin (HlyA) is the prototype toxin of a major family of exoproteins produced by Gram-negative bacteria known as "repeats in toxins." Only fatty acid-acylated HlyA molecules at residues Lys564 and Lys690 are able to damage the target cell membrane. Fatty acylation of pro-HlyA is dependent on the co-synthesized acyltransferase HlyC and the acylated form of acyl-carrier protein. By using a collection of hlyA and hlyC mutant strains, the processing of HlyC was investigated. HlyC was not detected by Western blot in an E. coli strain encoding hlyC and hlyA, but it was present in a strain encoding only hlyC. The hlyC mRNA pattern, however, was similar in both strains indicating that the turnover of HlyC does not occur at the transcriptional level. HlyC was detected in Western blots of cell lysates from an E. coli strain encoding HlyC and a HlyA derivative where both acylation sites were substituted. Similar results were obtained when HlyC was expressed in a hlyA mutant strain lacking part of a putative HlyC binding domain, indicating that this particular HlyA region affects HlyC stability. We did not detect HlyC in cell lysates from hlyC mutants with different abilities to acylate pro-HlyA, suggesting that the degradation of HlyC is not related to the HlyA acylation process. The protease systems ClpAP, ClpXP, and FtsH were found to be responsible for the HlyA-dependent processing of HlyC.

Solution structure of B. subtilis acyl carrier protein

BACKGROUND

Acyl carrier protein (ACP) is a fundamental component of fatty acid biosynthesis in which the fatty acid chain is elongated by the fatty acid synthetase system while attached to the 4'-phosphopantetheine prosthetic group (4'-PP) of ACP. Activation of ACP is mediated by holo-acyl carrier protein synthase (ACPS) when ACPS transfers the 4'-PP moiety from coenzyme A (CoA) to Ser36 of apo-ACP. Both ACP and ACPS have been identified as essential for E. coli viability and potential targets for development of antibiotics.

RESULTS

The solution structure of B. subtilis ACP (9 kDa) has been determined using two-dimensional and three-dimensional heteronuclear NMR spectroscopy. A total of 22 structures were calculated by means of hybrid distance geometry-simulated annealing using a total of 1,050 experimental NMR restraints. The atomic rmsd about the mean coordinate positions for the 22 structures is 0.45 +/- 0.08 A for the backbone atoms and 0.93 +/- 0.07 A for all atoms. The overall ACP structure consists of a four alpha-helical bundle in which 4'-PP is attached to the conserved Ser36 that is located in alpha helix II.

CONCLUSIONS

Structural data were collected for both the apo and holo forms of ACP that suggest that the two forms of ACP are essentially identical. Comparison of the published structures for E. coli ACP and actinorhodin polyketide synthase acyl carrier protein (act apo-ACP) from Streptomyces coelicolor A3(2) with B. subtilis ACP indicates similar secondary structure elements but an extremely large rmsd between the three ACP structures (>4.3 A). The structural difference between B. subtilis ACP and both E. coli and act apo-ACP is not attributed to an inherent difference in the proteins, but is probably a result of a limitation in the methodology available for the analysis for E. coli and act apo-ACP. Comparison of the structure of free ACP with the bound form of ACP in the ACP-ACPS complex reveals a displacement of helix II in the vicinity of Ser36. The induced perturbation of ACP by ACPS positions Ser36 proximal to coenzyme A and aligns the dipole of helix II to initiate transfer of 4'-PP to ACP.

Mass spectrometric analysis of recombinant adenylate cyclase toxin from Bordetella pertussis strain 18323/pHSP9

The adenylate cyclase toxin-hemolysin (ACT) is a key virulence factor of the whooping cough agent Bordetella pertussis (Bp). The major cytotoxic activity of this 1706-residue protein consists of its capacity to invade a variety of eukaryotic cells directly across their cytoplasmic membrane and to deliver into cells a catalytic adenylate cyclase domain. This causes impairment of immune effector cells and apoptosis of lung macrophages by uncontrolled conversion of ATP to cAMP. The adenylate cyclase toxin-hemolysin acquires biological activity upon post-translational amide-linked palmitoylation of the epsilon-amino group of lysine 983 (K983) by the accessory fatty acyltransferase, CyaC. However, an additional conserved acylation site can be identified in ACT at lysine 860 (K860) and this residue is palmitoylated when recombinant ACT is produced in Escherichia coli (r-Ec-ACT). In this paper we report the double acylation of r-Bp-ACT secreted by a recombinant Bp strain 18323/pHSP9. This strain overproduces ACT from an oligocopy plasmid carrying the entire cya locus of Bordetella pertussis 18323. Palmitoylation of both conserved lysines (K860 and K983) of r-Bp-ACT expressed by this Bp strain was found. In addition, an error in the deduced protein sequence was identified, with Leu being the real residue at position 1001 and not the Val residue given in the published gene sequence. We also discuss these results in comparison with those from recombinant ACT expressed in E. coli strain K12 XL1-Blue. The analytical approach for characterization of the fatty acylation of ACT from strain 18323/pHSP9 consisted of multiple proteolytic digestion procedures (trypsin, Asp-N), microcapillary liquid chromatography/tandem mass spectrometry and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry.

Eagle-type methicillin resistance: new phenotype of high methicillin resistance under mec regulator gene control

We report a novel phenotype of methicillin resistance, designated "Eagle-type" resistance, which is characteristic in its resistance to high concentrations of methicillin (64 to 512 microg/ml) and susceptibility to low concentrations of methicillin (2 to 16 microg/ml). The type of resistance was expressed in mutant strains selected with high concentrations (e.g., 128 to 512 microg/ml) of methicillin from the pre-methicillin-resistant Staphylococcus aureus strain N315, whose mecA gene transcription is strongly repressed by the mecI gene-encoded repressor protein MecI. The Eagle-type mutant strains harbored no mutation in the mecI gene or in the operator region of mecA gene to which MecI repressor is supposed to bind. In the representative Eagle-type strain h4, repression of mecA gene transcription and penicillin-binding protein 2' production were found to be released by exposing the cells to a high concentration (128 microg/ml) of methicillin but not to lower concentrations (1 and 8 microg/ml) of methicillin. The strain h4 expressed paradoxical susceptibility (Eagle effect) to the cytokilling activity of methicillin. Experimental deletion of mecI gene from the chromosome of h4 by mecI-specific gene substitution converted its Eagle-type resistance to homogeneously high methicillin resistance. We cloned two novel genes, designated hmrA and hmrB, from genomic library of h4, which conferred Eagle-type resistance to N315 when introduced into the cell in multiple copies. The genes were shown to confer homogeneous methicillin resistance to the heterogeneously methicillin-resistant strain LR5 when they were introduced into on multicopy plasmids. This result strongly indicated that the genetic alteration responsible for the expression of the Eagle phenotype is identical, or equivalent in its effect, to the genetic alteration underlying heterogeneous-to-homogeneous conversion of methicillin resistance in S. aureus.

Interaction of Escherichia coli hemolysin with biological membranes. A study using cysteine scanning mutagenesis

Escherichia coli hemolysin (HlyA) is a membrane-permeabilizing protein belonging to the family of RTX-toxins. Lytic activity depends on binding of Ca2(+) to the C-terminus of the molecule. The N-terminus of HlyA harbors hydrophobic sequences that are believed to constitute the membrane-inserting domain. In this study, 13 HlyA cysteine-replacement mutants were constructed and labeled with the polarity-sensitive fluorescent probe 6-bromoacetyl-2-dimethylaminonaphthalene (badan). The fluorescence emission of the label was examined in soluble and membrane-bound toxin. Binding effected a major blue shift in the emission of six residues within the N-terminal hydrophobic domain, indicating insertion of this domain into the lipid bilayer. The emission shifts occurred both in the presence and absence of Ca2(+), suggesting that Ca2(+) is not required for the toxin to enter membranes. However, binding of Ca2(+) to HlyA in solution effected conformational changes in both the C-terminal and N-terminal domain that paralleled activation. Our data indicate that binding of Ca2(+) to the toxin in solution effects a conformational change that is relayed to the N-terminal domain, rendering it capable of adopting the structure of a functional pore upon membrane binding.

Acylation of lysine 983 is sufficient for toxin activity of Bordetella pertussis adenylate cyclase. Substitutions of alanine 140 modulate acylation site selectivity of the toxin acyltransferase CyaC

The capacity of adenylate cyclase toxin (ACT) to penetrate into target cells depends on post-translational fatty-acylation by the acyltransferase CyaC, which can palmitoylate the conserved lysines 983 and 860 of ACT. Here, the in vivo acylating capacity of a set of mutated CyaC acyltransferases was characterized by two-dimensional gel electrophoresis and mass spectrometric analyses of the ACT product. Substitutions of the potentially catalytic serine 20 and histidine 33 residues ablated acylating activity of CyaC. Conservative replacements of alanine 140 by glycine (A140G) and valine (A140V) residues, however, affected selectivity of CyaC for the two acylation sites on ACT. Activation by the A140G variant of CyaC generated a mixture of bi- and monoacylated ACT molecules, modified either at both Lys-860 and Lys-983, or only at Lys-860, respectively. In contrast, the A140V CyaC produced a nearly 1:1 mixture of nonacylated pro-ACT with ACT monoacylated almost exclusively at Lys-983. The respective proportion of toxin molecules acylated at Lys-983 correlated well with the cell-invasive activity of both ACT mixtures, which was about half of that of ACT fully acylated on Lys-983 by intact CyaC. These results show that acylation of Lys-860 alone does not confer cell-invasive activity on ACT, whereas acylation of Lys-983 is necessary and sufficient.

Expression, Purification, and Characterization of BioI: A Carbon–Carbon Bond Cleaving Cytochrome P450 Involved in Biotin Biosynthesis in Bacillus subtilis

Pimelic acid formation for biotin biosynthesis in Bacillus subtilis has been proposed to involve a cytochrome P450 encoded by the gene bioI. We have subcloned biol and overexpressed the encoded protein, Biol. A purification protocol was developed utilizing ion exchange, gel filtration, and hydroxyapatite chromatography. Investigation of the purified BioI by UV-visible spectroscopy revealed spectral properties characteristic of a cytochrome P450 enzyme. BioI copurifies with acylated Escherichia coli acyl carrier protein (ACP), suggesting that in vivo a fatty acid substrate may be presented to BioI as an acyl-ACP. A combination of electrospray mass spectrometry of the intact acyl-ACP and GCMS indicated a range of fatty acids were bound to the ACP. A catalytically active system has been established employing E. coli flavodoxin reductase and a novel, heterologous flavodoxin as the redox partners for BioI. In this system, BioI cleaves a carbon-carbon bond of an acyl-ACP to generate a pimeloyl-ACP equivalent, from which pimelic acid is isolated after base-catalyzed saponification. A range of free fatty acids have also been explored as potential alternative substrates for BioI, with C16 binding most tightly to the enzyme. These fatty acids are also metabolized to dicarboxylic acids, but with less regiospecificity than is observed with acyl-ACPs. A possible mechanism for this transformation is discussed. These results strongly support the proposed role for BioI in biotin biosynthesis. In addition, the production of pimeloyl-ACP explains the ability of BioI to function as a pimeloyl CoA source in E. coli, which, unlike B. subtilis, is unable to utilize free pimelic acid for biotin production.

Escherichia coli alpha-hemolysin (HlyA) is heterogeneously acylated in vivo with 14-, 15-, and 17-carbon fatty acids

alpha-Hemolysin (HlyA) is a secreted protein virulence factor observed in certain uropathogenic strains of Escherichia coli. The active, mature form of HlyA is produced by posttranslational modification of the protoxin that is mediated by acyl carrier protein and an acyltransferase, HlyC. We have now shown using mass spectrometry that these modifications, when observed in protein isolated in vivo, consist of acylation at the epsilon-amino groups of two internal lysine residues, at positions 564 and 690, with saturated 14- (68%), 15- (26%), and 17- (6%) carbon amide-linked side chains. Thus, HlyA activated in vivo consists of a heterogeneous family of up to nine different covalent structures, and the substrate specificity of the HlyC acyltransferase appears to differ from that of the closely related CyaC acyltransferase expressed by Bordetella pertussis.

Secretion of RTX leukotoxin by Actinobacillus actinomycetemcomitans

Actinobacillus actinomycetemcomitans, the etiologic agent for localized juvenile periodontitis and certain other human infections, such as endocarditis, expresses a leukotoxin that acts on polymorphonuclear leukocytes and macrophages. Leukotoxin is a member of the highly conserved repeat toxin (RTX) family of bacterial toxins expressed by a variety of pathogenic bacteria. While the RTX toxins of other bacterial species are secreted, the leukotoxin of A. actinomycetemcomitans is thought to remain associated with the bacterial cell. We have examined leukotoxin production and localization in rough (adherent) and smooth (nonadherent) strains of A. actinomycetemcomitans. We found that leukotoxin expressed by the rough, adherent, clinical isolate CU1000N is indeed cell associated, as expected. However, we were surprised to find that smooth, nonadherent strains of A. actinomycetemcomitans, including Y4, JP2 (a strain expressing a high level of toxin), and CU1060N (an isogenic smooth variant of CU1000N), secrete an abundance of leukotoxin into the culture supernatants during early stages of growth. After longer times of incubation, leukotoxin disappears from the supernatants, and its loss is accompanied by the appearance of a number of low-molecular-weight polypeptides. The secreted leukotoxin is active, as evidenced by its ability to kill HL-60 cells in vitro. We found that the growth phase and initial pH of the growth medium significantly affect the abundance of secreted leukotoxin, and we have developed a rapid (<2 h) method to partially purify large amounts of leukotoxin. Remarkably, mutations in the tad genes, which are required for tight nonspecific adherence of A. actinomycetemcomitans to surfaces, cause leukotoxin to be released from the bacterial cell. These studies show that A. actinomycetemcomitans has the potential to secrete abundant leukotoxin. It is therefore appropriate to consider a possible role for leukotoxin secretion in the pathogenesis of A. actinomycetemcomitans.

Expression and purification of four different rhizobial acyl carrier proteins

In rhizobia, besides the constitutive acyl carrier protein (AcpP) involved in the biosynthesis and transfer of common fatty acids, there are at least three specialized acyl carrier proteins (ACPs): (1) the flavonoid-inducible nodulation protein NodF; (2) the RkpF protein, which is required for the biosynthesis of rhizobial capsular polysaccharides; and (3) AcpXL, which transfers 27-hydroxyoctacosanoic acid to a sugar backbone during lipid A biosynthesis. Whereas the nucleotide sequences encoding the three specialized ACPs are known, only the amino acid sequence of the AcpP of Sinorhizobium meliloti was available. In this study, using reverse genetics, the genes for the constitutive AcpPs of S. meliloti and of Rhizobium leguminosarum were cloned and sequenced. Previously, it had been shown that NodF and RkpF can be overproduced in Escherichia coli using the T7 polymerase expression system. Using the same system, the constitutive AcpPs of S. meliloti and of R. leguminosarum, together with the specialized ACP AcpXL, were overproduced and purified. All the known ACPs of rhizobia can be labelled in vivo during expression in E. coli with radioactive beta-alanine added to the growth medium due to their modification with a 4'-phosphopantetheine prosthetic group. The availability of all functionally different ACPs should help to unravel how different fatty acids are targeted towards different biosynthetic pathways in one organism.

Conformational studies of Actinobacillus actinomycetemcomitans leukotoxin: partial denaturation enhances toxicity

A 114 kDa, water-soluble, cytotoxin secreted by the Gram-negative bacterium Actinobacillus actinomycetemcomitans (Aa) is similar in sequence to Escherichia coli alpha-hemolysin, but is non-hemolytic, killing leukocytes of select species, including humans. In this work, we investigated aspects of the water-soluble conformation of Aa toxin which relate to its biological, pore-forming activity. The toxin has five native tryptophans and fluorescence spectra were monitored in aqueous solutions in the presence of varying denaturants. Significant changes in the fluorescence spectra, without significant wavelength shifts, were induced by small additions of denaturants and changes in the temperature or pH. The fluorescence changes suggested that small perturbations in the aqueous environment resulted in structural changes in the toxin related not to a large unfolding but to more subtle conformational changes. Analytical ultracentrifugation showed the toxin to be a globular monomer in dilute aqueous solution. Circular dichroism spectroscopy showed about 25% alpha-helical structure which is largely maintained up to a temperature (65 degrees C) known to deactivate toxin activity. Changes in the cytotoxic properties of the toxin were monitored with flow cytometric analysis following preincubation of the toxin under mild conditions similar to those used in the fluorescence studies. These experiments showed that the pretreated toxin exhibited enhanced cell-killing potency on toxin-sensitive cells. The correlation of cytotoxicity with the changes in Trp fluorescence is consistent with the idea that partial unfolding of Aa toxin is an early, obligate step in toxin-induced cell kill.

Actinobacillus species and their role in animal disease

Actinobacillus species are Gram-negative bacteria responsible for several quite distinct disease conditions of animals. The natural habitat of the organisms is primarily the upper respiratory tract and oral cavity. A. lignieresii is the cause of actinomycosis (wooden tongue) in cattle: a sporadic, insidiously-developing granulomatous infection. In sharp contrast is A. pleuropneumoniae which is responsible for a rapidly spreading often fatal pneumonia, common among intensively reared pigs. Detailed investigation of this organism has provided a much clearer picture of the bacterial factors involved in causing disease. A. equuli similarly causes a potent septicaemia in the neonatal foal; growing apparently unrestricted once infection occurs. Other members of the genus induce characteristic pathogenesis in their preferred host, with one, A. actinomycetemcomitans, being a cause of human periodontal disease. This article reviews recent understanding of the taxonomy and bacteriology of the organisms, and the aetiology, pathogenicity, diagnosis and control of animal disease caused by Actinobacillus species.

An ordered reaction mechanism for bacterial toxin acylation by the specialized acyltransferase HlyC: formation of a ternary complex with acylACP and protoxin substrates

The 110 kDa haemolysin protoxin (proHlyA) is activated in the Escherichia coli cytosol by acyl carrier protein-dependent fatty acylation of two internal lysine residues, directed by the co-synthesized protein HlyC. Using an in vitro maturation reaction containing purified protoxin peptides and acylACP, we show unambiguously that HlyC possesses an apparently unique acyltransferase activity fully described by Michaelis-Menten analysis. The Vmax of HlyC at saturating levels of both substrates was approximately 115 nmol acyl group min-1 mg-1 with KMacylACP of 260 nM and KMproHlyA of 27 nM, kinetic parameters sufficient to explain why in vivo HlyC is required at a concentration equimolar to proHlyA. HlyC bound the fatty acyl group from acylACP to generate an acylated HlyC intermediate that was depleted in the presence of proHlyA, but enriched in the presence of proHlyA derivatives lacking acylation target sites. HlyC was also able to bind in vivo 4'-phosphopantetheine. Substitution of conserved amino acids that could act as putative covalent attachment sites did not prevent binding of the fatty acyl or 4'-phosphopantetheine groups. These data and substrate variation analyses suggest that the unique acylation reaction does not involve covalent attachment of fatty acid to the acyltransferase, but rather that it proceeds via a sequential ordered Bi-Bi reaction mechanism, requiring the formation of a non-covalent ternary acylACP-HlyC-proHlyA complex.

Characterization of a Pseudomonas aeruginosa fatty acid biosynthetic gene cluster: purification of acyl carrier protein (ACP) and malonyl-coenzyme A:ACP transacylase (FabD)

A DNA fragment containing the Pseudomonas aeruginosa fabD (encoding malonyl-coenzyme A [CoA]:acyl carrier protein [ACP] transacylase), fabG (encoding beta-ketoacyl-ACP reductase), acpP (encoding ACP), and fabF (encoding beta-ketoacyl-ACP synthase II) genes was cloned and sequenced. This fab gene cluster is delimited by the plsX (encoding a poorly understood enzyme of phospholipid metabolism) and pabC (encoding 4-amino-4-deoxychorismate lyase) genes; the fabF and pabC genes seem to be translationally coupled. The fabH gene (encoding beta-ketoacyl-ACP synthase III), which in most gram-negative bacteria is located between plsX and fabD, is absent from this gene cluster. A chromosomal temperature-sensitive fabD mutant was obtained by site-directed mutagenesis that resulted in a W258Q change. A chromosomal fabF insertion mutant was generated, and the resulting mutant strain contained substantially reduced levels of cis-vaccenic acid. Multiple attempts aimed at disruption of the chromosomal fabG gene were unsuccessful. We purified FabD as a hexahistidine fusion protein (H6-FabD) and ACP in its native form via an ACP-intein-chitin binding domain fusion protein, using a novel expression and purification scheme that should be applicable to ACP from other bacteria. Matrix-assisted laser desorption-ionization spectroscopy, native polyacrylamide electrophoresis, and amino-terminal sequencing revealed that (i) most of the purified ACP was properly modified with its 4'-phosphopantetheine functional group, (ii) it was not acylated, and (iii) the amino-terminal methionine was removed. In an in vitro system, purified ACP functioned as acyl acceptor and H(6)-FabD exhibited malonyl-CoA:ACP transacylase activity.

The interaction between RTX toxins and target cells

RTX toxins are important virulence factors produced by a wide range of Gram-negative bacteria. They fall into two categories: the hemolysins, which affect a variety of cell types, and the leukotoxins, which are cell-type- and species-specific. These toxins offer interesting models for targeting, insertion and translocation of aqueous proteins into lipid membranes.

Identification of genetic determinants for the hemolytic activity of Streptococcus agalactiae by ISS1 transposition

Streptococcus agalactiae is a poorly transformable bacterium and studies of molecular mechanisms are difficult due to the limitations of genetic tools. Employing the novel pGh9:ISS1 transposition vector we generated plasmid-based mutant libraries of S. agalactiae strains O90R and AC475 by random chromosomal integration. A screen for mutants with a nonhemolytic phenotype on sheep blood agar led to the identification of a genetic locus harboring several genes that are essential for the hemolytic function and pigment production of S. agalactiae. Nucleotide sequence analysis of nonhemolytic mutants revealed that four mutants had distinct insertion sites in a single genetic locus of 7 kb that was subsequently designated cyl. Eight different open reading frames were identified: cylX, cylD, cylG, acpC, cylZ, cylA, cylB, and cylE, coding for predicted proteins with molecular masses of 11, 33, 26, 11, 15, 35, 32, and 78 kDa, respectively. The deduced amino acid sequence of the protein encoded by cylA harbors a conserved ATP-binding cassette (ABC) motif, and the predicted proteins encoded by cylA and cylB have significant similarities to the nucleotide binding and transmembrane proteins of typical ABC transporter systems. Transcription analysis by reverse transcription-PCR suggests that cylX to cylE are part of an operon. The requirement of acpC and cylZABE for hemolysin production of S. agalactiae was confirmed either by targeted mutagenesis with the vector pGh5, complementation studies with pAT28, or analysis of insertion elements in naturally occurring nonhemolytic mutants.

The conserved lysine 860 in the additional fatty-acylation site of Bordetella pertussis adenylate cyclase is crucial for toxin function independently of its acylation status

The Bordetella pertussis RTX (repeat in toxin family protein) adenylate cyclase toxin-hemolysin (ACT) acquires biological activity upon a single amide-linked palmitoylation of the epsilon-amino group of lysine 983 (Lys983) by the accessory fatty-acyltransferase CyaC. However, an additional conserved RTX acylation site can be identified in ACT at lysine 860 (Lys860), and this residue becomes palmitoylated when recombinant ACT (r-Ec-ACT) is produced together with CyaC in Escherichia coli K12. We have eliminated this additional acylation site by replacing Lys860 of ACT with arginine, leucine, and cysteine residues. Two-dimensional gel electrophoresis and microcapillary high performance liquid chromatography/tandem mass spectrometric analyses of mutant proteins confirmed that the two sites are acylated independently in vivo and that mutations of Lys860 did not affect the quantitative acylation of Lys983 by palmitoyl (C16:0) and palmitoleil (cis Delta9 C16:1) fatty-acyl groups. Nevertheless, even the most conservative substitution of lysine 860 by an arginine residue caused a 10-fold decrease of toxin activity. This resulted from a 5-fold reduction of cell association capacity and a further 2-fold reduction in cell penetration efficiency of the membrane-bound K860R toxin. These results suggest that lysine 860 plays by itself a crucial structural role in membrane insertion and translocation of the toxin, independently of its acylation status.

Vaccination and protection of pigs against pleuropneumonia with a vaccine strain of Actinobacillus pleuropneumoniae produced by site-specific mutagenesis of the ApxII operon

The production of toxin (Apx)-neutralizing antibodies during infection plays a major role in the induction of protective immunity to Actinobacillus pleuropneumoniae reinfection. In the present study, the gene encoding the ApxII-activating protein, apxIIC, was insertionally inactivated on the chromosome of a serovar 7 strain, HS93. Expression of the structural toxin, ApxIIA, and of the two genes required for its secretion, apxIB and apxID, still occurs in this strain. The resulting mutant strain, HS93C- Ampr, was found to secrete the unactivated toxin. Pigs vaccinated with live HS93C- Ampr via the intranasal route were protected against a cross-serovar challenge with a virulent serovar 1 strain of A. pleuropneumoniae. This is the first reported vaccine strain of A. pleuropneumoniae which can be delivered live to pigs and offers cross-serovar protection against porcine pleuropneumonia.

Characterization of the C-terminal domain essential for toxic activity of adenylate cyclase toxin

Adenylate cyclase toxin (CyaA) of Bordetella pertussis belongs to the RTX family of toxins. These toxins are characterized by a series of glycine- and aspartaterich nonapeptide repeats located at the C-terminal half of the toxin molecules. For activity, RTX toxins require Ca2+, which is bound through the repeat region. Here, we identified a stretch of 15 amino acids (block A) that is located C-terminally to the repeat and is essential for the toxic activity of CyaA. Block A is required for the insertion of CyaA into the plasma membranes of host cells. Mixing of a short polypeptide composed of block A and eight Ca2+ binding repeats with a mutant of CyaA lacking block A restores toxic activity fully. This in vitro interpolypeptide complementation is achieved only when block A is present together with the Ca2+ binding repeats on the same polypeptide. Neither a short polypeptide composed of block A only nor a polypeptide consisting of eight Ca2+ binding repeats, or a mixture of these two polypeptides, complement toxic activity. It is suggested that functional complementation occurs because of binding between the Ca2+ binding repeats of the short C-terminal polypeptide and the Ca2+ binding repeats of the CyaA mutant lacking block A.

Target cell specificity of the Pasteurella haemolytica leukotoxin is unaffected by the nature of the fatty-acyl group used to activate the toxin in vitro

The leukotoxin (LktA) of Pasteurella haemolytica is active only against cells of ruminant origin. It is synthesised as an inactive protoxin encoded by the lktA gene and post-translationally modified to the active toxin by the product of the lktC gene. The LktA and LktC proteins were expressed separately in Escherichia coli and partially purified. Active LktA was produced in vitro in the presence of LktC and acyl-acyl carrier protein (ACP) charged separately in vitro with a fatty-acyl group. The toxic activity and target cell specificity of LktA and adenylate cyclase toxin (CyaA), a toxin active against a wide variety of mammalian cells, were investigated after activation with ACP charged with different fatty acids. Palmitoyl-ACP produced the most active toxin in both cases and, although other fatty acids were also effective, the fatty acid preference was the same for the in vitro activation of both toxins. Activated LktA remained ruminant cell-specific whichever acyl group was used to acylate the A protoxin.

Host proteins can stimulate Tn7 transposition: a novel role for the ribosomal protein L29 and the acyl carrier protein

The bacterial transposon Tn7 is distinguished by its ability to insert at a high frequency into a specific site in the Escherichia coli chromosome called attTn7. Tn7 insertion into attTn7 requires four Tn7-encoded transposition proteins: TnsA, TnsB, TnsC and TnsD. The selection of attTn7 is determined by TnsD, a sequence-specific DNA-binding protein. TnsD binds attTn7 and interacts with TnsABC, the core transposition machinery, which facilitates the insertion of Tn7 into attTn7. In this work, we report the identification of two host proteins, the ribosomal protein L29 and the acyl carrier protein (ACP), which together stimulate the binding of TnsD to attTn7. The combination of L29 and ACP also stimulates Tn7 transposition in vitro. Interestingly, mutations in L29 drastically decrease Tn7 transposition in vivo, and this effect of L29 on Tn7 transposition is specific for TnsABC+D reactions.

Hemolytically active (acylated) alpha-hemolysin elicits interleukin-1beta (IL-1beta) but augments the lethality of Escherichia coli by an IL-1- and tumor necrosis factor-independent mechanism

Many pathogenic Escherichia coli produce the toxin alpha-hemolysin (Hly), and lipopolysaccharide (LPS), interleukin-1 (IL-1), and tumor necrosis factor (TNF) have all been recognized as important effector molecules during infections by gram-negative organisms. Despite the characterization of many in vitro effects of hemolysin, no direct relationship has been established between hemolysin, LPS, proinflammatory cytokine production, and E. coli-induced mortality. Previously, we have shown in vivo that hemolysin elicits a distinct IL-1alpha spike by 4 h into a lethal hemolytic E. coli infection. Using three transformed E. coli strains, WAF108, WAF270, and WAH540 (which produce no Hly [Hlynull], acylated Hly [Hlyactive], or nonacylated Hly [Hlyinactive], respectively), we sought to determine the specific roles of hemolysin acylation, LPS, IL-1, and TNF in mediating the lethality of E. coli infection in mice. WAF270 was 100% lethal in BALB/c, C3H/HeJ, and C57BL/6 mice; in mice pretreated with antibody to the type 1 IL-1 receptor; in type 1 IL-1 receptor-deficient mice; and in dual (type 1 IL-1 receptor-type 1 TNF receptor)-deficient mice at doses which were nonlethal (0%) with both WAF108 and WAH540. At lethal doses, WAF270 killed by 6 +/- 2.3 h while WAF108 and WAH540 killed at 36 +/- 9.4 and 36 +/- 13.8 h, respectively. These differences in mortality were not due to IL-1 or TNF release, and the enhanced expression of LPS, which corresponded to Hly expression, was not likely the primary factor causing mortality. We demonstrate that bacterial fatty acid acylation of hemolysin is required in order for it to elicit IL-1 release by monocytes and to confer its virulence on E. coli.

E. coli alpha-hemolysin: a membrane-active protein toxin

alpha-Hemolysin is synthesized as a 1024-amino acid polypeptide, then intracellularly activated by specific fatty acylation. A second activation step takes place in the extracellular medium through binding of Ca2+ ions. Even in the absence of fatty acids and Ca2+ HlyA is an amphipathic protein, with a tendency to self-aggregation. However, Ca(2+)-binding appears to expose hydrophobic patches on the protein surface, facilitating both self-aggregation and irreversible insertion into membranes. The protein may somehow bind membranes in the absence of divalent cations, but only when Ca2+ (or Sr2+, or Ba2+) is bound to the toxin in aqueous suspensions, i.e., prior to its interaction with bilayers, can alpha-hemolysin bind irreversibly model or cell membranes in such a way that the integrity of the membrane barrier is lost, and cell or vesicle leakage ensues. Leakage is not due to the formation of proteinaceous pores, but rather to the transient disruption of the bilayer, due to the protein insertion into the outer membrane monolayer, and subsequent perturbations in the bilayer lateral tension. Protein or glycoprotein receptors for alpha-hemolysin may exist on the cell surface, but the toxin is also active on pure lipid bilayers.

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.

Incomplete activation of Escherichia coli hemolysin (HlyA) due to mutations in the 3' region of hlyC

Mutational analysis of the carboxy-terminal region of Escherichia coli HlyC was performed by site-directed mutagenesis. Replacement of residue Val-127 or Lys-129 reduced the activity of HlyC to about 30 or 60%, respectively, of that of the wild type, while replacement of Gly-128 reduced the activity to less than 1% of the wild-type level. Complete inactivation of HlyC was caused by a double mutation, replacement of Gly-128 with valine and of Lys-129 with isoleucine. Analysis of culture supernatants from mutants with reduced hemolytic activity by two-dimensional gel electrophoresis revealed the production and simultaneous secretion of nonacylated, monoacylated, and fully acylated HlyA forms, demonstrating impairment of the acylation reaction, possibly due to a decreased affinity of HlyC for the individual HlyA acylation sites.

Hydrodynamic properties of Vibrio harveyi acyl carrier protein and its fatty-acylated derivatives

The amino acid sequence of Vibrio harveyi acyl carrier protein (ACP) is 86% identical to that of Escherichia coli ACP, although five nonconservative amino acid differences are concentrated in the loop region between helices I and II (residues 18-25). We have investigated the influence of these sequence differences on the hydrodynamic properties of the two ACPs and their fatty acylated derivatives. Hydropathy analysis suggests that V. harveyi ACP is more hydrophobic than E. coli ACP in the loop region, a prediction supported by stronger binding of V. harveyi acyl-ACPs (C12 to C16) to octyl-Sepharose. Gel filtration experiments indicated that both ACPs undergo a similar conformational expansion when pH was elevated from 7.5 (R(s) = 24 A) to 9.0 (R(s) = 30 A). Fatty acylation reversed this expansion: R(s) for 16:0-ACP was 12 A, independent of ACP source and pH. By contrast, V. harveyi and E. coli ACPs exhibited distinct gel electrophoretic properties. Fatty acylation of V. harveyi ACP produced a greater increase in mobility on a conformationally sensitive native gel system. Moreover, while both V. harveyi and E. coli ACPs migrated anomalously at 20 kDa on SDS-polyacrylamide gel electrophoresis, they exhibited strikingly different behavior on SDS gels upon acylation with longer chain fatty acids. These results indicate that E. coli and V. harveyi ACPs exhibit similar overall pH- and fatty acid-dependent conformational changes, but gel electrophoresis is more sensitive to structural differences due to variations of hydrophobicity and charge.

Generation of targeted nonpolar gene insertions and operon fusions in Pasteurella haemolytica and creation of a strain that produces and secretes inactive leukotoxin

An efficient method for targeted gene inactivation and generation of chromosomal gene fusions in Pasteurella haemolytica has been devised and used to create an lktC::cat operon fusion by allelic exchange at the leukotoxin gene cluster (lktCABD). A copy of the lktC gene was insertionally inactivated by using a nonpolar, promoterless cat cassette and then delivered into P. haemolytica on a shuttle vector. Plasmid incompatibility was used to detect clones where double recombination events had occurred at the chromosomal locus. The insertion in lktC did not affect expression of the downstream genes, and the mutant strain secreted an antigenic proleukotoxin that was neither leukotoxic nor hemolytic. Expression of the lktC gene in trans restored the wild-type phenotype, confirming that LktC is required for activation of the proleukotoxin to the mature leukotoxin. Construction of the lktC::cat operon fusion allowed us to quantitate leukotoxin promoter activity in P. haemolytica and to demonstrate that transcription was maximal during early logarithmic growth phase but was reduced following entry into late logarithmic phase. This allelic exchange system should be useful for future genetic studies in P. haemolytica and could potentially be applied to other members of Haemophilus-Actinobacillus-Pasteurella family, where genetic manipulation is limited.

Prelytic and lytic conformations of erythrocyte-associated Escherichia coli hemolysin

Flow cytometry was developed as a method to assess the conformation of erythrocyte-bound Escherichia coli hemolysin polypeptide (HlyA). Topology of membrane-associated hemolysin (HlyA(E)) was investigated by testing surface accessibility of HlyA regions in lytic and nonlytic bound states, using a panel of 12 anti-HlyA monoclonal antibodies (MAbs). Hemolysin associates nonlytically with erythrocytes at 0 to 2 degrees C. To test the hypothesis that the nonlytic HlyA(E) conformation at 0 to 2 degrees C differs from the lytic conformation at 23 degrees C, MAb epitope reactivity profiles at the two temperatures were compared by flow cytometry. Four MAbs have distinctly increased reactivity at 0 to 2 degrees C compared to 23 degrees C. HlyA requires HlyC-dependent acylation at lysine residues 563 and 689 for lytic function. Toxin with cysteine substitution mutations at each lysine (HlyA(K563C) and HlyA(K689C)) as well as the nonacylated form of hemolysin made in a HlyC-deficient strain were examined by flow cytometry at 0 to 2 and 23 degrees C. The three mutants bind erythrocytes at wild-type toxin levels, but there are conformational changes reflected by altered MAb epitope accessibility for six of the MAbs. To test further the surface accessibility of regions in the vicinity of MAb-reactive epitopes, HlyA(E) was proteolytically treated prior to testing for MAb reactivity. Differences in protease susceptibility at 0 to 2 degrees and 23 degrees C for the reactivities of three of the MAbs further support the model of two distinct conformations of cell-associated toxin.

Domains of Escherichia coli acyl carrier protein important for membrane-derived-oligosaccharide biosynthesis

Acyl carrier protein participates in a number of biosynthetic pathways in Escherichia coli: fatty acid biosynthesis, phospholipid biosynthesis, lipopolysaccharide biosynthesis, activation of prohemolysin, and membrane-derived oligosaccharide biosynthesis. The first four pathways require the protein's prosthetic group, phosphopantetheine, to assemble an acyl chain or to transfer an acyl group from the thioester linkage to a specific substrate. By contrast, the phosphopantetheine prosthetic group is not required for membrane-derived oligosaccharide biosynthesis, and the function of acyl carrier protein in this biosynthetic scheme is currently unknown. We have combined biochemical and molecular biological approaches to investigate domains of acyl carrier protein that are important for membrane-derived oligosaccharide biosynthesis. Proteolytic removal of the first 6 amino acids from acyl carrier protein or chemical synthesis of a partial peptide encompassing residues 26 to 50 resulted in losses of secondary and tertiary structure and consequent loss of activity in the membrane glucosyltransferase reaction of membrane-derived oligosaccharide biosynthesis. These peptide fragments, however, inhibited the action of intact acyl carrier protein in the enzymatic reaction. This suggests a role for the loop regions of the E. coli acyl carrier protein and the need for at least two regions of the protein for participation in the glucosyltransferase reaction. We have purified acyl carrier protein from eight species of Proteobacteria (including representatives from all four subgroups) and characterized the proteins as active or inhibitory in the membrane glucosyltransferase reaction. The complete or partial amino acid sequences of these acyl carrier proteins were determined. The results of site-directed mutagenesis to change amino acids conserved in active, and altered in inactive, acyl carrier proteins suggest the importance of residues Glu-4, Gln-14, Glu-21, and Asp-51. The first 3 of these residues define a face of acyl carrier protein that includes the beginning of the loop region, residues 16 to 36. Additionally, screening for membrane glucosyltransferase activity in membranes from bacterial species that had acyl carrier proteins that were active with E. coli membranes revealed the presence of glucosyltransferase activity only in the species most closely related to E. coli. Thus, it seems likely that only bacteria from the Proteobacteria subgroup gamma-3 have periplasmic glucans synthesized by the mechanism found in E. coli.

Why do mitochondria synthesize fatty acids? Evidence for involvement in lipoic acid production

The function of acyl carrier protein (ACP) in mitochondria isolated from pea leaves has been investigated. When pea leaf mitochondria were labeled with [2-14C] malonic acid in vitro, radioactivity was incorporated into fatty acids, and, simultaneously, ACP was acylated. [1-14C]Acetate was much less effective as a precursor for fatty acid synthesis, suggesting that mitochondria do not possess acetyl-CoA carboxylase. The incorporation of radioactivity from [2-14C]malonate into fatty acids and the labeling of ACP were inhibited by cerulenin and required ATP and Mg2+. These findings indicate that plant mitochondria contain not only ACP, but all enzymes required for de novo fatty acid synthesis. Over 30% of the radioactive products from pea mitochondria labeled with [2-14C]malonate were recovered in H protein, which is a subunit of glycine decarboxylase and contains lipoic acid as an essential constituent. In similar experiments, the H protein of Neurospora mitochondria was also labeled by [2-14C]malonate. The labeling of pea H protein was inhibited by addition of cerulenin into the assay medium. Together, these findings indicate that ACP is involved in the de novo synthesis of fatty acids in plant mitochondria and that a major function of this pathway is production of lipoic acid precursors.

Delayed-type hypersensitivity activity of the Brucella L7/L12 ribosomal protein depends on posttranslational modification

The ribosomal protein L7/L12 isolated from Brucella melitensis induces a delayed-type hypersensitivity (DTH) reaction in brucella-sensitized guinea pigs. Surprisingly, the recombinant brucella L7/L12 protein expressed in Escherichia coli as a fusion protein with a six-histidine tag cannot elicit such a reaction. The six histidines tagged to the recombinant L7/L12 protein were removed enzymatically, but the resulting protein did not induce a DTH reaction in sensitized animals. Incubation of the recombinant L7/L12 fusion protein in a B. melitensis lysate endowed the recombinant protein with a DTH activity, suggesting that the recombinant protein was modified by this treatment. Glycosylation or phosphorylation of the recombinant L7/L12 protein could not be detected. On the other hand, radiolabeled palmitic acid was found to be incorporated to the recombinant protein during its incubation in the brucella lysate. This incorporation was specific for the brucella L7/L12 protein and was inhibited when the brucella lysate was frozen and thawed before the incubation. The data reported here indicate that posttranslational modification of L7/L12 protein comprising at least an acylation step is required for the brucella L7/L12 DTH activity.

Bordetella pertussis adenylate cyclase toxin: proCyaA and CyaC proteins synthesised separately in Escherichia coli produce active toxin in vitro

Bordetella pertussis produces a cell-invasive adenylate cyclase toxin (CyaA) which is related to the RTX family of pore-forming toxins. Like all RTX toxins, CyaA is synthesised as a protoxin (proCyaA), encoded by the cyaA gene. Activation to the mature cell-invasive toxin involves palmitoylation of lysine 983 and is dependent on co-expression of cyaC. The role of the cyaC gene product in the acylation reaction has not been determined. We have developed an efficient T7 RNA polymerase system for over-expression of cyaA and cyaC separately in Escherichia coli. Each protein accumulated intracellularly in an insoluble form and could be collected by centrifugation of lysed cells. A single-step purification was achieved by extraction of the aggregated material with 8 M urea. Active cell-invasive CyaA was produced in vitro when the proCyaA and CyaC proteins were mixed with a cytosolic extract of either E. coli or B. pertussis. Activation was assumed to occur by an acylation reaction requiring acyl carrier protein (ACP) as cofactor, as the cytosolic factor required for toxin activation was lost if the S100 extract was dialysed before use and the cytosolic factor could be replaced in the in vitro reaction by ACP charged separately in vitro with palmitic acid, as reported previously for activation of the homologous E. coli haemolysin (HlyA). The in vitro activation system may be used to investigate the mechanism of the CyaC-dependent acylation of proCyaA and the effect of variation of the modifying fatty acyl group on target cell specificity and toxic activity of CyaA.

Cross-talk between bacterial pathogens and their host cells

A taxonomically diverse group of bacterial pathogens have evolved a variety of strategies to subvert host-cellular functions to their advantage. This often involves two-way biochemical interactions leading to responses in both the pathogen and host cell. Central to this interaction is the function of a specialized protein secretion system that directs the export and/or translocation into the host cells of a number of bacterial proteins that can induce or interfere with host-cell signal transduction pathways. The understanding of these bacterial/host-cell interactions will not only lead to novel therapeutic approaches but will also result in a better understanding of a variety of basic aspects of cell physiology and immunology.