Lipoprotein from the outer membrane of Escherichia coli: purification, paracrystallization, and some properties of its free form

Besides TLR2 and TLR4, NLRP3 is also involved in regulating Escherichia coli infection-induced inflammatory responses in mice

The host Toll-like Receptor-2 (TLR2) and Toll-like Receptor-4 (TLR4) play critical roles in defense against Escherichia coli (E. coli) infection is well-known. The NLR pyrin domain-containing 3 (NLRP3) inflammasome is also an important candidate during the host-recognized pathogen, while the roles of NLRP3 in the host inflammatory response to E. coli infection remains unclear. This study aimed to explore the roles of NLRP3 in regulating the inflammatory response in E. coli infection-induced mice. Our result indicated that compared to wild-type mice, the TLR2-deficient (TLR2^-/-), TLR4-deficient (TLR4^-/-), and NLRP3-deficient (NLRP3^-/-) mice had significant decrease in liver damage after stimulation with Lipopolysaccharide (LPS, 1 μg/mL), Braun lipoprotein (BLP, 1 μg/mL), or infected by WT E. coli (1 × 10⁷ CFU, MOI 5:1). Meanwhile, compared with wild-type mice, the TNF-α and IL-1β production in serum decreased in TLR2^-/-, TLR4^-/-, and NLRP3^-/- mice after LPS, BLP treatment, or WT E. coli infection. In macrophages from NLRP3^-/- mice showed significantly reduced secretion of TNF-α and IL-1β in response to stimulation with LPS, BLP, or WT E. coli infection compared with macrophages from wild-type mice. These results indicate that besides TLR2 and TLR4, NLRP3 also plays a critical role in host inflammatory responses to defense against E. coli infection, and might provide a therapeutic target in combating disease with bacterium infection.

Braun Lipoprotein Protects against Escherichia coli-Induced Inflammatory Responses and Lethality in Mice

Escherichia coli (E. coli), a Gram-negative bacterium, is an important pathogen that causes several mammalian diseases. The outer membrane components of E. coli, namely, lipopolysaccharide (LPS) and bacterial lipoprotein, can induce the host innate immune response through pattern recognition receptors (PRRs). However, the detailed roles of the E. coli Braun lipoprotein (BLP) in the regulation of host inflammatory response to E. coli infection remain unclear. In this study, we sought to determine the effects of BLP on E. coli-induced host inflammatory response and lethality using mouse models. Experiments using the E. coli DH5α strain (BLP-positive), E. coli JE5505 strain (BLP-negative), and E. coli JE5505 strain combined with BLP indicated that the presence of BLP could alleviate mortality and organ (liver and lung) damage and decrease proinflammatory cytokine (tumor necrosis factor alpha [TNF-α] and interleukin-1β [IL-1β]) and chemokine (regulated on activation normal T-cell expressed and secreted [RANTES]) production in mouse serum and organs. Conversely, E. coli JE5505, E. coli DH5α strain, and E. coli JE5505 combined with BLP treatment induce enhanced anti-inflammatory cytokine (interleukin 10 [IL-10]) production in mouse serum and organs. In addition, BLP could regulate the secretion of proinflammatory cytokines (TNF-α and IL-1β), chemokines (RANTES), and anti-inflammatory factors (IL-10) through mitogen-activated protein kinase (MAPK) and nuclear factor-kappaB (NF-κB) signaling pathways in macrophages. Altogether, our results demonstrate that the bacterial component BLP plays crucial and protective roles in E. coli-infected mice, which may influence the outcome of inflammation in host response to E. coli infection. IMPORTANCE In this study, we investigated the roles of bacterial outer membrane component BLP in regulating inflammatory responses and lethality in mice that were induced by a ubiquitous and serious pathogen, Escherichia coli. BLP could alleviate the mortality of mice and organ damage, as well as decrease proinflammatory cytokines and chemokine production and enhance anti-inflammatory cytokine production in mouse serum and organs. Overall, our results demonstrate that the bacterial component BLP plays crucial and protective roles in E. coli-infected mice through regulating the production of an inflammatory mediator, which may influence the outcome of inflammation in host response to E. coli infection. Our findings provide new information about the basic biology involved in immune responses to E. coli and host-bacterial interactions, which have the potential to translate into novel approaches for the diagnosis and treatment of E. coli-related medical conditions, such as bacteremia and sepsis.

Escherichia coli Braun Lipoprotein (BLP) exhibits endotoxemia - like pathology in Swiss albino mice

The endotoxin lipopolysaccharide (LPS) promotes sepsis, but bacterial peptides also promote inflammation leading to sepsis. We found, intraperitoneal administration of live or heat inactivated E. coli JE5505 lacking the abundant outer membrane protein, Braun lipoprotein (BLP), was less toxic than E. coli DH5α possessing BLP in Swiss albino mice. Injection of BLP free of LPS purified from E. coli DH5α induced massive infiltration of leukocytes in lungs and liver. BLP activated human polymorphonuclear cells (PMNs) ex vivo to adhere to denatured collagen in serum and polymyxin B independent fashion, a property distinct from LPS. Both LPS and BLP stimulated the synthesis of platelet activating factor (PAF), a potent lipid mediator, in human PMNs. In mouse macrophage cell line, RAW264.7, while both BLP and LPS similarly upregulated TNF-α and IL-1β mRNA; BLP was more potent in inducing cyclooxygenase-2 (COX-2) mRNA and protein expression. Peritoneal macrophages from TLR2^−/− mice significantly reduced the production of TNF-α in response to BLP in contrast to macrophages from wild type mice. We conclude, BLP acting through TLR2, is a potent inducer of inflammation with a response profile both common and distinct from LPS. Hence, BLP mediated pathway may also be considered as an effective target against sepsis.

Key Residues of Outer Membrane Protein OprI Involved in Hexamer Formation and Bacterial Susceptibility to Cationic Antimicrobial Peptides

Antimicrobial peptides (AMPs) are important components of the host innate defense mechanism against invading pathogens. Our previous studies have shown that the outer membrane protein, OprI from Pseudomonas aeruginosa or its homologue, plays a vital role in the susceptibility of Gram-negative bacteria to cationic α-helical AMPs (Y. M. Lin, S. J. Wu, T. W. Chang, C. F. Wang, C. S. Suen, M. J. Hwang, M. D. Chang, Y. T. Chen, Y. D. Liao, J Biol Chem 285:8985-8994, 2010, http://dx.doi.org/10.1074/jbc.M109.078725; T. W. Chang, Y. M. Lin, C. F. Wang, Y. D. Liao, J Biol Chem 287:418-428, 2012, http://dx.doi.org/10.1074/jbc.M111.290361). Here, we obtained two forms of recombinant OprI: rOprI-F, a hexamer composed of three disulfide-bridged dimers, was active in AMP binding, while rOprI-R, a trimer, was not. All the subunits predominantly consisted of α-helices and exhibited rigid structures with a melting point centered around 76°C. Interestingly, OprI tagged with Escherichia coli signal peptide was expressed in a hexamer, which was anchored on the surface of E. coli, possibly through lipid acids added at the N terminus of OprI and involved in the binding and susceptibility to AMP as native P. aeruginosa OprI. Deletion and mutation studies showed that Cys1 and Asp27 played a key role in hexamer formation and AMP binding, respectively. The increase of OprI hydrophobicity upon AMP binding revealed that it undergoes conformational changes for membrane fusion. Our results showed that OprI on bacterial surfaces is responsible for the recruitment and susceptibility to amphipathic α-helical AMPs and may be used to screen antimicrobials.

What is the total number of protein molecules per cell volume? A call to rethink some published values

Novel methods such as mass-spectrometry enable a view of the proteomes of cells in unprecedented detail. Recently, these efforts have culminated in quantitative measurements of the number of copies per cell for most expressed proteins in organisms ranging from bacteria to mammalian cells. Here, we estimate the expected total number of proteins per unit of cell volume using known parameters related to the composition of cells such as the fraction of cell mass that is protein, and the average protein length. Using simple arguments, we estimate a range of 2–4 million proteins per cubic micron (i.e. 1 fL) in bacteria, yeast, and mammalian cells. Interestingly, we find that measured values that are reported for fission yeast and mammalian cells are often about 3–10 times lower. We discuss this apparent discrepancy and how to use the estimate as benchmark to recalibrate proteome-wide quantitative censuses or to revisit assumptions about cell composition.

We estimate the expected total number of proteins per unit cell volume as 2–4 million proteins per cubic micron. Some reported values for fission yeast and mammalian cells using mass spectrometry are 3–10 times lower than these estimates. We discuss this apparent discrepancy and how to recalibrate proteome-wide quantitative censuses.

A new cloning system based on the OprI lipoprotein for the production of recombinant bacterial cell wall-derived immunogenic formulations

The conjugation of antigens with ligands of pattern recognition receptors (PRR) is emerging as a promising strategy for the modulation of specific immunity. Here, we describe a new Escherichia coli system for the cloning and expression of heterologous antigens in fusion with the OprI lipoprotein, a TLR ligand from the Pseudomonas aeruginosa outer membrane (OM). Analysis of the OprI expressed by this system reveals a triacylated lipid moiety mainly composed by palmitic acid residues. By offering a tight regulation of expression and allowing for antigen purification by metal affinity chromatography, the new system circumvents the major drawbacks of former versions. In addition, the anchoring of OprI to the OM of the host cell is further explored for the production of novel recombinant bacterial cell wall-derived formulations (OM fragments and OM vesicles) with distinct potential for PRR activation. As an example, the African swine fever virus ORF A104R was cloned and the recombinant antigen was obtained in the three formulations. Overall, our results validate a new system suitable for the production of immunogenic formulations that can be used for the development of experimental vaccines and for studies on the modulation of acquired immunity.

Outer membrane lipoprotein Lpp is Gram-negative bacterial cell surface receptor for cationic antimicrobial peptides

Antimicrobial peptides/proteins (AMPs) are important components of the host innate defense mechanisms. Here we demonstrate that the outer membrane lipoprotein, Lpp, of Enterobacteriaceae interacts with and promotes susceptibility to the bactericidal activities of AMPs. The oligomeric Lpp was specifically recognized by several cationic α-helical AMPs, including SMAP-29, CAP-18, and LL-37; AMP-mediated bactericidal activities were blocked by anti-Lpp antibody blocking. Blebbing of the outer membrane and increase in membrane permeability occurred in association with the coordinate internalization of Lpp and AMP. Interestingly, the specific binding of AMP to Lpp was resistant to divalent cations and salts, which were able to inhibit the bactericidal activities of some AMPs. Furthermore, using His-tagged Lpp as a ligand, we retrieved several characterized AMPs, including SMAP-29 and hRNase 7, from a peptide library containing crude mammalian cell lysates. Overall, this study explores a new mechanism and target of antimicrobial activity and provides a novel method for screening of antimicrobials for use against drug-resistant bacteria.

Outer membrane protein I of Pseudomonas aeruginosa is a target of cationic antimicrobial peptide/protein

Cationic antimicrobial peptides/proteins (AMPs) are important components of the host innate defense mechanisms against invading microorganisms. Here we demonstrate that OprI (outer membrane protein I) of Pseudomonas aeruginosa is responsible for its susceptibility to human ribonuclease 7 (hRNase 7) and alpha-helical cationic AMPs, instead of surface lipopolysaccharide, which is the initial binding site of cationic AMPs. The antimicrobial activities of hRNase 7 and alpha-helical cationic AMPs against P. aeruginosa were inhibited by the addition of exogenous OprI or anti-OprI antibody. On modification and internalization of OprI by hRNase 7 into cytosol, the bacterial membrane became permeable to metabolites. The lipoprotein was predicted to consist of an extended loop at the N terminus for hRNase 7/lipopolysaccharide binding, a trimeric alpha-helix, and a lysine residue at the C terminus for cell wall anchoring. Our findings highlight a novel mechanism of antimicrobial activity and document a previously unexplored target of alpha-helical cationic AMPs, which may be used for screening drugs to treat antibiotic-resistant bacterial infection.

Passive immunization to outer membrane proteins MLP and PAL does not protect mice from sepsis

Multiple older studies report that immunoglobulin directed to rough mutant bacteria, such as E. coli J5, provides broad protection against challenge with heterologous strains of Gram-negative bacteria. This protection was initially believed to occur through binding of immunoglobulin to bacterial lipopolysaccharide (LPS). However, hundreds of millions of dollars have been invested in attempting to develop clinically-effective anti-LPS monoclonal antibodies without success, and no study has shown that IgG from this antiserum binds LPS. Identification of the protective mechanism would facilitate development of broadly protective human monoclonal antibodies for treating sepsis. IgG from this antiserum binds 2 bacterial outer membrane proteins: murein lipoprotein (MLP) and peptidoglycan-associated lipoprotein (PAL). Both of these outer membrane proteins are highly conserved, have lipid domains that are anchored in the bacterial membrane, are shed from bacteria in blebs together with LPS, and activate cells through Toll-like receptor 2. Our goal in the current work was to determine if passive immunization directed to MLP and PAL protects mice from Gram-negative sepsis. Neither monoclonal nor polyclonal IgG directed to MLP or PAL conferred survival protection in 3 different models of sepsis: cecal ligation and puncture, an infected burn model, and an infected fibrin clot model mimicking peritonitis. Our results are not supportive of the hypothesis that either anti-MLP or anti-PAL IgG are the protective antibodies in the previously described anti-rough mutant bacterial antisera. These studies suggest that a different mechanism of protection is involved.

Passive immunization to outer membrane proteins MLP and PAL does not protect mice from sepsis

Multiple older studies report that immunoglobulin directed to rough mutant bacteria, such as E. coli J5, provides broad protection against challenge with heterologous strains of Gram-negative bacteria. This protection was initially believed to occur through binding of immunoglobulin to bacterial lipopolysaccharide (LPS). However, hundreds of millions of dollars have been invested in attempting to develop clinically-effective anti-LPS monoclonal antibodies without success, and no study has shown that IgG from this antiserum binds LPS. Identification of the protective mechanism would facilitate development of broadly protective human monoclonal antibodies for treating sepsis. IgG from this antiserum binds 2 bacterial outer membrane proteins: murein lipoprotein (MLP) and peptidoglycan-associated lipoprotein (PAL). Both of these outer membrane proteins are highly conserved, have lipid domains that are anchored in the bacterial membrane, are shed from bacteria in blebs together with LPS, and activate cells through Toll-like receptor 2. Our goal in the current work was to determine if passive immunization directed to MLP and PAL protects mice from Gram-negative sepsis. Neither monoclonal nor polyclonal IgG directed to MLP or PAL conferred survival protection in 3 different models of sepsis: cecal ligation and puncture, an infected burn model, and an infected fibrin clot model mimicking peritonitis. Our results are not supportive of the hypothesis that either anti-MLP or anti-PAL IgG are the protective antibodies in the previously described anti-rough mutant bacterial antisera. These studies suggest that a different mechanism of protection is involved.

Prolipoprotein signal peptidase of Escherichia coli requires a cysteine residue at the cleavage site

A signal peptidase specifically required for the secretion of the lipoprotein of the Escherichia coli outer membrane cleaves off the signal peptide at the bond between a glycine and a cysteine residue. This cysteine residue was altered to a glycine residue by guided site-specific mutagenesis using a synthetic oligonucleotide and a plasmid carrying an inducible lipoprotein gene. The induction of mutant lipoprotein production was lethal to the cells. A large amount of the prolipoprotein was accumulated in the outer membrane fraction. No protein of the size of the mature lipoprotein was detected. These results indicate that the prolipoprotein signal peptidase requires a glyceride modified cysteine residue at the cleavage site.

Escherichia coli Braun lipoprotein induces a lipopolysaccharide-like endotoxic response from primary human endothelial cells

All bacteria contain proteins in which their amino-terminal cysteine residue is modified with N-acyl S-diacylglycerol functions, and peptides and proteins bearing this modification are immunomodulatory. The major outer membrane lipoprotein of Escherichia coli, the Braun lipoprotein (BLP), is the prototypical triacylated cysteinyl-modified protein. We find it is as active as LPS in stimulating human endothelial cells to an inflammatory phenotype, and a BLP-negative mutant of E. coli was less inflammatory than its parental strain. While the lipid modification was essential, the lipidated protein was more potent than a lipid-modified peptide. BLP associates with CD14, but this interaction, unlike that with LPS, was not required to elicit endothelial cell activation. BLP stimulated endothelial cell E-selectin surface expression, IL-6 secretion, and up-regulation of the same battery of cytokine mRNAs induced by LPS. Quantitative microarray analysis of 4400 genes showed the same 30 genes were induced by BLP and LPS, and that there was near complete concordance in the level of gene induction. We conclude that the lipid modification of at least one abundant Gram-negative protein is essential for endotoxic activity, but that the protein component also influences activity. The equivalent potency of BLP and LPS, and their complete concordance in the nature and extent of endothelial cell activation show that E. coli endotoxic activity is not due to just LPS. The major outer membrane protein of E. coli is a fully active endotoxic agonist for endothelial cells.

Characterization of braun's lipoprotein and determination of its attachment sites to peptidoglycan by (252)Cf-PD and MALDI time-of-flight mass spectrometry

A strategy for the characterization of bacterial lipoprotein-in this case Braun's lipoprotein (an outer membrane 7-ku lipoprotein) isolated from Escherichia coli-is described by time-of-flight mass spectrometric (TOF/MS) techniques [(252)Cf plasma desorption (PD) TOF/MS and matrix-assisted laser desorption-ionization (MALDI) TOF/MS]. Covalent linkage of lipid at the N-terminal cysteine (posttranslationally modified to a S-[2,3-bis(acyloxy)-propyl]-N-acylcysteine) and, therefore, strict insolubility in aqueous solution constitute common features for this class of proteins. Relative molecular mass determination of the major molecular species of Braun's lipoprotein was obtained by selection of an appropriate mixture of organic solvents compatible with matrix/support materials useful for the mass spectrometric techniques applied. Minor components of this lipoprotein that differ only in the fatty acid composition of the lipid anchor were detected by PD TOF/MS after enzymatic release of the extremely hydrophobic N-terminal amino acid followed by selective extraction with chloroform. Part of the primary sequence of this lipoprotein was confirmed based on peptide fragment ions observed in the positive ion PD mass spectra of cyanogen bromide-generated peptide fragments that had been isolated previously by reverse phase high-performance liquid chromatography (HPLC). Peptidoglycan fragments that represent the attachment sites of lipoprotein to peptidoglycan were enzymatically released, separated by reverse phase HPLC, and finally characterized by time-of-flight mass spectrometric techniques ((252)Cf-PD TOF/MS, MALDI TOF/MS). The results obtained with both techniques differed only in the better sensitivity obtained with MALDI TOF/MS, which consumed a factor of 100 to 1000 less material than with PD TOF/MS.

TH1 cells trigger tumor necrosis factor alpha-mediated hypersensitivity to Pseudomonas aeruginosa after adoptive transfer into SCID mice

Recent experiments have shown that gamma interferon (IFN-gamma), either administered or induced in vivo, e.g., by certain bacteria, is a key mediator in inducing hypersensitivity to bacterial lipopolysaccharides. The source of endogenous IFN-gamma in this context (natural killer versus TH1 cells) has not been investigated yet. In order to investigate the role of antigen-specific, IFN-gamma-producing TH1 cells in murine Pseudomonas aeruginosa infection, a murine TH1 cell line was propagated in vitro by using recombinant P. aeruginosa outer membrane protein I. Adoptive transfer experiments were performed by intravenous injection of various amounts of TH1 cells into P. aeruginosa-challenged SCID mice. Adoptive transfer of 5 x 10(6) T cells into SCID mice followed by an intraperitoneal challenge with 1.4 x 10(6) CFU of live P. aeruginosa resulted in the rapid death of the animals within 12 h postchallenge, whereas transfer of lower T-cell doses and saline as a control did not cause any detrimental effects. After challenge with 2.8 x 10(6) CFU of P. aeruginosa, similar results were obtained 18 h postchallenge; however, at the end of the 72-h observation period, no significant differences in survival rates were obtained between the groups treated with different amounts of T cells. The rapid death of mice treated with 5 x 10(6) T cells was reflected by 860-fold-elevated levels of tumor necrosis factor alpha (TNF-alpha) present in serum 2 h postchallenge, whereas no significant differences in TNF-alpha serum levels were detectable in mice treated with lower doses of T cells or with saline. Pretreatment of T-cell-reconstituted SCID mice with neutralizing anti-IFN-gamma monoclonal antibodies completely protected mice from bacterial challenge and reduced TNF-alpha levels in serum. We conclude that under the experimental conditions described here, IFN-gamma- and interleukin-2-producing TH1 cells represent an important trigger mechanism inducing TNF-alpha-mediated hypersensitivity to bacterial endotoxin.

Reversible topology of a bifunctional transmembrane protein depends upon the charge balance around its transmembrane domain

Hybrid genes were constructed to express bifunctional hybrid proteins in which staphyloccal nuclease A with or without an amino-terminal OmpA signal sequence was fused with TEM beta-lactamase (at the carboxyl terminal side) using the signal peptide of the major outer membrane lipoprotein of Escherichia coli as an internal linker. The hybrid proteins were found to be inserted in the membrane. Orientation of the hybrid protein with the OmpA signal peptide showed that the nuclease was translocated into the periplasm and the beta-lactamase remained in the cytoplasm. This indicates that the cleavable OmpA signal peptide served as a secretory signal for nuclease and the internal lipoprotein signal served as the transmembrane anchor. In the absence of the OmpA signal sequence the topology of the hybrid protein was reversed indicating that the internal lipoprotein signal peptide initially served as the signal peptide for the secretion of the carboxy terminal beta-lactamase domain across the membrane and subsequently as a membrane anchoring signal. The role of charged amino acids in the translocation and transmembrane orientation of membrane proteins was also analysed by introducing charged amino acids to either or both sides of the internal lipoprotein signal sequence in the bifunctional hybrid proteins in the absence of the amino-terminal signal sequence. Introduction of two lysine residues at the carboxy-terminal side of the internal signal sequence reversed the topology of the transmembrane protein by translocating the amino-terminal nuclease domain across the membrane, leaving the carboxyl terminal beta-lactamase domain in the cytoplasm. When three more lysine residues were added to the amino-terminal side of the internal signal sequence of the same construct the membrane topology flipped back to the original orientation. A similar reversion of the topology could be obtained by introducing negatively charged residues at the amino-terminal side of the internal signal sequence. Present results demonstrate for the first time that a bifunctional transmembrane protein can be engineered to assume either of the two opposite orientations and that charge balance around the transmembrane domain is a major factor in controlling the topology of a transmembrane protein.

Protection of immunosuppressed mice against infection with Pseudomonas aeruginosa by recombinant P. aeruginosa lipoprotein I and lipoprotein I-specific monoclonal antibodies

Outer membrane protein I (OprI) is one of the major proteins of the outer membrane of Pseudomonas aeruginosa. The protective effect of OprI vaccination and that of three OprI-specific monoclonal antibodies (MAbs) against infection with P. aeruginosa were tested in immunosuppressed mice. The combination of Oprl and MAb 2A1 protected the mice against a challenge with a 96-fold 50% lethal dose. The binding site of MAb 2A1 was mapped, resulting in the identification of a protective epitope (amino acids 7 to 20).

Protection against experimental Pseudomonas aeruginosa infection by recombinant P. aeruginosa lipoprotein I expressed in Escherichia coli

Lipoprotein I (OprI) is one of the major proteins of the outer membrane of Pseudomonas aeruginosa. OprI is a candidate for a vaccine against P. aeruginosa, because it cross-reacts antigenically in all serotype strains of the International Antigenic Typing Scheme. We recently cloned and expressed the gene coding for OprI in Escherichia coli. This heterologously expressed OprI was used successfully to immunize mice against P. aeruginosa. In addition, OprI from serogroup 12 of P. aeruginosa was highly purified by preparative isoelectric focusing and used for immunization of mice. Both vaccines protected the mice against a challenge with a four- to fivefold 50% lethal dose of P. aeruginosa.

The antibody response to salmonellae in mice and humans studied by immunoblots and ELISA

The antibody response to salmonellae in mice and humans was studied by immunoblots and ELISA. Sera from mice infected with attenuated salmonellae (including an aroA live vaccine strain) recognized up to 45 different bands on immunoblots at the height of the response, including lipoprotein, OmpA protein, porins, a putative heat-shock protein and flagella. Adsorption of antisera with intact or sonicated smooth or rough salmonellae prior to immunoblotting showed that antibodies were directed against exposed, masked and intracellular antigens. Sera from H-2 congenic B10 mice which vary in their ability to clear salmonellae from the reticuloendothelial system (RES) showed a progressive increase in the intensity of the antibody response, which persisted longer in animals which failed to clear bacteria from the RES. The LPS response was much stronger in susceptible mice. Sera from 18 confirmed cases of human typhoid recognized similar antigens to mouse typhoid sera, with individual variations; there was no correlation between the immunoblot pattern and the titres of other serological tests for typhoid fever.

Stimulation of human and murine adherent cells by bacterial lipoprotein and synthetic lipopeptide analogues

Lipoprotein from the outer membrane of Escherichia coli and its synthetically prepared N-terminal lipopeptide segments Pam3Cys-Ser-Ser-Asn-Ala and Pam3Cys-Ser, as well as lipoprotein from other Enterobacteriaceae, constitute potent polyclonal B lymphocyte activators. Here, we demonstrate that these compounds were also able to stimulate human and murine leukocytes: in murine macrophages, we could show the induction of interleukin 1 release by the mitogens, as measured in the thymocyte proliferation assay. Moreover, murine peritoneal exudate cells were stimulated to secrete prostaglandins E2 (PGE2) and F2 alpha (PGF2 alpha). The effect of Pam3Cys-Ser on the murine macrophage cell line P388D1 was also tested: the compound induced an increase in proliferation, as measured by a thymidine incorporation assay. In addition, the cell line could be induced to release IL 1 into the supernatant. Correspondingly, induction of IL 1 release could also be demonstrated in human mononuclear cells. Our results demonstrate that the two novel synthetic lipopeptides are potent stimulators for human monocytes and murine macrophages. These findings may be important for the elucidation of the role of these bacterial surface components in the course of bacterial infections.

Nucleotide sequence analysis of the gene encoding the Deinococcus radiodurans surface protein, derived amino acid sequence, and complementary protein chemical studies

The complete nucleotide sequence of the gene encoding the surface (hexagonally packed intermediate [HPI])-layer polypeptide of Deinococcus radiodurans Sark was determined and found to encode a polypeptide of 1,036 amino acids. Amino acid sequence analysis of about 30% of the residues revealed that the mature polypeptide consists of at least 978 amino acids. The N terminus was blocked to Edman degradation. The results of proteolytic modification of the HPI layer in situ and Mr estimations of the HPI polypeptide expressed in Escherichia coli indicated that there is a leader sequence. The N-terminal region contained a very high percentage (29%) of threonine and serine, including a cluster of nine consecutive serine or threonine residues, whereas a stretch near the C terminus was extremely rich in aromatic amino acids (29%). The protein contained at least two disulfide bridges, as well as tightly bound reducing sugars and fatty acids.

Interaction of mitogenic bacterial lipoprotein and a synthetic analogue with mouse lymphocytes. Isolation and characterization of binding proteins

Lipoprotein from the outer membrane of Escherichia coli constitutes a potent mitogen and polyclonal activator for B lymphocytes of different species. The binding of lipoprotein to murine spleen cells was investigated using water-soluble 125I-labelled citraconylated lipoprotein from E. coli B/r. Our results indicate that the binding of this B-cell mitogen to splenocytes is a saturable, time- and dose-dependent, reversible process; about 9.7 X 10(8) lipoprotein molecules were bound to each cell. The mechanism of the binding of lipoprotein to lymphocytes was investigated by using the synthetic analogue of its N-terminal part, S-[2,3-bis(palmitoyloxy)-(2RS)-propyl]-N-palmitoyl-(R)-cysteinyl-( S)-seryl- (S)-seryl-(S)-asparaginyl-(S)-alanine (tripalmitoyl pentapeptide). This compound had been shown by us previously to be the molecular part of lipoprotein responsible for mitogenicity and exhibited, in all experiments performed, a stimulatory activity towards B lymphocytes comparable, or even superior, to native lipoprotein. Binding proteins for the synthetic N-terminus were enriched by affinity chromatography, using an affinity column prepared by coupling the mitogenic compound to CPG-aminopropyl controlled-pore glass beads by the carbodiimide method. [3H]Leucine-labelled murine spleen cells were solubilized by the nonionic detergent NP40 and applied to the affinity adsorbent. Proteins bound to the column were selectively eluted by a solution of tripalmitoyl pentapeptide, and the fractions were analyzed by sodium dodecyl sulphate/polyacrylamide gel electrophoresis and autoradiography. Our results indicate the presence of a major binding protein of Mr 35000 on mouse primary lymphocytes for the biologically active N-terminal structure of lipoprotein, which might play a role as membrane receptor in mitogenic B lymphocyte activation.

Induction kinetics and cell surface distribution of Escherichia coli lipoprotein under lac promoter control

The induction kinetics and surface accessibility of the outer membrane lipoprotein were studied in an Escherichia coli strain with the lpp gene under control of the lac promoter. Free lipoprotein appeared rapidly after induction with isopropyl-beta-D-thiogalactopyranoside and reached a steady-state level after 30 min. The newly induced lipoprotein was slowly bound to the peptidoglycan layer. Immunological methods were developed to detect lipoprotein accessible at the cell surface after various pretreatments as well as peptidoglycan-bound lipoprotein at the surface of isolated peptidoglycan sacculi with specific antibodies in combination with 125I-protein A. With these methods an increase in lipoprotein molecules at the cell surface and bound to the peptidoglycan sacculus could be detected following induction. The topology of newly synthesized lipoprotein was examined in thin sections as well as at the cell surface and the surface of the peptidoglycan sacculus with immunoelectron microscopy. Ultrathin cell sections, whole cells, and isolated peptidoglycan sacculi showed lipoprotein distributed homogeneously over the entire surface.

Demonstration of a peptidoglycan-linked lipoprotein and characterization of its trypsin fragment in the outer membrane of Brucella spp

The sodium dodecyl sulfate (SDS) extraction-trypsin digestion protocol used by Braun and Sieglin (V. Braun and U. Sieglin, Eur. J. Biochem. 13:336-346, 1970) to show the peptidoglycan-linked lipoprotein of Escherichia coli was applied to both Brucella abortus and E. coli. Whereas a single polypeptide of 8,000 molecular weight was obtained from E. coli, several proteins of apparent molecular weight lower than 35,000 were demonstrated by SDS-polyacrylamide gel electrophoresis in B. abortus. These results did not change when the trypsin digestion conditions were modified. On the other hand, when the SDS extractions were performed under conditions more stringent than those used for other gram-negative bacteria, only a polypeptide fragment of apparent molecular weight of 8,000 was obtained from B. abortus. This polypeptide was similar to the trypsin fragment of the E. coli lipoprotein with respect to its behavior in SDS-polyacrylamide gels, isoelectric point in urea, molecular weight, and presence of both ester- and amide-linked fatty acids. Moreover, the amino acid analysis showed an overall similarity with respect to the amino acid composition of E. coli lipoprotein. A polypeptide of the same molecular weight, isoelectric point, and amino acid composition was also obtained from Brucella ovis by the same method. These results demonstrated that B. abortus and B. ovis cell envelopes contain a lipoprotein and strongly support the hypothesis that it is the only major protein covalently linked to the peptidoglycan.

Ultrastructure of native lipoprotein from Escherichia coli envelopes

The free form of the major lipoprotein from Escherichia coli cells envelopes has been purified to homogeneity by gentle extraction procedures and conventional chromatographic separations in a non-ionic detergent. The morphology of paracrystals obtained from homogeneous protein was investigated by low-dose electron microscopy. Electron diffraction of the paracrystals was consistent with alpha-helices arranged perpendicularly to the main cross-band with a periodicity of 20 nm.

GTP-binding membrane protein of Escherichia coli with sequence homology to initiation factor 2 and elongation factors Tu and G

The amino acid sequence of LepA protein, which has been shown to be cotranscribed with signal peptidase I in Escherichia coli, was compared with greater than 2000 known protein sequences. It was revealed that, of the 598 amino acid residues contained in LepA, an amino-terminal domain of 112 residues is homologous to a domain of similar size found in initiation factor 2, elongation factor Tu, and elongation factor G (IF2, EF-Tu, and EF-G), factors required for translation in E. coli. In this domain, 46 and 34 residues align perfectly with the corresponding regions of EF-G and EF-Tu, respectively. If functionally conserved residues within this domain (19 for EF-G and 17 for EF-Tu) are included, the overall resemblance is 58% and 46%, respectively, for EF-G and EF-Tu. A similar domain exists internally in IF2, where there is 42% overall resemblance with the domain of LepA. Immediately adjacent to this region is a small sequence of limited similarity that exists not only in EF-G, EF-Tu, and IF2 but also in the protooncogene c-Ha-ras-1 (from human bladder) and other GTP-binding proteins. Given these homologies, GTP-photoaffinity labeling and subcellular fractionation experiments were undertaken, and it was found that LepA is indeed a membrane-bound GTP-binding protein.

The mitogenic principle of Escherichia coli lipoprotein: B-lymphocyte mitogenicity of the synthetic analogue palmitoyl-tetrapeptide (Pam-Ser-Ser-Asn-Ala)

N-Palmitoyl-(S)-seryl-(S)-seryl-(S)-asparaginyl-(S)-alanine (Palmitoyl-tetrapeptide) is an analogue of the N-terminal part of the lipoprotein from the outer membrane of Escherichia coli. It was prepared by chemical synthesis and tested for biological activity in in vitro lymphocyte culture systems. In spleen cells of the inbred mouse strains C3H/HeJ, C3H/He/Bom/ nunu , and Balb/c, the compound exhibited stimulatory activity towards B-lymphocytes comparable to the effect of native lipoprotein, as measured by the incorporation of 3H-thymidine and 3H-uridine, and by a hemolytic plaque assay. The B-lymphocyte tumor cell line BCL1 was also activated by the compound. The results demonstrate, that the N-terminal tetrapeptide moiety of lipoprotein, linked to a lipophilic molecule, constitutes by itself a novel B-lymphocyte mitogen.

Synthetic analogues of the N-terminal lipid part of bacterial lipoprotein are B-lymphocyte mitogens in vitro and in vivo

Two analogues of the N-terminal part of bacterial lipoprotein, S-[2,3-bis(palmitoyloxy)-(2RS)propyl]-N-palmitoyl-(R)-cysteine methyl ester ("tripalmitoyl cysteine") and S [-2,3-bis(palmitoyloxy)-(2RS)-propyl]-N-palmitoyl-(R)-cysteinyl-(S)-seryl-(S)-seryl-(S)-asparaginyl-(S)-alanine ("tripalmitoyl pentapeptide") were synthesized and tested for mitogenic activity. The compounds were potent mitogens towards mouse spleen cell cultures, as measured by 3H-thymidine incorporation and by hemolytic plaque assays. This activity was not dependent on the presence of serum. Tripalmitoyl pentapeptide had little, if any, effect on thymocytes. When injected intravenously into Balb/c mice, the synthetic compounds induced splenomegaly and polyclonal B-cell activation, the latter was evident from an increase in the number of plaque-forming cells against trinitrophenylated sheep red blood cells. Thus, a synthetic fragment of a bacterial surface component was shown to exhibit marked biological activity in vitro as well as in vivo.

Effects of inserting eight amino acid residues into the major lipoprotein on its assembly in the outer membrane of Escherichia coli

A DNA sequence consisting of 24 base pairs was inserted into the structural gene (lpp) coding for the major lipoprotein of the Escherichia coli outer membrane which was carried on a high-copy-number plasmid in which expression was regulated through a lac promoter-operator region. This modification resulted in the insertion of eight amino acid residues, Glu-Glu-Phe-Leu-Glu-Glu-Phe-Leu, between the glutamine residue at position 9 and the leucine residue at position 10 of the wild-type lipoprotein sequence. When production of the mutant lipoprotein was induced by a lac inducer, the cells became swollen, showed unusual morphology, and eventually lysed. When the membrane fraction was analyzed after the induction, the mutant lipoprotein was found to have been normally secreted across the cytoplasmic membrane and assembled in the outer membrane. This lipoprotein was modified with glycerol and palmitic acid and even formed the bound form, which was linked covalently to peptidoglycan. The major difference between the membrane-associated mutant lipoprotein and the wild-type lipoprotein was that the mutant lipoprotein became sensitive to trypsin treatment. These results indicate that the substantial alteration in mutant lipoprotein structure near the amino-terminal end does not interfere with modification of the amino-terminal cysteine residue or cleavage of the signal peptide by the prolipoprotein-specific signal peptidase. However, this mutant lipoprotein assembled in the outer membrane appears to have deleterious effects with respect to envelope structure and cellular morphology and viability.

Inhibition of secretion of a mutant lipoprotein across the cytoplasmic membrane by the wild-type lipoprotein of the Escherichia coli outer membrane

A globomycin-resistant mutant of Escherichia coli was found to produce a precursor of the major outer membrane lipoprotein (prolipoprotein), in which the glycine residue at position 14 within the signal peptide was replaced by an aspartic acid residue. The same mutation has been reported by Lin et al. (Proc. Natl. Acad. Sci. U.S.A. 175:4891-4895, 1978). The structural gene of the mutant prolipoprotein was inserted into an inducible expression cloning vehicle. When the mutant prolipoprotein was produced in lipoprotein-minus host cells, 82% of the unprocessed protein was found in the membrane fraction, with the remaining 18% localized in the soluble fraction. However, when the production of the mutant prolipoprotein was induced in the wild-type lpp+ host cells, only 31% of the mutant prolipoprotein was found in the membrane fraction, leaving the remaining 69% in the soluble, cytoplasmic fraction. In addition, the assembly of the wild-type lipoprotein in these cells was not affected, whether the mutant prolipoprotein was produced or not. These results suggest that secretions of both mutant and wild-type prolipoproteins utilize the same component(s) responsible for the initial stages of secretion across the cytoplasmic membrane. However, it appears that the wild-type lipoprotein has a higher affinity for these components than does the mutant lipoprotein.

Alteration of the immunoglobulin G subclass responses in mice to lipopolysaccharide: effects of nonbacterial proteins and bacterial membrane phospholipids or outer membrane proteins of Proteus mirabilis

The immunoglobulin M (IgM) and the IgG1, IgG2ab, and IgG3 subclasses of plaque-forming cells (PFC) specific for lipopolysaccharide (LPS) were measured after immunization of mice with LPS alone and compared with the responses to LPS in combination with nonbacterial proteins and with bacterial membrane phospholipid vesicles or two major outer membrane proteins from Proteus mirabilis. The relative numbers of IgG PFC belonging to the IgG1, IgG2, or IgG3 subclasses induced by immunization with LPS alone depended upon the type of LPS administered. Phospholipids and the proteins effected characteristic alterations in not only the strength but also the subclass of the IgG responses to LPS. The results suggest that the hydrophobic-hydrophilic nature or state of aggregation of the preparations plays a role in the induction of IgG1 and IgG2 subclasses of PFC specific for LPS. Complex formation with LPS and adjuvant was apparently necessary to obtain these effects.

Synthesis of outer membrane proteins in cpxA cpxB mutants of Escherichia coli K-12

Two major proteins, the murein lipoprotein and the OmpF matrix porin, are deficient in the outer membrane of cpxA cpxB mutants of Escherichia coli K-12. We present evidence that the cpx mutations prevent or retard the translocation of these proteins to the outer membrane. The mutations had no effect on the rate of lipoprotein synthesis. Mutant cells labeled for 5 min with radioactive arginine accumulated as much lipoprotein as otherwise isogenic cpxA+ cpxB+ cells. This lipoprotein accumulated as such; no material synthesized in mutant cells and reactive with antilipoprotein antibodies had the electrophoretic mobility of prolipoprotein. Hence, the initial stages of prolipoprotein insertion into the inner membrane leading to its cleavage to lipoprotein appeared normal. However, after a long labeling interval, mutant cells were deficient in free lipoprotein and lacked lipoprotein covalently bound to peptidoglycan, suggesting that little if any of the lipoprotein synthesized in mutant cells reaches the outer membrane. Immunoreactive OmpF protein could also be detected in extracts of mutant cells labeled for 5 min, but the amount that accumulated was severalfold less in mutant cells than in cpxA+ cpxB+ cells. Analysis of beta-galactosidase synthesis from ompF-lacZ fusion genes showed this difference to be the result of a reduced rate of ompF transcription in mutant cells. Even so, little or none of the ompF protein synthesized in mutant cells was incorporated into the outer membrane.

Polyclonal activation of B-lymphocytes in vivo by Salmonella typhimurium lipoprotein

Lipoprotein prepared from the outer membrane of Salmonella typhimurium is a polyclonal activator of murine B-lymphocytes. It was shown to be mitogenic for splenic cultures, stimulating increased incorporation of [3H]thymidine into DNA. When injected intravenously into mice, the lipoprotein induced splenomegaly and polyclonal B-cell activation. The latter was evident from an increase in the number of plaque-forming cells against trinitrophenylated sheep erythrocytes. Similar results were obtained with Escherichia coli lipoprotein.

Association of the prolipoprotein accumulated in the presence of globomycin with the outer membrane of Escherichia coli

The prolipoprotein, a secretory precursor of the outer membrane lipoprotein of Escherichia coli, is known to be accumulated in the cell envelope when cells are grown in the presence of a cyclic antibiotic, globomycin. The prolipoprotein was localized in the cytoplasmic membrane when it was separated from the outer membrane by sucrose-density gradient centrifugation. However, when the envelope fraction was treated with sodium sarcosinate, the prolipoprotein was found almost exclusively in the sarcosinate-insoluble outer membrane fraction. The prolipoprotein separated in the cytoplasmic membrane by sucrose-density gradient centrifugation was soluble in sarcosinate and could not form a complex with the outer membrane once solubilized in sarcosinate. Labeling of the two lysine residues at positions 2 and 5 of the prolipoprotein with [3H]dinitrophenylfluorobenzene was enhanced 26-fold when the cells were disrupted by sonication. On the other hand, a tryptic fragment of the ompA protein, which is known to exist in the periplasmic space, increased its susceptibility to [3H]dinitrophenylfluorobenzene only 5.3-times upon disruption of the cell structure. These results indicate that the prolipoprotein accumulated in the presence of globomycin is translocated across the cytoplasmic membrane and interacts with the outer membrane. At the same time, it is attached to the cytoplasmic membrane with its amino-terminal signal peptide in such a way that the amino-terminal portion of the signal peptide containing two lysine residues is left inside the cytoplasm.

Crystal formation induced by uropathogenic bacteria. An in vitro study in human urine

Human urine was studied in regard to precipitation of crystals under various experimental conditions. These included alkalization of the urine by adding NaOH or by inoculation with Proteus bacteria. The contents of calcium and magnesium in the urinary supernatant after incubation for 24 hours at 37 degrees C were significantly lower after inoculation with Proteus bacteria than in noninoculated urine. Microscopy of the sediments showed distinct crystalline aggregations after the bacterial inoculation. Inoculation with killed Proteus organisms did not lead to crystalline precipitation in otherwise identical experiments. The effect of other live bacteria was also studied. The mechanism of crystal formation in bacteria-inoculated urine is discussed.

Mutations in genes cpxA and cpxB alter the protein composition of Escherichia coli inner and outer membranes

Mutations in chromosomal genes cpxA and cpxB altered the protein composition of the inner and outer bacterial membranes. Electrophoretic analyses of membrane proteins from isogenic strains differing only at their cpx loci and of spontaneous cpxA+ revertants of a cpxA cpxB double mutant showed that the alterations define a pattern that is uniquely attributable to the cpx mutations. Two major outer membrane proteins, the OmpF matrix porin and the murein lipoprotein, were deficient or absent from the outer membrane of mutant cells, whereas the quantities of two other major outer membrane proteins, the OmpC matrix porin and the OmpA protein, were not significantly altered. The cpx mutations did not generally alter the functional or chemical properties of the cell envelope. In the electron microscope, mutant cells appeared ovoid, but individual cells showed no surface irregularities to suggest gross defects in the cell envelope. These observations suggest that the primary effect of the mutations is to alter selectively the synthesis or translocation of certain envelope proteins.

Role of positive charge on the amino-terminal region of the signal peptide in protein secretion across the membrane

The positively charged amino-terminal region of the signal peptide has been proposed to have an important role at an initial step of protein secretion across the membrane (loop model). To test this hypothesis, the charge on the amino-terminal region of the signal peptide of the prolipoprotein of the Escherichia coli outer membrane was altered by using synthetic oligonucleotides from +2 to +1, 0, and -1 by guided site specific mutagenesis of a plasmid DNA carrying an inducible lipoprotein gene. The wild-type sequence of this sectio, Met-Lys-Ala-Thr-Lys (+2), was thus changed to Met-Lys-Asp-Thr-Lys (I-1; +1), Met-Ala-Thr-Lys (I-2; +1), Met-Asp-Thr-Lys (I-3; 0), and Met-Glu-Asp-Thr-Lys (I-4; -1). After induction of lipoprotein production, cells were pulse labeled with [35S]methionine for 10 sec. The lipoprotein of I-1, I-2, and I-3 was assembled in the membrane, although the rates of lipoprotein production progressively decreased as the charge on the signal peptide became more negative. Conversely, in the case of I-4, only a small amount of lipoprotein assembled in the membrane while a large amount of glycerol-unmodified prolipoprotein accumulated in the cytoplasm. This soluble prolipoprotein was gradually and posttranslationally secreted across the membrane to be modified and assembled in the membrane. These results indicate that the positively charged amino-terminal region of the signal peptide plays an important role in efficient protein secretion across the membrane.

Characterization of a new murein-associated lipoprotein in the outer membrane of Proteus mirabilis

A murein-associated outer membrane protein from Proteus mirabilis has been isolated. Since the protein carries ester- as well as amide-linked fatty acids it can be classified as a second outer membrane lipoprotein. An apparent molecular weight of 15,000 for this protein was determined from amino acid analysis and sodium dodecylsulfate/polyacrylamide gel electrophoresis. The amino acid composition, however, does not show similarities with the amino acid composition of the lipoprotein covalently linked to murein, which has a molecular weight of 7,300 as described previously in Proteus mirabilis.

Identification of different forms of the murein-bound lipoprotein found in isolated outer membranes of Escherichia coli

The identification of the free and murein-bound forms of the Escherichia coli lipoprotein on dodecylsulphate-polyacrylamide gels was systematically investigated by analyzing the low-molecular-weight proteins (Mr less than 20 000) of both cytoplasmic and outer membranes. The free form of the lipoprotein was identified on 15% polyacrylamide gels as the fastest migrating component (Mr = 7200-7500) of isolated outer membranes; it could be separated from a small cytoplasmic membrane protein (Mr = 6500) which was probably identical to the dicyclohexylcarbodiimide binding proteolipid of the membrane-bound ATPase. Lysozyme treatment of both outer membranes and murein sacculi failed to convert the murein-bound lipoprotein into a fragment of uniform size; instead the bound form appeared as a series of bands consisting of lipoprotein bound to one, two,...eight murein subunits. The composition of this ladder depended on the method used to isolate outer membranes. Beside these lipoprotein bands the outer membrane contained two other proteins, III and V; the relation of these proteins to previously described proteins is discussed.

Mitogenicity of a lipid-deficient lipoprotein from a mutant Escherichia coli strain

From the mutant bacterial strain Escherichia coli JE5511 lpp lpm, muropeptide-containing and muropeptide-free lipoproteins were prepared. By gas chromatography and by infrared spectroscopy we showed that the products were deficient in the two ester-bound N-terminal fatty acids, but still carried the amide-linked fatty acid. Mutant lipoproteins were tested for mitogenicity in lipopolysaccharide nonresponder C3H/HeJ mice by incorporation of [3H]thymidine and [3H]uridine and by hemolytic plaque assays for immunoglobulin-secreting plasma cells. Our results showed that the mutant lipoproteins still exhibited marked mitogenicity toward mouse B-lymphocytes, although the activity of the products was reduced in comparison to the wild-type lipoprotein. Thus, the presence of one fatty acid in the N-terminal part of lipoprotein is sufficient to bring about mitogenicity.